This PDF file contains the front matter associated with SPIE Proceedings Volume 11345, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Polarization, the path along which light’s electric field oscillates, is a key property of electromagnetic radiation. In this work, we motivate a mathematical framework—Matrix Fourier optics—that enables a simple description of light’s interaction with diffractive optics that spatially modify polarization. This formalism generalizes a large body of past work in metasurface polarization optics. We show how Matrix Fourier optics allows for the design of arbitrary polarization-analyzing metasurface gratings. These gratings can be used as the single polarization component in a compact full-Stokes polarization camera. We demonstrate practical, real-time polarization photography with this camera, which may find application in machine vision and remote sensing.

A new approach of a highly efficient counter propagating optical fiber based tweezers using 3D printed Fresnel lenses at the fiber facets will be presented. In contrast to conventional fiber tip tweezers the emitted beams of the Fresnel lens fibers are converging resulting in a significantly enhanced trapping efficiency in both axial and transverse direction. The used polymer diffractive Fresnel lens structures are fabricated by femtosecond two photo lithography using a commercial system. The measured trapping efficiencies are found to be up to 90 times higher than for similar results using chemically wet-etched fiber tips. A short outlook on further concepts of enhanced optical fiber tweezers based on 3D diffractive structures will also be given.

We present a set of experiments in which the backscattering spectra of 4 μm single TiO₂ particles are probed with circularly polarized vortex beams. The experiment is carried out with a tunable laser at λ = 760 - 810nm. We observe that the use of vortex beams enables us to tailor the backscattering in different ways. Given a certain backscattering of a particle (induced by a Gaussian beam or a plane wave), we observe that vortex beams can tune it and sharpen it. Moreover, we also observe that the level of conservation of helicty can be increased thanks to the use of vortex beams. We explain the mechanisms that give rise to these effects using Mie Theory. Our method of backscattering control can be experimentally implemented in most of microscopy set-ups. In addition, if brought to its limits, the method can be used to excite single multipolar modes from spheres. We believe that our method could find application in the levitation of particles or the excitation of whispering gallery modes.

The concept of bichromatic photonic lattice is shown to allow a very easy design of high-performance optical cavities with applications in nonlinear optics, integrated photonics and optomechanics.

The standard method for manipulating small particles is the optical tweezer, which relies on the strong optical gradient found at the focus of highly focused beams. However, this is not easily scalable for control of several particles simultaneously. Our recent work explores the possibility of achieving optical forces for manipulation of particles without using focused illumination, instead relying on near-field interactions when particles near a surface are illuminated with a simple plane wave, to provide repulsive and lateral forces, which can be controlled via the illuminating light’s polarisation, enabling fast modulation speeds. The use of plane waves would allow massively-parallel control, movement and sorting of vast numbers of particles simultaneously. A crucial requirement is active levitation because electrically polarised particles are known to strongly attract towards most uncharged surfaces naturally, hindering manipulation. Previous concepts for overcoming this obstacle have involved epsilon-near-zero metamaterials or multi-layered structures that introduce a degree of difficulty into the fabrication. In this work, we show that particles with an optical magnetic dipole resonance naturally repel from a plasmonic surface, such as any metal below its plasma frequency. Optical magnetic resonances have already been suggested as a means for novel nanophotonic applications, including Huygens metasurfaces and plasmon-assisted tractor beams. The repulsion is driven by the optical gradients found in the particle’s backscattering from the surface. We have conducted numerical proof-of-principle simulations that show this repulsion occurring for a realistic core-shell particle (designed to emit pure magnetic dipole scattering) near a bulk gold surface, showing clear evidence that the repulsion is driven by the particle’s magnetic resonance. We therefore propose a straightforward experiment that can demonstrate active levitation of a nanoparticle without structured light. We anticipate new nanomechanical devices deriving from this principle.

The mechanisms of absorption, emission, and scattering of photons form the foundations of optical interactions between light and matter. In the vast majority of such interactions there is a significant interplay and energy exchange between the radiation field and the material components. In absorption for example, modes of the field are depopulated by photons whose energy is at resonance with a molecular transition producing excited material states. In all such optical phenomena, the initial state of the radiation field differs in mode occupation to its final state. However, certain optical processes can involve off-resonance laser beams that are unchanged on interaction with the material: the output light, after interaction, is identical to the laser input. Such off-resonance interactions include forward Rayleigh scattering, responsible for the wellknown gradient force in optical trapping, and the laser-induced intermolecular interaction commonly termed optical binding; in both processes, an intense beam delivers its effect without suffering change. It is possible for beams detuned from resonance to provide not only techniques for optomechanical and optical manipulation, but also to passively influence other important and functional interactions such as absorption from a resonant beam, or energy transfer. Such effects can be grouped under the banner of ‘optical catalysis’, since they can significantly influence resonant processes. Furthermore, off-resonance photonics affords a potential to impact on chemical interactions, as in the passive modification of rotational constants and phase transitions. To date, apart from optical manipulation, the potential applicability of passive photonics, particularly in the realm of chemical physics and materials science, has received little attention. Here we open up this field, highlighting the distinct and novel role that off-resonance laser beams and the ensuing photonics can play.

Plasmonic nanowires are key building-blocks for plasmonic based devices, such as nano-sources and nano-sensors. They can take advantage of the propagative nature of surface plasmon polaritons (SPP) in a guided way, and the strong field enhancement at the nanowire tips. Here, the proof-of-concept of a SPP-mediated remote Raman effect with a coaxial nanowire is reported [1]. The remote Raman spectroscopy consists in probing a species at a distanced place from the excitation. The metallic nanowire geometry promotes a guiding of the surface plasmon polaritons along several micrometers, while the required momentum matching for exciting SPP is allowed by the discontinuity at the nanowire tip. The proposed systems are cylindrical coaxial nanowires consisting of a gold core to propagate SPP and a Raman-emitting shell of poly(3,4-ethylene-dioxythiophene) (PEDOT) grown only at one tip of the gold nanowire. A second challenge has been tackled, dealing with the weakness of the Raman signal to be detected. It was proposed to enlarge the nanowire tips in order to enhance the in-coupling of an excitation optical signal with the nanowire, and the out-coupling of the plasmon-mediated signal at a remote location. It has been achieved by transforming the gold nanowire tips with dry laser heating treatments to obtain dumbbell-like nanowires. The plasmonic properties of these original nanowires have been determined by an EELS-STEM study and interpreted by finite-element modeling. The benefits of these enlarged tips on the optical signal has been investigated by a Rayleigh backscattering study. [1] D. Funes-Hernando, M. Peláez-Fernández, D. Winterauer, J.-Y. Mevellec, R. Arenal, T. Batten, B. Humbert and J.L. Duvail Nanoscale (2018) 10, 6437 – 6444

A noble metal nanoparticle (MNP)- quantum emitter (QE) composite nanostructure operates as a nanoscale counterpart of a conventional laser, and serves as an ultra-compact coherent source of surface plasmons, which holds the potential of bolstering device miniaturization. Equivalent to a standard laser, the MNP acts as the resonator, and the gain medium consists of pumped QEs. In this work, we demonstrate the possibility of engineering the emission statistics of such a plasmonic laser, to meet prerequisites set by its intended application. We perform a comprehensive analysis on plasmonic statistics of a spheroidal MNP-QE composite nanostructure, through reduced density matrix formalism, and examine the tunability of the key observable quantities against various system parameters including the geometry of the MNP, the rate of excitation, and the dielectric constant of the submerging medium, to gather the insights for customizing and optimizing this composite nanostructure for a particular application. For a given frequency of operation, our simulations offer a guide for the most suitable shape of the resonator, and provides the estimations for the expected energy output and its coherence, for a range of input power. Furthermore, it can be extended to assist in making the material choices. In essence, our work facilitates tailoring efficient, coherent and tunable plasmonic laser devices to power-up many promising nanoscale applications.

