This PDF file contains the front matter associated with SPIE Proceedings Volume 10926, including the Title Page, Copyright information, Table of Contents, Introduction, Author and Conference Committee lists

Accurate measurement of time and position has been based on the interrogation of atoms, leading to exquisite lab-based systems with increasingly better performance. Practical measurements in portable systems have been less successful. We will discuss progress towards tactical atomic physics that will have a revolutionary impact for many military and commercial applications.

The future acceleration of many computational workloads is expected to depend on novel architectures, circuits, and devices. We describe an effort utilizing the analog and non-volatile nature of memristor crossbar arrays to accelerate matrix operations, which underpin many applications in image and signal processing, neural networks, and scientific computations. Significant performance gains and energy reductions over purely digital systems are forecasted based on our work. We describe our studies in the understanding of tantalum oxide-based memristors, integration with CMOS circuits, fine programming control over memristors, compact SPICE models of the device dynamics, and experimental implementations of matrix-heavy operations, including machine learning. We also describe applications that take advantage of the neuron-like behavior in memristor devices, in addition to the above synapse-like functions. This work begins with investigations of niobium oxide-based systems, and interesting dynamics observed based on positive feedback coupling to the electronic transport. These devices are utilized in the construction of a computing system to solve optimization problems such as the traveling salesmen problem.

Photonic techniques emulating the brain’s powerful computational capabilities are attracting considerable research interest as these offer promise for ultrafast operation speeds. In this talk we will review our approaches for ultrafast photonic neuronal models based upon Semiconductor Lasers, the very same devices used to transmit internet data traffic over fiber-optic telecommunication networks. We will show that a wide range of neuronal computational features, including spike activation, spiking inhibition, bursting, etc., can be optically reproduced with these devices in a controllable and reproducible way at sub-nanosecond time scales (up to 9 orders of magnitude faster than the millisecond timescales of biological neurons). Moreover, all our results are obtained using off-the-shelf, inexpensive Vertical-Cavity Surface Emitting Lasers (VCSELs) emitting at 1310 nm and 1550 nm; hence making our approach fully compatible with current optical communication technologies. Further, we will also introduce our recent work demonstrating the successful communication of sub-nanosecond spiking signals between interconnected artificial VCSEL-based photonic neurons and the potential of these systems for the ultrafast emulation of basic cortical neuronal circuits. These early results offer great prospects for novel neuromorphic (brain-like) photonic networks for brain-inspired ultrafast information processing systems going beyond traditional digital computing platforms.

Heart attacks and strokes are the leading causes of death in USA. Research indicates that high blood viscosity and turbulence in blood circulation are the keys to trigger these vascular diseases. High blood viscosity and turbulence place much heavier work load on the heart. Turbulence in blood circulation can further rapture blood vessels and develop atherosclerotic plaque. Therefore, reducing blood viscosity and suppressing turbulence is the key to prevent heart attacks and strokes. On the other hand, these two tasks conflict each other. Presently, the only method to reduce the blood viscosity is to take medicine, such as aspirin. However, besides of heavy side effect, using medicine to reduce the blood viscosity only makes the turbulence worse because the Reynolds number goes up with the viscosity reduction. Moreover, neither medicine nor method is available presently to suppress turbulence in blood circulation. Here we report our new discovery with magnetorheology (MR): application of a strong magnetic field to blood along its flow direction, red blood cells are polarized in the magnetic field and aggregated into short chains along the flow direction. The blood viscosity becomes anisotropic: Along the flow direction the viscosity is significantly reduced, but in the directions perpendicular to the flow the viscosity is considerably increased. In this way, the blood flow becomes laminar, turbulence is suppressed, the blood circulation is greatly improved, and the risk for heart attacks is reduced. While these effects are not permanent, they last for about 24 hours after one magnetic therapy treatment. Our lab experiments and tests with mice confirm that this technology can successfully prevent development of atherosclerotic plaque. Our clinical trials further confirm that this MR technology can effectively cure hypertension for human and help people to prevent heart attack and stroke.

A non-invasive blood glucose measurement method using quantum cascade lasers (QCLs) oscillated in mid-infrared region is introduced. A mid-infrared spectroscopy system equipped with a combination of QCLs as light sources and hollow-optical fibers as beam delivery media is developed. As a wavelength of QCL, the wavenumber of 1152 cm^-1 that exhibited strong correlation with blood glucose in the optical absorption of human lips is firstly chosen. Then 1186 cm^-1 where glucose shows no absorption is also chosen as a reference to remove effects of the baseline variation. By using the QCL-based system, it is shown that the differential absorption between these wavelengths exhibits high correlation with blood glucose level.

Advanced optoelectronic systems that intimately integrate with the brain and the peripheral nervous system have the potential to accelerate progress in neuroscience research and to spawn new therapies in clinical medicine. Specifically, capabilities for injecting electronics, light sources, photodetectors, multiplexed sensors, programmable microfluidic networks and other components into precise locations of the deep brain and for softly laminating them onto targeted regions of the surfaces of the brain or the peripheral nerves will open up unique and important opportunities in stimulation, inhibition and real-time monitoring of neural circuits. In this talk, we will describe foundational concepts in materials science and assembly processes for these types of technologies.

Hydrocarbon detection in the gas phase can be a powerful tool to guide downstream operations for the oil and gas industry. This application requires highly sensitive, selective and robust spectroscopic techniques. In this work we present: i) a quartz-enhanced photoacoustic (QEPAS) sensor that can individually detect methane and ethane in the part per billion range and propane in the ppm range by employing a single interband cascade laser emitting at 3345 nm; ii) a QEPAS sensor detecting ¹²CH₄ and ¹³CH₄ isotopes at the part-per billion sensitivity level, by employing a quantum cascade laser emitting at 7730 nm.

We report on the performance of new quartz tuning fork (QTF) designs optimized for quartz-enhanced photoacoustic spectroscopy (QEPAS). We investigated the impact on resonance properties of prong geometries differing from the standard rectangular one. We proposed a QTF with T-shaped prongs and a QTF with prongs having rectangular grooves carved on the surface. QTFs were implemented in a QEPAS sensor and performances were compared in terms of signalto-noise ratio (SNR). Then, QTFs were acoustically coupled with single- and dual-tube micro-resonator systems. A record x60 SNR enhancement factor with respect to the bare QTF was achieved with QTF having T-shaped prongs.

We have successfully fabricated extended SWIR photodetectors with the cutoff wavelength of 2.5 μm by using InGaAs (-0.3 %)/GaAsSb(+0.3 %) strain compensated type-II quantum wells as an absorption layer. The 250-pair InGaAs/GaAsSb quantum wells were grown on an InP substrate by metal organic vapor phase epitaxy. The p-n junction was formed in the absorption layer by selective zinc diffusion. Dark current was low and showed diffusion current limited mode. Quantum efficiency in the wavelength region between 2.0 μm and 2.5 μm which corresponds to the type-II absorption became twice as high as that of the normal lattice-matched InGaAs/GaAsSb type-II quantum wells.

In this work, the electronic band structure of the InAs/GaSb superlattice (SL) is calculated using a commercial 8-band k⸳p solver and the electrical performance of longwave nBp device structure evaluated with Atlas from Silvaco software. By taking into account an InSb interface layer and the interface matrix (formulated by P.C. Klisptein), the model can predict the measured energy band gap of different InAs/GaSb SLs having different period composition and thickness (7/4, 10/4, 12/4, 14/4 and 14/7 SLs) within an error corresponding to the ±𝑘_𝐵𝑇 deviation range. The effective mass is then extracted from the electronic band structure calculation and discussed for numerous SL designs. In particular, we compare a 14/7 SL and a 12/2 SL having an energy band gap equal to 0.122 and 0.118 eV at 77K, respectively. The electron-hole wavefunction overlap for the 12/2 SL has been estimated to be ~74%, which is almost twice the value calculated for the 14/7 SL (~40%). This arises from the delocalization of carriers in a thinner SL period also leading to smaller carrier effective masses in the 12/2 SL. The dark-current of a nBp structure has been calculated for both SLs. For the 14/7 SL, the dark-current level has been found to be higher by a factor of over 3 than the Rule07 benchmark, whereas for the 12/2 SL, it is lower by a factor of 0.77, demonstrating that the SL design can be used to improve the device performances.

Type-II superlattices (T2SLs) have several fundamental advantages over bulk infrared-sensitive materials due to larger band edge effective masses and the ability to have their band structures engineered to suppress Auger recombination, leading to lowering tunneling currents, longer carrier lifetimes and higher ideal sensitivity. Realizing in practice the potential performance gains relies heavily on reducing the number or efficacy of defects that form Shockley-Read-Hall (SRH) recombination centers, which otherwise limit carrier lifetimes. InAs/GaInSb T2SLs typically have relatively short minority carrier lifetimes in comparison with bulk HgCdTe, which has limited the detectivities of photodetectors based on these T2SLs at both cryogenic and ambient operating temperatures. Studies have shown that InAs/InAsSb T2SLs lattice matched to GaSb substrates are comparable in ideal photodiode performance to InAs/GaInSb ones. Reducing the electrical activity of defects by passivating them with hydrogen is equivalent to lowering their density, and has proven successful in other semiconductor systems. We report here results from Ga-free and Ga-containing T2SLs exposed to inductively-coupled plasmas (ICPs). Our technical approach consisted of characterizing the basic material properties of LWIR InAs/InAsSb T2SL wafers and device performance of LWIR InAs/GaSb T2SL photodiodes that were bulk-passivated with atomic hydrogen, and comparing with unpassivated samples. On average, the in-plane Hall electron mobility increased from 1800 cm2/Vs to 6800 cm²/Vs after hydrogenation. ICP hydrogenation also improved the minority carrier lifetime for each of the explored ICP conditions. Lifetime values increased from an average of 80 ns before hydrogenation to almost 200 ns, a relative increase of over 200%, suggest that some recombination-mediating defects have been at least partially passivated. The Hall mobility improvements were found to be rather stable over the considered short periods of room temperature storage.

