This PDF file contains the front matter associated with SPIE Proceedings Volume 10531, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Utilizing nano-antennas and plasmonic light concentrators in photoconductive terahertz sources has proven to offer significantly higher terahertz radiation powers by enhancing the photoconductor quantum efficiency while maintaining ultrafast operation. This is because the use of nano-antennas and plasmonic light concentrators in a photoconductive source reduces the average transport path of photocarriers to the terahertz radiating elements, increasing the ultrafast photocurrent that contributes to terahertz radiation generation. In this talk I will present an overview of some of the recent advancements in photoconductive terahertz sources based on plasmonic contact electrodes, enabling significant enhancement in efficiency and output power of photoconductive terahertz sources. I show that the significant performance enhancement offered by plasmonic contact electrodes can be utilized to achieve record-high optical-to-terahertz conversion efficiencies as high as 7.5% and milliwatt terahertz power levels in both continuous-wave and pulsed operation at optical pump wavelengths ranging from 800 nm to 1550 nm.

Terahertz (THz) electric field pulses containing frequency components across an ultra-wide spectrum are important for spectroscopy investigations, and are valuable to improving the application of THz radiation to the security, medical, and communication industries. We perform 2D finite-difference time-domain simulations of sub-wavelength LiNbO₃ (LN) waveguides (i.e. waveguides having core dimensions that are sub-wavelength with respect to the femtosecond optical pump pulse). The sub-wavelength aspect of these waveguiding structures maximizes the intensity of the pump pulse in the LN core, while also minimizing the LN reststrahlen band absorption. Notably, Cherenkov radiation is generated at frequencies between 0.18 and 106 THz, where the sub-wavelength nature of the waveguides allows for Cherenkov emission at ~47° over the entire frequency spectrum. Additionally, we show how a 100 μm×1 mm×500 nm waveguide (pumped by a 780 nm, 7 fs, 1 nJ femtosecond pulse) produces a 140 fJ THz electric field pulse.

Our understanding of the physics/chemistry of the interstellar medium increased since we got the capacity to develop heterodyne spectroscopy tools in the THz frequency range. For instance, an example of an important emission line in astronomy is the fine structure of the molecular deuterated hydrogen at 2.675 THz. Heterodyne detection requires local oscillator sources that operate a few GHz away from the frequency of interest. THz quantum cascade lasers (QCL) emerge therefore as suitable sources. The combination of quantum cascade laser as local oscillator and ultra-sensitive hot electron bolometers for the mixing is so far the sole solution available in order to realise a compact and ultra-sensitive heterodyne detection system. The first building block of our heterodyne detector is a spectrally single mode, low power consumption THz QCL operating at a specified target frequency. We developed devices with low threshold driving currents (<30mA). Their power dissipation, when operated in CW mode, stays below 250mW over the whole operation range. These characteristics make the components compatible for compact integration. Despite the small beam divergence of the 3rdorder DFB architecture employed, the emission pattern is not perfectly Gaussian. We have therefore developed a solution to re-shape the QCL’s output beam into a Gaussian one, using a dielectric hollow waveguide (DHW). We have realized a full study to perfect the coupling between the QCL and the DHW, as the coupling losses are the limiting factor. This solution stands out as the most efficient for our heterodyne system. Finally, the low-power-dissipation QCL was combined with a hot-electron superconducting bolometer, to yield an ultra-compact heterodyne detector. Characterization of the heterodyne detector unit, obtained with a hot and a cold blackbody calibration set-up, will be presented during the talk.

The inverse spin Hall effect (ISHE) can be used to generate broadband terahertz (THz) radiation. This has been demonstrated recently [1 – 3]. We report on efficient generation of pulsed broadband terahertz radiation utilizing the inverse spin hall effect in Fe/Pt bilayers on MgO and sapphire substrates. The magnetic and nonmagnetic layers were epitaxially grown on MgO and sapphire substrates. The emitter was optimized with respect to layer thickness, growth parameters, substrates and geometrical arrangement. Using the device in a counterintuitive orientation a hyperhemispherical Si lens was attached to increase the collection efficiency of the emitter. In this arrangement multiple reflections of the THz pulses from the substrate surfaces are avoided as the metallic layers act as an antireflection coating [4]. The experimentally determined dependence of the THz signal on the layer thicknesses was in qualitative agreement with simulations of the ISHE in the Fe-Pt bilayer. An optimum layer thicknesses of 2 nm and 3 nm were found for Fe and Pt, respectively. The optimized emitter provided a bandwidth of up to 8 THz for both the sapphire and MgO substrates which was mainly limited by the GaAs photoconductive antenna used as detector. The dynamic range reached 60 dB for the MgO substrate at a frequency of 1.5 THz. The pulse length was as short as 220 fs for a pump pulse length of the 800 nm pump laser of about 50 fs. In the case of MgO substrates strong THz absorption of MgO reduced the dynamic range above 3 THz considerably. Average pump powers as low as 25 mW (at a repetition rate of 80 MHz) have been used for terahertz generation. This and the general performance makes the spintronic terahertz emitter compatible with established emitters using nonlinear generation methods. References [1] T. Seifert et al., Nature Photonics 10, 483 (2016) [2] D. Yang, et al., Advanced Optical Materials. doi:10.1002/adom.201600270 (2016) [3] Y. Wu, Adv. Mater. doi:10.1002/adma.201603031 (2016) [4] J. Kröll et al., Optics Express 15, 6552 (2007)