An overview on our latest research on UV plasmonics with Rh and Ga metal nanostructures is presented. We will pay attention to their plasmonic performance and UV tunability. For Ga, its polymorphism will be analyzed and for Rh three characteristic geometries will be studied: tripod star, nanocube and tetrahedron. As an alternative to metals, low heat generation materials for bio applications will be analyzed. A numerical analysis of several candidate low loss dielectric materials that show HRI properties in the UV will be presented. In particular, this analysis will focus on the near-field enhancement and scattering directionality above 3 eV.

Plasmonic-metal nanostructures offer unique opportunities for solar fuels, due to their tunable light absorption characteristics and ability to generate hot carriers. In this talk I will show examples of functional devices and discuss indepth the underlying physical mechanisms. In particular, I will report our recent results on ultrafast dynamics of hot-holes as well as solid-state investigation of copper-based systems.

Cathodoluminescence spectroscopy (CL) is a unique technique to probe optical modes at the nanoscale. The electric field surrounding a 10-30 keV electron beam dynamically polarizes matter, creating optical excitations over the 0-10 eV spectral range, that are then detected in the far field. CL can measure the dispersion of plasmonic and dielectric nanostructures at deep-subwavelength spatial resolution. So far, CL has probed the angle-dependent spectrum and polarization of nanoscale emitters. However, detecting the phase of the emitted plasmon scattering wavefronts has remained elusive. Here, we introduce Fourier-transform CL holography as a method to determine the far-field phase distribution of scattered plasmonic fields. To do so, we measure the interference between two fields: (1) the electron-induced CL emitted by a plasmonic nanoscatterer and (2) a broadband reference field created by transition radiation induced by the same electron. From the angular interference patterns we directly reconstruct angle-resolved phase and intensity distributions. Taking the 6 (x-y-z) plasmonic electric and magnetic dipoles as a complete orthogonal set of scatterers we directly derive from the amplitude and phase data the relative strength and phase of all scattering dipoles, as they are excited by electron-beam. We investigate the resonant scattering of 30 keV electron-beam excited surface plasmon polaritons (SPPs) off single-crystalline Ag nanocubes and find dominant scattering from the z-oriented electric dipole plasmon. In contrast, SPP scattering from nanoscale holes made in a Ag film induces an in-plane x-oriented electric dipole with the concomitant y-oriented magnetic dipole; both interfering in the far field creating a strongly beamed plasmon scattering distribution. Using a newly developed CL energy-momentum spectroscopy configuration we derive the phase of scattered fields as a function of frequency. The data are fully consistent with the plasmon polariton dispersion and the pi phase flip across the scattering resonance is directly observed in the measured phase fronts. Fourier-transform CL holography opens up a new world of coherent light scattering and surface wave studies at nanoscale spatial resolution. It also opens up novel ways to investigate the temporal and spatial coherence of electron beam wavefonts and addresses fundamental questions regarding the collapse of the electron wavefunction as it excites surface plasmons.

Modal expansion techniques have long been used as an efficient way to calculate radiation of sources in closed cavities. With one set of cavity modes, calculated once and for all, the solution for any arbitrary configu-ration of sources can be generated almost instantaneously, providing clear physical insight into the spatial variation of Greens function and thus the local density of states. Nanophotonics research has recently generated an explo-sion of interest in generalizing modal expansion methods to open systems, for example using quasinormal mode / resonant state expansion [1]. Yet one major practical obstacle remains: numerical generation of resonator modes is slow and unreliable, often requiring considerable skill and hand guiding. Here, we present a practical numerical method for generating suitable modes, possessing the trifecta of traits: speed, accuracy, and reliability. Our method is capable of handling arbitrarily-shaped lossy resonators in open systems. It extends existing methods that expand modes of the target struc-ture using modes of a simpler analytically solvable geometry as a basis [1]. This process is guaranteed to succeed due to completeness, but is ordinarily inefficient because optical structures are usually piecewise uniform, so the resulting field discontinuities cripple convergence rates. Our key innovation is use of a new minimal set of basis modes that are inherently discontinuous, yet remarkably simple. We choose to implement our method for the General-ized Normal Mode Expansion (GENOME) [2] which unlike its alternatives [1], is valid for any source configuration, including the important case of sources exterior to the scatterer. We achieve rapid exponential convergence, with 4 accurate digits after only 16 basis modes, far more than is necessary. This also means lightning-speed simulation results, faster by 2-3 orders of mag-nitude compared to mode generation using COMSOL. Finally, our method is extremely reliable, as it culminates in a small dense linear eigensystem. No modes go missing, nor are there spurious modes that need to be manually discarded, which is critical to the success of modal expansion methods. [1] M. B. Doost et al., Phys. Rev. A 90, 013834 (2014), C. Sauvan et al., Phys. Rev. Lett., 110 237401, (2013) [2] P. Chen, D. Bergman and Y. Sivan, Phys. Rev. Appl., accepted (2018).

The availability of low cost, integrated, radiation sources in the infrared with a narrow-band emission spectrum is of great importance in a variety of applications such as infrared sensing, thermophotovoltaics, radiation cooling, and thermal circuits [1]. An easy way to obtain infrared radiation is to take advantage of thermal emission form a heated body. However, the spectral and directional control of thermal emission is a challenging task due to the incoherent behavior (both spatially and temporally) of the thermal radiation. We investigate the possibility of spatially and spectrally controlling the thermal infrared emission by exploitation of the Yagi–Uda antenna design. Hybrid antennas composed of alternating SiC and Au elements are considered. As a starting point we considered SiC dipole antennas as feeders, operating at 400 K. The length and section of the rods have between designed in order to have a single resonance in the wavelength range around 12 micron. We used Au elements to work as the reflector and the directors elements. Indeed, gold is a low emissivity material and the mirror and directors are supposed to be slightly detuned with respect to the emitting wavelength. However, all the elements are at the same temperature and contribute to the final bandwidth of emission and efficiency. The numerical study was performed by modifying a previously developed model based on the fluctuational electrodynamics approach and on the discretization of the resulting volume integral equation to calculate relative emissivity and spatial emission pattern of nanoparticle ensembles [2]. We show that despite of the chaotic nature of thermal radiation it is possible to obtain efficient highly directional and narrow bandwidth antennas in the mid to far IR by adapting the Yagi-Uda scheme with a combination of metallic and polar materials. We compare the performances obtained with hybrid antennas with respect to all metallic antennas and discuss how to improve specific features for different kind of applications. References [1] A. Lochbaum et al. “On-Chip Narrowband Thermal Emitter for Mid-IR Optical Gas Sensing”, ACS Photonics, 4, 1371−1380, (2017) [2] M. Centini et al., “Midinfrared thermal emission properties of finite arrays of gold dipole nanoantennas”, Phys. Rev. B. 92 205411 (2015)

Evanescent and propagating field distributions from a strongly focused wave beam with subwavelength waist w_a << λ as a function of polar angle and distance are investigated. Exact amplitudes and intensities of evanescent E_ev and propagating E_p fields, including interference terms, are presented both in near and far field regions. It is shown that the amplitude of E_ev decays as exp(-r/w_a) in near-field region and evanescent waves do not contribute to the far field in the forward direction as well as in the transverse directions θ = π/2, even though the oscillating evanescent field of the same strength, but opposite in sign to the propagating field, exists in the transverse sheet. A far-field method for characterizing apertures based on the relationship between the relative intensity of propagating modes and a subwavelength aperture diameter is proposed.