To date, virtually all techniques used to image the thermal properties of 2D materials and thin films at the nanoscale have required to position the sample in contact with probes that act as undesirable thermal sinks and dramatically affect the measurements. Thermoreflectivity, an optical technique in which thermal transport properties are measured by contactlessly probing the heat-induced changes in reflectivity at the air-sample interface, has been utilized to image and map the thermal conductivity of solids at the macroscopic and microscopic level, but, so far, has been diffraction-limited in its applicability at the nanoscale. In this paper, we show how our group has tackled such an issue by coupling thermoreflectivity mapping with near-field scanning optical microscopy (NSOM) in a pump-probe nano-optical technique [Nanoscale 9 (2017) 4097]. We show that our technique is successful in investigating the local impact on the thermal conductivity of edges and wrinkles of non-ideal domains of 2D materials. Further on, we investigate the thermal properties of a graphene thin film decorated with copper particles and demonstrate that contactless near-field scanning thermoreflectance imaging can map the electron-phonon coupling in graphene-based nanocomposites.

The combination of the AFM technique and the sphere-mediated microscopy (SMM) opens a new opportunity to the Atomic Force Microscopy (AFM). With the help of a tipless AFM cantilever is possible to place and scan a microspheres (MS) close to the surface. From the optical point of view, when a MS is close to a surface act as high NA nanolenses whose optical characteristics define the maximum attainable resolution. By using the stages of a standard AFM, the microsphere can easily scan over the surface. The deflection of the cantilever could still be used to control the distance between microsphere and sample. With an optical detector and a low N.A. objective is so possible to obtain optical high resolution maps synchronized with the topography ones. Despite microspheres do not to break the Abbe diffraction limit or produce super-resolution, they can be used as portable and cheap optical elements that can enhance the effective NA of a system. A systematic optical characterization of the system will be presented in parallel with some preliminary results of forthcoming applications of SMM in nanolithography, micro or nano Raman spectroscopy and Near Filed Optical Microscopy (SNOM).

Interference with or without entanglement has been recognized as a key resource for quantum computing and quantum communications systems, as for example discussed by Nielsen and Chuang¹ and numerous other works. Multiple paths between sources and detectors require an understanding of the underpinning wave-particle duality issue in the interference effects. Recently a new axiom (particle and its wave function φ(r, t) cannot be coincident or co-located at space-time point (r_k, t_k) unless φ(r, t) = δ(r-r_k, t-t_k) the Dirac delta function) has been suggested² and justified, which explains duality without Niels Bohr’s complementarity principle, thus eliminating the role of the observer, avoiding complicated “which way” (welcher-weg) considerations and observer subjectivity. This greatly simplifies analysis and design of multi-path quantum systems and restores objectivity. The same paper also suggested in the context of entanglement new concepts of (a) “total causality” that includes entanglement as a cause to locally and causally explain “action at a distance”, and (b) “partial causality” that excludes entanglement as a cause and thereby introduces the perception of strange phenomena of non-locality, retro-causality and quantum erasure, which are nevertheless very important. This paper reviews and then applies the axiom to bring much needed clarity to certain confusing and much debated aspects of developments in non-interaction measurements, counterfactual communications and quantum computers. These potential clarifications and simplifications of analysis and design of multi path systems may help developers of future quantum communication and quantum computer systems.

Working within the framework of the classical theory of electrodynamics, we derive an exact mathematical solution to the problem of self-force (or radiation reaction) of an accelerated point-charge traveling in free space. In addition to deriving relativistic expressions for self electromagnetic fields, we obtain exact formulas for the rates of radiated energy and linear momentum without the need to renormalize the particle’s mass – or to discard undesirable infinities. The relativistic expression of self-force known as the Abraham- Lorentz-Dirac equation is derived in two different ways. Certain properties of the self-force are examined, and an approximate formula for the self-force, first proposed by Landau and Lifshitz, is discussed in some detail.

Systems with the ability to observe and manipulate individual quantum states have been brought to applications that include among others satellite-free navigation and high-precision gravimetric sensing. Fundamentally, the applicability of quantum technology is limited by the complexity and financial burden of light sources required for such systems. These sources need to feature high optical power combined with compromised beam quality and frequency-stabilized narrow-linewidths. These parameters directly influence the performance of the quantum technology measurement system. Semiconductor devices are able to provide high brightness over broad spectral regions through band-gap engineering. InGaN-based laser sources can be engineered to operate from 380nm to 530 nm. This aligns well with the transitions of atomic species such as strontium, magnesium and ytterbium. However, a challenge remains to offer the narrow-linewidths (<1 MHz) and the high powers (>100 mW) required for many of these applications. We will present our development of GaN based narrow-linewidth seed and tapered amplifiers to operate at 461nm for first stage strontium cooling. This includes growth of custom optimised GaN epitaxy for operation at 461 nm, a robust ECDL geometry, a novel tapered amplifier design and important work in characterising and minimising the surface reflectivity to identify suitable working parameters. A comprehensive characterization of the device will be presented.

Quantum technologies containing key GaN laser components will enable a new generation of precision sensors, optical atomic clocks and secure communication systems for many applications such as next generation navigation, gravity mapping and timing since the AlGaInN material system allows for laser diodes to be fabricated over a wide range of wavelengths from the u.v. to the visible. We report our latest results on a range of AlGaInN diode-lasers targeted to meet the linewidth, wavelength and power requirements suitable for optical clocks and cold-atom interferometry systems. This includes the [5s²S_1/2-5p²P_1/2] cooling transition in strontium⁺ ion optical clocks at 422 nm, the [5s₂¹S₀-5p¹P1] cooling transition in neutral strontium clocks at 461 nm and the [5s²s_1/2-6p²P_3/2] transition in rubidium at 420 nm. Several approaches are taken to achieve the required linewidth, wavelength and power, including an extended cavity laser diode (ECLD) system and an on-chip grating, distributed feedback (DFB) GaN laser diode.

Sensors are measuring tools. In any measurement, we have at least two different kinds of interactants. We never know all there are to know about any one of these interactants and the interaction processes that are mostly invisible. Yet, our engineering innovation driven evolution is persisting for over five million years. It is then important to articulate explicitly our Interaction Process Mapping Thinking (IPM-T) that we keep applying in the real world without formally recognizing it. We present how the systematic application of IPM-T removes century old wave-particle duality by introducing a model of hybrid photon. It seamlessly bridges the quantum and the classical worlds. Photons are discrete energy packets only at the moment of emission; then they evolve diffractively and propagate as classical waves. Thus, "interference of a single indivisible photon" is only a non-causal assertion. We apply IPM-T to improve the photoelectric equation and we obtain Non-Interaction of Wave (NIW) amplitudes. Note that Huygens explicitly articulated NIW by postulating his "secondary spherical wavelets." Later Fresnel incorporated this postulate in the now famous Huygens- Fresnel (HF) diffraction integral. Most modern optical science and engineering are based upon propagating EM waves through optical devices and systems using this integral in some form or other. Maxwell’s wave equation accepts HF integral as its solution. Systematic application of IPM-T to our causal and working mathematical equations, along with NIW in interferometric experiments, reveal that Superposition Effects can emerge only when the interacting material dipoles respond, whether classically or quantum mechanically, to the joint stimulations due to all the simultaneously superposed waves. This indicates the non-causality of our belief that a single indivisible photon can interfere by itself. We would not have a causally evolving universe had any stable elementary particle were to change itself through selfinterference. Further, our working superposition equations always contain two or more terms representing two or more independently evolving entities. That is why we need physical instruments with two or more independent channels of propagation along with appropriately placed detectors to generate physical superposition effects. Nature does not violate causality. Otherwise, our causally framed equations would not have been working so elegantly.

Currently, there is no SWIR camera that can detect a single photon above cryogenic temperatures. We present the latest results of our Electron Injection (EI) cameras, which show a clear path toward achieving such a formidable goal. We present a fundamental relation between the capacitance of EI injector and its sensitivity to photons, and show experimental results that strongly support it. We also demonstrate the weak temperature dependency of the EI detectors, which allows operation with compact thermoelectric coolers. We also present our latest results for ultra-fast SWIR modulators, as fast optical shutter, and their applications in time of flight (ToF) SWIR 3D imaging. Our results show 3D imaging at video rates, with an unprecedented depth resolution due to the low energy consumption in the modulator. Our measurements show our new SWIR modulators consume only ~1 femto-Joule/bit, and with a timing accuracy of ~4 pico-seconds.