We present a photoconductive terahertz source that offers broadband pulsed terahertz radiation with enhanced optical-to-terahertz conversion efficiencies compared to photoconductive terahertz sources based on short-carrier-lifetime semiconductors. The performance enhancement is achieved by utilizing a plasmonic nanocavity that tightly confines optical pump photons inside a photoconductive layer near the terahertz radiating elements. The plasmonic nanocavity is implemented by sandwiching the photoconductive layer between a distributed Bragg reflector and plasmonic metallic structures, which are optimized to be resonant at the optical pump wavelength. The plasmonic structures are also designed as a broadband terahertz nanoantenna array. A thin undoped GaAs film is used as the photoconductive layer offering much higher carrier drift velocities compared to short-carrier-lifetime GaAs substrates. The tight confinement of the optical pump photons and the use of a low-defect photoconductive semiconductor layer allow drift of almost all of the photo-generated carriers to the terahertz nanoantennas in a sub-picosecond time scale to efficiently contribute to pulsed terahertz radiation. We experimentally demonstrate that the presented terahertz source offers 60 times higher optical-to-terahertz conversion efficiency compared to a similar terahertz nanoantenna array fabricated on a short-carrier-lifetime semiconductor. We demonstrate pulsed terahertz radiation with powers exceeding 4 mW over 0.1-4 THz frequency range.

We demonstrate a high-average-power, single-longitudinal-mode, and tunable terahertz-wave (THz-wave) source based on difference frequency generation (DFG) in a MgO:LiNbO₃ (MgO:LN) crystal. The DFG waves are generated using a pair of Yb-doped, pulsed fiber lasers with a master oscillator power fiber amplifier configuration. The average power of the THz-wave output reaches 1.35 mW at 1.0 THz (300 μm) at a linewidth of 7.2 GHz, and the tunability ranges from 0.34 to 1.25 THz under a pulse repetition frequency of 500 kHz. With this scheme, we constructed a compact THz-wave generation head for imaging applications. The combination of MgO:LN-DFG and the stable and robust fiber laser sources is highly promising for developing high-average-power THz-wave sources, particularly in the high-transmission sub-THz region. This approach may enable new applications of THz-wave spectroscopy in imaging and remote sensing.

The applications of the terahertz (THz) technology include detection, sensing, and imaging of biological objects and chemical agents. The traditional approaches to sensing, and imaging of biological objects and chemical agents have been based on measuring THz absorption or reflection from biological tissues and compiling spectroscopic signatures of different chemicals. An extreme sensitivity of THz absorption and reflection to the water content allows for distinguishing cancer and healthy cells. Chemical changes, changes in polarizability or density or conformation might be detected as well. One of the limitations of the traditional THz absorption/reflection studies is related to a wavelength limited resolution. A more recent THz technology based on plasmonics and metamaterials has the resolution determined by the plasmonic detector feature size, i.e. down to the nanometer scale. Such detectors operating in a resonant regime are expected to have a large sensitivity at the plasma frequencies to even small changes of the dielectric properties at the detector surfaces. While remaining high-risk and high payoff technology, THz biological and chemical sensing is poised for breakthrough developments due to the recent progress in THz electronics including emerging nanoscale Si CMOS based sub-THz ICs.

We demonstrate an efficient terahertz (THz) detector based on an optical hybrid cavity, which consists of an optically thin photoconductive layer between a distributed Bragg reflector (DBR) and an array of electrically isolated nanoantennas. Using a combination of numerical simulations and optical experiments, we find a hybrid cavity design which absorbs <75% of incident light with a 50 nm photoconductive layer. By integrating this optical hybrid cavity design into a THz detector, we see enhanced detection sensitivity at the operation wavelength (~815 nm) over designs which do not include the nanoantenna array.

Antenna-integrated Schottky diode is fabricated for the detection of IR waves. The high current voltage nonlinearity, reflected by a current sensitivity of 15 A/W allows it to achieve a maximum rectification responsivity of 0.40x10^-8 A/W/cm² towards 28.3 THz radiations at lower DC bias. Simultaneously, the modulation of the cutoff wavelength of the diode by DC bias allows it to photo-detect 10.6 μm radiations at higher DC bias. The integration of optical antenna with the diode results in light intensity enhancement of three orders of magnitude, validated by COMSOL finite element simulation and experimental measurements. The high intensity causes a significantly high photocurrent, quantified by a higher than unity external quantum efficiency, η_e; approximately 3 orders of magnitude higher than the highest external quantum efficiency demonstrated by 10.6 μm Schottky barrier photodetectors. The maximum responsivity of the device is comparable to the highest values demonstrated by the current state of the art 10.6 μm detectors.

BaSnO_3 (BSO) is a transparent conductive oxide. This category of materials is interesting for applications such as optically transparent electrodes in solar cells and displays. This perovskite-material possesses many interesting properties including a wide bandgap, 3 eV, and a high electrical conductivity (exceeding 10^4 S/cm at room-temperature), which make it very interesting for visible-transparent applications. The DC conductivity in BSO can be superior to that in ITO, which is a commonly used transparent conductive oxide. Thin films used in our study were grown by molecular beam epitaxy (MBE) on LSAT substrates. The epitaxial structure of the samples consist of 45 nm of La-doped BSO on top of a 45 nm thick undoped BSO film grown on LSAT. The BSO films were characterized by means of terahertz spectroscopy. The terahertz-extracted optical conductivity was ~0.8x10^3 S/cm in the 0.1 to 2 THz frequency range. Using these films, upon patterning into stripes, we demonstrate a terahertz polarizer. The polarizer is transparent at visible wavelengths, and functional at terahertz wavelengths; it achieves 96% transmission for terahertz polarization parallel to the stripes and 16% transmission for the perpendicular polarization. Furthermore, we also show that resonant structures, such as cross resonators, are also realizable in this material. The large optical conductivity in BSO films at terahertz frequencies, together with being transparent at visible wavelengths, makes it a very good candidate for developing visible-transparent electromagnetic structures responding in the terahertz frequency range.

