This PDF file contains the front matter associated with SPIE Proceedings Volume 11771, including the Title Page, Copyright information, and Table of Contents.

Welcome to the SPIE Quantum Optics and Photon Counting 2021, a part of Optics + Optoelectronics Digital Forum 2021! Conference co-chairs Ivan Prochazka, Roman Sobolewski, Martin Stefanak, Aurel Gabris and our Program Committee members, we all thank you for joining us in this exciting event and sharing your cutting-edge research with the colleagues from around the world.

Superconducting single-photon detectors (SSPDs) have developed into a mature device technology and excel due outstanding performance metrics, in particular high detection efficiency combined with high time resolution and low dark count rate for a wide wavelength range from the visible to the mid-infrared. In addition to commercially available systems with devices coupled to optical fibers, SSPDs can be integrated with photonic circuits using scalable nanofabrication technologies. Here, we will present recent progress on SSPDs based on NbTiN thin films and their integration on different photonic material platforms. Our process for NbTiN growth at room temperature will be described, using magnetron reactive co-sputtering to achieve high-quality superconducting layers down to thicknesses of few nanometres. Optimized SSPD devices are realized by tuning the superconducting properties of NbTiN thin films, adjusting the material composition and nanocrystalline structure. The realization of different types of detectors and geometries will be shown, including nanofabrication techniques for achieving fully suspended nanowire structures. Furthermore, we will discuss challenges and prospects for scaling-up SSPD device technology as well as detector systems. Multiplexing schemes such as dispersion engineering of superconducting transmission lines will be highlighted as powerful approach to address multiple detectors and reduce the number of required feedthroughs and electrical lines in the cryostat. Eventually, exemplary applications of SSPDs for photon counting in quantum optics and light detection and ranging (LIDAR) will be outlined.

Superconducting Single-Photon Detectors (SSPD) invented two decades ago have evolved to a mature technology and have become devices of choice in the advanced applications of quantum optics, such as quantum cryptography and optical quantum computing. In these applications SSPDs are coupled to single-mode fibers and feature almost unity detection efficiency, negligible dark counts, picosecond timing jitter and MHz photon count rate. Meanwhile, there are great many applications requiring coupling to multi-mode fibers or free space. `Classical’ SSPDs with 100-nm-wide superconducting strip and covering area of about 100 µm2 are not suitable for further scaling due to degradation of performance and low fabrication yield. Recently we have demonstrated single-photon counting in micron-wide superconducting bridges and strips. Here we present our approach to the realization of practical photon-counting detectors of large enough area to be efficiently coupled to multi-mode fibers or free space. The detector is either a meander or a spiral of 1-µm-wide strip covering an area of 50x50 µm2. Being operated at 1.7K temperature it demonstrates the saturated detection efficiency (i.e. limited by the absorption in the detector) up to 1550 nm wavelength, about 10 ns dead time and timing jitter in range 50-100 ps.

Single-photon counting has become an essential tool in quantum optics experiments, as well as remote sensing and life science applications. However conventional technologies such as single-photon avalanche diodes, as well as the availability of standard telecom optical components, has limited much of this work to the near infrared/telecom wavelength range. Superconducting nanowire single photon detectors (SNSPDs) have emerged in recent years as the gold standard in photon counting applications due to their low dark count rates, fast timing resolution and high efficiency [1]. SNSPDs have also demonstrated photon counting efficiency out to much greater wavelengths which enables us to explore new experimental possibilities in the mid-infrared [2]. In this work we design and fabricate mid-infrared SNSPDs and deploy them in a variety of photon counting experiments [3,4]. The devices are based on a NbTiN superconducting film integrated into an optical cavity to enhance absorption in the mid-infrared. We characterise these devices using an optical parametric oscillator, tuneable between 1.5 m and 4.2 m. We then deploy these in a proof-of-principle tabletop light detection and ranging (LIDAR) experiment at 2.3 m. LIDAR in the mid-infrared is attractive due to spectral regions of low atmospheric absorption and reduced solar background photon flux, when compared to telecom wavelengths. We also present results from a photon-pair source operating at 2 m. This is a key resource for extending quantum optics and quantum secure communications to the mid infrared domain. Pairs are generated using a custom lithium niobate crystal and detected using SNSPDs. We demonstrate two-photon interference and polarisation entanglement of the photon pairs at 2 m. This work opens the pathway to future development of quantum optics and quantum technologies in the mid-infrared spectral region. References [1] Gol’tsman et al Applied Physics Letters 79 705 (2001) [2] Marisli et al Nano Letters 12 (9) 4799 (2012) [3] G. G. Taylor et al Optics Express 26 (27) 38147 (2018) [4] S. Prabhakar et al Science Advances 6 (13) eaay5195 (2019)