Deep learning is a class of machine learning techniques that uses multi-layered artificial neural networks for automated analysis of signals or data. The name comes from the general structure of deep neural networks, which consist of several layers of artificial neurons, each performing a nonlinear operation, stacked over each other. Beyond its main stream applications such as the recognition and labeling of specific features in images, deep learning holds numerous opportunities for revolutionizing image formation, reconstruction and sensing fields. In this presentation, I will provide an overview of some of our recent work on the use of deep neural networks in advancing computational microscopy and sensing systems, also covering their biomedical applications.

A fast and reliable three dimensional monitoring of the environment is indispensable for robotics, automation engineering or autonomous driving. For these applications LiDAR is a key sensor technology. Normally a light source in the near infrared range is used, which is invisible to human eyes. High ambient light compared to the laser source intensity is a major problem for these systems. Therefore, a measurement concept to reduce the impact of ambient light is necessary. In this paper we present a measurement concept in which the full distance range is scanned and the probability to detect events from far objects is improved. The general problem is that a photon of the background illumination can be detected instead of the reflected laser signal which stops the measurement. The concept allows us to detect the received laser pulse buried in the superimposed background light easier and improve the measurement quality. This is possible due to the delayed start of the measurement and thus the selection of different measurement windows in which an earlier detection of the laser generated events is accessible. In consequence, the probability for receiving an unwanted ambient photon is reduced. For this technique no prior information about the object conditions or its rough distance is required and it can be applied in all situations of the direct time-of-flight measurement to cope with high ambient light. Hence it allows a reliable distance measurement at various ambient and target conditions.

High-dynamic-range imaging enables recording scenes with both very dark and bright subscenes. When coupled with photon-counting, the highest signal-to-noise ratio is obtained. Solid-state photon-counting can be implemented either with single-photon avalanche diodes (SPADs), or with conventional CMOS image sensors featuring high conversion gain and low readout noise. In SPAD-based imagers, the pixel dead time (Tdead) limits the dynamic range to the maximum counting rate, i.e. 1/Tdead, as well as a nonlinear photon response. Conventional CMOS image sensors with photon-counting capability are limited in dynamic range due to low full-well capacity, while oversampling in time and/or space (like in the quanta image sensor approach) increases the readout noise, thereby deteriorating the photon-counting capability. We present a quantitative analysis on how to use the SPAD photon response nonlinearity and count saturation to actually extend the optical dynamic range far beyond 1/Tdead. Theory and simulations are compared to measurements of the photon response, standard deviation and signal-to-noise ratio for different SPAD recharging (or resetting in CMOS imagers) mechanisms. We also quantify the decrease in signal-to-noise ratio when applying linearization corrections. Results show that by applying active clock recharge, one can extend the optical dynamic range by a factor of 2.8 over 1/Tdead, and by more than 16× over 1/Tdead with active event-driven recharge. It has to be noted that this methodology can be applied to any photon-counting array. Further, we discuss high-resolution image sensor architectures enabling photon counting with single exposure >120dB dynamic range.

Non-mechanical beam steering for optical systems has been important for decades, but with the dramatic increase in commercial applications, such as the driverless car, it is becoming even more important. In this paper, three decades of progress in optical beam steering is discussed, along with a comparison of approaches being developed. Non-mechanical optical beam steering is compared against mechanical steering approaches. Unique space fed beam steering approaches are discussed using different active electro optical media. Some electro optical beam steering systems are explained in detail and some methods to improve the efficiency are presented.

This manuscript describes some of the new advances in active mid-infrared photonic integrated circuits enabled by new quantum cascade laser technologies. This includes monolithic beam steering which was achieved via the integration of a widely tunable QCL and a tapered grating outcoupler. A record 17.9 degrees of steering with a low divergence beam (0.5 degrees) was achieved. In addition, the use of surface emitting architectures is proposed as a means to reduce the manufacturing cost of next-generation QCLs. A reflective outcoupler is demonstrated which can allow for stable surface emission from a quantum cascade laser and has potential for cost-effective wafer-scale manufacturing. This outcoupler is integrated with an amplified, electrically tunable laser architecture to demonstrate high power surface emission at a wavelength near 4.9 μm. Single mode peak power up to 6.7 W is demonstrated with >6 W available over a 90 cm^-1 (215 nm) spectral range. All of this is achieved while maintaining a high quality output beam, similar to a standard edge emitter.

Quantum Cascade Lasers (QCL) are highly efficient, compact, and wavelength-agile devices in Mid-Wavelength InfraRed (MWIR – generally 3.7 – 4.8 microns wavelength) and Long-Wavelength InfraRed (LWIR – generally 8 – 12 microns wavelength). Wall-plug efficiencies have been shown to be above 20% [1,2]. which make them highly desirable for any application where SWaP (Size, Weight, and Power) are critical to fielded systems. Multiple applications exist for both pulsed and continuous-wave (CW) format QCLs in communications, remote sensing, and electronic warfare. Specifically, there are few competitive comparable alternatives in CW to these products. They also are capable of operation close to the diffraction limit, making them practical as long-range devices beyond that of a laboratory device. Single QCL emitter CW power levels tend to be limited to around 1 W in the lower part of the MWIR and in LWIR and higher in the upper part of the MWIR. Under a Navy Small Business Innovative Research contract and funding from the Office of Naval Research, Naval Research Laboratory (NRL) has worked with Forward Photonics, LLC to increase power, improve spectral output, and reduce SWaP for these devices. Among the technologies explored have been spectral beam combining [3-6], polarization beam combining, spectral tailoring, and innovative ideas in laser packaging. As a result, QCL-based laser systems can be scaled in power and efficiently packaged to deliver adequate power on target at significant ranges. Both air-cooled and water-cooled versions of higher power devices in MWIR have been successfully field tested by NRL. Plans are underway to duplicate this success in LWIR and to further reduce SWaP and cost for practical, affordable, production devices.

This work was funded under a grant from the Office of Naval Research Code 312.

The combination of broadly tunable quantum cascade laser chips in an external cavity (EC-QCL) with a micro- electromechanical system (MEMS) scanner with integrated diffraction grating as wavelength-selective element allows for the development of extremely compact and robust spectroscopy systems. Resonant MOEMS grating scanners enable spectral tuning rates of hundreds of wavenumbers per millisecond and consequently broad-band spectroscopy with millisecond temporal resolution. Also non-resonant (quasi-static) MOEMS grating scanners are possible, providing scan rates of tens of Hz as well as static setting of arbitrary wavelengths, as common for mechanically driven EC lasers, while keeping the small MOEMS footprint, ruggedness, and low power consumption. Here, we give a progress report on the latest developments on MOEMS-based EC-QCLs made by Fraunhofer IAF and IPMS. We will highlight two of our latest developments: A non-resonant MOEMS EC-QCL version that allows arbitrary scan frequencies up to few ten Hertz, as well as static operation. Furthermore, we present the application of a resonantly driven cw-MOEMS-EC-QCL with cavity-length control to enable fast high-resolution spectroscopy over a spectral range of >100 cm^-1, offering new possibilities for spectroscopy on complex gas mixtures.

Spin-polarized lasers such as spin-polarized vertical-cavity surface-emitting laser (spin-VCSELs) are prospective devices in which the radiative recombination of spin-polarized carriers results in an emission of circularly-polarized photons. Nevertheless, additional linear in-plane anisotropies in the cavity generally lead in preferential linearlypolarized laser emission and to possible coupling between modes. Optimization of room-temperature spinVCSELs thus relies on a proper modeling method and on a good understanding of these anisotropies that may reveal (i) a local linear birefringence due to strain fields at the surface or (ii) a birefringence in quantum wells (QWs) due to phase-amplitude coupling originating from the reduction of the biaxial D_2d to the C_2v symmetry group at the III-V ternary semiconductor interfaces. We present a novel method for the modeling of steady-state and dynamical properties of generally anisotropic multilayer semiconductor lasers containing multiple QWs active region. In order to solve the dynamical properties of spin-VCSELs, we combine here optical Bloch equations for a 4-level system with the scattering-matrix formalism, which treats VCSELs as a multilayer structure containing classical active dipole layers [T. F¨ord¨os et al., Phys. Rev. A 96, 043828 (2017)]. The method is then demonstrated on real semiconductor laser structures with InGaAs/GaAsP quantum wells. It is used for calculation of the laser resonance condition, the polarization properties of eigenmodes, the electromagnetic-field distribution inside the laser cavity, and time-dependent properties of the emitted light.

Free space optical (FSO) communications are a potential application envisioned for Quantum Cascade Lasers technology. In this paper, we sketch out the main market scenarii for FSO communications, where data channels should reach up to Tbit/s over distances that range from 0.1 to 36000 km. We analyze where QCLs can be relevant, and what standards they must meet to be relevant at the industrial level. The following topics are discussed: competing technologies, atmospheric transmission physics, signal processing and multiplexing. We try and translate the constraints of the system-level into challenges for device-level research and development.

