Quantum Optics and Photon Counting 2021

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.

We are reviewing the high-average and high peak-power fs-laser sources and experimental areas currently in operation and preparation for user operation. This includes the 1 kHz, 15fs, 50mJ, Allegra laser based on OPCPA-technology. Short pulse 5ps-CPA thin disc lasers pump a series of OPCPA crystals ensuring a high contrast output. The Allegra laser enters the experimental area E1 with a number of end-stations for user experiments. The HAPLS (sub-30fs, Ti: Sapphire) laser pumped by a high-average power frequency converted DPSSL is currently delivering 500 TW, 3.3 Hz pulses via a stable vacuum beam transport system with a pointing stability around 1rad to the experimental areas for plasma physics experiments (E3) and ion acceleration (E4) with the ELIMAIA station. Both areas are fully equipped with target chambers and focusing optics for experimental operation and user assisted commissioning. The Nd:Glass laser Aton provides 1.5 kJ pulses and is currently being compressed to 10 PW in a large compressor tank. A second oscillator allows shaped pulse ns-operation at kJ level or future combination of 1 PW pulses and kJ shaped ns-pulses for advanced WDM or fusion experiments in the E3 area. A new laser disc liquid cooling technology enables repetition rates of 1 shot/minute allowing a much higher data acquisition for this kind of experiments. Furthermore we will report on the first experiments and the future experimental plans as well as on the prospects for user operation.

The Extreme Light Infrastructure – Attosecond Light Pulse Source (ELI-ALPS), the Hungarian pillar of ELI, is the first of its kind that operates by the principle of a user facility, supporting laser based fundamental and applied researches in physical, biological, chemical, medical and materials sciences at extreme short time scales. This goal is realized by the combination of specialized primary lasers which drive nonlinear frequency conversion and acceleration processes in more than twelve different secondary sources. Any light pulse source can act as a research tool by itself or, with femtosecond synchronization, in combination with any other of the sources. Thus a uniquely broad spectral range of the highest power and shortest light pulses becomes available for the study of dynamic processes on the attosecond time scale in atoms, molecules, condensed matter and plasmas. The ground-breaking laser systems together with the subsequent outstanding secondary sources generate the highest possible peak power at the highest possible repetition rate in a spectral range from the E-UV through visible and near infrared to THz. The facility – besides the regular scientific staff - will provide accessible research infrastructure for the international scientific community user groups from all around the world. The attosecond secondary sources are based on advanced techniques of Higher-order Harmonic Generation (HHG). Other secondary sources provide particle beams for plasma physics and radiobiology. A set of state-of-the-art endstations will be accessible to those users who do not have access or do not wish to bring along their own equipment. Step by step the lasers are now commissioned, trialed and handed over for user operation. References S. Kuhn et al., “The ELI-ALPS facility: the next generation of attosecond sources.”, Topical Review, Journal of Physics B, 50 (2017) 132002

Founded by the European Strategy Forum on Research Infrastructure (ESFRI), three state-of-art laser-based institutes in Romania, Hungary, and the Czech Republic were commissioned in the Extreme Light Infrastructure (ELI). Construction for the three sites started in 2012 and, as of 2020, all sites are operational. ELI-NP (Extreme Light Infrastructure: Nuclear Physics) is located 10km south of Bucharest in Romania. Its flagship installation is two beams of 10 PW, each providing 230 J output energy at a 23 fs laser pulse width. The capability to provide a 10 PW output was recently demonstrated in a live performance. We were able to show that the 10 PW laser shots can be delivered for 10 minutes at a rate of one shot every minute. A total of 230 Zoom participants worldwide, including Prof G Mourou and Prof D Strickland, the Physics Nobel Laureates in 2018, witnessed this breakthrough demonstration. An early experiment at the 100 TW laser station at ELI-NP has already been completed. We successfully demonstrated an electron acceleration of up to 300 MeV, either resulting in monoenergetic or broadband spectra, depending on the well controllable experimental conditions we set. Operations at the 1 PW and 10 PW experimental stations will start soon. External user access will be tested with the early and commissioning experiments and will be formulated coherently within the framework of the IMPULSE project guided by ELI-DC. Reference Current status and highlights of the ELI-NP program research program, KA Tanaka, K Spohr, D Balabanski, et al., Matter Rad. Extremes, 5, 024402 (2020): doi.10.1063/1.5093535

