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Frequency analysis of the photoacoustic radiofrequency signals and oxygen saturation estimates were used to monitor the in-vivo response of a novel, thermosensitive liposome treatment. The liposome encapsulated doxorubicin (HaT-DOX) releasing it rapidly (Photoacoustic imaging (VevoLAZR, 750/850 nm, 40 MHz) of EMT-6 breast cancer tumors was performed 30 min pre- and post-treatment and up to 7 days post-treatment (at 2/5/24 h timepoints). HaT-DOX-treatment responders exhibited on average a 22% drop in oxygen saturation 2 h post-treatment and a decrease (45% at 750 nm and 73% at 850 nm) in the slope of the normalized PA frequency spectra. The spectral slope parameter correlated with treatment-induced hemorrhaging which increased the optical absorber effective size via interstitial red blood cell leakage. Combining frequency analysis and oxygen saturation estimates differentiated treatment responders from non-responders/control animals by probing the treatment-induced structural changes of blood vessel.

In ultrasound imaging, fully-developed speckle arises from the spatiotemporal superposition of pressure waves backscattered by randomly distributed scatterers. Speckle appearance is affected by the imaging system characteristics (lateral and axial resolution) and the random-like nature of the underlying tissue structure. In this work, we examine speckle formation in acoustic-resolution photoacoustic (PA) imaging using simulations and experiments. Numerical and physical phantoms were constructed to demonstrate that PA speckle carries information related to unresolved absorber structure in a manner similar to ultrasound speckle and unresolved scattering structures. A fractal-based model of the tumor vasculature was used to study PA speckle from unresolved cylindrical vessels. We show that speckle characteristics and the frequency content of PA signals can be used to monitor changes in average vessel size, linked to tumor growth. Experimental validation on murine tumors demonstrates that PA speckle can be utilized to characterize the unresolved vasculature in acoustic-resolution photoacoustic imaging. Keywords: Photoacoustic speckle, Ultrasound speckle, Spatial resolution, Autocovarience function, Tumor vasculature, In-vivo imaging, Vascular trees

A new topology that generates a DC voltage whose value is depending on the phase difference of two input RF signals is presented. It is implemented by the mixer concept. With the use of microwave photonics, the mixer-based phase detector can be operated over a very wide frequency range. It also has the advantages of high phase detection resolution, small phase detection error, and small DC offset and phase offset. The phase detector can be constructed using off-the-shelf components including a single integrated optical modulator and a low-speed balanced detector. Measured results on the phase detector demonstrate a 0° to 180° RF signal phase difference detection range for 3.5 GHz to 18 GHz input RF signal frequencies, less than ±3° phase detection error and 2° phase detection resolution.

Van der Waals (vdW) heterostructures based on transition metal dichalcogenides (TMDs) generally possess a type-II band alignment that facilitates the formation of interlayer excitons between constituent monolayers. Manipulation of the interlayer excitons in TMD vdW heterostructures holds great promise for the development of excitonic integrated circuits that serve as the counterpart of electronic integrated circuits, which allows the photons and excitons to transform into each other and thus bridges optical communication and signal processing at the integrated circuit. As a consequence, numerous studies have been carried out to obtain deep insight into the physical properties of interlayer excitons, including revealing their ultrafast formation, long population recombination lifetimes, and intriguing spin-valley dynamics. These outstanding properties ensure interlayer excitons with good transport characteristics, and may pave the way for their potential applications in efficient excitonic devices based on TMD vdW heterostructures. At present, a systematic and comprehensive overview of interlayer exciton formation, relaxation, transport, and potential applications is still lacking. In this review, we give a comprehensive description and discussion of these frontier topics for interlayer excitons in TMD vdW heterostructures to provide valuable guidance for researchers in this field.

An iterative pairwise maximum likelihood (ML) receiver is proposed for asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) in visible light communications (VLC). With the aid of the pairwise ML detection, half of the received ACO-OFDM samples are forced to zero in time domain. However, when the signal-to-noise ratio (SNR) is low, the pairwise ML detection may cause misjudgment. With the iterative pairwise ML detection, the ACO-OFDM symbols are detected in frequency domain and the SNR gain of 3 dB could be available. Simulation results show that the pairwise ML receiver without iteration provides the SNR improvement of 1.5∼2.7 dB for 7% forward error correction (FEC) limit. The proposed iterative receiver gives a SNR gain of 2.6∼2.9 dB with 7% FEC limit.

In order to achieve high-speed, high-sensitivity optical communication over long distances and high attenuation channels, we propose a photon-counting communication system based on the detection of multichannel single-photon avalanche diodes (SPADs). First, the ideal pulse position modulation (PPM) bit error rate (BER) expression for a multichannel detection system is derived, and then the effect of the detector's dead time is taken into account. The BER of the multichannel detection system was analyzed by means of a Monte Carlo simulation, after which we set up a four-channel receiving experimental verification system with a 1 × 4 fiber splitter and four single-photon counting modules (SPCMs). The simulation and experimental results demonstrated that the BER of the multichannel detection system could be greatly reduced and the communication speed substantially improved. Finally, a reliable photon-counting communication system with a rate of 50 Mbps and sensitivity of 4 photons/bit was developed using serially-concatenated pulse position modulation (SCPPM) technology.