Rare-earth doped upconverting nanoparticles like NaYF₄:Yb,Er absorb infra-red light and emit visible radiation. We use light at the absorption resonance, namely 975 nm, to optically trap the nanoparticles and subsequently study the effect on the trapping parameters like the trap stiffness. We trap hexagonal shaped upconverting nanoparticles and find that these align with their side along the tweezers beam. We also place a polarizers to study the backscatter emission features and find that the emission spectra vary depending upon the orientation of the polarizer with respect to the side axis of the particle. We use this to find the rotational Brownian motion along the yaw sense. We also use the fluorescent emission to ascertain the axial Brownian motion of the particle. This can be used to ascertain the motion of the particle while trapped on resonance in tweezers without actually accessing the infra-red trapping light.

Plasmonic devices are of great interest for different applications including chemical sensing for food and water contaminants. Their properties to confine high electromagnetic fields strictly depend on the size, shape and, more in general, on the geometry of their basic constituents. In this work we fabricate and characterize two-dimensional periodic arrangements of novel plasmonic supra-molecular cells with different minimum intercell distance and in both nanopillars and nanocavities geometries. For patterns based on nanopillars we evaluate the bulk sensitivity associated to their localized surface plasmon resonance which results to have a value up to 408 nm/RIU while for the patterns based on nanocavities we found a SERS enhancement factor up to 2.1x10⁶. We tested the sensing performance of these nanostructures analyzing different concentrations in water of fipronil pesticide using both LSPR and SERS tools. Our results suggest that these plasmonic patterns are promising to develop nanosensors for a dual-sensing detection of water contaminants with high sensitivity.

Magneto-luminescent materials that do not exist in nature, can find a wide application in biomedical and environmental fields. Here we describe magneto-optical properties of core-shell-shell nanocomposites consisting of a Fe₃O₄ superparamagnetic iron oxide nanoparticle (SPION) covered with quantum-sized brightly luminescent CdSe layer surrounded by ZnS passivating shell. The synthesized nanocomposites demonstrate excitonic bands in their absorption and photoluminescent (PL) spectra centered at 585 nm and 603 nm, respectively. The PL quantum yield of nanocomposites has been increased by 5 times due to their passivation with ZnS shell. The analysis of magneto-optical properties of the synthesized Fe₃O₄/CdSe/ZnS nanocomposites has shown that their magnetic circular dichroism (MCD) spectrum is characterized with the bands centered at 430 nm, 350 nm and 303 nm corresponding to ⁶A₁ → ⁴E, ⁴A₁(⁴G); ⁶A₁ → ⁴E(⁴D) and ⁶A₁ → ⁴T₁(⁴P) electronic transitions, respectively. It has been found that the synthesized core-shell SPIONs demonstrate excellent colloidal stability, magneto-optical properties typical for SPIONs and bright photoluminescence

We present a theoretical study of a nanostructured guided-mode resonant (GMR) spectral filter operating in the long-wave infrared (LWIR) wavelength range. The component is made of III-V semiconductors: heavily n-doped InAsSb for the grating and GaSb for the waveguide of the GMR resonator. In order to study the tolerance for the fabrication process and to adjust the resonance of the filter, a geometric study is also presented.

Detection of a single light nano-source by photoluminescence microscopy reveals many properties which would be hidden by ensemble averaging. However many applications use light sources packed in a dense layer, for instance light-emission diodes (LEDs) using quantum dots or nanoplatelets. Increasing attention has been brought to such dense systems, showing that they behave differently from single isolated emitters because of interactions, charge transport and energy diffusion between neighboring particles. Semiconductor nanoplatelets present especially large interactions as they tend to self-assemble with very large areas facing each other and very small separation distances. In particular, by Förster-resonant energy transfer (FRET), an exciton in a given platelet can recombine and create another exciton in the neighboring platelet with a theoretical rate of (3 ps)-1, much larger than radiative recombination. By using adequate solvent and ligands, we have assembled linear chains of hundreds of CdSe 7x21 nm² nanoplatelets, constituting an exceptional model system for analyzing the interaction and collective behavior of stacked nanoparticles. We focused a laser beam onto one spot of the nanoplatelets chain. Photoluminescence imaging showed light emission from a much larger portion of the chain, with energy propagation over typically 150 nm. A diffusion equation model of this system shows that such propagation distances correspond to a FRET time of a few picoseconds, in good agreement with theoretical estimates. Ongoing experiments are considering how such fast transfer affects the photophysics of nanoplatelets (blinking, multi-exciton recombination) and induces collective effects. References : Fu Feng et al., ACS Photonics 5, 1994 (2018) Fu Feng et al., Nano Research 11, 3593 (2018) S. Jana et al., Science Advances 3, e1701483 (2017)

Forster Resonance Energy Transfer (FRET) is a major interparticle energy transfer mechanism used in a wide range of modern-day applications. Hence, enhancing the FRET rate by different mechanisms has been extensively studied in the literature. Obtaining Plasmonic enhanced FRET by placing a metal nanoparticle (MNP) in the vicinity of energy exchanging molecules is one such mechanism. Here we present a model to elucidate the effects of extraneous surface charges present on such a vicinal MNP on the FRET rate considering the nonlocal response of the MNP. This model is based on the well established extended Mie theory of Bohren and Hunt along with the idea of introducing an effective dielectric function for the charged MNP. Our results indicate that the excess surface charges will lead to a blueshift in the resonance frequency and greater enhancements in the FRET rate for both local and nonlocal response based methods. Furthermore, we propose potential substitutes for noble metals that are conventionally used in plasmonic enhanced FRET.

Atom-field interactions near optical interfaces have a wide range of applications in quantum technology. Motivated by this, this paper revisits the spontaneous emission of atomic dipoles in the presence of a two sided semi-transparent mirror. First we review the main properties of the quantised electromagnetic field near a semitransparent mirror. To do so, we employ a quantum mirror image detector method which maps the experimental setup which we consider here onto analogous free space scenarios. We emphasise that the local density of states of the electromagnetic field depends on the reflection rates of both sides of the mirror surface. Hence it is not surprising that also the spontaneous decay rate of an atomic dipole in front of a semi-transparent mirror depends on both reflectance rates. Although the effect which we describe here only holds for relatively short atom-mirror distances, it can aid the design of novel photonics devices.