We’ve come a long way since 1609, from spectacle lenses to mirrors in space, from twitching frog legs to the Event Horizon Telescope observing a black hole. But far more is possible. On the ground, a new generation of optical telescopes is under construction, up to 39 m in diameter. Adaptive optics compensates for the turbulent atmosphere, but could work far better with an orbiting reference beacon in space. Bright chemiluminescent emission lines in the upper atmosphere interfere with observations, but could be blocked by fiber optic filters. Energy-resolving photon counting detectors promise far greater sensitivity. New ways of making mirrors offer far better resolution for space X-ray telescopes. Coronagraphs can suppress starlight enough to reveal exoplanets in direct imaging, or starshades can cast star shadows on telescopes to do the same thing. New generations of far IR detectors with large cryogenic telescopes in space can reveal the cool and cold universe. Radio telescopes on the quiet far side of the Moon can overcome the limits of the ionosphere and intense local interference to see events in the early universe as it heated up again after the Big Bang expansion cooled everything. Neutrino telescopes can see stars being shredded by black holes, and gravitational wave detectors see merging neutron stars and black holes. Atom wave gravimeters can measure the internal structure of planets and asteroids, and sample return missions are already bring back distant bits of the solar system. What will happen next? I don’t know but it will be glorious.

The nascent field of semiconductor core fibres is attracting increased interest as a means to exploit the excellent optical and optoelectronic functionality of the semiconductor material directly within the fibre geometry. Compared to their planar counterparts, this new class of waveguide retains many advantageous properties of the fibre platforms such as flexibility, cylindrical symmetry, and long waveguide lengths. Furthermore, owing to the robust glass cladding it is also possible to employ standard fibre post-processing procedures to tailor the waveguide dimensions and reduce the optical losses over a broad wavelength range, of particular use for nonlinear applications. This presentation will review progress in the development of nonlinear devices from the semiconductor core fibre platform and outline exciting future prospects for the field.

X-ray free-electron lasers, delivering x-ray pulses of femtosecond duration, are available for experiments for more than a decade and allow for hitherto unachievable x-ray intensities on sample, reaching up to 1021 W/cm2 for hard x-rays. At these intensities, the probability of a single atom or molecule to absorb a photon of an impinging x-ray pulse reaches unity. Moreover, several interactions of photons and matter within the duration of the x-ray pulse – nonlinear x-ray matter interactions – become possible, opening the pathway to nonlinear x-ray optics. For a macroscopic ensemble of atoms, molecules, nanometer-sized clusters or a solid, the interaction with a strongly focused x-ray beam can create macroscopic, highly excited states of matter, far from equilibrium. In particular, saturated absorption with a high-intensity x-ray pulse can result in transient states, present for roughly one femtosecond, with the characteristic feature, that every single atom in the interaction region is in a population inverted state with missing population in the innermost electronic shell. This macroscopic population inversion can lead to collective radiative decay mechanisms, such as amplified spontaneous emission or superfluorescence. In this presentation I will give you an overview over our experimental and theoretical investigations of these single-pass x-ray laser amplifiers in the x-ray spectral domain. I will address applications of this phenomenon in the area of chemical x-ray emission spectroscopy, a new concept of an x-ray laser oscillator, and will highlight recent theoretical developments to describe collective spontaneous emission in the x-ray spectral domain.

The Matter in Extreme Conditions (MEC) instrument at LCLS pioneered the use of the hard X-ray free electron laser (XFEL) in combination with high-power optical lasers to advance high energy density science. Commissioned in 2012 as an open-access scientific capability, this application of the powerful XFEL diagnostic has driven a rich array of high-profile scientific results, providing new insight into atomic and structural properties of dynamic plasma and high-pressure material states. Aided in part by the success of MEC and other high power laser facilities, there has been a strong call from the research community over the past 5 years for increased national investments in high power lasers combined with existing national lab infrastructure. In response to a mission need statement from the US Department of Energy, Fusion Energy Sciences, SLAC has developed a conceptual design for a project to build a new HED science facility combining high rep-rate (10Hz) petawatt laser systems and high energy (1kJ) long pulse lasers with the LCLS XFEL. Combined with flexible and high efficiency experimental systems, this facility will enable a world-unique set of scientific capabilities complementing the new emerging generation of high-power laser facilities, including the pillars of ELI and new HED end stations at European XFEL and SACLA. In this talk, I will present an overview of the facility conceptual design and place it in the context of the growing field of high-power laser science.