We report the design, fabrication and characterization of a 1.55-μm dual-wavelength Distributed Bragg Reflector (DBR) laser photonic integrated circuit (PIC) chip for continuous-waves (CW) terahertz generation. The PIC chip consists of a pair of parallel DBR lasers with integrated semiconductor optical amplifiers (SOA), and a multimode interference (MMI) coupler for combining the two wavelength channels into a single output waveguide. A room temperature continuous wave THz tuning range of 0.06 THz – 0.71 THz was obtained through optical heterodyne technique.

Electromagnetic metasurface cloaks provide an alternative paradigm toward rendering arbitrarily shaped scatterers invisible. Most transformation-optics (TO) cloaks intrinsically need wavelength-scale volume/thickness, such that the incoming waves could have enough long paths to interact with structured meta-atoms in the cloak region and consequently restore the wavefront. Other challenges of TO cloaks include the polarization-dependent operation to avoid singular parameters of composite cloaking materials and limitations of canonical geometries, e.g., circular, elliptical, trapezoidal, and triangular shapes. Here, we report for the first time a conformal-skin metasurface carpet cloak, enabling to work under arbitrary states of polarization (SOP) at Poincaré sphere for the incident light and arbitrary conformal platform of the object to be cloaked. By exploiting the foundry three-dimensional (3D) printing techniques to fabricate judiciously designed meta-atoms on the external surface of a conformal object, the spatial distributions of intensity and polarization of its scattered lights can be reconstructed exactly the same as if the scattering wavefront were deflected from a flat ground at any SOP, concealing targets under polarization-scanning detections. Two conformal-skin carpet cloaks working for partial- and full-azimuth plane operation are respectively fabricated on trapezoid and pyramid platforms via 3D printing. Experimental results are in good agreement with numerical simulations and both demonstrate the polarization-insensitive cloaking within a desirable bandwidth. Our approach paves a deterministic and robust step forward to the realization of interfacial, free-form, and full-polarization cloaking for a realistic arbitrary-shape target in real-world applications.

We utilize singular spectrum analysis (SSA) for post-processing distance measurement data obtained by a dual-comb femtosecond laser rangefinder. Signal denoising technique based on SSA allows detection of low amplitude vibrations from time series of distance data with limited signal-to-noise ratio (SNR). Various one-dimensional vibrations, including harmonic, amplitude modulated harmonic, and sinc-function vibrations, with micrometer amplitude comparable with measurement uncertainty of laser rangefinder, have been imposed on the target mirror by a piezo positioner. These vibrations have been extracted by SSA and the obtained vibration amplitudes agree with that imposed on the target very well. A commercial interferometer is used to evaluate the accuracy of SSA. Standard deviations of residuals between the SSA extractions and interferometer measurements are smaller than 0.6 $\mu$ m. This technique is promising for complex moving pattern detection in object tracking and remote sensing.

We report the design and fabrication of a high-speed GeSn normal-incidence p-i-n photodetector. To realize high-speed detection in all telecommunication bands, we optimize the Sn content in the absorption layer, the absorption-layer thickness, and the device size. The responsivity of the 18-μm-diameter device at 1550 nm reaches 0.32 A/W with an extended cutoff wavelength of 1700 nm and a 3-dB bandwidth as high as 28 GHz under −3 V bias, clear open eye diagrams are also obtained under zero bias at 1630 nm. All the results indicate that the device has a significant potential for applications in Si-based optical telecommunication in all telecommunication bands.

In this paper, we propose a dynamic key technique based on Cellular Neural Network (CNN) for security improvement in the orthogonal frequency division multiplexing passive optical network (OFDM-PON). To enhance the encryption scheme security, a six-dimensional CNN hyperchaotic system is employed to encrypt the data. And, the keys are divided into the dynamic and static. The dynamic key is randomly extracted from a key set by incorporating the random feature of the input data. Then, the chaotic sequence generated by the dynamic key is served as the synchronous sequence for encryption. Moreover, the chaotic sequences generated by the static keys are used to resist the chosen-plaintext attacks (CPAs) and scramble the phase of QAM symbols on the frequency domain. With these processing techniques, the multi-fold data encryption can create a key space of ∼10315 to protect against the exhaustive trial. The transmission of 10-Gb/s encrypted 16-QAM-based OFDM signal is demonstrated over 20-km single mode fiber (SMF) by experiment. The results show that our proposed scheme can provide excellent confidentiality of data transmission against the CPAs and brute-force attack.

A systematic comparison of fiber-based gas pressure sensors using different types of hollow-core photonic crystal fibers (PCFs) is conducted. The sensor was fabricated by splicing a segment of hollow-core silica capillary between a single mode fiber (SMF) and a hollow-core PCF. A Fabry-Perot (FP) cavity configuration is thus formed with the capillary tube as the sensing cavity, while the PCF acts as gas passage to the external environment. External pressure change would lead to the variation in the refractive index of air in the sensing cavity, resulting in a wavelength shift of the interference dips. The pressure sensitivity is measured to be around 4 nm/MPa, with a high linearity of 99.7% or above, regardless of the type of the PCFs we used as gas inlet. Among the four different PCFs, the large-mode-area (LMA) PCF shows the highest fringe contrast in the reflection spectrum. Modal analysis reveals that this is due to the high reflectivity caused by the solid core of LMA-PCF. Our experimental results also indicate that the length of the sensing cavity, as well as the offset fusion splice will influence the fringe contrast. The sensor has potential application in gas pressure sensing for advantages of high sensitivity, compact size and ease of fabrication.