Semiconductor quantum dots (QDs) are widely used in photovoltaic and optoelectronic devices due to their unique optical properties. Photoluminescence (PL) properties of QDs can be significantly improved by their electromagnetic coupling with plasmonic nanoparticles (PNPs). The excitation of resonant localized plasmon modes leads to the enhancement of the density of photon states and increase of electromagnetic field near the surface of PNPs, what boosts the acceleration of the exciton radiative decay, known as the Purcell effect. To study the dependence of the degree of acceleration of radiative decay rate (Purcell factor) on the distance between QDs and PNPs, we fabricated thin-film hybrid structures based on CdSe(core)/ZnS/CdS/ZnS(multishell) QDs and silver or gold PNPs with a controllable distance between these components. The change in the radiative decay rate of excitons was calculated from the PL intensities and lifetimes before and after the deposition of PNPs on top of the QD thin film covered by a poly(methyl methacrylate) (PMMA) spacer. For both PNP types, the PL lifetime of underlying QDs decreased, whereas the PL intensity of the latter decreased only slightly for gold PNPs and even increased for silver PNPs. This indicates the acceleration of QDs radiative decay (Purcell effect) mediated by exciton-plasmon interaction. The Purcell factor was higher for silver PNPs than that for gold PNPs, what can be explained by the better spectral overlap between the QDs PL band and silver PNPs absorbance and the absence of interband absorption in silver at the wavelength of QDs PL. The results of this study provide better understanding of the Purcell effects in hybrid materials based on QDs and PNPs.

The impact of prolonged irradiation of SPIONs/CdSe/ZnS nanocomposites by visible light on nanocomposite luminescence has been studied. It has been shown that prolonged irradiation of the nanocomposites with 405 nm laser can triple their photoluminescence quantum yield. It has been demonstrated that the efficiency of photoinduced processes on the nanocomposite surface correlates very well with the concentration of the nanocomposite surface ligand in our samples. We have also found that the quantum sized CdSe shell of SPIONs/CdSe/ZnS nanocomposites demonstrates the QD-like dependence of photoluminescence quantum yield on visible light dose and this nanocomposite property can be efficiently used to brighten their photoluminescence.

Tapered nanowire antennas have emerged as a versatile solid-state platform for quantum optics. These broadband photonic structures efficiently funnel the spontaneous emission of an embedded quantum dot into a directive free-space beam. They find application in the realization of bright sources of quantum light, and enable the implementation of giant optical non-linearities, at the single-photon level. In this work, we discuss advances aiming at further optimizing this light-matter interface. In particular, recent measurements revealed that the thermal excitation of a single nanowire vibration mode can have a sizeable influence on the quantum dot optical linewidth. This motivated a comprehensive theoretical analysis, which shows that the thermally-driven vibrations of the nanowire have a major impact on the quantum dot light emission spectrum. Even at liquid helium temperatures, these prevent the emission of indistinguishable photons. To overcome this intrinsic limitation, we propose several designs that restore photon indistinguishability thanks to a specific engineering of the mechanical properties of the nanowire. We anticipate that such a mechanical optimization will also play a key role in the development of other high-performance light-matter interfaces based on nanostructures.

In this work, we report a Multi-Walled Carbon Nanotubes (MWCNTs) MIR source for operation with MEMS spectrometers. We designed a miniaturized source that consists of a micro-machined joule heater on a highly doped silicon substrate, where the heater surrounds an active area. The micro heater filament is made of a platinum thin film on top of silicon with a thin titanium layer, which is used as an adhesive layer between the Silicon dioxide (SiO₂) and the platinum. After dicing the silicon chips, a multi-layered thin film of solution-based MWCNTs is plotted within the active area using a micro-plotting machine with a layer dimensions of 4x4 mm² and a layer thickness of about 1μm. Finally, the device is thermally annealed to improve the morphology of the MWCNTs thin film surface. The micro-machined platinum structure is joule heated by means of applying a voltage difference to the designated pads on the chip allowing a uniform surface heating of the active area containing the MWCNTs thin film. In order to measure the emitted radiation, a MEMS MIR FTIR spectrometer is used to measure the emitted power spectral density from the source with and without the plotting of the MWCNTs thin film, in the MIR range from 2.5μm up to 4.8μm while applying different voltages. The recorded results show that the plotting of the MWCNTs over the silicon substrate improved the recorded PSD of the spectrometer for the all applied voltages. The thermal distribution of the active area is also captured by means of infrared camera at different voltages showing maximum temperatures of 251 °C and 296 °C, while applying 25 and 30 Volts, respectively.

Dielectric nanostructures designed in sub-wavelength scale can be tuned to achieve high-Q resonances in the wavelength region of interest with a high concentration of field in and around the structure, which can be used to achieve enhanced light-matter interaction. Such dielectric metasurfaces are potentially conducive platforms for exploiting nonlinear photonic devices at lower input power levels. In this work, we design, fabricate and experimentally demonstrate one-dimensional silicon nitride based guided-mode resonant structure, which exhibits inherently low nonlinear optical effects for enhancing third harmonic signals from a conformal layer of ultra-thin amorphous silicon coated over the gratings. The GMR structures studied here consist of an etched silicon dioxide layer deposited on top of a glass substrate, followed by the deposition of a silicon nitride layer. The thickness of the silicon nitride layer is chosen (~ 160 nm) to achieve GMR resonances around 1550 nm wavelength. The resonance is found to redshift to 1580 nm in presence of the 10 nm amorphous silicon layer. THG studies on the above amorphous-silicon deposited GMR structures shows resonant enhancement of ~ 18x on-grating when compared to off-grating at the peak of the GMR resonance. The present work demonstrates the use of a silicon-processing compatible material stack to realize separately GMR resonances and nonlinear medium to achieve resonant nonlinear enhancement, thus paving the way for silicon-compatible layered nonlinear metasurfaces.

We propose and numerically simulate a new and highly compact integrated 4x4 mode coupler based on two single-mode waveguides exploiting both forward and backward propagating directions to double the number of modes. The two parallel waveguides are coupled via long and short-period gratings to the co- and counterpropagating directions, respectively, of a single cladding mode of the device which acts as a bus between the waveguides. By connecting all end facets to optical circulators we construct a device with four input and output ports but only using two single-mode waveguides. Such a device can be fabricated in a single micromachined silica ridge structure. A photosensitive raised index layer is used for vertical confinement that supports multiple modes horizontally. We UV-write the waveguides and the Bragg gratings and provide a tilt angle to improve coupling. We have demonstrated this technology before for a polarizing waveguide-to-waveguide coupler and have simulated other unidirectional devices. We use coupled mode theory to simulate the system. By tailoring the grating parameters, we can achieve a wide variety of coupling ratios. Analytically, we find a set of solutions in which no light escapes via the cladding modes through the ends of the device and we have calculated device parameters to achieve a wide range of splitting ratios including coupling light from one input port equally into all output ports. Moreover, we derived analytically a set of parameters to implement a Walsh-Hadamard transformation and are investigating further options to implement a universal 4x4 mode-coupler on this platform. We envisage that the device can be used for quantum information processing where two qubits are encoded in the waveguides using a photon in each propagation direction.

The most common way to enhance interaction of electromagnetic waves with matter at the nanoscale is to use microresonators and resonant optical nanoantennas. In virtue of small size their optical properties are well described in terms of multipole decomposition, namely, by first several terms in the multipole expansion. The specific multipole content of the mode is completely determined by its symmetry and shape of the resonator. Here, we classify eigenmodes in resonators of the simplest shapes depending on their symmetry group. For each type of mode, we found its multipole content. As an illustrative example, we apply the developed formalism to the analysis of dielectric triangular prism and demonstrate the formation of high-Q resonances originated due to suppression of the scattering through the main multipole channel.