We present the result of the creation and investigation of the multi-element superconducting single photon detectors, which can recognize the number of photons (up to six) in a short pulse of the radiation at telecommunication wavelengths range. The best receivers coupled with single-mode fiber have the system quantum efficiency of ⁓85%. The receivers have a 100 ps time resolution and a few nanoseconds dead time that allows them to operate at megahertz counting rate. Implementation of the multi-element architecture for creation of the superconducting single photon detectors with increased sensitive area allows to create the high efficiency receivers coupled with multi-mode fibers and with preserving of the all advantages of superconducting photon counters.

Quantum communication is a fast-growing field that takes advantage of the quantum physics laws to protect and secure sensitive data. This work takes part of the European project UNIQORN (Affordable Quantum Communication for Everyone: Revolutionizing the Ecosystem from Fabrication to Application) whose aim is to develop a Quantum System on Chip (QSoC) for telecom application. The Integrated Circuit (IC) designed contributes in the QRNG block of the system, tailored to communicate with the integrated non-linear optics circuit. Such detector is a 32×1 linear array based on Single-Photon Avalanche-Diode (SPAD) detectors for the generation of a raw random number, by revealing the position on the array of the single photon impinging on it, realized in a BCD 0.16 μm technology. The linear array architecture consists of 32 pixels, pitched at 125 μm, each made by 4 SPADs with different diameter (5 μm, 10 μm, 20 μm, 50 μm). Two operation modes are implemented: Single-Hit Mode, needed to reveal the (5-bit) position of the pixel triggered by a single photon, representing a random number, in a time window synchronous with the laser emission. Multi Hit Mode, used to identify a coincidence of a certain number of photons(more than one, two, three or four) detected within a specified time window, thanks to a multi threshold coincidence detection logic.

Microscopy resolution below the diffraction limit can be achieved by exploiting quantum light properties. NitrogenVacancy (NV) color centers in diamond, dye molecules and quantum dots are examples of single-photon emitters, whose antibunching property allows super-resolution imaging through the measurement of high-order autocorrelation functions. In this work, we present a novel Single Photon Avalanche Diode (SPAD) array architecture optimized for n-fold photon coincidence counting, in each point across the whole sensitive area. It is implemented in a 160 nm Bipolar-CMOS-DMOS (BCD) technology, and it includes 24 × 24 SPAD pixels with 50-μm pixel pitch and 10-μm SPAD diameter. Multi-photon coincidences (within time windows ranging from 2 ns to 500 ns) are identified by post-processing of the in-pixel timing data. Given the expected low photon rate on the detector in quantum imaging applications, on-chip logic discards unwanted information to limit readout throughput and data storage. In fact, reading the whole array would take 3 µs, while skipping rows detecting no photon reduces the readout time to 240 ns in case of no photon detected over the entire array. Moreover, we implemented a multi-gate approach, which avoids halting the array during readout, thus enabling multiple data acquisitions. Thanks to these power-saving expedients and efficient readout, the architecture is scalable towards multiple modules, such as 48 × 48 or 96 × 96-pixel arrays. Finally, it features the possibility of being coupled with a micro-lens array to reach a 78% equivalent fill-factor.

An innovative general-purpose Digital Silicon-PhotoMultiplier (dSiPM) with 32 × 32 SPADs, designed in 160 nm BCD technology, is presented. The main goals of this device are to enhance the dynamic range, still keeping the single-photon resolution, and minimize the timing jitter. Both an analog and a digital approach are used to distinguish between 1 to ~300 incoming photons. A voltage-controlled current generator converts the pixel’s digital output pulse in a current pulse, tunable in amplitude (10 μA ÷ 350 μA) and duration (from 1 ns to the SPAD holdoff time). The digital option is useful in low photon flux applications. Instead, in high photon flux applications, the digital output misses information, due to an overlap among the photon pulses, so the analog option is to be preferred. Moreover, a double threshold algorithm is implemented in order to reduce the timing jitter of the output. Basically, the concept behind this procedure is to refer the timing measurement to the crossing of the lower threshold, while the higher threshold is only used as a validation for the measurement. Finally, a Time-to-Digital Converter (TDC), with a resolution of 75 ps, is integrated to provide the timing information. The SPAD frontend design works in a free running photon detection modality, and there is the possibility to enable or disable the pixels individually. Thanks to its programmable number of photon resolution and the improved timing performance, the detector can be exploited in many different scientific applications.