We report the effect of the double Rayleigh backscattering (DRB) noise, in which the interference of the signal light with the double Rayleigh backscattered light, on the fiber-optic radio-frequency (RF) transfer system. Here, we theoretically analyze and experimentally demonstrate that the DRB noise within the fiber link will become the dominative noise on the fiber-optic RF transfer system once narrow linewidth lasers are used. The strong DRB noise is experimentally confirmed through the measurement of the effect of the coherent interference term of the signal light and the double Rayleigh backscattered light on the 100 km RF transfer system. Our results provide evidence that the conventional RF transfer technique with the telecommunications-grade lasers, in which the chromatic dispersion effect in the single-mode fiber is compensated by dispersion compensation fibers (DCFs), could be the better choice for high-performance RF transfer.

Fiber loop ring down spectroscopy has been developed into a high-sensitivity technique for sensing many important characteristics of samples, such as refractive index. In contrast to the previous demonstrations by using mode-locked fiber laser, this work is demonstrated to use chaotic laser combined with fiber loop ring down sensor. The formation and performance of fiber tapers was monitored by combining chaotic fiber ring down technology on line and in real time. Two sets of refractive index sensing experiments are performed and further analyzed by using the proposed setup. The results are shown that the detection limit of the chaotic correlation fiber loop ring down system is 3.45 × 10−4 RIU and the sensitivity is 0.0682 ns−1RIU−1 in 6.7 m, respectively. The performance of our system can be drastically improved by increasing the length of the etched fiber tapers to be tens of centimeters using the chaotic correlation fiber loop ring down system with the smaller fiber ring lengths.

Graphene waveguide plays an important role in modulating optical signal. But it is hard to make a tradeoff between low propagating loss and high field confinement. Here, we propose a bilayer graphene waveguide in a thin topas film and a high refractive index material-germanium cladded with thin metallic film. The influences of structural parameter and chemical potential of graphene are studied to optimize the dimension and working mode. Simulation reveals that our structure can make a balance between high figure of merit (FOM) and low propagation loss, and it reaches a high modulation depth of $\text{2.5 dB}/ \mu\text{m}$ . The design can work with FOM over 200, propagation loss lower than $\text{0.2 dB}/\mu\text{m} $ , and propagation length beyond $\text{30}\;\mu\text{m}$ in a wide band from 6 THz to 18 THz. Compared with previous graphene waveguides, our structure operates from terahertz band to mid-infrared band, and it has longer propagation length due to the existence of bilayer graphene. Besides, benefiting from the thin metallic film, our structure can be integrated on chip.

A subwavelength gratings (SWG) assisted directional coupler (DC) is proposed to work as beam splitter (BS) for 850 nm chip-based optical coherence tomography (OCT). The bandwidth of the SWG assisted DC is maximized to 110 nm by selecting appropriate SWG geometries including its duty cycle and period. The axial point spread functions (PSFs) derived from the BS transmittances proves the optimized bandwidth is broad enough to support axial resolution higher than $4 \;\mu m$ . Meanwhile, the SWG period can be manipulated to match the center wavelength between BS and light source so that the wavelength mismatching induced resolution degradation is effectively mitigated. Furthermore, a statistical analysis is performed to demonstrate the SWG assisted DC is a high fabrication-tolerant device in terms of axial resolution, even the SWG geometries suffer from a fabrication jitter in 20 nm variance.

We propose a photonic mixer with image rejection capability using a broadband optical source (BOS) and polarization processing. Intermediate frequency (IF) signals with image rejection are generated from the local oscillator-modulated and the time-delayed radio frequency (RF)-modulated BOS light. The RF operating frequency is flexibly tunable over a wide range by simply adjusting the time delay. In addition, the proposed system is free from dispersion-induced mixing power fading and bias drift problem. Experimental results exhibit an image rejection ratio of more than 44.6 dB for RF signals from 2 to 18 GHz, while the IF signal is fixed at 800 MHz. Image rejection mixing operation at different IF frequencies and signal power is also confirmed.

The applicability of resonant waveguide grating (RWG)-based structures as filters to control the spectral response in an optical communication system is investigated. A new physical model for the structure is established on the basis of the Fabry–Pérot (FP) etalon model and coupled leaky mode theory (CLMT). It is found that the flat-top spectral response of the filter is achieved by the combined effect of the guided-mode resonance of an RWG and its Fabry–Pérot resonance (FPR). The bandwidth-tunable spectral response of the filter can be varied according to the change in the eigenvalues of the RWG by changing the structural parameters such as strip width of the grating and incident angle. The flat-top and bandwidth-tunable RWG-based resonant filter is a promising application for high-performance optical communication systems.

In this paper, we present a novel transmission line architecture in thin-film lithium niobate (TFLN) platforms to improve the velocity match between the microwave and the optical wave. Compared to conventional coplanar waveguide (CPW), the microwave index ($n_m$) of the proposed slotted electrodes can be optimized from 2.1 to 3 while maintaining the high modulation efficiency and 50-$\Omega$ impedance match. Equivalent-circuit model analysis and finite-element simulation are performed. The simulated half-wave voltage (V$_\pi$) of 1.2 V and E-O modulation bandwidth greater than 80 GHz is obtained for a 2-cm-long modulator. By utilizing the slotted slow-wave electrode, TFLN Mach-Zehnder modulators with CMOS-compatible operating voltage and 3-dB modulation bandwidth greater than 100 GHz are potentialized.