Sb-based materials rely on the GaSb, InAs, AlSb, InSb binary compounds and their quaternary or pentanary alloys (AlGaAsSb, GaInAsSb, AlGaInAsSb,.. ). This technology exhibits several distinctive properties as compared to other semiconductors: type-I to type-III band alignments, giant band offsets, low effective masses of electrons and holes, direct bandgaps between 0.15 and 1.7 eV. Conventional laser diodes (LDs) rely essentially on GaInAsSb type-I quantum wells (QWs) confined by AlGa(In)AsSb barrier layers. Low threshold currents and high T0 have been demonstrated between 1.5 and 3.4 µm. The AlGaInAsSb pentanary barrier is needed to extend the wavelength beyond 3 µm while keeping a type-I band alignment [3] even though it makes the epitaxial growth complex. Single mode operation has been achieved with both DFB lasers and VCSELs using the same active zone. At longer wavelength, interband cascade lasers (ICLs) based on GaInSb/InAs type-II p-n junctions stacked in series exhibit room temperature cw emission between 3.5 and 5 µm, including single mode operation of DFB lasers. At still longer wavelength InAs/AlSb quantum cascade lasers (QCLs) benefit from the low InAs effective mass and giant conduction band offset. High performance have been demonstrated all the way from 2.6 µm up to 25 µm, particularly at long wavelength which is an asset of this technology. The evolution toward smart, integrated, sensors requires integrating III-V optoelectronic devices with Si-based platforms. The epitaxial growth of III-V compounds on Si has thus been the focus of renewed attention for about a decade now. We have shown that the Si substrate preparation and the III-Sb nucleation on Si are crucial steps. This allowed us demonstrating a variety of epitaxially integrated optoelectronic devices such as laser diodes, photodetectors and the first ever QCL grown on Si. In this presentation we review the recent results obtained on the integration of antimonide-based QCLs epitaxially grown on Si substrates. We will show that this technology is very attractive for future III-V on Si integration, and we will discuss future integration schemes.

Quantum cascade lasers (QCLs) are optical sources exploiting radiative intersubband transitions within the conduction band of semiconductor heterostructures.¹ The opportunity given by the broad span of wavelengths that QCLs can achieve, from mid-infrared to terahertz, leads to a wide number of applications such as absorption spectroscopy, optical countermeasures and free-space communications requiring stable single-mode operation with a narrow linewidth and high output power.² One of the parameters of paramount importance for studying the high-speed and nonlinear dynamical properties of QCLs is the linewidth enhancement factor (LEF). The LEF quantifies the coupling between the gain and the refractive index of the QCL or, in a similar manner, the coupling between the phase and the amplitude of the electrical field.³ Prior work focused on experimental studies of the LEF for pump currents above threshold but without exceeding 12% of the threshold current at 283K⁴ and 56% of the threshold current at 82K.5 In this work, we use the Hakki-Paoli method6 to retrieve the LEF for current biases below threshold. We complement our findings using the self-mixing interferometry technique⁵ to obtain LEFs for current biases up to more than 100% of the threshold current. These insets are meaningful to understand the behavior of QCLs, which exhibit a strongly temperature sensitive chaotic bubble when subject to external optical feedback.⁷

Novel optical sensors the most often require thin films or surface structures with strictly controlled properties, playing a critical role in them by initiating or modifying their sensorial responses. Selected results of research on atomic layer deposited (ALD) metallic oxides will be shown, regarding their applicability for thin functional coatings in lossy mode resonance (LMR) and long period grating (LPG) optical fiber sensors. Basically amorphous films of tantalum oxide (TaxOy), zirconium oxide (ZrxOy) and hafnium oxide (HfxOy) below 200 nm were deposited at relatively low temperature (LT) of 100°C. The optical, structural, topographical, tribological, hydrophilic and chemical stability properties of the films and their technological controllability were analysed. The TaxOy was selected and successfully applied as an oxide coating in LPG sensor. As chemically robust in alkali environment (pH over 9) it allowed to gain a potential for fabrication of regenerable/reusable biosensor. Additionally, ALD technique was tested as a tool for tailoring sensorial properties of LMR sensors. The double-layer coatings composed of two different materials were experimentally tested for the first time; the coatings were composed of plasma-enhanced chemical vapour deposited (PECVD) silicon nitride (SixNy) followed by much thinner ALD TaxOy. That approach yielded operating devices, ensuring fast overlay fabrication and easy tuning of the resonant wavelength at the same time. The LT ALD TaxOy films turned out to be slightly overstoichiometric (y/x approx. 2.75). Therefore, the issue of TaxOy chemical composition was studied by secondary ion mass spectroscopy, Rutherford backscattering spectrometry and x-ray photoelectron spectrometry.

We present an innovative approach for the growth of crystalline silicon on GaAs using plasma-enhanced chemical vapor deposition (PECVD). In this process the substrate is kept at low temperature (175 °C) and epitaxial growth is obtained via the impact of charged silicon clusters which are accelerated towards the substrate by the plasma-potential and melt upon impact. Therefore, this is a nanometer size epitaxial process where the local temperature (nm scale) rises above the melting temperature of silicon for extremely short times (in the range from ps to ns). This allows obtaining epitaxial growth even on relatively rough GaAs films, which have been cleaned in-situ using a SiF₄ plasma etching. We present in-plane X-Ray Diffraction (XRD) measurements which are consistent with the hypothesis that the epitaxial growth happens at a local high temperature. Indeed, the tetragonal structure observed and the low in-plane lattice parameter determined from XRD can only be explained by the thermal mismatch induced by a high growth temperature. The effect of the plasma on the underlying GaAs properties, in particular the formation of hydrogen complexes with GaAs dopants (C, Si, Te) is studied in view of the integration of the c-Si epi-layers into devices.

This paper reviews recent development of two-inch gallium nitride (GaN) substrates fabricated by the near equilibrium ammonothermal (NEAT) method. The NEAT method utilizes a low driving force to achieve consistent crystal growth over a long period of time (> 90 days) while maintaining high quality microstructure. Through refinement of growth conditions for the NEAT method and proper preparation of seed crystals we have achieved 2" GaN substrates with excellent microstructure. Currently, 2" GaN wafers sliced from bulk GaN crystals typically have a full width half maximum (FWHM) of the 002 X-ray rocking curve of 50 arcsec or less, a dislocation density of mid-10⁵ cm^-2 or less, and an electron density of about 2 x 10¹⁹ cm^-3. The high electron density is attributed to an oxygen impurity in the crystal. Due to high oxygen concentration, GaN crystal grown in the ammonothermal method tends to show a brownish color. Through process refinement, we successfully reduced oxygen concentration to 7 x 10¹⁸ cm^-3, which resulted in optical absorption coefficient of 5.6 cm^-1 at 450 nm. This progress ensures feasibility of the NEAT method for producing GaN wafers usable for various optoelectronic devices, power transistors and high-frequency transistors.

The use of photo-mixing techniques for THz emission offers attractive performances such as tunability and modulation bandwidth, that are suitable for bio-medical sensing and imaging, communications, or security. We will present the state-of-the-art performances of a vertical-external-cavity surface-emitting laser that operates on two transverse modes to ensure a stable continuous-wave and coherent (longitudinal, transverse and polarization) dual-frequency operation. THz emission is subsequently obtained by excitation of an uni-traveling-carrier photodiode (UTC-PD). The stability of the dual-frequency operation is achieved thanks to different types of functionalized surfaces involving the micro-fabrication of integrated III-V absorbing metallic masks or metamaterial phase masks by e-beam lithography. These functionalized surfaces allow to shape the optical and THz performances in terms of power, tunability and coherence. The latter will be specifically detailed in terms of longitudinal coherence, showing a THz frequency noise that is orders of magnitude lower than the optical one thanks to a significant correlation of technical noise. Tunable emission will be demonstrated from 50 GHz up to few THz with a linewidth of 150 kHz (during 3-ms), for a power of 1 W at 260 GHz that is limited by the UTC-PD for an optical excitation at 1064 nm at room temperature. We will discuss on the possibility to improve such a power significantly by taking advantage of the involved high-order transverse mode, offering possible intrinsically coherent networks of photo-emitters, thus paving the way to compact and agile coherent THz sources offering an output power over few mWs at frequencies of 100s of GHz.

Despite their ultrafast dynamics, modelocking of THz QCLs has been shown recently as a robust method to generate short THz pulses with pulse trains as short as ~4ps at a central frequency of ~2.5THz. This opens up applications of QCLs to synthesizer controlled frequency combs as well as operation of QCLs in more exotic modelocked regimes. In this work we demonstrate for the first time harmonic active modelocking of QCLs where pulses are generated at a harmonic of the round trip frequency. This is achieved by active modelocking at the second harmonic of the QCL fundamental round trip frequency, generating a short pulse train with two pulses per round-trip. This realisation potentially allows for higher average power from modelocked QCLs, modulated at multiple harmonics of the fundamental frequency, and is ideally adapted to the QCLs ultrafast dynamics with modulation beyond 60 GHz attainable.