We present the development and the validation of two SPAD camera systems, based on two SPAD array chips (Figure 1), respectively with 8x8 and 128x1 high-performance CMOS SPAD pixels, able to acquire both photon-counting 2D “intensity” images and photon-timing 3D “time-resolved” (hence, also distance-resolved) maps. Each pixel integrates a 30 μm SPAD detector, an 8-bit in-pixel counter (to counts the number of photons detected during user-selectable timeslots in the nanoseconds and microsecond range), and a 12-bit Time-to-Digital Converter (to timestamp the arrival time of the first photon detected by each SPAD, with sub-nanosecond resolution). In addition, the two array chips have the capability of actively gating the SPADs, driving the SPAD bias voltage above or below breakdown, with subnanosecond transitions allowing efficient time-domain filtering of incoming light. Active gating can be enabling in applications such as non-line of sight 3D ranging and time-domain functional Near-Infrared Spectroscopy (fNIRS), since it allows hiding unwelcome reflections, stray rays, or luminescence/fluorescence excitation signals. In fact, this feature allows the user to selectively avoid “early” photons, for instance those reflected by the sample surface, while measuring only the useful “late” photons, for instance those which interacted with the deeper biological tissue’s layers, preventing the triggering due to the strong reflections, which would saturate the SPADs. For optimizing chip operation in many different applications, both systems are extremely versatile and allow the user to customize the cameras for various measurement setups. The cameras quantum sensitivity allows the reconstruction of faint optical signals through the Time-Correlated Single Photon Counting ⁽¹⁾ (TCSPC) technique. In addition, they enable many quantum experiments where information on each photon arrival time is required for example to identify time-coincident events with entangled photons. The 128x1 linear array is perfectly suited for spectroscopy applications, particularly for advanced Raman techniques, thanks to on-chip time-gating and time-tagging capabilities.

We present a fully reconfigurable single-photon camera, able to operate in both analog and digital modalities so to exploit the best performance out of the two features while solving usual issues that affect SPAD imagers. At first, one of the problems in SPAD arrays is the presence of hot pixels, with much higher dark count rate (DCR) compared to others, that impair system performance and are inevitable because of the presence of trapped charge or impurities in the semiconductor. Then, analog SiPMs provide photon-number resolved analog output, but they give no information about where photons hit the active area. Instead, digital SiPMs (dSiPMs) are position sensitive, can integrate also on-chip electronics (e.g., to count and to time-stamp single photons) and can enable/disable each single pixel, depending on their DCR or the user application. The latter feature can be mandatory in applications where the background is comparable to the dark-count rate of the SPAD microcell. Instead, applications such as Light Detection and Ranging (LiDAR) may not care about hot pixels because usually ambient light background is much higher (e.g., up to 100 klux). The novel camera employs a versatile dSiPM based on a 5 × 5 SPAD array imager, configured for photoncounting applications: each SPAD has both an individual digital output pulse and a common analog output with programmable pulse-width. Thus, it provides all advantages of SPAD arrays, since each SPAD can count the photons detected therein, by means of FPGA-based programmable counters with integration period ranging from 2 ms to 500 ms. Moreover, the chip also provides the advantages of analog SiPMs, since it detects coincidences in an analog way, by using current pulses generated by the triggered SPADs and summing them together to provide a common output analog current. The pulse widths are adjustable in amplitude and in time duration, from 1 ns to 10 ns, so to select the desired coincidence window. A comparator signals when more than a user-selectable number N of photons get concurrently detected. Such a feature has been profitably exploited in LiDAR and Quantum Imaging applications. Furthermore, the availability of the 25 digital outputs allows to extract also the position of the detected photons. The developed camera is also suitable for TOF measurements. Feeding both the output of the comparator and the sync signal coming from a laser illuminator to a Time-to-Digital Converter (TDC), single point distance measurement can be performed. The great advantage of using coincidences is the reduction of the pile-up effect and the unwelcome ambient light triggering, thus drastically improving background rejection in most applications.