A 365 nm AlxGa1-xN-based ultraviolet light-emitting diodes (LEDs) with double Al composition gradient last quantum barrier and hole supply layer structure has been studied. Experimental results show that the proposed structure enhances the carrier injection efficiency and suppresses the overflow of electrons. The introduction of three-dimensional hole gas further enhances the hole injection efficiency. As a result, the wall plug efficiency and electroluminescence intensity are significantly improved. In addition, the carrier concentration and the radiative recombination rate in the active region of the quantum well increases. Looking at the energy band diagram, one sees that the double Al composition gradient structure leads to higher electron blocking and alleviates the hole blocking effect. Overall, we conclude that the double Al composition gradient structure provides a pathway to achieve high-efficiency ultraviolet LEDs.

We present a comprehensive systematic framework for optimizing adiabaticity in photonic lanterns. The framework considers the effects of both the cross-sectional and longitudinal device parameters on adiabaticity. Photonic lanterns are adiabatic photonic devices and are therefore known to have large device lengths. A Shortcuts-to-adiabaticity (STA) protocol is employed to optimize the adiabatic taper profile in all-fibre photonic lanterns. The method tailors adiabatic propagation of light in the system by appropriately correcting the local slope of the taper profile. A quantifiable measure of adiabaticity is established, based on the adiabaticity criterion. This measure relates inversely to the device length and is a useful parameter in taper optimization. We apply the protocol to reported photonic lantern devices to obtain optimal adiabatic taper profiles having shorter device lengths. The optimized taper profile refers to that taper profile which corresponds to the minimum local inter-modal coupling at every point along the quasi-adiabatic transition for a given device length. While optimizing the adiabatic transition by minimizing inter-modal coupling is the basic aim of this work, length optimization is an extremely useful by-product. This procedure can be used to either reduce the device length or to reduce the mode-coupling losses or both. This study analyses reported three and six-core mode selective photonic lanterns as examples. We also discuss ways to make the optimum profiles practically realizable.

We investigate the monolithicintegration of RF antennas onto a silicon-based integrated microwave photonics (IMWP) chip for short-range millimeter-wave (mmW) communication. The unification of antenna with photonic integrated circuits (PICs) reduces system loss for high data rate communication by eliminating parasitic interconnects. This integration of electronics (antenna) with photonics will be a key milestone leading to increased bandwidth capability and ubiquitous wireless links for emerging applications such as 5G, Internet of Things (IoT), autonomous vehicles, high data rate point-to-point communication, and wireless sensors. Through simulation above 20 GHz, we compare the transmission of three on-chip antenna structures designed in a commercial silicon photonics (SiPh) process and consider them for both inter and intra-chip communication. Results provide insight on the transmission gain variations relative to the antenna orientation from their distinct radiation pattern. The folded monopole structure provides superior gain, smaller footprint with layout flexibility, and good transmission spectrum. The analysis supports the idea of a monolithic mmW transmitter integrated with on-chip antennas on IMWP chip.

This paper proposes a novel refractive index profile design based on few-mode fibers (FMFs) which can support 4 linear polarization (LP) modes. We first present a FMF whose core is dually assisted by a nano-hole (NH) and a high-index-ring (HIR), and then substitute the dual assistance by an innovative graded concave HIR (GC-HIR) assisted structure. Using the finite element method (FEM), the parameters of the NH-HIR and GC-HIR optical fiber are adjusted to investigate their respective minimum effective refractive index difference (minΔneff), which is 2.012 × 10−3 for the former, and 2.532 × 10−3 for the latter. Both optical fibers have a significant improvement on the crosstalk suppression effect, and the GC-HIR optical fiber is even better. The proposed GC-HIR FMF with special refractive index profile has potential application prospects in mode division multiplexing (MDM) optical fiber communication system, and can provide theoretical foundation for the design and analysis of subsequent optical fibers and key components.

In this work, the design and fabrication of a Cu/p-Si/Pt Schottky photodetector with a simple structure is investigated. The mechanism of electron flow is explained using internal photoemission theory, which is further applied to make the fabricated devices reach a high responsivity of 0.542 mA/W at 0–V bias. The investigation revealed that the rapid thermal annealing process could significantly influence the device characteristics. Variations in Schottky barrier height and series resistance of the photodetectors were also analyzed and correlated with the device responsivity. With proper conditions to improve the Pt/Si ohmic contact, the device performance can be enhanced.

Photonics-assisted frequency conversion based on cascaded modulators is an effective way to satisfy the requirements of distributed antenna array applications. A filter-free photonic image-rejection down-converter with immunity to dispersion-induced power fading is proposed and demonstrated. The radio frequency (RF) signal from the distributed antenna unit is injected into a dual-drive Mach–Zehnder modulator (DDMZM) through an electrical 90° hybrid coupler (HC), and a single-sideband (SSB) signal is realized. It is then transmitted to the center office by an optical fiber link. At the center office, the modulated RF signal is injected into a dual-polarization dual-parallel MZM (DP-DPMZM) after a polarization controller (PC). Afterwards, an LO signal remodulates it through the Y-DPMZM, and carrier-suppressed SSB modulation is fulfilled via another electrical 90° HC. After the power splitting, quadrature I and Q IF signals can be obtained by a photonics-based continuously adjustable phase shifter. Through a combination of the I/Q IF signals with a low-frequency electrical 90° HC, a photonics image-rejection down-converter based on phase cancellation is realized. Moreover, no optical filter is required; thus, the image-rejection down-converter can work with a large bandwidth. The proposed filter-free photonic image-rejection down-converter is theoretically analyzed and experimentally verified.