Within the last decade, photoconductive terahertz (THz) systems have become well-established tools in scientific laboratories and industrial R&D departments. In particular, the exploitation of telecommunication technology around 1.5 μm wavelength enabled this development. Continuous wave (cw) THz systems benefit especially from telecom technology since the required optical components are already available. As no femtosecond fiber-laser is needed, photonic integration may lead to extremely compact cw THz devices. We present a fully fiber-coupled cw THz system in combination with optimized InGaAs-based emitter and detector antennas and an optical phase modulator. This system can be employed as both, a highly precise spectroscopic tool and a high-speed measurement system for non-destructive testing. In addition, we present recent results on heterodyne cw THz spectroscopy. This is a prerequisite for future broadband, wireless telecommunication systems using THz carrier frequencies. The fiber-coupled heterodyne receiver is able to detect THz signals up to 1 THz with an intermediate frequency of 2.2 GHz. These are the highest values reported for photoconductive heterodyne receivers so far.

Engineering of novel structures with high strength to weight ratio for applications in aerospace, renewable energy and naval industries has resulted in an increased popularity of sandwich structured composites. A sandwich-structured composite is fabricated by bonding a thick lightweight core between two stiff, thin skins such as Glass Fiber Reinforced Plastic (GFRP). Balsawood is a type of homogeneous core which is widely used for renewable energy structures, such as wind turbine blades. In this paper, a GFRP-balsawood sandwich structure is evaluated non-destructively for internal defects such as holes, using a CW Terahertz system in transmission mode. Internal defects will give rise to differential THz transmission and hence can be identified using THz imaging. The imaging studies are carried out with a central frequency of 0.35 THz and the sample is raster scanned using 2-D translational stages controlled by high precision stepper motors in x-y directions to obtain the THz image. The image acquired using CW THz system clearly identifies the defects in the GFRP-balsawood composite structure with good contrast demonstrating the potential of THz imaging for non-destructive testing of sandwich composite structures.

This talk advertises scattering-type scanning near-field infrared micro-spectroscopy (s-SNIM) in the spectral range of 75 to 1.3 THz [1], as provided by the free-electron laser FELBE, the narrow-band laser-light source at Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Germany. We demonstrate the -independent s-SNIM resolution of a few 10 nm by exploring structured Au samples, Graphene-transistors, meta-materials [2], and local ferroelectric phase-transitions down to LHe [3]. s-SNIM secondly was integrated into a THz pump-probe experiment for the inspection of excited states in structured SiGe samples. We developed a novel demodulation technique with high temporal resolution [4] hence achieving an excellent Signal-to-Noise Ratio. Thirdly using the super-radiant TELBE light source [5], HZDR recently extended the wavelength range down to 100 GHz radiation. We adapted our s-SNIM to this TELBE photon-source as well, achieving an equally high spatial resolution as with FELBE. Moreover, the superb 30-fs temporal resolution of TELBE will allow us to study a multitude of physical phenomena with sub-cycle resolution [5,6], such as spin-structures, magnons and phonon polaritons. [1] F. Kuschewski et al., Appl. Phys. Lett. 108 (2016) 113102. [2] S.C. Kehr et al., ACS Photonics 3 (2016) 20. [3] J. Döring et al., Appl. Phys. Lett. 105 (2014) 053109. [4] F. Kuschewski et al., Sci. Rep. 5 (2015) 12582. [5] B. Green et al., Sci. Rep. 6 (2016) 22256. [6] S. Kovalev et al., Struct. Dyn. 4 (2017) 024301.

Remnant radiation from the early universe, known as the Cosmic Microwave Background (CMB), has been redshifted and cooled, and today has a blackbody spectrum peaking at millimetre wavelengths. The QUBIC (Q&U Bolometric Interferometer for Cosmology) instrument is designed to map the very faint polaristion structure in the CMB. QUBIC is based on the novel concept of bolometric interferometry in conjunction with synthetic imaging. It will have a large array of input feedhorns, which creates a large number of interferometric baselines.

The beam from each feedhorn is passed through an optical combiner, with an off-axis compensated Gregorian design, to allow the generation of the synthetic image. The optical-combiner will operate in two frequency bands (150 and 220 GHz with 25% and 18.2 % bandwidth respectively) while cryogenically cooled TES bolometers provide the sensitivity required at the image plane.

The QUBIC Technical Demonstrator (TD), a proof of technology instrument that contains 64 input feed-horns, is currently being built and will be installed in the Alto Chorrillos region of Argentina. The plan is then for the full QUBIC instrument (400 feed-horns) to be deployed in Argentina and obtain cosmologically significant results.

In this paper we will examine the output of the manufactered feed-horns in comparison to the nominal design. We will show the results of optical modelling that has been performed in anticipation of alignment and calibration of the TD in Paris, in particular testing the validity of real laboratory environments. We show the output of large calibrator sources (50 ° full width haf max Gaussian beams) and the importance of accurate mirror definitions when modelling large beams. Finally we describe the tolerance on errors of the position and orientation of mirrors in the optical combiner.

Terahertz radiation has many unique applications in imaging and sensing currently limited by low efficiency, complexity, and bulky nature of existing terahertz emitters. In this study, we propose an innovative terahertz emitter based on plasmonic nanowire light absorbers that can convert optical beam to terahertz radiation with unprecedented conversion efficiencies. By utilizing nanowire arrays integrated with plasmonic nano-antennae, we confine the majority of the incident optical photons within nanoscale distances from metal contacts. As a result, the majority of the photo-generated carriers quickly drift to the plasmonic nano-antennae in a sub-picosecond time-scale and contribute to efficient terahertz generation. It is predicted that bias-free terahertz emitters based on this novel device architecture can achieve tens of mW terahertz radiation power levels and optical-to-terahertz conversion efficiencies as high as 24%. Additionally, the proposed terahertz emitters operate at telecommunication optical wavelengths and are monolithically integrated on Si or InP substrates, offering a compact and low-cost device platform.