Light Detection and Ranging (LiDAR) is a technique that can be applied to identify the position of objects in an industrial environment, which usually suffer by strong background illumination. In this work we present a novel architecture of a Single Photon Avalanche Diode (SPAD) array optimized for a direct Time Of Flight (dTOF) single-point rangefinder system, with a distance range of about 2 m and a resolution of a few centimeters. The ASIC has been implemented in a 0.16 µm Bipolar-CMOS-DMOS (BCD) technology and includes 10 × 40 pixels, 80 Time-to-Digital Converters (TDCs), and a histogram builder. The peculiarity of this work is the ability of performing a Region-Of-Interest (ROI) selection of just those pixels illuminated by the laser spot, as well as a smart sharing of timing electronics. ROI selection is performed through SPADconnected up/down counters, that are decremented whenever the connected SPAD is triggered within the time window where the laser spot is expected, whereas they are incremented when the connected SPAD is triggered within a time window where the laser pulse is not present. If the counter stores a negative value, the pixel is considered to be within the laser spot, and just those pixels might trigger a TDC during the following 500 samples frame. Each TDC is shared among 5 non-adjacent pixels that should not be hit at the same time, considering the expected laser spot dimension. The implemented TDCs have 75 ps resolution and 19.2 ns Full Scale Range (FSR).

Based on the typical performance indicators in single photon detectors, a novel figure of merit (FoM) is proposed to quantify the overall performance of SPADs (Single Photon Avalanche Diodes). The overall performance comparison exists not only between different devices but also between the same devices under different operating conditions. In this paper, the same device under different operating conditions is used as an example. To verify the validity of the novel figure of merit, a fuzzy mathematical model is introduced from the perspective of statistical mathematics. A compromise fuzzy decision-making method is used to analyze when a device achieves optimal performance under different operating conditions. The weights of the performance indicators required by the method are obtained by the combination weighting approach which combines the analytic hierarchy process and entropy weight method. The results show that the optimal operating conditions obtained by fuzzy mathematical analysis are consistent with the results obtained from the new FoM, and thus the novel FoM we proposed is feasible.

We are reporting on the concept, design, construction, and critical operating parameters of a new photon-counting detector package. It was developed based on silicon SPADs manufactured using K14 technology. Four detection chips with an active area diameter of 25 microns are used. The active quenching electronics enable the detection chips' operation in a bias range of 0.5 to 2.5 Volts above their breakdown voltages in a continuous counting mode. The entire design and construction are prepared for long-term operation in space conditions. Our operation experience of K14 detection chips and all the electronics in numerous space missions was taken into account when designing the device. It can be operated in an extensive temperature range of −55 to +50°C without any active temperature stabilization. The built-in SPAD biase power supply voltage is following the SPAD breakdown voltage temperature dependence. This way, the detection chips are biased fixed bias above their breakdown voltage over the entire temperature range. The critical detector parameters depend on a selected bias above a breakdown voltage. For selected configuration, every single detector's parameters are as follows: photon detection probability at 800 nm is 30%, the maximum count rate is 2 MHz, the timing resolution is better than 80 ps FWHM, detection delay temperature drift is within the range of ±0.3 ps/K. The dark count rate is typically < 50 kHz at +25°C. It may be reduced one order of magnitude, lowering the operating temperature to 0°C. The entire detector package power consumption is well below 1 Watt; its mass will be below 100 grams.