Two new photonics-based microwave frequency translators are presented. They are based on Serrodyne modulation to frequency translate an optical carrier, which beats with a microwave signal sideband at a photodetector to generate a frequency translated microwave signal. The proposed frequency translators have a simple structure, a wide bandwidth and a large spurious signal suppression ratio. Experimental results are presented for the novel structures, which demonstrate the realisation of microwave signal frequency translation with >38 dB spurious signal suppression over a wide microwave signal frequency range of 2 to 18 GHz and with >35 dB spurious signal suppression for a translation rate of 500 Hz to 3 MHz. Frequency translation of an 80 MHz bandwidth frequency modulated continuous wave signal at 5.88 GHz frequency with >34 dB spurious signal suppression is also demonstrated.

The scattering from air–glass interfaces within solid-core polarization-maintaining photonic crystal fiber (PM–PCF) will increase the fiber attenuation coefficient, which may lead to high transmission loss. Therefore, it is necessary to describe scattering properties to guide research into reducing fiber loss. In this paper, the loss resulting from roughness scattering at multi-hole interfaces within PM–PCF was theoretically and experimentally analyzed. A PM–PCF scattering model was established to explore the scattering distribution. On the other hand, a fully automatic testing device was built to enable the measurement of a three-dimensional scattering sphere. Simulations were in good agreement with experimental measurements. Moreover, this new proposed measurement method could apply to other PCFs and it will be a useful tool for further scattering research.

Polarizing beamsplitters have numerous applications in optical systems, such as systems for freeform surface metrology. They are classically manufactured from birefringent materials or with stacks of dielectric coatings. We present a binary subwavelength-structured form-birefringent diffraction grating, which acts as a polarizing beamsplitter for a wide range of incidence angles −30∘…+30∘. We refine the general design method for such hybrid gratings. We furthermore demonstrate the manufacturing steps with Soft-UV-Nanoimprint-Lithography, as well as the experimental verification, that the structure reliably acts as a polarizing beamsplitter. The experimental results show a contrast in efficiency for TE- and TM-polarization of up to 1:18 in the first order, and 34:1 in the zeroth order. The grating potentially enables us to realize integrated compact optical measurement systems, such as common-path interferometers.

Vector beams, non-separable in spatial mode and polarisation, have emerged as enabling tools in many diverse applications, from communication to imaging. This applicability has been achieved by sophisticated laser designs controlling the spin and orbital angular momentum, but so far is restricted to only two-dimensional states. Here we demonstrate the first vectorially structured light created and fully controlled in eight dimensions, a new state-of-the-art. We externally modulate our beam to control, for the first time, the complete set of classical Greenberger–Horne–Zeilinger (GHZ) states in paraxial structured light beams, in analogy with high-dimensional multi-partite quantum entangled states, and introduce a new tomography method to verify their fidelity. Our complete theoretical framework reveals a rich parameter space for further extending the dimensionality and degrees of freedom, opening new pathways for vectorially structured light in the classical and quantum regimes.

Optical tweezers and associated manipulation tools in the far field have had a major impact on scientific and engineering research by offering precise manipulation of small objects. More recently, the possibility of performing manipulation with surface plasmons has opened opportunities not feasible with conventional far-field optical methods. The use of surface plasmon techniques enables excitation of hotspots much smaller than the free-space wavelength; with this confinement, the plasmonic field facilitates trapping of various nanostructures and materials with higher precision. The successful manipulation of small particles has fostered numerous and expanding applications. In this paper, we review the principles of and developments in plasmonic tweezers techniques, including both nanostructure-assisted platforms and structureless systems. Construction methods and evaluation criteria of the techniques are presented, aiming to provide a guide for the design and optimization of the systems. The most common novel applications of plasmonic tweezers, namely, sorting and transport, sensing and imaging, and especially those in a biological context, are critically discussed. Finally, we consider the future of the development and new potential applications of this technique and discuss prospects for its impact on science., Optical manipulation: plasmonic tweezers Plasmonic tweezers exploit sub-wavelength confinement of light to allow trapping and manipulation of small particles with far greater precision than usual. Yuquan Zhang and coworkers from Shenzhen University in China and Delft University of Technology in The Netherlands have now reviewed the principles of operation, benefits and potential applications of such tweezers. They document the popular designs of plasmonic nanostructures that have been used to create tweezers to date and the theories behind the generation of surface plasmon polaritons and the forces that they induce. They also discuss the opportunities for improving performance of the tweezers in the future and their applications in the areas of manipulation, sorting, characterization and sensing of objects, especially biological entities such as viruses, DNA, biomolecules and cells.

Self-homodyne optical orthogonal frequency division multiplexing (OFDM) transmission system is a variant of self-coherent systems with a relaxed laser linewidth requirement as opposed to a full coherent system. The system is designed by transmitting a carrier together with the subcarriers, ensuring a strong phase-correlation between them at a receiver compared to the coherent system where only the subcarriers are transmitted. Despite the strong phase-correlation, the laser phase noise (PN) can still degrade the self-homodyne system performance due to the interaction of the PN with a chromatic dispersion (CD) induces a phase-modulation to intensity-modulation (PM-to-IM) conversion noise which cannot be canceled using a balanced-receiver. This imposes a high noise pedestal underneath each subcarrier, with the highest pedestal's peak imposed beneath the subcarriers that are farthest away from the carrier. This in turn produces a phase-decorrelation between the carrier and subcarriers which degrades the Q-factor between the lowest and highest subcarrier frequencies. In this work, the effect of the PM-to-IM conversion noise is reduced for the transmission of up to 174 Gb/s using a simple delay-line assigned to realign the phase-decorrelation of the subcarriers. With this technique, the subcarriers Q-degradation can be improved and the total transmission length can be tripled with Q-improvements of 5.4, 5.3, and 5-dB for 16-quadrature amplitude modulation (QAM) over 1360 km, 32- QAM over 800 km, and 64-QAM over 480 km, respectively, using 10-MHz LW with a 15-GHz total signal bandwidth. For 29 GHz signal bandwidth, the phase-walk-off of the high frequency subcarrier becomes more severe where the introduced delay can increase the transmission length by 2.5 times than without the delay, which increased the signal tolerance to the conversion noise.