Meet colleagues interested in this conference topic for small group discussion and networking, hosted by the conference chairs. While this informal session emphasizes conversation rather than talk delivery, it is also an entry point into two-way discussions during this virtual conference. Hosted by: Ivan Prochazka, , Czech Technical Univ. in Prague, Roman Sobolewski, Univ. of Rochester, Martin Štefaňák, Czech Technical Univ. in Prague, and Aurél Gábris, Czech Technical Univ. in Prague and Wigner Research Ctr. for Physics. This session is not recorded. There will be no on-demand recording of this event.

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)

The rapid development of quantum technologies, which is taking place in recent years, significantly increases the interest in ultra-sensitive detectors that can effectively register single photons. Superconducting single-photon detectors (SSPD) was developed in 2001 [1] and all the faster and more confidently conquer new niches of application. Even nowadays their characteristics are significantly higher than avalanche photodiodes (APD) and photomultiplier tubes (PMT) [2,3]. Further research has determined a need for SSPD improvements via the production of multi-element devices. Such a configuration can recognize the number of photons contained in one short optical pulse of radiation (PNR). In this paper, we experimentally investigated the possibility of optimizing the topology of PNR SSPDs to increase their efficiency and the number of resolved photons in the radiation pulse. Developed topology allowed us to create NbN PNR SSPDs with a resolution of up to 8 photons in a radiation pulse and with the very high quantum efficiency which reached of 90% at a wavelength of 1550 nm for receiver based on PNR SSPD and with a single-mode fiber as radiation input. It's worth mentioning, that the main limiting factor for the further development of superconducting single-photon detectors is their small input aperture, i.e. currently all receivers use single-mode fiber as an optical input of radiation. Unfortunately, the development of a single-element SSPD with an increased sensitive area has many fundamental problems and has not been yet implemented. Using a new approach, we got record results of superconducting single-photon detectors with large active area of 40x40 µm2 and coupled to multi-mode fiber with system quantum efficiency >50%.The other performances of the devices are only marginally inferior to that of conventional detectors coupled to SM fiber, but explicitly exceeds previously obtained results for detectors with a large active area. The created superconducting single-photon detectors with a large input aperture and PNR SSPD have multiple promising applications: in quantum cryptography [4], quantum computing (LOQC) [5], for fast communication in air and space [6], LIDAR technologies, biomedicine, etc. [1] G. Gol’tsman et al. Appl. Phys. Lett.79(6), 705-707 (2001). [2] K. Smirnov et al. Superconductor Science and Technology 31(3) 035011 (2018) [3] F. Marsili et al. Nature Photonics 7, 210-214 (2013). [4] A. Divochiy et al. Nature Photonics 2, 302-306(2008). [5] E. Knill et al. Nature, vol. 409, no.6816, p. 46, (2001). [6] D. V. Murphy et al. Proc. SPIE 8971, 89710V (2014).

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. Nitrogen-Vacancy (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) of 32 × 32 SPADs, designed in 0.16 µm 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 readout. Both an analog and a digital approach are used to distinguish between 1 to ⁓300 incoming photons. Voltage-Controlled Current Generator converts the pixel digital output 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 all the photon generated pulses, so the analog option is required. 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 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 namely 75 ps, is integrated to provide as output 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 flexible number of photon detections and the introduced features to increase the timing performance, the detector can be exploited in a lot of scientific applications.

We present the development and the validation of two SPAD camera systems, based on two SPAD array chips, respectively with 8x8 and 128x1 high-performance CMOS SPAD pixels, able to acquire both on-chip photon-counting 2D “intensity” images and photon-timing 3D “time-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), and a 12-bit Time-to-Digital Converter (to timestamp the arrival time of the first photon detected by each SPAD). The two arrays have the additional capability of actively gating the SPADs, driving the SPAD bias voltage above or below breakdown, with sub-nanosecond transitions allowing efficient time-domain filtering of incoming light. This feature allows the user to selectively avoid “early” photons while measuring only the useful “late” photons preventing the triggering due to the strong first reflection, which would saturate the SPADs. 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 hides unwelcome reflections, stray rays, or luminescence/fluorescence excited signals. For optimizing chip operation in many different applications, both systems are extremely versatile and allow the user to customize the cameras to various measurement setups. The cameras quantum sensitivity allows the reconstruction of faint optical signals by means of the Time-Correlated Single Photon Counting (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 its time-gating and time-tagging capabilities.