We report on quantum dynamical studies of ultrafast photo-induced energy and charge transfer in organic semiconductor materials, complementing time-resolved spectroscopic observations that underscore the coherent nature of the ultrafast elementary transfer events in these molecular aggregate systems. Our approach combines first-principles parametrized Hamiltonians with accurate quantum dynamics simulations using multiconfigurational methods, along with semiclassical approaches. This paper focuses on the elementary mechanism of coherent exciton migration and creation of charge-transfer excitons in polythiophene type materials, representative of the poly(3-hexylthiophene) (P3HT) polymer. Special emphasis is placed on the interplay of trapping due to high-frequency phonon modes, and thermal activation due to low-frequency ”soft” modes which drive a diffusive dynamics.

We report a comparative experimental and theoretical study of second harmonic generation from a 20nm-thick indium tin oxide nanolayer in the proximity of the epsilon-near-zero condition. We record the efficiency of the second harmonic signal both as a function of wavelength as well as of the angle of incidence around the epsilon-near-zero crossing point. We compare our experimental results with numerical simulations based on a hydrodynamical model able to capture all major physical mechanisms driving the electrodynamic behavior of conductive oxide layers, with unique aspects of the different nonlinear sources. We found a very good quantitative and qualitative agreement between experiment and theory.

Semiconductor nanocrystals (SNCs), in particular, quantum dots (QDs) and nanoplatelets (NPLs), have orders of magnitude higher two-photon absorption cross-sections (TPACS) than organic dyes, what paves the way to their advanced applications in bioimaging, sensing, and optoelectronics. Traditionally, z-scan and two-photon photoluminescence (PL) excitation spectroscopy are used to determine the TPACS values. The main disadvantage of both methods is the necessity to know the exact sample concentration. In this study, we describe an approach to the TPACS determination from the analysis of two-photon-excited (TPE) PL saturation in CdSe(core)/ZnS/CdS/ZnS(multishell) QDs and CdSe NPLs. The results obtained for NPLs using developed approach are significantly smaller than those obtained by the z-scan method and are close to the values obtained for QDs. We assume that this discrepancy occurs due to the fact, that unlike the z-scan technique, the TPE PL saturation method measures the TPACS only for single-exciton states because of the low PL quantum yields of multiexciton states. Therefore, there is no need to know the concentration, which eliminates the corresponding estimation error. Thus, the measurement of TPE PL saturation in SNCs makes it possible to determine the absolute values of the TPACS of single-exciton states, which are more informative for applications of TPE PL than the TPACS of mixed multiexciton states.

Three-dimensional (3D) metamaterials show the potential for realizing efficient nonlinear nanoscale devices. Despite the recent progress, the nonlinear metamaterials lack in terms of conversion efficiencies when compared against conventional nonlinear materials that rely on phase-matching techniques. Here, we demonstrate how the nonlinear responses of 3D metamaterials can be improved by stacking metasurfaces on top of each other and by applying phase-matching techniques. We demonstrate this by successfully fabricating phase-matched metamaterials consisting of stacked metasurfaces. Especially, we observe a 25-fold enhancement of second harmonic generation emission from a device consisting of five metasurfaces.

In this work, quite different from many previous studies in the ultrafast region, we study the thermo-optical nonlinearity of a single metal nanoparticle and many-nanoparticle composite under continuous-wave illumination. For single metal nanoparticle system, we show that the thermal effect is able to qualitatively explain the experimental results of the strong nonlinear scattering from sufficiently small Au nanoparticle. To characterize the thermo-optical nonlinearity of single nanoparticle of finite size, we use the best experimentally measured data of the temperature dependent permittivities of bulk gold and calculate the temperature and scattering cross-section of the nanoparticle. We show that, quite counterintuitively, the particle temperature changes with its size non-monotonically. Furthermore, our numerical model shows much better agreement with the nonlinear scattering measurement results than the previous studies. The results of the single nanoparticle system are then used to study the thermo-optical nonlinearity of many-nanoparticle composites. Specifically, the temperature distribution of the many-nanoparticle composite is calculated by properly summing the heat generated by all nanoparticles in the composite as well as modeling by simulation. We show that, in contrast to the case of a single nanoparticle, the temperature distribution and thus the thermo-optical nonlinearity of the composite are weakly dependent on the illumination wavelength, nanoparticle size, and density, but is strongly sensitive to the beam size and the thermal conductivity of the host material. These results are critical for the optimization of the photo-thermal effect in many applications. More importantly, since photo-thermal effects were shown to be responsible for observations of faster chemical reactions, our results can be used to interpret correctly the differences in chemical reaction enhancements originating from the thermo-optical nonlinearity at different illumination intensities.

The possibility of surface plasmon polaritons (SPPs) amplification and generation in a waveguiding system containing single-walled carbon nanotube (CNT) and dielectric substrate is investigated. The SPP amplification is created by applying direct current in the CNT. Using numerical simulation, the effect of the substrate on the SPP characteristics in CNT was studied and it was shown that for a substrate with a high refractive index, the SPP amplification is realized at a lower drift velocity of free carriers of the pump current. For realization of the feedback in the CNT it is proposed to use the substrate with a periodic modulation of the dielectric function.

In this paper, the DFT calculations for three stilbene derivatives, of 4-aminostilbene, 4,4'-diaminostilbene and 4-amino- 4'-(N,N-diethylamino)stilbene, are proposed enabling to assign the SERS spectra reliably, including in the «hot spots» conditions. The choice of the best model is made based on more than 50 calculations of molecule*Agn clusters. We studied, how the cluster size (4, 6 and 14 silver atoms), charge (positive localized, non-localized and neutral) and solvent accounting affect on the results of calculation. The obtained theoretical data is compared with the SERS experimental results and with the similar systems from literature. The best correspondence is obtained for the systems with Ag14 complexes (both, in the «bridge way» and «single-end» absorption). Accounting for the solvent brings the calculated data substantially closer to the experimental as well. Charge localization affects on the calculated results for asymmetric complexes only at a given level of theory.

In our work, transverse Anderson localization is introduced for the first time in a simple wedge-type optical waveguide, which is formed by a triangular air hole imbedded into a fused silica material via a conventional fiber drawing technique. The micro tube is filled with a polymeric medium consisting of fluorescent dye molecules and naturally formed air inclusions caused by the capillary effect to offer a scattering medium for photons to localize the interfered electromagnetic waves. Anderson localization is explored through various single modes at different emission wavelengths within the photoluminescence spectral bandwidth of dye molecules. The photonic design of the optical waveguide allows the guidance of a single Anderson localized mode and suppression of the other modes to enable investigation of the spontaneous emission rate of the emitters, which are principally coupled into a single Anderson localized mode. The physical mechanism behind the changes in the emission dynamics of the fluorescent emitters is investigated by the time-resolved spectroscopy, which is found to be on resonance dependent with a particular cavity mode. The fastest decay rate of the light emission from the excited dye molecules is attributed to be due to the photons that couple into the localized optical modes without any spectral detuning. The enhancement of the spontaneous emission rate by a factor of 2.2 is achieved as the majority of the photons are coupled into an Anderson localized mode. Thus, a simple wedge-type optical waveguide is demonstrated to provide an opportunity to enhance light-matter interaction and opens new avenues to understand the nature of the spontaneous emission dynamics of the fluorescent emitters that are trapped in quasi optical cavities.