The surface of a topological insulator harbors exotic topological states, protected against backscattering from disorder by time reversal symmetry. The study of these exotic quantum states not only provides an opportunity to explore fundamental phenomena in condensed matter physics, such as the spin Hall effect, but also lays the foundation for applications from quantum computing to spintronics. Conventional electrical measurements suffer from substantial bulk interference, making it difficult to clearly distinguish topological surface states from bulk states. Employing terahertz time-domain spectroscopy, we study the temperature-dependent optical behavior of a 23-quintuple-thick film of bismuth selenide (Bi₂Se₃) allowing for the deconvolution of the surface state response from the bulk. Our measurement of carrier dynamics give an optical mobility exceeding 2100 cm²/V•s at 4 K, indicative of a surface-dominated response, and a scattering lifetime of ~0.18 ps and a carrier density of 6×10¹² cm^-2 at 4 K for the Bi₂Se₃ film. The sample was further processed into a Hall bar device using two different etching techniques, a wet chemical etching and Ar⁺ ion milling, which resulting in a reduced Hall mobility. Even so, the magneto-conductance transport reveals weak antilocalization behavior in our Bi₂Se3 sample, consistent with the presence of a single topological surface state mode.

A modified Bakman Technologies PB7200-2000-T portable THz spectrometer mounted to a consumer drone was employed to measure water vapor absorptions ten meters above the ground.

Guided terahertz pulse reflectometry uses a near field probe to couple a wave in a waveguide. The signal is then reflected depending on the impedance at the end of the waveguide. We experimentally use a near field antenna from Protemics which have two photoconductive antennas on the same chip. The first is excited with laser on frond side et the second on the back side. We associate to this antenna a waveguide to couple a wave and we determine the localization of the end. We observe at this position a change of the signal depending on the impedance. This topology can be used for remote imaging without lens or optics of for non destructive test of 3D electronics packages. Finite Differente in Time Domain simulation were also used to evaluate coupling and origin of losses at each transition.

We present terahertz (THz) time-domain spectroscopy (TDS) as a versatile tool for applications in non-destructive testing. Due to fiber-coupled THz systems, which exploit the advantages of mature telecommunication technology, THz- TDS is a promising tool for industrial process control. As an example, we demonstrate thickness measurements on multilayered plastic pipes by combining THz reflection measurements with a transfer matrix method for data evaluation. Furthermore, we show the potential of THz-TDS for time resolved 2D imaging. For this, we combine a photoconductive near-field probe with a commercially available fiber-coupled THz TDS system. Due to the coherent measurement scheme, which provides amplitude and phase information at each sampling point, in combination with an acquisition rate of 40 pulse traces per second, dynamic processes on the picosecond timescale can be monitored with unprecedented resolution. Exemplarily, we visualize the propagation of a THz-wave on the surface a of photoconductive THz emitter with a lateral resolution of 20 μm and sub-picosecond temporal resolution.

We report on our development of a millimeter-wave radar system for multilayer thickness inspection.

sequences. Sizing, surface and volume rendering can be extracted and compared with targeted dimensions. In this work, we use millimeter wave systems tomographic system (100 and 300 GHz), frequency modulated systems (100 and 300 GHz), and pulse time domain systems (100 GHz to 4 THz) for non destructive characterization of 3D printed additive manufacturing parts. The aim of this talk to to show the advantages and disadvantages of several techniques to define their application aera. This work is associated to a data processing analysis using automated segmentation, extracting of the different volumes of interest (VOI) composing the sample. A mesh is performed for each VOI to numerically calculate the dimensions, surfaces and volume which leads to 3D visualization and dimensional measurements. Overall sequence is implemented onto unique software and validated through different sample analysis.

We report on our recent industrial development projects on millimeter-wave and terahertz imaging solutions for non-destructive testing. This involves system realizations as well as their integration in industrial environments.

Recent innovations in photonics and nanotechnology are now enabling terahertz (THz) research to be applied in many industrial fields such as homeland security, information and communications technology (ICT), biology and medical science, non-destructive tests or quality control of food and agricultural products. Still many challenges are to be addressed, the main one being to provide THz systems with sufficient signal to noise ratio when operated in real industrials conditions. In addition, cost is a key lock that hampers the spread of this technology but it is clear that cost-effective sources and detectors compatible with standard microelectronics will drive down the overall cost, and in particular will make THz imaging accessible for industrial use. In order to bring THz imaging to industry, Leti has been developing over the past decade complementary CMOS-compatible uncooled imaging 2D-array technologies: antenna-coupled bolometers and Field Effect Transistor detectors. In addition, CEATech built a test platform dedicated to the development of industrial prototypes of photonics technologies. In particular, in collaboration with i2S, this platform includes the TZCAM camera equipped with Leti’s 320×240 bolometric pixel array and gives access to a full industrial THz imaging chain that is essential for maturation of this emerging technology. This paper gives an overview of these developments and illustrates industrial applications with examples of uncooled THz imaging tests, e.g. opaque object 2D inspection or 3D tomography.

We demonstrated experimentally a new method for generation of linearly chirped light waves with almost perfect linearity over a broad range of about 800 GHz. The external modulation method that we adopt can maintain frequency jitters at a very low level by avoiding relaxation oscillation effects which are an intrinsic property in intra-cavity modulation methods. The linearly chirped light could provide an excellent time-frequency mapping tool for wide-range applications.