We present a fully reconfigurable single-photon camera, able to operate in both analog and digital modalities and to exploit the best performance out of the two features. The camera employs a versatile 5 × 5 array of SPAD detectors, exploited as a digital SiPM device when configured for photon-counting applications: the output of each SPAD is fed to 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 so to output an 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 gives 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.

Light Detection and Ranging (LiDAR) is a technique that, among its many applications, can be applied to identify the position of objects in an industrial environment, usually denoted by strong background illumination. In this work we present the 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 SPAD-connected 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.

We studied the transmission of photons in mouse brain in both with and without intact skull states using Monte Carlo method. Photon track, optical absorption density and fluence rate were used as indicators for analysis. We found that, the photon distribution without intact skull goes farther in both longitudinal and transverse directions compared with that of with intact skull. The distribution of optical absorption density and fluence rate was fusiform and rounder on the whole with intact skull. This study will provide reference and theoretical guidance for the optical imaging of mouse brain and the study of the mouse and human brain.

Bio-nanophotonics is a recently emerged, but already well-defined, truly interdisciplinary field of science and technology aimed at establishing and using the peculiar properties of light and nanoscale light-matter interaction with an emphasis on life science applications. The aim of the present work is a spectroscopic and thermodynamic study of DNA catalytic properties in the following processes: a) reduction; b) formation of inter-strand crosslinks; c) performing of photodynamic effects; d) nanoscale resonance radiationless electron excitation energy transfer. The most attention is paid to the latter, as it is truly nanoscale method in its origin. The nanoscale method of laser-induced fluorescence resonance energy transfer (FRET) to donor (acridine orange) - acceptor (ethidium bromide) intercalator pair for quantitative and qualitative study of stability quality DNA double helix in solution in real time is used. The FRET method allows to estimate the concentration of double helix areas with high quality stability applicable for intercalation in DNA after it is subjected to stress effect. It gives the opportunity to compare various types of DNAs with: different origins; various degrees of damage; being in various functional states. Reduction of silver ions on a double helix of 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. It is shown that the process of both absorption and photo-desorption of reduced silver atoms on a double helix of DNA is characterized by an S-shaped transition. It is counted the oscillatory force of the complex DNA-Ag(0) for a silver atom which is equal to f (434.3 nm) = 0.33 Acknowledgements. The work was partly supported by Shota Rustaveli National Science Foundation of Georgia (SRNSFG) YS-19-2047.

A new optical pumping (OP) method is proposed and realized in the NIM6 cesium atomic fountain clock with two overlapping π polarized laser beams on resonance with |F=3>- |F’=3> and |F=4>- |F’=3> transition respectively, directly preparing most atoms in the |F=3,mF=0> state, to increase the detected atom populations and replace the magnetic state selection process.

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 effects 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, for a given value of the Ohmicity parameter, quantum coherence effects diminish as we increase the logarithmic perturbations in the environment. 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 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 behavior at longer time scales. Conventional ohmic-like spectral density functions, model this behavior 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 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/In0.15Ga0.85As 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 optical and structural properties of multilayer InAs/GaAs Stranski-Krastanov (SK) quantum dots (QDs) electronically coupled to Sub monolayer (SML) QDs grown by Molecular Beam Epitaxy (MBE) has been investigated. The In0.15Ga0.85As material has a lattice constant between that of InAs and GaAs, which 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. The electronic coupling between SK and SML QDs allows carriers to tunnel from SML QDs to SK QDs. The photoluminescence efficiency of these multilayer structures has been greatly enhanced and it is found that the PL peak intensity for the hepta layer structure is almost ~100 times high as compared to that of the single layer structure. The FWHM and the activation energy has been calculated and the best values came out to be ~38 nm and 311 meV, respectively for the hepta layer structure. High Resolution X-Ray Diffraction (HRXRD) measurements have been done on these structures in order to know the structural characteristics of these 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 hepta layer structure is optimal one which can be used in various optoelectronic device applications.