Photon counting imaging detectors (PCD) has paved the way for the emergence of Spectral X-ray Computed Tomography (SCT), which simultaneously measures a material’s linear attenuation coefficient (LAC) at multiple energies defined by the energy thresholds. In previous work SCT data was analysed with the SIMCAD method for material classifications. The method measures system-independent material properties such as electron density, ρ_e and effective atomic number, Z_eff to identify materials in security applications. The method employs a spectral correction algorithm that reduce the primary spectral distortions from the raw data that arise from the detector response: charge sharing and weighting potential cross-talk, fluorescence radiation, scattering radiation, pulse pile up and incomplete charge collection. In this work, using real experimental data we analyze the influence of the spectral correction on material classification performance in security applications. We use a vectorial total variation (L∞-VTV) as a convex regularizer for image reconstruction of the spectral sinogram. This reconstruction algorithm employs a L∞ norm to penalize the violation of the inter energy bin dependency, resulting in strong coupling among energy bins. Due to the strong inter-bin correlation, L∞-VTV leads to noticeably better performance compared to bin-by-bin reconstructions including SIRT and total variation (TV) reconstruction algorithms. The image quality was evaluated with the correlation coefficient that is computed relative to ground-truth images. A positive weighting parameter defines the strength of the L∞-VTV regularization term and thus controls the trade-off between a good match to spectral sinogram data and a smooth reconstruction in both the spatial and spectral dimension. The classification accuracy both for raw and corrected data is analyzed over a set of weighting parameters. For material classification, we used 20 different materials for calibrating the SIMCAD method and 15 additional materials in the range of 6 ≤ Zeff ≤ 15 for evaluating the classification performance. We show that the correction algorithm accurately reconstructs the measured attenuation curve, and thus gives higher detection rates. We show that using the spectral correction leads to an accuracy increase of 1.6 and 3.8 times in estimating ρ_e and Z_eff, respectively

The brain is composed of the cerebrum, cerebellum, diencephalon and brainstem. The cerebrum is the superlative part of the central nervous system and also the main part of the brain. There are differences and similarities between humans and mouse. The study of mouse brain model is helpful to understand the process in clinical trials and also has reference significance for the study of human brain. Therefore, the study of mouse brain is particularly important. As the skull has a large scattering effect on light, it is difficult for us to image the brain through the skull directly. Therefore, we often use methods such as optical clearing or thin skull to reduce or remove the influence of the skull on imaging. In this paper, the transmission of photons in mouse brain was studied using Monte Carlo method. In the study of photon trajectories, the photon distribution without intact skull went farther in both longitudinal and transverse directions compared with that of with intact skull. In terms of the optical absorption density and fluence rate. On the condition of with intact skull, the distribution of optical absorption density and fluence rate was fusiform and rounder on the whole. The radial distribution range of optical absorption density and fluence rate was 0.25 cm, which was approximately 2.5 times of that of with intact skull. In the depth direction, due to the strong scattering and absorption of the scalp and skull, the optical absorption density dropped sharply from 0.890 cm^-1 to 0.415 cm^-1. When the photons arrived at the gray matter layer, only a few photons were reserved. Due to the strong absorption and scattering effect of the gray matter layer, only a few photons left, the optical absorption density increased from 0.415 cm^-1 to 0.592 cm^-1, and then decreased again. When the depth was 1.35 cm, the optical absorption density dropped to 0 cm^-1. After removing the skull, due to the weak absorption and scattering effect of normal saline and cerebrospinal fluid, the optical absorption density was low (0.119 cm^-1) and dropped slowly. When the photons arrived at the gray matter layer, most of the photons were reserved. Due to the strong absorption and scattering effect of the gray matter layer, the optical absorption density increased from 0.117 cm^-1 to 0.812 cm^-1, then the optical absorption density decreased to 0 cm^-1 at a depth of 1.35 cm. The distribution of radiant fluence rate is similar to that of optical absorption density. This study will provide reference and theoretical guidance for the optical imaging of mouse brain and the study of the mouse and human brain.

The main goal of the research is to study the catalytic properties of the DNA in the reactions of reduction of silver ions and photo-induced processes due to the unique structural properties and capabilities of molecular recognition of DNA using spectroscopic and thermodynamic methods. Reduction of silver ions (Ag⁺) on a double helix of the DNA is based on inter-strand crosslink formation model consisting of several adsorption processes. It is shown that inter-strand crosslink is an absorption process consisting of several simple adsorption processes and the time of this absorption process is the sum of adsorption times. The conformational transitions of DNA caused by photo-irradiation of silver atoms (Ag⁰) on the DNA molecule have been studied. Photo-induced desorption of silver atoms from DNA double helix is a complex and multiphase process, which, for its part, causes conformational transitions in DNA. In particular, we have two states of bonding of silver atoms the first state before irradiation and the second state after irradiation. Kinetic study of the photo-desorption of silver atoms from the DNA-Ag⁰ complex obtained desorption rate constant 𝑘𝑑 and adsorption heat Q_a, which are k_d ≅ 1.8 × 10⁻⁴ s ⁻¹ and Q_a ≥ 85 kJ/M (Ag0) for silver atoms which are bound to the DNA. The oscillatory force of the DNA-Ag⁰ complex for a silver atom is counted to by equal to f _{(434.3 nm)} = 0.33.