Nyquist pulses, which are defined as responses of a Nyquist filter, can be used in time-division multiplexing transmission which can simultaneously achieve ultrahigh data rate and spectral efficiency. Generally, the methods of Nyquist pulse generation are based on optical Nyquist filters, optical parametric amplifier effect and electro-optical (EO) modulation. In this paper, we focus on the method of EO modulation. Traditionally the limitation of this method is the complex structure and driven signal synchronization between multiple EO modulators when cascaded EO modulators or special modulator structures are using to generate Nyquist pulses. To address this issue, we proposed a novel setup in which only one EO intensity modulator and an electrical arbitrary waveform generator (AWG) are employed. With this method, it is required less on devices. Furthermore, duty cycles of the ideal Nyquist pulses generated by this new method can be changed by using different tones number to drive the EO modulator. The duty cycles of Nyquist pulses we generated can set at 21%, 16% and 12.5% at the repetition of 2.5 GHz by programming the tones number at 2, 3 and 4 on the AWG. The narrowest pulse full width at half maximum is 50.2 ps, which the measured bandwidth is 22.5 GHz by the optical spectrum analyzer, are generated using only one EO intensity modulator with lower bandwidth down to 10 GHz. This method has a potential benefit to reduce the duty cycle further if we use a modulator with bandwidth more than 10 GHz.

Compound semiconductor mid-wavelength infrared photodetectors operating at room temperature are the sensors of choice for demanding applications such as thermal imaging, heat-seeking, and spectroscopy. However, those detectors suffer from high dark current and thus normally require additional cooling accessories. In this work, we argue for the fundamental feasibility that by using nanowires coupled with plasmonic nano-antennae as photoabsorbers, the dark current can be largely reduced compared with typical planar devices. To demonstrate the idea, we simulate the device characteristics, such as dark current, responsivity, and detectivity, of InAsSb_0.07 nanowire photodetectors, and compare those properties with the best research InAs photovoltaic diodes. The results show that the designed nanowire detectors offer over one-order lower dark current and enable a peak detectivity of 7.0×10¹⁰ cm Hz^1/2W^-1 at 3.5 μm. We believe this work will provide a guidance to the design of nanowire-based MWIR photodetectors and stimulate additional experimental and theoretical research studies.

Precise measurements of the dipole moment of the neutron test the standard model of particle physics. Typical experiments detect the evolution of neutrons in magnetic and electric fields. Achieving high sensitivity requires stable and homogeneous fields. We are investigating nitrogen-vacancy diamonds for sensing electric fields. As a first step we have measured electric fields by optically-detected magnetic resonance. Near avoided crossings a first-order Stark effect is observed. Line positions can be measured to about 5 kHz, allowing electric fields to be measured to about 2 kV/cm. Extending the technique to use electromagnetically-induced transparency will allow for an all-optical probe, but may introduce issues of systematic errors in the electric field measurement.

We report on THz MEMS sensors suitable for large focal plane arrays and readout schemes compatible with real-time imaging. Terahertz absorption near 100 %, optimized to particular monochromatic quantum cascade laser (QCL) illumination sources, was achieved using metal-dielectric metasurfaces. MEMS devices were designed using metasurface absorbers as structural components, allowing for streamlined fabrication of very efficient detectors in two different configurations. In the first scheme, bi-material sensors were used, where the heat from the absorbers is converted into mechanical deformation. The angular displacement, proportional to the absorbed THz radiation, was then optically probed. In the second configuration, THz to IR conversion was achieved whereas the front side of the metasurface absorbs THz and the backside served as an efficient infrared emitter, allowing its temperature to be probed directly by a commercial, thermal (infrared) camera. The devices are comprised of ultrathin films of silicon-rich silicon oxide and aluminum, deposited on silicon substrates and were fabricated standard MEMS processes including Bosh deep reactive ion etching to remove the substrate. The sensors were fabricated in a matrix configuration and individually characterized. The main figures of merit, such as spectral response, thermal time constant and sensitivity are controlled by the geometry and can be modified by design according to the application demands.

A numerical and experimental study of a tunable single-layered tunable fishnet metamaterials (TFMM) infiltrated with polymer dispersed liquid crystal (PDLC) in THz range is reported. The retrieving procedure of refractive index takes antiresonance phenomenon into account and is verified by the assumption in effective medium theory. An encapsulating technique that covers PDLC with a 1.5μm polyimide layer (skin layer) was applied to prevent volatile liquid crystal from escaping the PDLC and enable the deposition of metal layer while minimizing the Fabry-Perot effect of the skin layer on the TFMM measurements. Concept of tuning TFMM infiltrated with PDLC (PDLC-TFMM) was proven by measuring the frequency shift in the reflection coefficient (0.01THz), with an observed minimum negative refractive index of -15 at 0.55THz.

The terahertz (THz) wave applications at frequencies from 100 GHz to 10 THz has attracted much attention, especially in a broadband wireless communication. In the THz broadband wireless devices, photo mixing by using the unitraveling- carrier photodiode (UTC-PD) on the InP substrate is a critical issue, which is down-converted to the optical signal to THz wave. In this situation, the loss in the THz section is a serious problem. Therefore, antennas and transition lines should be fabricated on the same substrate. In our previous research, 1 x 4 and 4 x 4 planar array antennas using one-sided directional slot dipole antenna elements and branched coplanar waveguide (CPW) connected to the output of UTC-PD on the InP substrate is designed. In this paper, 4 x 4 phased array antenna on InP substrate for 300 GHz broadband telecommunication is demonstrated. The total antenna size is 1,930 μm x 2,000 μm x 18 μm. Four 1 x 4 subarray antenna are stacked planarly, and each subarray antenna is connected to the UTC-PD through the CPW. Each antenna element is arranged at the distance of half wavelength in order to sharpen the directivity. By changing the delay line attached to the optical fiber, the phase difference of each subarray can be obtained. From the phase difference between each antenna elements, our proposed phased array antenna has a sharp beam and beam steering characteristics.