Fabrication of approx. 3 THz Al0.15Ga0.85As/GaAs QCLs grown by Molecular Beam Epitaxy equipped with Ta/Cu or Ti/Cu waveguide claddings will be presented. Our previous studies showed that copper layers as the waveguide claddings are most promising in THz QCLs technology. The theoretical predictions showed that lasers with Ti/Cu or Ta/Cu claddings (where Ti and Ta play the role of diffusion barriers and improve adhesion) show the smallest waveguide losses when compared with other metals. The main important issue of the presentation will be the wafer bonding of the QCL active region and GaAs receptor wafer. We will compare the results of ex-situ and in-situ bonding technology. The structures were tested by optical microscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDXS). Our studies show that it was necessary to apply at least 5 nm-thick diffusion-barrier layers, as well as to keep all of the process temperatures below 400C in order to ensure the barrier tightness. The next important issue was control of composition of metallic claddings, in order to provide the control of the refractive index profiles of the claddings. The ridge structure lasers were fabricated with ridge width in the range 100 – 140 µm, formed by dry plasma etching in BCl3/Cl2/Ar mixture in ICP RIE system. The lasers operated with threshold current densities of approx. 1.2 kA/cm2 at 77 K and the Tmax = 130 K, when fed by 100-300 ns current pulses supplied with 0.3-1 kHz repetition frequencies. *This research is supported by The National Centre for Research and Development (bilateral cooperation, project no. 1/POLTUR-1/2016) and TUBITAK (Scientific and Technical Research Council of Turkey) project number 215E113.

Plasmonic nanostructures made of novel low-loss plasmonic materials are emerging as prime components for single-photon sources and nanoscale sensors with unique properties. Plasmon-enhanced single-photon emitters possess the advantages of GHz photon production rates, nanoscale footprint, room-temperature operation and strong tolerance to the cavity resonance frequency shifts. We present several realizations of quantum emitters coupled to plasmonic nanoantennas made of crystalline silver, using both random and deterministic assembly methods, focusing on single-photon production and the optical readout of single electronic spin states.

Quantum optical nonlinearities have received growing interest for their key role in quantum information science, quantum simulations, and other quantum technologies. Unfortunately, most materials exhibit very weak optical nonlinearities, virtually non-existent at the single photon level. Nano-scale optical resonators can store light for a long period of time in cubic-wavelength scale volume, and thus present a unique opportunity to enhance the light-matter interaction. Additionally, breakthroughs in materials science allow us to engineer inherently strong nonlinear materials. In this talk, I will present our theoretical and experimental efforts in nonlinear nanophotonics, integrated with atomically thin 2D materials, specifically transition metal dichalcogenides and solution-processed quantum dots. By confining both light and matter in the wavelength scale, we aim to reach the nonlinear regime, where single photons start repelling each other. I will also elaborate on the possibility of scaling this platform to multiple single photon quantum nodes with the possibility of creating a correlated quantum fluid of light.

Metasurfaces are planar optical devices consisting of artificially fabricated photonic meta-atoms with unique optical responses, and have been extensively studied for thier extraordinary abilities to modulate electromagnetic waves. The field of metasurfaces have focused on the wavefront shaping, and many approaches have been proposed to implement novel optical devices with various versatility. In particular, metasurfaces have been shown to be able to implement high-quality holographic reconstructions of light with both spatial amplitude and phase information at the subwavelength-scale resolution, expected to be applied for the next generation imaging technology such as three-dimensional holographic imaging and optical data storage. In addition, metasurface lenses, called metalenses, have such powerful features as flatness, high numerical aperture, and multi-functions that do not appear in convensional optical lenses. Based on these meta-optics, recent advances in metasurfaces have led to the development of new optical applications using these meta-optics, and several studies have begun to be reported recently. In this talk, several optical metasurface platforms for manipulation of electromagnetic waves will be presented. We will outline the physical mechanisms and concepts of the optical metasurfaces. A variety of metasurfaces providing amplitude, phase and polarization modulations will be introduced and discussed about their features. Then, we will discuss their recent states for optical applications such as holography, microscope, and augmented reality. Especially, our recent works that show their feasiblity in augmented reality imaging with ultrawide field of view not shown in convensional optics. Finally, our perspectives in this area will be discussed.

Optical holography became a powerful tool for arbitrarily manipulating the wavefronts of light. With the recent development of metasurface holography it became possible to tailor all the fundamental properties of light (amplitude, phase, polarization, wave vector and frequency) within a thin slab of material. However, for exploring the full capability of the information storage of metasurface holograms and enhance the encryption security, smart multiplexing techniques together with suitable metasurface designs are required. Here, we demonstrate a novel method for achieving multichannel vectorial holography and show its potential for obtaining dynamic displays and high-security applications. We explore birefringent metasurfaces for the complete control of polarization channels with the freedom of designing both the polarization dependent phase shift and polarization rotation matrix. We show that although the target holographic phase profiles have quantified phase relations they can process very different information within different polarization manipulation channels. For our metasurface holograms, we demonstrate high fidelity, large efficiency, broadband operation, and a total of twelve polarization channels. Such multichannel polarization multiplexing can be used for dynamic vectorial holographic display and provide triple protection to the optical security devices. The concept is appealing for applications of arbitrary spin to angular momentum conversion and various phase modulation/beam shaping elements.

Dielectric and semiconductor structures at the nanoscale are increasingly being applied in nanophotonic applications such as enhanced sensing, magnetic field enhancements, and metasurfaces. In contrast to their traditional metal plasmon counterparts -- such as Au and Ag -- dielectric materials benefit from low losses and CMOS compatibility. Here we explore of 3D dielectric structures on the nanometer and micron scales via a new patterning method, which employs both 3D, direct laser write (DLW) and reactive ion etching (RIE). Polymer structures, which are controlled down to the submicron scale both laterally and in height are printed using DLW onto various dielectric materials and are sub sequentially, etched using RIE. By tuning the etch ratio of the dielectric and polymer, the 3D printed pattern is transferred into the dielectric. By patterning a range of different 3D geometries onto Si, SiC, and hBN, we show that this method is applicable to a range of dielectric and semiconductor materials and to a range of different microstructures and nanostructures. Further, we show the possibility of selectively removing the polymer mask without damaging the underlying dielectric material, which enables the possibility of additional fabrication methods, such as for etching thin films.

The efficient harvesting of electromagnetic (EM) waves by sub-wavelength nanostructures can result in perfect light absorption in the narrow or broad frequency range. These metamaterial based perfect light absorbers are of particular interest in many applications, including thermal photovoltaics, photovoltaics, sensing, filtering, and photodetection applications. Although advances in nanofabrication have provided the opportunity to observe strong light-matter interaction in various optical nanostructures, the repeatability and upscaling of these nano units have remained a challenge for their use in large scale applications. Thus, in recent years, the concept of lithography-free planar light perfect absorbers has attracted much attention in different parts of the EM spectrum, owing to their ease of fabrication and high functionality. In this talk, we will explore the material and architecture requirements for the realization of light perfect absorption using these multilayer metamaterial designs from ultraviolet (UV) to far-infrared (FIR) wavelength regimes. We will provide a general theoretical formulation to find the ideal condition for achieving near unity light absorption. Later, these theoretical estimations will be coupled with findings of recent studies on light perfect absorbers to explore the physical phenomena and the limits of different materials and design architectures. These studies are categorized in three main class of materials; metals, semiconductors, and other types of materials. We will show that, by the use of proper material and design configuration, it is possible to realize these lithography-free light perfect absorbers in every portion of the EM spectrum. This, in turn, opens up the opportunity of the practical application of these perfect absorbers in large scale dimensions. In the last part of the talk, we will discuss the progress, challenges, and outlook of this field to outline its future direction.

All optical quantum information processing is an important developing field for understanding and exploiting quantum entanglement and offers opportunities in quantum computation, quantum sensing and quantum communication. The presentation will describe work on laser written devices in a silica on silicon platform. The devices can be used to create single photon sources, waveguide circuitry and number resolving detectors. Applications of the technology include boson sampling, quantum teleportation, on-chip number resolving detection and quantum simulation. The use of laser writing allows for rapid prototyping, and by including high quality Bragg gratings, it is possible to provide accurate measurements of device properties including losses and coupler variability. Furthermore, the ability to make accurate measurements of material properties such as dispersion and birefringence allows the creation of tailored devices with optimal properties for identical photon sources.

We will discuss 2D and 3D photonic integrated microsystems designed for coherent synthesis, processing, and detection of optical waves in temporal, spectral, and spatial domains. The talk will be in three parts addressing applications in future communication, computing, and imaging systems. The 3D photonic integrated circuit (PIC) platform exploits direct inscribing of arbitrarily shaped waveguides using femtosecond lasers. In one example, such a 3D PIC allows arbitrary beam forming for spatial division multiplexing using orbital angular momentum states. We will further discuss 2D integrated photonic technologies based on InP, silicon, and silica material platforms. Examples at UC Davis include > THz coherent optical transmitters and receivers on a monolithically integrated InP platform, silicon photonic RF-photonic lattice filters, silicon-photonic optical routers, and photonic-lanterns. Dynamic optical arbitrary waveform generation and measurement (OAWG and OAWM) technologies enable > THz scale coherent optical transmission systems with arbitrary modulation formats and bandwidth. Integrated 3D devices can realize arbitrary spatial beam forming and beam steering. We will also demonstrate a silicon-photonic switch which realizes all-to-all interconnection of > 1000 computing nodes, and discuss a compact interferometric imaging system realized on 2D/3D photonic integrated circuits. Finally, we will address future prospects of the new 2D/3D photonic integration technologies towards realizing future communication, computing, and imaging systems.