In this paper, a new optical pumping method, redistributing the atom populations among the Zeeman magnetic sublevels of the ground state |F=3> is proposed as the state preparation process to improve the signal to noise ratio (SNR) of the atomic fountain clock with a larger atom population on the |F=3, m_F=0>clock state. A preliminary experimental result of state preparation efficiency exceeding 60% is obtained on the NIM6 cesium (Cs) fountain clock.

Resonance energy transfer between an excited donor and a potential acceptor is a highly researched area in science. Multiple theories have been introduced in the literature to understand and simulate this energy transfer. The formulation of quantum master equation incorporating full polaron transformation approach is one of the approximation methods for simulating dynamics of the coherent resonance energy transfer. Full polaron based quantum master equation is well known for undergoing infrared divergence for Ohmic and sub-Ohmic environments where the spectral density function scales linearly or sub-linearly at low frequencies. Our objective of this paper is to study an environment where logarithmic perturbations can be experienced with a full polaron based quantum master equation and gauge its performance. In doing so, we study how a perturbation in the frequency domain affects the overall quantum coherence of the energy transfer. Our results demonstrate that for larger system bath coupling strengths, full polaron based quantum master equation is unable to provide accurate results whereas for weaker system bath coupling strengths, it performs better. Further, for a given system bath coupling strength, as logarithmic perturbations are increasing, the damping characteristics of the coherent energy transfer are also increasing. In addition, we show that smaller values of the Ohmicity parameter can suffer severe distortions even for a small logarithmic perturbation. Doing so, we show that full polaron transformation-based quantum master equation is capable of undergoing infrared divergence even for a super Ohmic environment, when higher orders logarithmic perturbations are present.

Excitations Energy transfer occurring among an excited donor chromophore and potential acceptor chromophores has gained prime research interest owing to the highly efficient nature of the energy transferring process. One of the more popular approximation methods in simulating this energy transfer is the multi-site exciton full polaron transformation-based quantum master equation which has shown the ability to interpolate between weak and strong system bath coupling regimes. It has been shown that decay processes in many physical processes follow the well-known exponential decay laws with inverse power law behaviour at longer time scales. Conventional ohmic-like spectral density functions, model this behaviour well. However, it has been shown quantum mechanically that the long-term relaxation of such systems also has a significant inverse logarithmic term that is not captured by ohmic-like SDF models. Therefore, logarithmic decays and logarithmic factors are not rare in the literature with respect to excitations energy transfer. Recently introduced Ohmic-like spectral density function that can account for slight perturbations in the frequency domain has used these logarithmic factors to model this perturbation. Our objective of this paper is to study the energy transfer of a multi-site exciton system attached to an environment where these logarithmic perturbations could be experienced, with a full polaron based quantum master equation. Our results reveal that, when system bath coupling strength is larger the derived multi-exciton full polaron transformation-based quantum master equation is unable to simulate accurate dynamics where in some scenarios the well-known phenomena of infrared divergence occur. On the other hand, when the system bath coupling strength is weak, derived equation conveys better results. In addition, results show that smaller Ohmicity values can suffer from acute distortions even for a smaller logarithmic perturbation. Also, we show that when logarithmic perturbations are increased, damping characteristics of the energy transfer are also increased in general.