We designed and fabricated a low-bias operational uni-travelling carrier photodiode (UTC-PD) structure, which can be operated at over 100 GHz. The main structure of the device consisted of p-doped InGaAs for the photo-absorption layer and non-doped InP for the carrier collector layer, to obtain both a high electron drift velocity at a low bias and a small CR time constant based on the pn-junction capacitance. Through an on-wafer probing test, the frequency response was measured up to 210 GHz using a 1 mm coaxial connecter type (DC-110GHz), W-band (75–110 GHz) and G-band (140–220 GHz) waveguide probe with a spectrum analyzer. In the measurement results, a large bandwidth of 10 MHz-110 GHz could be obtained with good flatness within ±1 dB. When the W-band and G-band performance were characterized, the high-power characteristic of -3.8 dBm could be achieved at 106 GHz. and the output power level of - 19.8 dBm could be confirmed at 210 GHz as well.

Integrated optical beamforming networks (OBFNs) are critical for photonics-assisted wideband microwave phased array antennas. Here, a 1x4 integrated OBFN based on low loss silicon nitride waveguide technology was demonstrated for millimeter wave (mmW) communications. Three cascaded optical ring resonators serve as tunable true time delays (TTDs) for each channel, which overcomes the beam squint issue associated with wideband communication. The tuning of rings was carefully calibrated using the lossless ring delay response theoretical model, and experiments were performed verifying the tuning accuracy. Theoretical simulations were performed to optimize the delay response of the OBFN using a genetic algorithm, which revealed tradeoffs among the delay response flatness, absolute delay value, and TTD bandwidth. Two topologies of the OBFN were theoretically and experimentally compared. A lookup table of the optimized ring parameters was generated, based on which single-delay channels with dynamic tuning ranges of 208.7 ps and 172.4 ps for TTD bandwidths of 6.3 GHz and 8.7 GHz were achieved, which correspond to phase shifts of 37.5π and 31π for a 90-GHz signal, respectively. Moreover, all four channels were tuned to a delay distribution with a differential delay around 4.2 ps, which is equivalent to a 49° beamsteering angle for a 90 GHz half-wavelength dipole antenna array. Using heterodyne upconversion and a single delay path, a 41 GHz mmW signal with 3-Gbps NRZ OOK data modulation was generated and delayed. Future work will focus on higher frequencies into the W-band and on beamsteering experiments.

An optical coherent receiver for the down conversion of radio frequency (RF) signals from 10-18 GHz to 2 GHz is presented. Light from a distributed feedback semiconductor laser is split between two lithium niobate Mach-Zehnder modulators driven either by a tunable local oscillator (LO) tone or a RF signal coming, for example, from a receiving antenna. The modulated light signals are combined with an optical coupler and filtered by two fiber Bragg gratings (FBG) that select one optical sideband from each signal. Detection of the filtered light by a balanced photo-detector produces an electrical signal at an intermediate frequency equal to the beat difference between the RF and LO frequencies.

Most current RF photonic systems are made from individually packaged devices that are interconnected with fiber-optic cables. In order to reduce size and weight and make the coherent receiver suitable for use in smaller airborne and mobile platforms, optical and opto-electronic components are packaged within a common enclosure where light routing is performed by micro-optics. A printed circuit board (PCB) is included within the module. It comprises a micro-processor to control and monitor the laser, the FBGs and thermo-electric coolers to ensure a robust operation over time and fluctuating environmental conditions. The module including the PCB, laser, modulators, optics, optical filters and balanced detector has a size of 89 x 64 x 32 mm³.

We present a theoretical and experimental study of novel architectures of optoelectronic oscillators that have a nonlinear response in the electric branch. We perform a nonlinear analysis and obtain a good comparison between our theoretical results and the experimental measurements.

Optical antennas are known as components that enhance interaction between energy of guided modes of optical waveguides and optical free space modes to grow the efficiency of optoelectronic devices. In optical wireless communications, radiation specifications such as efficiency and directivity, and impedance matching are crucial parameters of optical antennas. In addition, compatibility between optical antennas and waveguides that feed them is significant consideration. In some works, optical antennas are designed to transfer energy to/from plasmonic waveguides based on leaky wave concept. In some investigations, hybrid plasmonic leaky-wave optical antennas were designed and optimized to radiate efficiently in conventional plasmonic hybrid structures (a material with low refractive index (SiO₂) is located between a metal, and another material with higher refractive index (Si)). For further improvement, by perturbing conventional structures and adding an extra layer such as silicon carbide (SiC) or nickel silicide (NiSi₂) between SiO₂ and Si, controlling radiation confinements were improved. In this work, a novel structure of hybrid plasmonic leaky-wave optical antenna is proposed that has improvement in impedance matching and directivity between 192 and 197 THz compare with previous works. Here, Electron Beam Lithography (EBL) nanopatterning procedure is suggested for fabrication process to achieve multicomponent-multilayer hybrid plasmonic leaky-wave optical antenna.

THz encoders have distinct advantages for position sensing compared with other types of encoders, such as those based on optical and inductive sensors. A polarization-dependent metamaterial absorber reflects one polarization while absorbs the other, which makes it an ideal building block for the barcode of a THz encoder system. In this paper, we present the design, fabrication, and experiments of a THz polarization-dependent metamaterial absorber, and its application to a polarimetric sensing system.