Mid-infrared racetrack resonators are demonstrated working at 8μm wavelength. The devices are based on a graded SiGe platform providing low propagation loss on a large wavelength range in the mid-IR. Different resonators designs have been fabricated, with varying gap distances in the directional coupler. Q factors of more than 3000 have been experimentally demonstrated. These results pave the way towards compact mid-IR sensors or efficient active devices.

Mid-infrared trace gas sensing is a rapidly developing field with wide range of applications. Although CRDS, TDLAS, FTIR and others, can provide parts per billion and in some cases, parts per trillion sensitivities, these systems require bulky and expensive optical elements and, furthermore, are very sensitive to beam alignment and have significant size and weight that place constrains on their applications in the field, particularly for airborne or handheld platforms. Monolithic integration of light sources and detectors with an optically transparent passive photonics platform is required to enable a compact trace gas sensing system that is robust to vibrations and physical stress. Since the most efficient quantum cascade lasers (QCLs) demonstrated are in the InP platform, the choice of InGaAs-InP for passive photonics eliminates the need for costly wafer bonding versus silicon, germanium of GaAs, that would require optically absorbing bonding interfaces. The InGaAs-InP material platform can potentially cover the entire λ=3-15μm molecular fingerprint region. In this paper, we experimentally demonstrate monolithic integration of QCL, quantum cascade detector (QCD) and suspended membrane sub-wavelength waveguides in a fully monolithic InGaAs/InP material system. The transverse magnetic polarized QCL emission is efficiently coupled into an underlying InGaAs suspended membrane subwavelength waveguide. In addition to low-loss compact waveguide bends, the suspended membrane architecture offers a high analyte overlap integral with the analyte. The propagating light is absorbed at the peak absorbance wavelength of the selected analyte gas and the transduced signal is detected by the integrated QCD. Gas sensing will be demonstrated.

Optical communication and remote sensing (on the ground and in space) including astronomy requesting high-dynamic range observations are the next frontiers in high-bandwidth communication and civil space surveillance technologies. Each requires very precise glass mirror technology, which has not kept pace with corresponding optical and infrared sensor advances. Consequently communication and remote sensing systems are currently limited by the cost and manufacturing restrictions of their high-quality optics. We are developing a new and interdisciplinary technology for creating extremely lightweight diffractionlimited meta-material-based optical systems with exceptional optical quality spectacularly lower cost and production time — Live-Mirror. Notably such new technology is crucial to the development of dedicated high angular resolution and high-contrast telescope concept – The ExoLife Finder (ELF) Telescope – to the exoplanets studies and related science such as detecting life and even civilizations on Earth-like planets.

A range of high precision laser fabrication tasks can be implemented using a combination of ultrashort pulsed lasers and tight focusing with a high numerical aperture lens. These methods can create material modifications confined within three dimensions with sub-micrometer scales. Dynamic optical elements, such as deformable mirrors or liquid crystal spatial light modulators can enhance the performance of laser fabrication systems through adaptive beam control. We explain how these methods are being used to improve fabrication of photonic devices, through aberration correction and parallelization for three-dimensional structuring of materials. In particular, aberration correction enables precise focusing deep inside high refractive index materials, where refraction at the surface introduces spherical aberration that increases with focusing depth. This causes distortion of the focus and hence a reduction in precision and efficiency of fabrication. We have used adaptive laser fabrication to enable a range of devices: applications include waveguide circuits for quantum optics, waveguides and Bragg gratings in diamond, targeted creation of colour centres in diamond, novel polymer/liquid crystal structures and diamond-based radiation detectors. These devices for building blocks for applications in quantum optics and sensing.

Doubly-clamped pre-stressed silicon nitride string resonators excel as high Q nanomechanical systems enabling room temperature quality factors of several 100,000 in the 10 MHz eigenfrequency range. Dielectric transduction ideally complements the silicon nitride strings, providing an all-electrical control scheme while retaining the large mechanical quality factor [1,2]. It is mediated by an inhomogeneous electric field created between adjacent electrodes. The resulting gradient field provides an integrated platform for actuation, displacement detection, frequency tuning as well as strong mode. Dielectrically controlled silicon nitride strings are an ideal testbed to explore a variety of dynamical phenomena ranging from multimode coupling to coherent control. Here I will focus on the nonlinear dynamics of a strongly driven string [3,4]. While the response of the string is described by the cubic nonlinearity of the well-established Duffing model, the power spectrum reveals a series of interesting satellite peaks. I will show how they reflect the dynamics of the driven string in the presence of thermal noise, enabling insights into the squeezing of thermal fluctuations as well as nonlinear switching phenomena. [1] Q. P. Unterreithmeier et al., Nature 458, 1001 (2009). [2] J. Rieger et al., Appl. Phys. Lett. 101, 103110 (2012). [3] M. Seitner et al., Phys. Rev. Lett. 118, 254301 (2017). [4] J. Huber et al., in preparation

Graphene is a very attractive material for broadband photodetection in hyperspectral imaging and sensing systems. However, its potential use has been hindered by tradeoffs between the responsivity, bandwidth, and operation speed of existing graphene photodetectors. Here, we present engineered photoconductive nanostructures based on gold-patched graphene nano-stripes, which enable simultaneous broadband and ultrafast photodetection with high responsivity. These nanostructures merge the advantages of broadband optical absorption, ultrafast photocarrier transport, and carrier multiplication in graphene nano-stripes with the ultrafast transport of photocarriers to the gold patches before recombination. Through this approach, high-responsivity operation is achieved without the use of bandwidth- and speed-limiting quantum dots, defect states, or tunneling barriers. We demonstrate high-responsivity photodetection from the visible to the infrared regime (0.6 A/W at 0.8 μm and 11.5 A/W at 20 μm) with operation speeds exceeding 50 GHz. Our results demonstrate an improvement of the response times by more than seven orders of magnitude and an increase in bandwidths of one order of magnitude compared to those of higher-responsivity graphene photodetectors based on quantum dots and tunneling barriers.

For diffusion limited nBn detectors, using an absorption layer much thinner than the optical attenuation length and minority carrier diffusion length can improve the dark current. As the absorber thickness decreases, the lower dark current increases the signal-to-noise ratio to provide greater sensitivity or higher temperature operation. However, if the quantum efficiency (QE) also decreases with absorber thickness, the advantage of reduced dark current is eliminated. Here we discuss the use of a metallic grating to couple the incident light into laterally-propagating surface plasmon polariton (SPP) modes, so as to increase the effective absorption length. We fabricate the gratings using a deposited Ge layer, which provides a uniform grating profile without increasing the dark current. Using this process in conjunction with a 0.5 μm-thick InAsSb absorber lattice-matched to GaSb, we demonstrate an external QE of 34% for T = 78–240 K. An nBn structure with an InAs_0.8Sb_0.2 absorber that is grown metamorphically on GaSb using a step-graded InGaSb buffer has a peak external QE of 39% at 100 K, which decreases to 32% by 240 K. Finally, we demonstrate that a grating with SPP resonance near the bandgap extends the absorption band, and can potentially reduce the dark current by a factor of 3–8 in addition to the 5× reduction due to the thinner absorber.

We proposed and demonstrated a new modulation scheme of surface plasmon resonance (SPR) sensor for higher sensitivity. We fabricated novel SPR transducers with wedge membrane structure on the Au thin film (Otto configuration) for gas sensing application. The transducers are composed of SiO₂ / gas flowing layer / Au trilayers, fabricated by wafer bonding of SiO₂ substrate on Au thin film on Si substrate via the ball spacers to define the thickness (t) of the gas flowing layer. There is variation of t within the sample owing to the inhomogeneous bonding force during the adhesion process, leading to variation of t and modulating the SPR within the single transducer. The SPR was measured with attenuated total reflection with a glass prism. We measured SPR for different positions having different t by images detected by a camera with focusing and objective lenses. Asymmetric intensity distributions in the images were fitted with the simulated reflectivity, meaning that the obtained images correspond to the SPR near the cut-off condition in Otto configuration. We observed the shift of the intensity distribution upon the change of the excitation condition through different t. Based on the new modulation scheme, higher sensitivity is expected through analysis of the observed images.

We present in this work the study of metal insulator metal (MIM) structures on waveguides for hydrogen leak sensors. The configuration is based on a transducer layer deposited on the core of a multimode fiber optic. The reference transducer layer is a multilayer stack based on a silver, a silica and a palladium layer. The spectral modulation of the light transmitted by the fiber allows to detect hydrogen. The sensor is only sensitive to the transverse magnetic polarized light and the transverse electric polarized light can be used as a reference signal. The multilayer thickness defines the sensor performances in terms of sensitivity, SNR and time response. The silica thickness tunes the resonant wavelength, the silver (or gold) supports the plasmon and the palladium detects the hydrogen gas in the environment. This study synthesizes the sensor performances as a function of different parameters such as the sensitive materials, different thicknesses, numerical aperture, etc. and goes towards very promising nano-detectors based on the use of original nanoparticles.