We propose a scheme for ultra-broadband and polarization-insensitive metamaterial perfect absorber (PA) by utilizing a thin metal-insulator-metal (MIM) stack, which is comprised of an array of cross-shaped titanium resonators, a silica dielectric spacer, and an opaque titanium slab. The bandwidth of absorption (A) > 90% is up to 2100 nm, ranging from the visible to near infrared region. At the wavelength range of 400-2500 nm, this metamaterial absorber shows strong absorption of electromagnetic waves. The spectral average absorptivity reaches 93.8% and the maximal absorptivity is up to 99.8%. In addition, the simulations show that the absorption remains high over a broad range of incident angles. Additionally, we have investigated the influences of geometries, structural parameters and material features on the absorption properties. The utilization of titanium rather than the noble metals efficiently lowers the fabrication cost and enhances the thermal stability and biocompatibility, thus paving a way to numerous applications such as solar energy harvesting, imaging, infrared detection and bio-medical techniques.

The paper considers methods of countering speech synthesis attacks on voice biometric systems in banking. Voicebiometrics security is a large-scale problem significantly raised over the past few years. Automatic speaker verificationsystems (ASV) are vulnerable to various types of spoofing attacks: impersonation, replay attacks, voice conversion, and speech synthesis attacks. Speech synthesis attacks are the most dangerous as the technologies of speech synthesisare developing rapidly (GAN, Unit selection, RNN, etc.). Anti-spoofing approaches can be based on searching forphase and tone frequency anomalies appearing during speech synthesis and on a preliminary knowledge of the acoustic differences of specific speech synthesizers. ASV security remains an unsolved problem, because there is no universalsolution that does not depend on the speech synthesis methods used by the attacker. In this paper, we provide the analysisof existing speech synthesis technologies and the most promising attacks detection methods for banking and financialorganizations. Identification features should include emotional state and cepstral characteristics of voice. It is necessary to adjust the user’s voiceprint regularly. Analyzed signal should not be too smooth and containing unnatural noisesor sharp interruptions changes in the signal level. Analysis of speech intelligibility and semantics are also important.Dynamic passwords database should contain words that are difficult to synthesize and pronounce. The proposed approach could be used for design and development of authentication systems for banking and financial organizations resistantto speech synthesis attacks.

To overcome the limitations of QKD, post-quantum cryptography, and computational security-based cryptography protocols in this paper, an atmospheric turbulence-controlled cryptosystem is proposed. The proposed encryption scheme employs the traditional scheme to utilize the atmospheric turbulence as the common source of randomness only in the initialization stage to determine the common parameters to be used in the proposed encryption scheme. To overcome low secret-key rates of traditional scheme, dictated by the long coherence time Tc of turbulence channel, the proposed encryption scheme updates the parameters of gamma-gamma distribution, used to generate irradiance samples for cumulative distribution function-based determination of the key, every Tc seconds and as such the final key is shaped by the atmospheric turbulence channel. We also describe a scheme that randomly selects one of several available paths in which the simultaneously measured irradiance samples, after interleaving, are used to generate the raw key. The secret-key rates of the proposed schemes are orders of magnitude higher compared to corresponding traditional QKD and source type physical-layer security schemes and are comparable with the state-of-the-art optical communication data rates.

Herein, we investigate the influence of an AlGaN metasurface on AlGaN-based deep-ultraviolet light-emitting diodes' light-extraction efficiency by utilizing the 3D finite-difference time-domain method. As the first step in this study, we optimize an AlGaN metasurface to maximize the transmittance from LED to air. Then, six different flip-chip structures' light-extraction efficiencies are compared with each other to judge the usefulness of the metasurface's adoption on AlGaN-based LED. Considered structures are classified into two categories, with and without inclined sidewall, respectively. Each classified structure is subdivided again by three factors: flat interface, roughed cones, and metasurface on n-AlGaN. Extracted results show that the combination of inclined sidewall and the AlGaN metasurface shows synergetic results positively increasing light-extraction efficiency.

In this work, by utilizing ionic liquid as the gain medium solvent, which is a new category of materials, we demonstrated a droplet-based dye laser system in a co-flowing microfluidic device. We characterized the droplet laser system and achieved a lasing threshold of 40.1 μJ/mm2. Owning to the unique properties of ionic liquids, such as negligible vapor pressure and good thermal and chemical stability, they offer great potentials for the development of on-chip light sources, tunable optofluidic devices, and sensitive biochemical analysis.

We consider the problem of calculating a refractive element with two surfaces, forming a flat front and a given distribution of illumination. The supporting quadrics method is formulated for calculating a given optical element and it is shown that this method coincides with the gradient method for some functional related to the problem of the Monge-Kantorovich mass transfer problem. This enables adaptive selection of the step in the supporting quadric method. At the end of the article a design example is given.