In order for Terahertz (THz) technology to enter the forefront of the industry, technology to generate THz wave with simple structure and low cost is required. This paper suggests compact and portable THz imaging system by the use of Single Mode Fabry-Perot Laser Diodes (SMFPLD). The existing THz system has a disadvantage that it is bulky, complicated, expensive, and difficult to use. To compensate for this, we use an inexpensive and simple FPLD combined with an external cavity to construct a THz imaging system. SMFPLD shows a steady longitudinal mode state through self-injection locking by means of a built-in aspherical lens. The position of the dominant mode changes as the current and temperature are controlled, and the dual mode is also generated under certain conditions. This characteristic is a great advantage in the photomixing method that requires two lasers. Because a laser alone can create a THz wave, and it creates two frequencies in a single family, it is not necessary to consider polarization or phase differences. Based on these advantages, we have constructed a laser module and combined with PCA to create a THz imaging system. A THz imaging systems demonstrated in this paper are superior to ordinary systems due to their inexpensive and compact structure.

Ordinary Portland Cement (OPC) primarily constitutes Tricalcium Silicate (C3S) and Dicalcium Silicate (C2S) making up 60–70 % and 20–30 % of the cement matrix respectively. During cement hydration, C3S starts to react faster contributing to early stage strength in comparison to C2S, which reacts slowly and is responsible for long term strength development of concrete. C2S is manufactured at lower temperatures compared to C3S, resulting in lesser emission of carbon dioxide as compared to C3S. Moreover, C2S produces less Ca(OH)₂ than C3S, which is an undesirable hydration product. Thus, incorporation of greater percentages of C2S in cement matrix will be highly beneficial, provided it’s early stage reactivity can be increased. One of the key methods to increase reactivity of C2S is incorporating nanosilica which accelerates the hydration along with the formation of greater amount of calcium silicate hydrate (C-S-H) which is responsible for the strength development of concrete. Hence, understanding the acceleration in hydration dynamics of the nanosilica incorporated β-C2S can help in optimizing the percentages of C3S and C2S in cement. In this study, Terahertz spectroscopy has been employed to track the acceleration of hydration of C2S due to the addition of nanosilica. Results show early stage reduction in peak height of the resonance around 520 cm^-1 in nanosilica incorporated sample which indicates faster hydration of C2S during hydration. Furthermore, early stage formation of a prominent resonance around 453 cm^-1 for the nanosilica incorporated C2S sample implies formation of C-S-H like structures confirming the accelerated hydration rate.

We have numerically demonstrated refractive index sensing using terahertz metamaterials comprised of single split gap resonators. Sensing capabilities of odd and even order resonance modes are precisely investigated. In this scheme the top surface of metamaterials array is covered with a thin layer of polyimide whose refractive could be changed manually. The sensitivity and corresponding figure of merit (FoM) of several lowest order modes are examined with respect to the different thicknesses of the coated polyimide film. We have investigated the electric field distributions at different resonances for the MMs. Although we have mainly focused on refractive index sensing but this study could be extremely useful for the development of metamaterials based sensing devices, bio-sensors etc.

Linear and nonlinear terahertz (THz) phenomenon are studied in a cadmium silicon phosphide, CdSiP₂, crystal. The ~2 THz phonon mode of the CdSiP₂ crystal is probed via THz spectroscopy experiments, allowing phonon-polariton dispersion to be observed in the recorded time-domain signals. In the frequency range of 0.5-2.9 THz, the refractive indices of this uniaxial crystal are determined (allowing the material’s birefringence to be calculated), along with the material’s extinction coefficients. Using a 780 nm central-wavelength pump pulse having a duration of 50 fs, THz generation is achieved in the non-centrosymmetric CdSiP₂ crystal. The resulting THz electric field pulse has a bandwidth ranging from 0.07-6 THz. The results of this study suggest CdSiP₂ has the potential to find use as low-loss THz waveplates in the frequency range of 0.5-1.9 THz, as well as a broadband THz source.

Rochester Institute of Technology (RIT) and its collaborators at the University of Rochester and Harris Corporation are developing a room-temperature imaging Terahertz (THz) frequency detector using Si-MOSFET (Silicon Metal Oxide Semiconductor Field Effect Transistor) CMOS devices. They are implemented into a focal plane imaging array for use in many applications, such as transmission or penetration imaging and spectroscopy. Technology for THz detection is often extremely costly, due to either expensive detector materials or cryogenic cooling systems. However, the devices tested here are low-cost due to the use of conventional room temperature silicon CMOS technology. The devices operate from 170 to 250 GHz with an additional detector design has been fabricated for 30 THz (10 microns wavelength). Results are presented for the initial testing of single test structure FETs. These devices were designed with several different antenna configurations and a range of MOSFET design variations for evaluation. The primary goal of the work presented here is to determine the optimized detector design for the subsequent focal plane array implementation based on the largest responsivities and lowest noise-equivalent power (NEP). Transmission testing of the devices yields responsivities of about 100 to 1000 V/W and a NEP of about 0.5 to 10 nW·Hz^-1/2. Through this evaluation and by utilizing signal amplification on the chip, signal modulation at higher frequencies, and smaller process sizes the performance of these devices will continue to improve in future designs.

We represent a high speed evanescently coupled waveguide UTC-PD. It is shown that a layer of polymer layer under the electrical pads can enlarge the response bandwidth of a UTC-PD greatly. The effects of the polymer layer are also studied with numerical simulations, which show the same trend.

We experimentally investigate the impact of relevant parameters such as dispersion regime, and coupling ratio between the two loops on the phase noise performances of a 10 GHz coupled optoelectronic oscillator (COEO). The setup is based on a mode-locked semi-conductor laser at 1.55μm combined to a classical OEO. Optimization of these parameters leads to ultra-low phase noise at close-to-carrier frequencies (-100 dBc/Hz at 100 Hz and -125 dBc/Hz at 1 kHz).