The influence of the Sb composition both on the band-alignment and the optical characteristics of strain-coupled vertically aligned InAs/GaAsSb Stranski-Krastanov (SK) quantum dots (QDs) embedded on six stack InAs/In_0.15Ga_0.85As Sub-monolayer (SML) matrix has been studied using nextnano simulation tool. A ten-layer strain-coupled InAs SK QDs electronically coupled to six stack SML QDs which has been the optimized structure is utilized in this study. Four different structures with Sb composition of 10%, 14%, 18% and 22% are chosen as a capping layer over InAs QDs and it is found that a transition in the band-alignment from type-I to type-II occurs when the Sb composition is increased above 14%. The optical characteristics have been simulated for these heterostructures which showed a red shift in the photoluminescence (PL) peak values with increase in the Sb composition. The PL peak value of ~1035 nm has been validated with the experimental PL data for the ten-layer InAs/GaAs SK QDs grown on six stack SML QDs without GaAsSb capping. With the similar dot size, the PL peak occurred at ~1115 nm, ~1159 nm, ~1209 nm and ~1284 nm, respectively, for 10%, 14%, 18% and 22% Sb composition structures. Investigation of electron and hole eigen states has been done for these structures. The usage of GaAsSb capping layer (strain reducing layer: SRL) over the InAs SK QDs allows an undulated strain transition from one SK QD layer to the other. The hydrostatic and the biaxial parts of the strain are estimated and a decrease in the hydrostatic compressive strain in the QDs has been observed with increase in the Sb composition. An increase in the biaxial strain with Sb composition has been noticed which result in lowering of the energy band gap and a red shift in the PL emission wavelength. Along with type-II band alignment, the low hydrostatic strain with 22% Sb composition facilitates lower dark current and also a red shifted PL results from ~1035 nm to ~1284 nm shows a promising direction for the realization of several optoelectronic device applications.

The impact of GaAs1-xNx as a capping layer of InAs quantum dots using digital alloy approach has been investigated. GaAsN capping layer helps in homogeneous distribution of dots on the surface due to formation of nitrogen induced point defects, which helps in minimizing overall compressive strain within the QDs and hence increased dot size. The capping layer of thickness 10 nm with nitrogen composition of 1.8% (Sample A) is considered for analog alloy or conventional approach. The short-period-superlattice (SPS) or sub-divided capping concept has been taken in digital alloy approach for depositing the capping layer. Each SPS is having 2.5 nm thickness and different nitrogen content (1.2%, 1.4%, 1.6%, and 1.8% in each SPS from QD towards top GaAs layer) (Sample D). The hydrostatic and biaxial strain have been computed using Nextnano simulation software and compared for both the mentioned approach. The hydrostatic and biaxial strain in sample D is improved by 0.329% and 0.093% respectively as compare to that in sample A. The observed emission PL wavelength is computed to be 1508 nm and 1430 nm for samples A and D respectively. The simulated PL of sample D is less as compare to that of sample A because of same dot size has been considered for simulation. But as we grow the digital sample using GaAsN material as capping layer, the dot size is increased which in turn helps in red shift in emission PL. Thus, digital alloy approach helps in making devices for future optoelectronic applications.

The effect of thin In0.15Ga0.85As strain-reducing layer on the structural and optical characteristics of multilayer InAs/In(Ga)As Stranski-Krastanov (SK) quantum dots (QDs) electronically coupled to Sub monolayer (SML) QDs has been investigated. The capping of In_0.15Ga_0.85As material over the InAs SK QDs reduces the out-diffusion of Indium atoms from the InAs dot resulting in an increased QD size. Moreover, the In_0.15Ga_0.85As material has a lattice constant between that of InAs and GaAs that aids in undulated transition of strain from the dot to the capping material and the GaAs spacer helping the growth of multilayer QD structure with high crystalline quality. Five different heterostructures are used in this study by varying the number of SK QD layers i.e., single layer (x1), bi layer (x2), penta layer (x5), hepta layer (x7) and deca layer (x10) which are grown on the same six stack SML QDs. A growth strategy has been employed while growing these multilayer SK QDs such that similar size QDs are grown even for deca layer structure with superior dot size homogeneity. The emission full width at half maxima computed at a temperature of 19 K came out to be ~50 nm for the penta layer structure indicating formation of uniform size QDs. The SK and SML QD sizes are chosen such that the ground eigen state of SML QDs coincide with excited states of SK QDs allowing the possibility of carriers to tunnel from the SML to SK QDs. The use of In0.15Ga0.85As capping led to a red shift in the PL peak position as compared to the structure without In0.15Ga0.85As capping. High-Resolution X-Ray Diffraction (HRXRD) measurements are carried out on these structures to understand the structural characteristics of multilayer SK on SML structures. The HRXRD results show that the hepta layer and the deca layer structures have the minimum strain with best crystalline quality. Considering both the optical and the structural characteristics, it has been concluded that the growth strategy helps in growing similar size QDs for the multilayer structures which can be used in various optoelectronic device applications.