The absorption of solid state materials in complex photonic and optoelectronic devices overlap in the visible spectrum. Due to the overlap of spectral features, ultrafast measurements of charge carrier dynamics and transport is obscured. Here, the element specificity of transient extreme ultraviolet (XUV) spectroscopy is advanced as a probe for studying photoexcited charge transport in multiple-material junctions. The core-hole excited by the XUV transitions also imparts structural information on to the probed electronic transition. Transient XUV can therefore measure electron and averaged phonon dynamics for each elemental species in a junction. Application to polaron measurement in α-Fe₂O₃, valley-specific scattering in Si, and charge transfer in a nanoscale Ni-TiO₂-Si junction will be discussed.

In two-dimensional (2D) materials, such as black phosphorus, the hysteresis attributed to surface and interfacial disorder can severely limit applications in electronics. In this work, we characterize the hysteresis in Al2O3-encapsulated black phosphorous samples by studying conductivity switching transients in response to an applied step gate bias. Using the dispersive diffusion model for relaxation in disordered systems, the so-called bimolecular and unimolecular recombination limits were observed in low-disorder pristine and high-disorder oxidized BP samples, respectively. Two different heavy-tail lineshapes ( the algebraic decay and the stretched exponential relaxation ) were clearly distinguished in the low- and high-disorder limits, respectively. The parameterization of these transients allows temperature dependence of the line-fit parameters to be tracked. If interpreted under the continuous time random walk model, the observed temperature dependence of the dispersion parameter beta would result from a disorder-induced tail of localized trap states.

First discovered at the beginning of the 20th century but still only partially understood today, organic semiconductors combine the electrical and optical properties typical of inorganic semiconductors with properties such as flexibility, low cost, and structural tunability via chemical modification. They are of significant interest due to their potential for optoelectronic applications such as displays, photosensors and solar cells. Crystalline organic charge-transfer compounds, combinations of two or more organic molecules in which one species acts as a donor of electric charge and the other as an acceptor, could provide new properties or improved performance to increase the range of application of organic semiconductors. Because of the hierarchy of bonding in these molecular crystals, the subtle interplay of electronic and vibrational states has far more influence on their properties than on those of covalent inorganic crystals. The further development of many applications of such compounds is limited by the lack of understanding of exciton dissociation and charge recombination processes and how these processes depend on the electronic and electron-vibration interactions. The charge-transfer states formed at the donor-acceptor interface play a key role, and both experimental and theoretical analyses depend on the arrangement of the donor and acceptor molecules at the nanoscale. By combining optical and transport measurements such as resonant Raman scattering, transient absorption and photocurrent with quantumchemical calculations it is possible to advance our understanding of the physics of these complex materials, paving the way for their application in 21st-century opto-electronic devices.

All-inorganic cesium lead halide perovskite nanocrystals (CsPbX3 NCs, X = Cl, Br, I) have attracted much attention recently due to their high photoluminescence quantum yields (50-90%) and narrow emission bands with wide tunability. They combine the advantages of perovskites and quantum dots creating an exceptional material for low-cost optoelectronic and photovoltaic devices. Conducting low-voltage electron energy loss spectroscopy (EELS) on individual NCs, we provide novel insights regarding three important aspects of their microscopic behavior: (i) we explicitly demonstrate the relation between NC size and shape with their bandgap, and that the effective coupling between proximal NCs causes band structure modifications [1]; (ii) the synthesis of CsPbX3 NCs inevitably yields simultaneous formation of other nanostructures, insulating Cs4PbBr6 nanohexagons and hybrid nanospheres [2]; and (iii) drop-casted NCs merge spontaneously at room conditions by seamless stitching of aligned NCs, it can be accelerated by humidity and mild-temperature treatments, while arrested with electron beam irradiation [3]. Further, by using high-resolution induced absorption and emission spectroscopies, we obtain detailed information on carrier dynamics in perovskite NCs [4], their water-resistant encapsulation [5], and on energy exchange within their ensembles [6]. Finally, we will report on the on-going quest of carrier multiplication in these materials. [1] J. Lin et al. Nano Lett. 2016, 16, 7198. [2] C. de Weerd et al. J. Phys. Chem. C 2017, 121, 19490. [3] L. Gomez et al. ACS Applied Materials & Interfaces 2018, 40, 5984. [4] E. M. L. D. de Jong et al. J. Phys. Chem. C 2017, 121, 1941. [5] L. Gomez et al. Nanoscale 2017, 9, 631. [6] C. de Weerd et al. J. Phys. Chem. C 2016, 120, 13310.

Since the author conducted researches on quantum remote sensing (QRS) in March 2000 and on quantum spectral imaging and quantum detection in June 2003, some difficulties occur. What’s the basic theory and how to realize the technical application of QRS passive imaging? What’s the basic theory of quantum spectral theory? How to apply quantum light field entanglement state during long-distance quantum detection into use? These question urged the author to research and reflect the basic theory of light quantum (photon) future technology. Therefore, in 2006, the author began to conduct relevant exploration, and proposed the key concept of lightstring theory. Besides, after referring to some theory in modern optics and physics, namely, the optical frequency comb and the string theory, he put forward to make light quantum concept concrete based on the nature of light in lightstring theory , and on which, he proposed the generation mechanism of string-light effect. The past ten years has witnessed some preliminary progress and there will be more discoveries and progress be made in the future. From the research requirements, the paper firstly elaborates the significance of light quantum future technology, as well as its research history and status with the emphasis on its progress. The main content includes the concept of lightstring, its research method and target. Finally, the future study plan is briefly introduced. The author tries to find a new way to study the light in nature, its generation mechanism and theoretical basis for its future technology.

This paper implements a simple optical method to forecast power output of superlattice multipliers which are subjected to an external GHz-THz field. These results complement a recent study which addressed the harmonic conversion efficiency in semiconductor superlattices by interface roughness design. Applying a strong ac field on such a device pumps energy into the system, which is then converted to radiation at harmonics of the pump frequency. Here we investigate the odd harmonics generation in an unbiased superlattice at room temperature, after excitation by input signals in a wide frequency range which can provided by realistic devices.

Quantum cascade laser (QCL) frequency combs are electrically pumped and have a small footprint which makes them an ideal candidate for an all-solid-state MIR spectrometer integrated at the chip-level. However, optical feedback is fatal for frequency comb generation in QCLs by destroying intermodal coherence, which limits the versatility and possible degree of integration of QCL frequency combs. In this work, we show how QCL frequency combs can be stabilized. Thereby, the frequency comb state becomes more robust against drift and noise, as well as virtually insensitive against static and dynamic optical feedback. The stability of the comb states are experimentally checked by shifted wave interference Fourier transform spectroscopy (SWIFTS), as well as by the multi-heterodyne signal using a second comb. An optimized RF compatible QWIP and phase retrieval algorithm enable the robust measurement of intermodal coherence and phase of the comb state from single shot SWIFTS interferograms in FTIR rapid-scan. The presented results pave the way to miniaturized and potentially single chip MIR spectrometers. To demonstrate that dual-comb spectrometers can be realized on a small footprint, we demonstrate a first prototype.

Integrated mid-infrared frequency combs promise to revolutionize chemical sensing. A technologically important approach is to employ a III-V semiconductor in a microresonator-based nonlinear comb. For a comb centered on 4.5 μm, a suitable waveguide material is InGaAs on InP. However, this approach also introduces pronounced higher-order group velocity dispersion that can make it difficult to achieve stable broadband output. One way to stabilize multiple solitons and the repetition rate is to pump simultaneously at two wavelengths separated by one or a few free spectral ranges of the microresonator. Here we show theoretically that this pumping scheme can lead to stable soliton crystals and calculate the required ranges of pump powers and wavelengths. We also show that this stabilization effect occurs only over a limited range of pump power and detuning parameters. For example, for the large detunings needed to isolate soliton pulses, the relative pump power is constrained to r_cr⪅P₂/P₁⪅R_cr with r_cr≈40% and R_cr≈97%. The stable parameter ranges are similar for third-order, sixth-order, and all-order dispersion.

Recently, on-chip quantum-cascade-laser-based frequency combs are gaining increasing attention both in the Mid-IR and in the THz spectral regions. THz devices offer the possibility of filling the gap of comb sources in a spectral region were no table top comb is available. We report on THz comb emission from homogeneous quantum cascade laser structure with a new active region design. It features a very low threshold current density (< 100 A/cm2), extremely wide bandwidth (>1 THz) extending from 3 THz to 4.2 THz and peculiar broadband behavior in the NDR region suggesting the presence of field domains. Time resolved spectral measurements employing an hot electron bolometer demonstrate the simultaneous lasing of all the modes in the NDR region and beatnote spectroscopy based on self-mixing proves the coherent nature of the broadband laser emission. Active control of the repetition rate is also demonstrated by using an external cavity scheme employing a piezo actuator. This active region is very promising for the future integration in an heterogeneous structure for octave spanning comb operation and also as an homogenous comb device

We report on a comparison between two quartz tuning forks (QTFs) employed for quartz-enhanced photoacoustic spectroscopy (QEPAS) having quadrupole and octupole electrode pattern configurations. With respect to the quadrupole, the implementation of the octupole pattern suppresses the fundamental mode and reduces by a factor of ~ 4.4 the electrical resistance for the first overtone mode with negligible variations of the related Q-factors. Both QTFs operating at the first overtone mode were implemented in a QEPAS sensor and the results showed that the octupole configuration provides a ~2.3 signal enhancement factor.