Generalized Brewster effect is a phenomenon where light of both TE (S-) and TM (P-) polarization transmit through a surface with no reflection for a particular incident angle. Generalized Brewster angle (GBA) in visible and near-infrared (NIR) wavelength region is very useful in many scientific and technical areas of applications. However, it is very rare to find a material having this effect as it demands both dielectric and magnetic response in that wavelength range and usually magnetic response is extremely weak in the optical wavelengths. Here we demonstrate the GBA effect of an anisotropic material composed of highly ordered high aspect ratio aluminium doped zinc oxide (AZO) nanopillar arrays. Along with the experimental demonstration, we also provide a proper numerical analysis to investigate the origin of this effect in the pillar array system which will be useful for many conventional as well as new applications in photonics including protein sensing.

Since the development of genome editing tools like CRISPR/Cas9, it is possible to modify the sequences of genes in a very specific manner. The molecular delivery into plant protoplasts to improve the quality of agricultural crops represents a major bottleneck in the routine application of CRISPR/Cas9 in modern plant breeding. To approach this need, we suppose using gold nanoparticle mediated (GNOME) laser transfection for delivery of CRISPR/Cas9 ribonucleoproteins (RNP) into potato protoplasts with high-throughput. As a proof-of-concept, we aim to reduce the toxic steroidal glykoalkaloid α-solanine in potatoes. GNOME laser transfection utilizes a picosecond Nd:YAG laser operating at 532 nm to excite surface plasmon resonance of membrane-attached gold nanoparticles. The strong absorption of laser light results in a temperature increase, leading to vaporization of the surrounding medium and to the formation of cavitation bubbles, which causes a transient permeabilization of the cell membrane. The challenges modifying protoplasts, in contrast to mammalian cells, include their sensitivity to osmolality stress, the lack of adherence to culture surfaces, the absence of commercial antibodies for nanoparticle targeting, and the low adherence of the applied nanoparticles to the protoplast’s membrane. Viability in respect to different conditions was evaluated using a resazurin assay and the delivery of molecules by FITC-dextrane. To facilitate the binding of the nanoparticles, a combination of a cell membrane binding lectin and a linker molecule was investigated. Furthermore, we demonstrate the prototype of a bench-top laser transfection device, which allows conducting the complete workflow within a biological laboratory environment.

Starting from optical Dirac equation, an alternative form of Maxwell's equations, I introduce a statistical operator and derive the optical analogue of von Neumann equation, which is the dynamical equation of the operator. I also found one of its stationary solution, a thermal state. According to the asymptotic analysis of the thermal state, optical coherence is sustainable at the zero temperature, while unsustainable at the high temperature limit. We can regard this as decoherence of classical optical field.

This work emphasizes on the comparative study of tin based group IV single and multiple quantum well photodetector in absence of light. Initially, the designs of the single quantum well infrared photodetector (SQWIP) and multiple quantum well infrared photodetector (MQWIP) are proposed and explained along with considerations. Dark current and detectvity is calculated by using rate equations considering carrier transfer mechanism in MQWIP and SQWIP. The result reveals that dark current in the order of microampere is obtained for SQWIP but it can be reduced by increasing number wells. Significant peak detectivity in the range of 10⁹ cm Hz^1/2 W^-1 is obtained for MQWIP at lower bias which is higher than that of SQWIP. However judicious selection of proper bias and number of well is required for optimized operation of MQWIP.

In this paper, the interband light absorption and photoluminescence of coupled vertical cylindrical quantum dots with double modified Pöschl–Teller potential in terahertz range made of InAs are studied. Expressions for the particle energy spectrum, wavefunction, absorption and PL coefficients and dependencies the geometrical sizes of quantum dot are obtained. The selection rules corresponding to different transitions between quantum levels are found.

The article presents the results of the SERS study of fluorine-containing dye 6G (R6G) adsorbed onto quartz surfaces modified with gold nanoparticles (NPs). A new technique for quartz glass modifying with hydrosols of gold NPs of various shapes has been developed. The possibilities of its application to implement SERS effect for R6G molecules have been shown. In this work, we synthesized hydrosols of spherical gold NPs (nanospheres) and rod-shaped NPs (nanorods (NRs)) and studied their optical and morphological properties. The SERS spectra of R6G molecules on NP modified quartz glasses have been obtained as well as the SERS enhancement factor has been calculated.

In realizing excellent plasmonic devices, a methodology based on flexibility and simplicity in fabrication, minimal sensitiveness to smaller nanoscale errors, larger dielectric layer thickness with superior device characteristics, and low-cost process is critically crucial for next-generation devices with multiple applications. One such attractive device is a plasmonic nanocavity, with numerous reports been already reported resulting in superior localized surface plasmon resonance (LSPR) and enhancement properties. The conventional spherical NP on a metallic mirror (NPOM) nanostructure’s plasmonic characteristics deteriorates with minor changes in dielectric layer thickness (t ≤ 6 nm). Alternatives like nanocube on mirror (NCOM), nanodisk on mirror (NDOM), provided better options towards LSPR tuning and near field enhancement. In recent times there are few reports based on faceted spherical NPOM design emerged. But however, the so far reported FNPOM nanostructures lacked the following: “facet width control, a clear SEM/TEM image of full geometry, and larger “t” with superior plasmonic characteristics”. In this work, for the first time, we report a clear FNPOM nanostructure with better control in facet fabrication using reactive thermal annealing (RTA) method. We used Ag NP on an Au mirror with SiO2 as a dielectric layer with different NP diameters of 50 nm, 70 nm and 100 nm with a precise facet width control (from 90% sphere to hemisphere). We employed a larger “t” ranging between 10 nm – 40 nm to display superior properties. From our dark field and LSPR mapping measurements, 70% of the sample are showed similar plasmonic characteristics from a 1 cm x 1 cm substrate. Our experiment results showed that it is possible to tune the LSPR resonance wavelength till 40 nm dielectric thickness reflecting it as a superior plasmonic nanocavity device. The reason behind this enhanced plasmonic characteristics is due to the introduction of facet in NPs and our three-dimensional finite difference time domain (3D FDTD) simulations results agreed well with experiment. For a final comparison, we checked our hemispherical shaped FNPOM versus NCOM design for NPs with diameter of 100 nm, where we find our FNPOM nanostructures showcased superior plasmonic properties.

In this work, we report the feasibility of the silver nanowire (AgNW) foils as highly-sensitive, reproducible and facile detection tools for surface-enhanced Raman spectroscopy (SERS) applications. These flexible and free-standing AgNW foils, fabricated by vacuum filtration method following a modified polyol synthesis of AgNWs, are adequately structured for both biological specimen filtering and trace amount of molecule detection simultaneously. The compatibility of AgNW foil in SERS is investigated by using a Raman active molecule of different steric volumes across the filter cross-section. We have shown that AgNW foils exhibit extremely strong SERS activity with detection limit up to 10^-9 M of crystal violet (CV) molecule with 20% variation over ~cm², revealing reliable homogeneity of the acquired signal. While naturally occurring polyvinylpyrrolidone (PVP) layer during polyol synthesis contribute to controlled aggregation, oxidation prevention, and size - shape control purposes, it also creates a major challenge for obtaining enormous enhancement factors. However, controlled thickness of aluminum oxide (Al₂O₃) coating on PVP@AgNW foils affords to achieve higher enhancement factors than the uncoated ones. What is interesting is that the maximum intensity is achieved from two cycles of Al2O3 deposited on AgNW foils. This is attributed to the two different origins: first, a higher adsorption affinity of CV molecules to Al2O3 layer than PVP layer; second, tunneling barrier formation against quantum tunneling effects.