We present a Silicon-photonic-assisted RF signal processing, in particular, frequency up conversion and RF pulse generation. For frequency up-conversion, we use compact silicon PN micro-ring modulator to frequency double the input RF signal. Experimentally we report an up-conversion of the baseband to a maximum of 12 GHz covering L, S, C and X band. We achieve a maximum suppression of 30 dB of the baseband at the output. The extinction could be improved by operating the ring modulator at the critical coupling. We present a detailed study on the effect of the optical carrier offset from the resonance wavelength and its effect on the suppression and the upconverted RF linewidth. Using the same platform, we also demonstrate RF-pulse generation using a PN Mach-Zehnder modulator. An RF modulated optical carrier, and an RF signal can be fed to the MZM to create pulsed RF of desired pulse width and rate. As a demonstrator, we show that RF-signal of frequency between L-X band could be fed and pulsed at various rates from 100-0.1 millisecond. The two examples demonstrate the feasibility of the Silicon Photonics Platform for building photonic assisted RF technology.

Due to very high frequency and large time bandwidth product, photonic generation, and processing of arbitrary microwave waveforms has been an interesting topic in the area of remote sensing application. Here, a photonic technique is proposed for the generation of a dual non-linear chirp microwave waveform in Ku-band. This methodology is based on the principle of optical external modulation through cascading of two Mach-Zehnder Modulators. Its application has been investigated in modern radar system in terms of range-Doppler resolution. A comparative study has been done on the performance analysis of a dual chirp signal with nonlinear and linear chirping capability.

A novel scheme for the generation and stabilization of the millimeter-wave (mmW) signal is theoretically analyzed and experimentally demonstrated. By using the microwave photonics frequency-quadrupling technology and phase-locked optoelectronic oscillator, we generate the millimeter-wave signal with low phase noise and high stability without the frequency limitation of the electrical phase detector and the voltage-controlled microwave phase shifter. Finally, a 40-GHz mmW signal with the stability of 1.38 × 10⁻¹² at the average time of 100 s is generated. The spurious suppression ratio reaches 97 dB, and the measured single-sideband phase noise is lower than -103 dBc/Hz at 10-kHz offset frequency.

A photonic integrated circuit developed in a generic foundry platform for continuously tunable microwave generation is presented. On this chip, two single-wavelength DFB laser diodes generate a two-tone emission through one optical waveguide coupler as combiner. The two-tone emission spectrum with >50-dB suppression ratio exhibits a continuous wavelength offset tunability up to ~4.3 nm, corresponding to 0.55-THz. With an external photodiode, a RF beat note at 10 GHz is experimentally demonstrated..

This study investigated a polyethylene terephthalate (PET) substrate and the effect of indium tin oxide (ITO) thin-film interference on the electromagnetic resonance of distorted metamaterials. The photoresist was developed on a PET substrate and swollen using isopropyl alcohol. The SRRs had various total lengths, gaps, and line widths. In addition, each of these three dimensions varied greatly and thus the distorted SRRs exhibited a broadband resonance spectrum. An ITO thin film was coated on the back of the PET substrate with a distorted metamaterials sample, and the terahertz spectrum was measured. The experimental results revealed that the ITO thin film can flatten the spectrum of the SRR sample. To determine the underlying reason, we varied the sheet resistance of the ITO film and observed the differences among the corresponding spectra. The flattened spectrum of the ITO films enhanced the thin-film interference effect of the PET substrate; consequently, the distorted metamaterials exhibited a flattened spectrum. These distorted metamaterials can be applied in terahertz imaging, terahertz communication systems, and optoelectronic integrated circuits.

The pharmaceutical industry requires a simple nondestructive quantitative inspection technique that does not change the characteristic properties of the target of inspection. Terahertz (THz) spectroscopy has potential as a nondestructive inspection technique because of the high transmittance of inspected materials and the absorption peaks related to some active pharmaceutical ingredients (APIs) in THz frequency regions. We have developed a compact terahertz spectroscopy system that uses an injection-seeded terahertz parametric generator (is-TPG) as a THz light source for the inspection and quantitative analysis of pharmaceutical tablets. Using this system, acetylsalicylic acid, acetaminophen, tranexamic acid, loxoprofen sodium, and caffeine included in over-the-counter (OTC) medicine tablets were identified as APIs by their specific absorption peaks. We also conducted a quantitative analysis of acetaminophen in one of the medicines by performing multivariate analyses. The root-mean-squared error of cross-validation (RMSECV) was 0.297 wt%. The limit of detection (LOD) and limit of quantitation (LOQ) were 0.990 wt% and 3.00 wt%, respectively. These results indicate that this is-TPG system could be applied as an inspection and quantitative analysis technique for OTC medicine tablets.

Art conservation terahertz (THz) diagnostics is an increasing field since THz imaging systems are commercially available. Since most of these experiment are done using femtosecond laser base time domain systems, we present in this paper how we can use frequency modulated continuous wave system to evaluate painting during restoration process.

We design and simulate planar antenna structure on the high- resistivity silicon substrate(ρ=1000Ω·cm) for the Nb5N6 micro- bolometer at the frequency range from 0.265 THz to 0.365 THz by CST Studio Suite. We have obtained the center frequency of the antenna at 0.3 THz by optimizing parameters of the antenna structure and the antenna has the very good radiation directivity. And the maximum directivity of the antenna is around 8.634 dBi at 0.3THz. The measured best voltage response of the Nb₅N₆ micro-bolometer detector is at 0.307 THz. The measured response frequency and the simulated S-parameter are in substantial agreement.