Simultaneous detection of different trace gases is an important topic in many applications, such as breath analysis, environmental monitoring and oil exploration. Quartz-enhanced photoacoustic spectroscopy offers a good solution of this problem, due to the possibility to operate the quartz resonators at the fundamental and overtone resonance frequencies by simultaneously exciting two different antinodes with two different laser sources. In this work we employed a custom spectrophone coupled with a diode laser and a quantum cascade laser for simultaneous detection of water vapor and methane/nitrous oxide with minimum detection limits in the ppb range.

Detection of DNA hybridization by Silicon Nanowire Optical Rectangular Waveguide (SNORW) using full vectorial finite element method is presented. Waveguide is designed to detect DNA hybridization through change in refractive index of single strand and double strand DNA. SNORW having high surface to volume ratio with optical confinement inside low index region permits a compact sensor. Waveguide sensing characteristics such as change in effective refractive index, waveguide sensitivity and power confinement is evaluated for optimized silicon wired waveguide.

We use the current-voltage characteristics to calculate the photo-responsivity and efficiency of the metal-insulator-metal photodiode with one insulator sandwiched between two different metals. The rectified operation of the device is based quantum mechanical tunneling theory.

Mid-infrared sensing with a Quantum Cascade Laser (QCL) as a light source is expected to offer a high sensitivity, a short measurement time, and a good portability compared to conventional methods. However, commercially available QCLs have high power-consumption, leading to the necessity for a large cooling system. Therefore, a portable sensor using a QCL have not been realized. To address this issue, recently we had developed a low power-consumption DFB-QCL which enables continuous wave (CW) operation up to 80 ◦C. In this study, we performed gas sensing using our QCL mounted on a Φ 5.6 mm TO-CAN package under uncooled condition. For example, even when the package temperature rose to room temperature +18 ◦C by injecting 180mA current (1.9 W power-consumption) into the uncooled QCL, it could CW operation, and emit output power of 9 mW. Lasing wavelength were stable when the power consumption of the QCL was below 2.2 W, and in this stable wavelength range, about 20 nm wavelength tuning range was obtained by sweeping injection current. We performed mid- infrared gas sensing of methane around 7.4 μm wavelength using a measurement system consisting of the QCL, a gas cell and a thermopile. For this measurement, the QCL was kept uncooled and was driven by CW current, which made a lasing wavelength sweep sufficient for sensing possible. Measured absorption wavelength, intensity, and width under uncooled operation were agreed well with the HITRAN simulation. Sensitivity was obtained about 2.3 ppb under uncooled operation, which was comparable to under cooled operation.

Commercial gas sensors based on quantum cascade laser (QCL) spectroscopy usually rely on a single measurement technique, such as the direct absorption approach, the photoacoustic method, or measuring the dispersion. While each method has its advantages/disadvantages, it is obvious to combine different techniques in a sensor to improve its overall performance which might include its precision, limit of detection, speed but also the concentration range accessible by a given sensor set-up. In this regards a very promising combination of 2f wavelength modulation spectroscopy (2f-WMS) with heterodyne phase sensitive dispersion spectroscopy (HPSDS) is presented. WMS is a direct absorption based technique - within the constraints of the Lambert-Beer law - utilizing laser modulation in order to generate higher harmonics of the absorption signal, thus shifting the measured signal bandwidth to higher frequencies and reducing 1/f noise. HPSDS in contrast relies on quantifying the dispersion induced by the anomalous dispersion close to absorption lines of the target analyte. In contrast to 2f-WMS, HPSDS is independent of the light intensity reaching the detector, which allows to cover a significantly larger concentration range. The simplicity and the benefits of combining these two techniques are demonstrated for carbon monoxide (CO). Furthermore, the sensor is equipped with three additional QCLs for quantifying the analytes nitric oxide (NO), nitrous oxide (NO₂) and sulfur dioxide (SO₂) in the ppbv range using 2fWMS, making it perfectly suited for ambient air measurements.

An optical setup for cavity enhanced mid-infrared photoacoustic spectroscopy is introduced. Optical feedback from a Brewster window cavity was utilized for efficient coupling of optical power from a distributed feedback quantum cascade laser into an optical cavity with a finesse of 1900, which led to intra-cavity optical power enhancement by a factor of 300. A custom quartz tuning fork was placed into the center of the cavity for intra-cavity quartz enhanced photoacoustic spectroscopy of CO at a wavelength of 4.59 μm. As compared to neighboring absorption lines of water, photoacoustic signals of CO were very weak due to slow vibrational to translational energy transfer. Signals increased sharply with increasing humidity. For humidified CO in N₂ at near ambient pressure, a limit of detection (3 ο) of 8.4 ppbHz^-1/2 was estimated.

Electron injector (EI) technology has already been proven capable of achieving unprecedented sensitivity in the shortwave infrared (SWIR), surpassing the current performance of commercial cameras. As on-chip optical interconnects have drawn increasing attention over the past few years, the need for energy-efficient (<10 fJ/bit) and fast (<10 Gbps) IR receivers has spurred new interest in detectors that can meet such requirements. However, heterojunction phototransistors typically suffer from a large power dependence of the gain-bandwidth product, which constitutes an intrinsic limitation to the realization of high-sensitivity, high-bandwidth photodetectors. We present a comprehensive analysis of the gain and bandwidth of the EI detectors as a function of optical power, for different device architectures. At low light level, as the optical power level increases, the recombination centers in the base are saturated by the higher excess carrier density, and as a result the gain-bandwidth product increases. At higher light level, however, the gain of the phototransistor drastically drops due to Kirk effect. As a result, the gain-bandwidth product peaks at a given power level, which is dependent on the band alignment, doping and defect density in the base. The presented results demonstrate a wide tunability of the EI detectors gain-bandwidth product as a function of the device architecture, and hence constitute a valuable platform for the design of novel detectors that can simultaneously achieve high sensitivity and high bandwidth at the desired optical power level depending on the envisaged application.

We describe plasmonic switches consisting of 1-D arrays of plasmonic nanostructures such that they have thin films of vanadium-dioxide (VO₂) in the vicinity of the plasmonic nanostructures. A multi-wavelength plasmonic switch is presented based on one dimensional plasmonic, asymmetric narrow-groove nanogratings (ANGN), coated with a thin layer of VO₂. Incident optical radiation is coupled into plasmonic waveguide modes in metallic narrow-groove nanogratings leading to a localization of electromagnetic fields inside the narrow grooves. The switching is exhibited due to coating of a thin layer of VO₂ ⎯ a material whose phase changes from semiconductor to metal on exposure to heat, IR radiation or voltage. As the phase of VO₂ changes, it undergoes a change in its dielectric and optical properties. This phase transition in the thin layer of VO₂ coated on the nanograting changes the overall optical response from the nanograting, thus exhibiting a switching in the reflectance spectra. The switchability is analyzed through the differential reflectance spectrum which is obtained by subtracting the reflectance spectra of VO₂ (M) coated ANGNs from the reflectance spectra of VO₂ (S) coated ANGNs. Asymmetry is created in these narrow-groove nanogratings by choosing different values for the narrow-groove gaps. Rigorous coupled wave analysis (RCWA) and finite difference time domain (FDTD) modeling demonstrates that ⎯ due to the presence of asymmetric groove widths ⎯ the incident light is coupled into plasmonic modes in all the grooves at different resonant wavelengths. The presence of several resonant wavelengths in reflectance spectra of ANGNs gives rise to multiple dips and peaks in the differential reflectance spectra, thus exhibiting multiple switching wavelengths. Thus, these asymmetric plasmonic narrow-groove nanogratings can be employed for switching at multiple wavelengths.

Single-photon avalanche diodes (SPADs) are direct photon-to-digital detectors that enable scalable arrays with Poisson-limited signal-to-noise ratio and picosecond timing resolution. However, SPAD detectors require a guard-ring structure to prevent lateral edge breakdown. The guard ring, in addition to pixel electronics, reduces the sensitive area within the pixel, often below 50%. We present the simulation, design and characterization of microlens structures to increase the effective fill factor and SPAD photon detection efficiency. The main challenges in designing microlenses for SPADs are a relatively large SPAD pitch and a low native fill factor, requiring high microlens efficiency over a wide angular distribution of light. In addition, we addressed the requirements of several designs in the same technology, featuring native fill factors which range from 10.5% to 28%, by carrying out the microlens fabrication at wafer reticle level. The fabrication process starts with creating a photoresist microlens master, used to fabricate a mould for microlens imprints. After dispensing a UV curable hybrid polymer on top of the SPAD array, the mould is used to imprint the microlens array shape, and then cured with UV exposure. By using microlenses, we were able to increase the initial fill factor to more than 84% effective fill factor for a 28.5 μm pixel pitch. We also explore the influence of the passivation layer on the SPAD photon detection efficiency.