Surgeons need surgical navigation and intraoperative imaging for accurate surgery. However, current clinical surgical navigation systems have many drawbacks, including the need of fiducial markers, the lack of real-time registration update, and lack of functional imaging. We have developed a novel Real-time Fluorescence Imaging Topography Scanning (RFITS) system that delivers simultaneous real-time multimodal optical imaging, CT-to-Optical image registration, and dynamic fiducial-free surgical navigation. The RFITS prototype was characterized to evaluate system performance and used in biological models for fluorescence-guided surgery. We also performed multimodal imaging of a casting life-sized human cranial model and investigated image registration between intraoperative optical image data and preoperative tomographic image data. The RFITS prototype successfully integrated real-time intraoperative optical imaging and dynamic CT-based surgical navigation, offering complementary functional and structural information for surgical guidance. Our results demonstrated that the RFITS system can provide accurate multimodal surgical guidance in preclinical studies. To the best of our knowledge, this is the first report of a system capable of concurrent intraoperative fluorescence imaging and dynamic, real-time, CT-based navigation, whereas multiple modalities are accurately registered.

To advancement in security applications, for the first time, a bio-chaotic key generated from the emerging biometric trait “finger knuckle print (FKP)” has been proposed for data security and authentication. In the system, the feasibility to generate multiple keys from the single FKP image has been exploited to secure multiple information, which results in significant enhancement of the security strength. The encryption and decryption processes keep the user-specificity and perform better in reducing space for key storage and transmission. The research contribution is to facilitate a simplified optical implementation. Simulation results show the validity and effectiveness of the proposed scheme.

To improve the performance of optoelectronic devices, a high performance wide angle SiO2/Ti3O5 Distributed Bragg Reflector (DBR) mirror was designed and fabricated on double polished sapphire substrate. 17-pairs gradually varied DBR structure was applied in order to acquire a broad stopband reflectivity over 95% from 400 nm to 800 nm, covering the whole visible light region. The DBR mirrors deposited at the backside of optoelectronics devices are also demonstrated to remain a high reflectivity at wide incident angles, which improves the emission efficiency of LEDs and light absorption efficiency of InGaN/GaN photoelectrochemical cell (PEC).

The paper presents analysis of the features of robotic technological assembly lines and methods of their organization. The flexibility increasing of assembly robotic technological lines is currently provided mainly by the block-modular organization of technological lines and by various methods of technological equipment movement and structural transformation of manipulator links. For the assembly of products or their components from parts of complex configuration with various weight and size characteristics, it is recommended to use gripping devices of manipulators, which design gives the possibility to adapt the setting forces depending on the weight and dimensions of the gripped part. The growth of technological flexibility can be achieved through organizational, technical and design capabilities using a special technological assembly module of the carousel type, which main nodes are two turntables. The one turntable is designed for assembly operations. An example of a technological assembly module with three robots is given. Their interaction is carried out using computer control. In such a module, robots are arranged in a circle and the parts to be assembled are moved along a circle arc.

Subject of study. The authors propose an integrated approach to the problem of determining the aperture diameter of a probe laser that forms a laser reference star used in ground-based adaptive optoelectronic systems. The relevance of this study is due to the fact that modern large-aperture optical systems for tracking natural stars and artificial objects (spacecraft or fragments of space debris) widely implement the technology of forming laser stars and use them as reference sources for correcting phase distortions of a turbulent atmosphere. The choice of the energy and spatiotemporal characteristics of laser guide stars is related both to the parameters of the probe laser (radiation power and aperture diameter), which forms the guide star, and to the spatiotemporal characteristics of the atmosphere. Method. The diameter of the probe laser aperture (for the near and far radiation zones) is estimated taking into account the spatial coherence radius of the atmosphere r0, the radiation intensity and angular divergence of the laser beam, its random root-mean-square angular deviation (jitter) with respect to the calculated direction to the space objects. Main results. The estimation of the angular divergence of a laser beam is based on a comparative analysis and generalization of theoretical results obtained by calculating the optical resolution in systems for acquiring images of natural space objects. It is shown, in particular, that when determining the size of the aperture along with the value of the angular divergence of the probe beam, it is necessary to take into consideration the decrease in its radiation intensity with an increase in the diameter with respect to the radius of atmosphere coherence. The practical significance. The results are essential, firstly, for the development of ground-based adaptive optoelectronic systems for tracking artificial space objects, and secondly, for determining the geographic locations of the optoelectronic systems taking into account the astronomical climate.

The use of neural networks to detect differences in radiographic images of patients with pneumonia and COVID-19 is demonstrated For the optimal selection of resize and neural network architecture parameters, hyperparameters, and adaptive image brightness adjustment, precision, re-call, and f1-score metrics are used The high values of these metrics of classification quality (> 0 91) strongly indicate a reliable difference between radiographic images of patients with pneumonia and patients with COVID-19, which opens up the possibility of creating a model with good predictive ability without involving ready-to-use complex models and without pre-training on third-party data, which is promising for the development of sensitive and reliable COVID-19 ex-press-diagnostic methods © 2021, Institution of Russian Academy of Sciences All rights reserved