Paper performs results of SERS-active surfaces fabrication for Raman bacterial cells analysis. Based on FDTD simulation, the synthesis of colloidal gold nanoparticles (NPs) with the size range of 10 – 100 nm has been performed by the following methods: a) femtosecond laser ablation of a plate in a liquid; b) chemical reduction from tetrachloroauric acid trihydrate (HAuCl₄). Optimal sizes and shapes of the particles with a maximum of plasmon absorption in the range 500 – 800 nm have been determined by numerical simulation. For NPs deposited on quartz glass with rodamine 6G (R6G) and E. Coli bacterial cells, SERS solutions have been tested.

In this work, we numerically investigate a dielectric nanocavity composed of gallium phosphide nanocylinders. Our results demonstrate that proposed structures allow to increase the emission rate into zero phonon line of NV-center by a factor of 10. We compare properties of cavities made of crystalline silicon and gallium phosphide. Obtained parameters of the nanocavity are suitable for nanodiamonds with NV or SiV color centers and adopted for the existing lithography methods. We believe the proposed system is perspective for creation of a quantum nanophotonic chip for application in quantum telecommunication and quantum computing.

The Ge1-xSnx material system has been introduced as a potential solution for low-cost high-performance photodetector for short-wave infrared towards mid-infrared detections. An investigation of GeSn/SiGeSn nanostructure layer is reported for sensors for near and mid-infrared applications. Physics-based models will be developed for SiGeSn/GeSn based nanostructured sensors considering the carrier dynamics at hetero-interface, misfit dislocation and strain at the interface. We analyze the effect of biaxial strain on SiGeSn/GeSn alloys and determine the range of wavelength for the possible application in near and midinfrared range.

The paper presents the results of FDTD (Finite-Difference Time-Domain) mathematical modeling of electromagnetic fields distortion near the surfaces of multilayered spherical gold nanoparticles (NPs). NPs were functionalized by two shells: water as a model substance for a drug and SiO₂, as a capsuling polymer. The field values were converted into the coefficient of the effective signal for Surface-Enhanced Raman Scattering (SERS). During the simulation, parameters such as NP size, thickness of surface layers, wavelength of exciting radiation and the dependence of the effective amplification of the electromagnetic field on the thickness of the polymer and water layers were studied. The prospects of the theoretical approach of nanocomplexes for problems of theranostics have been shown. The presented approach could be applied as a basis for performing methods of controlled chemical synthesis of colloidal theranostics NPs.

We propose plasmonic nanostructures - a 2D array of circular ‘nanopillars inside square nanoholes’ - as polarization independent SERS substrates for portable detection of chemical and biological molecules. These substrates were fabricated in a reproducible and controllable manner on a wafer-scale using a combination of deep-UV lithography, reactive ion etching (RIE) and E-Beam evaporation. The SERS spectra were measured using a portable Raman spectrometer to demonstrate portable SERS based sensing and the limit of detection was found to be ~ 13.14 femtogram for the detection of 2,4-DNT. Furthermore, numerical modeling of the proposed substrates was carried out using Finite Difference Time Domain (FDTD) modeling to study the effect of structural parameters on the electromagnetic enhancement factor and the resonance wavelength. Moreover, based on numerical simulations and experimental results, it was found that the SERS signal from these SERS substrates is only slightly dependent on polarization. Thus, the proposed SERS substrate can be employed for polarization independent SERS-based trace detection of chemical and biological molecules in real-time field settings using a portable Raman spectrometer.

The hybrid systems consisting of noble metal nanoparticles with plasmon resonances and organic cyanine dye, which are able to delocalize and migrate the energy of excitons on a large number of aggregated molecules of the structure, can be used to study the plasmon-exciton interaction. The position as well as the region of overlap of the absorption of the exciton band and the plasmon resonance of the nanoparticles in the form of island film allows observing both weak and strong coupling. The influence of overlap between the J-aggregate band of cyanine dyes with different length of the conjugation chain and nanoparticle plasmon resonance on the optical properties of hybrid structures was studied. Inhomogeneous ensembles of noble nanoparticles were obtained as an island films on the sapphire substrates by thermal deposition in vacuum. The homologous series of dicarbocyanine, monocarbocyanine and pseudoisocyanine dyes was used for forming J-aggregates in ethanol solution without adding salt by the original technique. Dye solution was spincoated on the island film to obtained hybrid structure. The plasmon resonance of the island film was broadened due to the large dispersion of the nanoparticles in size. So strong and weak plasmon-exciton coupling can be observed in hybrid structures due to overlap of the absorption bands of island film and exciton-transition in J-aggregates. The influence of near field of noble nanoparticles on enhancement of dye molecule absorption is observed in hybrid films with dicarbocyanine and monocarbocyanine. The surface molecular concentration was monitored, in the experiment there was no significant increase of the number of adsorbed molecules on island film in comparison with clean dielectric substrate. Spectral dip at the wavelength of the maximum absorption of the J-aggregate was observed for hybrid films with pseudoisocyanine.

In this work we propose a simple one-step method for creation of hybrid Au/Si micro- and nanostructures with strong nonlinear response. We demonstrate that such structures depending on laser structuring parameters can produce strong enhancement of second harmonic signal compared to the initial Au/Si films or/and broadband white-light photoluminescence in the visible optical range. To explain this dependence, the studies of the fabricated structures were carried out by the Raman spectroscopy demonstrating strong correlation between phase composition of the structures and their nonlinear properties. We believe that proposed structures can be used as efficient nonlinear souses for different applications in bioimaging and nanospectroscopy.

We report stable and reproducible optical trapping of Eu-doped NaYF4 anisotropic nanorods using single fiber tip optical tweezers for investigating the orientation resolved emission spectra. The nanorods were elaborated by the hydrothermal process followed by annealing and centrifugation steps, resulting in a well-defined size distribution. Nanorod trapping was observed at two positions, first, in fiber tip contact and second, at a finite distance of 4 - 6 µm away from fiber tip in the axial direction. The nanorod trapped with tip contact was highly stable and stay trapped for several hours. They were aligned with the fiber axis with a residual angular distribution width of 4° at a light power of 34.8 mW. Moreover, we have determined trap stiffness of the off-tip trapped nanorod by applying the Boltzmann statistics and power spectra analysis of position fluctuations. Subsequently, the trapped NaYF4:Eu nanorods were used for studying the Eu3+ emission spectra in two orthogonal directions: perpendicular and parallel to the nanorod axis. The influence of anisotropic polarization was observed in the perpendicular direction whereas emission remained isotropic polarization in the parallel direction. The observed emission spectra have been analyzed for the well-defined peaks at 590 and 614 nm corresponds to the magnetic and electric dipole transitions. The experimental investigations were completed by studying the polarization-dependent emission spectra in the perpendicular direction.

Properties of self-assembled III-V quantum dot (QD) heterostructures for optoelectronic devices mainly rely on growth parameters and also on substrate used. The research community mainly preferred GaAs substrate instead of Si substrate for optoelectronics. However, the low cost and abundance of Si impels the researchers and industrialists to use Si for the commercial application using Si_xGe_1-x graded layer and Migration Enhanced Epitaxy (MEE) layer. Here we have studied the optical and structural study of Stranski–Krastanov (S-K) InAs quantum dots grown on Ge substrate with 6° offcut toward the (110) plane (Sample A) without MEE layer, which may be easy to integrate on Si. Starting from the thick GaAs buffer layer, AlAs/GaAs super-lattice buffer layers followed by three consecutive layers of 2.7 ML S-K InAs QDs with 50 nm GaAs capping were grown. Another sample (B) with the same heterostructure was grown on GaAs substrate for comparison. Low temperature photoluminescence (PL) for the sample (A) is blue-shifted as compared to sample B, which might be due to smaller dots formation. The bi-modal dot size distribution of the sample A and sample B was confirmed from the power dependent PL. In the low temperature PL spectrum, full width half maxima (FWHM) of the sample A is very close to that of the sample B. Rocking curve obtained from high resolution X-ray diffraction (HRXRD) for the sample A, shows Ge substrate peak and GaAs peak from the GaAs layer grown on the Ge substrate. The strain calculated from the HRXRD rocking curve for the sample A and sample B is -4.12x10^-3 and -2.0x10^-3 respectively. Strain value indicates crystalline quality of sample A is good and comparable to the same in sample B, grown on the GaAs substrate. The optical properties for the sample A can be enhanced further via monolayer coverage of the dots, capping material, capping thickness and ex-situ annealing techniques.

In this study, we have discussed the effect of strain distribution and optical properties on In_0.14Ga_0.86As matrix thickness variation (t_mat) in self-assembled InAs quantum dot (QD) heterostructure using temperature and power-dependent photoluminescence (PL) measurements. The calculated ground-state transition energies are 1.12, 1.14 and 1.09 eV for t_mat of 2, 4 and 6 ML (monolayer) In_0.14Ga_0.86As matrix thickness respectively. We also discern that the full-width at half-maximum (FWHM) broadens gradually as temperatures increases due to electron-phonon scattering. The calculated activation energy (E_a) values are 231, 302 and 98 meV for increasing t_mat. The partial strain relief due to varying In_0.14Ga_0.86As layer thickness occurs due to QD size tunability by preventing Indium (In) segregation effect, that sets the possibility to understand about InAs inter-band and intersubband transitions of PL emission. This has been validated with HRXRD results where strain decreases linearly with increasing t_mat. Here In_0.14Ga_0.86As layer acts as a strain-reducing layer (SRL) in QD heterostructure as well. Thus helps in reducing the hydrostatic strain (∊_hyd) of InAs QDs, while the lower InGaAs layer increases the QD density, leading to a remarkable rise in PL intensity due to state filling of carriers. The effect of strain distribution for varying t_mat in the heterostructure was also studied using nextnano++ simulations. The relative percentage change in hydrostatic (biaxial) strain was calculated to be 5.5% (8%) respectively. Thus, the results so obtained can help in tuning matrix thickness on the PL emission properties of QDs and therefore in the realization of several optoelectronic devices.

This study examines the photoluminescence (PL) properties and strain distribution in InAs/InGaAs heterostructure for varying number of sub-monolayer (SML) quantum dot stacks (nSML). High resolution x-ray diffraction (HRXRD) probes the strain effects, whereas PL spectroscopy evaluated the optical response. The ground-state transition energies calculated from PL experiments were found to be 1.19, 1.13, 1.11, 1.12 eV for 4, 6, 8 and 10 stacks respectively. It was observed that, with the increasing nSML, the PL peak emission energy has an initial blue shift and later a red shift, due to build-up of strain energy propagating from the bottom layers of InAs quantum dots (QDs). The activation energies (E_a) calculated from temperature-dependent PL (TDPL) measurements are 414, 279, 260 and 231 meV for 4, 6, 8 and 10 stacks respectively. The Raman characterization results explores on the strain relaxation effects by observing the shift and broadening in TO and LO phonon peaks of GaAs bulk material. The strain energy distribution along the growth direction (z-direction) was studied using nextnano++ simulations. The relative change in hydrostatic and biaxial strain at a particular z - position was calculated to be 3.2% and 5.5% respectively These strain components are of prime importance in understanding the position of conduction and valence band energy levels and finally the band gap energy. Thus, with these articulated results, we conclude that sample with 6 SML stacks is the optimum choice for fabricating optoelectronic devices operating in long range infrared telecommunication regime.

We have investigated the effects of varying facet angle of the truncated pyramidal QDs on the strain and energy band profile. The conventional truncated QD pyramid has a height of 3 nm with the facet angle of 51˚ along with base and top width of 15 and 11 nm respectively, as observed from the previously reported transmission electron microscopy images. Also, the simulated results were validated with reported experimental data for reliability of our work. Five different angle variations viz. 41˚, 45˚, 51˚, 56˚, 61˚ were considered, referred as structure A, B, C, D and E. It is observed that the magnitude of hydrostatic strain is reduced by 1.22% for the structure E (facet angle of 61˚) and increased by 1.507% for structure A (facet angle of 41˚), when compared with the conventional structure C. Therefore, the carrier confinement would be better in case of structure E as compared to its counterparts. In comparison to structure C, the biaxial strain is 1.39% higher and 2.203% lower in case of structure E and A respectively. Higher biaxial strain would inculcate red shift in emission wavelength because of the movement of heavy-holes upward in energy, which is reflected from the computed photoluminescence peaks. The emission wavelength obtained for structures A, B, C, D and E were approximately 1140, 1151, 1155, 1161 and 1183 nm respectively. Thus, among all, structure E offered longest emission wavelength and optimum strain distribution within the heterostructure.

In this study, the concept of analog and digital alloy capping with ternary alloy In_xGa_1-xAs has been utilized in capping layer of InAs QDs. The composition of indium (In) in the capping layer having thickness of 8 nm, was kept constant in case of analog alloy technique. While, in digital alloy technique, In composition was varied from 0.45 to 0.15 in steps of 0.10 (Structure D1) having a thickness of 2 nm each. Validation of simulated results with previous experimental data was carried out to check accuracy of the present work. The hydrostatic and biaxial strains in growth direction were computed and compared. The magnitude of hydrostatic strain is reduced by 4.115% and biaxial strain is increased by 0.62% in the QD region of structure D1 as compared to structure A1. This indicates better carrier confinement and longer emission wavelength in structure D1 as compared to structure A1. Structure D1 provided an increment of hydrostatic strain by 3.41% and a decrement of biaxial strain by 2.56% in the QD region as compared to structure A2. Structure D1 with digital capping alloy provides a gradual strain relaxation in the capping layer due to systematic variation of In composition, ensures strain relaxation profile throughout the heterostructure by minimizing the lattice disparity in subcapping layers. The computed emission wavelength of structure A1, A2, and D1 were 1224, 1313, and 1287 nm respectively. The digital alloy technique would help in minimizing strain distribution within the heterostructure and results in defect and dislocation free heterostructure.