Subject of Research. The paper presents an approach to obtaining images of objects based on indirect laser location data. The concept of recording and processing photometric data about an object is developed, which implies the joint usage of an optical quantum generator and a camera with a photodetector based on single-photon avalanche diodes. Registration of an optical radiation scattered by an object is performed if the object is placed outside the sight line of the probing equipment, for example, behind a light-tight obstacle. Photometric data processing is aimed at the object shape modeling. Methods. Indirect laser location involves irradiation of an object with a series of light pulses and registration of scattered optical radiation. Since the object is located outside the sight line of the probing equipment, light pulses are propagated from the optical quantum generator to the surface that re-reflects these pulses in the direction of the object under study. Photometric data are formed as a registration result of optical radiation scattered by the object, propagating in the opposite direction. Vertical and horizontal scanning of the image is provided by the laser beam movement during the scanning of space. The object image is formed as a result of a series of sequential operations to extract photometric data about the object from the scattered optical radiation, calculate brightness values, and determine the positions of the raster voxels in the space of a rectangular coordinate system. Main Results. A conceptual model of indirect laser location is developed based on: a pulsed laser, scanning and focusing optical systems, a camera for scattered optical radiation recording, time synchronization device for generation and registration of light pulses. An approach is proposed to the processing of indirect laser location data, which provides three-dimensional images of objects. An experiment results are presented on the formation of an object image based on open materials of laser location. Practical Relevance. The indirect laser location system application gives the possibility to get object images in the absence of direct eye contact. The results of the image formation experiment demonstrate that when an object is placed behind a light-tight obstacle, it is possible to get a reliable idea of its structure and shape. The image obtained in accordance with the proposed approach is characterized by high graphic similarity and is a source of information about objects that are difficult to access.

For non-contact 3D measurements in hydrodynamic tunnels by photogrammetry methods, it is necessary to refine the standard model of image formation in the camera by taking into account an effect of refraction of rays at the boundaries of optical media, namely, at an air-glass boundary and glass-working fluid boundary. The article presents a model of image formation for shooting in a working environment that includes various optical media and methods for calibrating an optical system for 3D measurements of the coordinates of scene objects, while taking into account the real boundaries of the optical media. Experimental results on calibrating the system of three-dimensional measurements when an object image is formed by rays passing through two optical boundaries are discussed.

We propose and demonstrate a 100 Gbit/s silicon photonic (SiPh) coherent receiver (Rx) with advanced digital signal processing (DSP) for dual-polarization quadrature-phase-shift-keying (DP-QPSK) transmission. It employs integrated 2D polarization-splitting grating couplers (2D-GCs), modified hybrid mixers and high speed silicon-germanium (SiGe) photodiodes (PDs). The 2D-GC replaced the polarization beam splitters and rotators needed in traditional coherent Rx. Moreover, the SiGe PD has the bandwidth >50 GHz.

With the development of economy, the distribution problem of logistics becomes more and more complex. Based on the traffic network data, this study analyzed the vehicle routing problem (VRP), designed a dynamic vehicle routing problem with time window (DVRPTW) model, and solved it with genetic algorithm (GA). In order to improve the performance of the algorithm, the genetic operation was improved, and the output solution was further optimized by hill climbing algorithm. The analysis of example showed that the improved GA algorithm had better performance in path optimization planning, the total cost of planning results was 31.44 % less than that of GA algorithm, and the total cost of planning results increased by 11.48 % considering the traffic network data. The experimental results show that the improved GA algorithm has good performance and can significantly reduce the cost of distribution and that research on VRP based on the traffic network data is more in line with the actual situation of logistics distribution, which is conducive to the further application of the improved GA algorithm in VRP.

A broadband and compact TE-pass/TM-stop polarizer is presented based on hybrid plasmonic grating (HPG) on an x-cut Lithium-Niobate-on-isolator (LNOI) platform. By comprehensively analyzing the effects of metal width on mode effective index, mode similarity, and mode conversion, we demonstrate the structure with a narrow metal layer. The simulation results indicate that the polarizer with a compact length of 9 μm achieves an extinction ratio over 20 dB within the wavelength range from 1470 nm to 1700 nm. The insertion loss is below 2.3 dB in C-band. Furthermore, the polarizer exhibits large fabrication tolerance to current fabrication technology.

A method to calibrate the errors of polarization information measured by division-of-focal-plane polarimeters is demonstrated. The principle of this method is explained, and its feasibility is verified by theoretical simulation and experiment. The simulation and experimental results show that this method can obviously improve the measuring precision of polarization information, and compared with other approaches, the proposed method just uses normal light sources, but can achieve comparable performance.

Recently weakly-coupled mode-division multiplexing (MDM) transmission systems and networks based on ultralow-modal-crosstalk few-mode fiber (FMF) have attracted much interest, for which optical amplification technologies with low modal crosstalk and two-dimension mode-wavelength gain flattening are highly desired. In this paper, we propose for the first time a design method for few-mode Erbium-doped fiber (FM-EDF) with multi-ring-core (MRC) structures of both index and Erbium doping, which has high compatibility with the transmission MRC-FMF and low differential modal gain (DMG) with simple pump injection requirement. By sharing the boundaries of ring areas for both index and Erbium doping, the design method also helps to reduce the complexity of preform fabrication. We show by simulation that at 1550-nm signal wavelength, a 6-mode MRC-EDF can achieve a minimum DMG of 0.35-dB with only LP11 pump light injection and a 10-mode MRC-EDF can achieve a minimum DMG of 0.60-dB with LP01 and LP03 pump lights injection. Besides, the tolerance of DMG to the variation of fiber parameters induced by fabrication is analyzed. The proposed FM-EDF is beneficial for the practical applications of weakly-coupled MDM transmission systems and networks.