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Optical spectrum analysis provides a wealth of information about the physical world. Throughout the development of optical spectrum analysis, sensitivity has been one of the major topics and has become essential in applications dealing with faint light. Various high-sensitivity optical detection technologies have been applied in optical spectrum analysis to enhance its sensitivity to single-photon level. As an emerging single-photon detection technology, superconducting nanowire single-photon detectors (SNSPDs) have many impressive features such as high detection efficiency, broad operation bandwidth, small timing jitter, and so on, which make them promising for enhancing the performance of optical spectral analysis. Diverse schemes for photon-counting spectrometers based on SNSPDs have been demonstrated. This article reviews these impressive works and prospects for the future development of this technology. Further breakthroughs can be expected in its theories, device performance, applications, and combinations with in-sensor computing, promoting it to be a mature and versatile solution for optical spectrum analysis on ultra-faint light.

Abstract Metasurfaces with tunable functionalities are greatly desired for modern optical system and various applications. To increase the operating channels of polarization-multiplexed metasurfaces, we proposed a structure of N cascaded dual-channel metasurfaces to achieve 2 N electrically switchable channels without intrinsic loss or cross-talk for certain functionalities, including beam steering, vortex beam generation, lens, etc. As proof of principles, we have implemented a 3-layer setup to achieve 8 channels. In success, we have demonstrated two typical functionalities of vortex beam generation with switchable topological charge of l = −3 ~ +4 or l = −1 ~ −8, and beam steering with the deflection direction switchable in an 8×1 line or a 4×2 grid. We believe that our proposal would provide a practical way to significantly increase the scalability and extend the functionality of polarization-multiplexed metasurfaces. Although this method is not universal, it is potential for the applications of LiDAR, glasses-free 3D display, OAM (de)multiplexing, and varifocal meta-lens.

Abstract The photonic Ising machine is a new paradigm of optical computing that takes advantage of the unique properties of light wave propagation, parallel processing, and low-loss transmission. Thus, the process of solving combinatorial optimization problems can be accelerated through photonic/optoelectronic devices, but implementing photonic Ising machines that can solve arbitrary large-scale Ising problems with fast speed remains challenging. In this work, we have proposed and demonstrated the Phase Encoding and Intensity Detection Ising Annealer (PEIDIA) capable of solving arbitrary Ising problems on demand. The PEIDIA employs the heuristic algorithm and requires only one step of optical linear transformation with simplified Hamiltonian calculation by encoding the Ising spins on the phase term of the optical field and performing intensity detection during the solving process. As a proof of principle, several 20 and 30-spin Ising problems have been solved with high ground state probability (≥0.97/0.85 for the 20/30-spin Ising model).

Abstract Topological photonics provides a new degree of freedom to robustly control electromagnetic fields. To date, most of established topological states in photonics have been employed in Euclidean space. Motivated by unique properties of hyperbolic lattices, which are regular tessellations in non-Euclidean space with a constant negative curvature, the boundary-dominated hyperbolic topological states have been proposed. However, limited by highly crowded boundary resonators and complicated site couplings, the hyperbolic topological insulator has only been experimentally constructed in electric circuits. How to achieve hyperbolic photonic topological insulators is still an open question. Here, we report the experimental realization of hyperbolic photonic topological insulators using coupled ring resonators on silicon chips. Boundary-dominated one-way edge states with pseudospin-dependent propagation directions have been observed. Furthermore, the robustness of edge states in hyperbolic photonic topological insulators is also verified. Our findings have potential applications in the field of designing high-efficient topological photonic devices with enhanced boundary responses.

ABSTRACT: High-sensitivity mass sensors under ambient conditions are essential in various fields such as biological research, gas sensing and environmental monitoring. In the current work, a phonon lasing enhanced mass sensor was proposed based on an optomechanical crystal cavity under ambient conditions. The phonon lasing was harnessed to achieve ultra-high resolution since it resulted in an extremely narrow mechanical linewidth (less than 10 kHz). Masses with different weights were deposited on the cavity, it is predicted that the maximum resolution for mass sensing can be 65 ± 19 zg, which approaches the mass order of a protein and an oligonucleotide. This implies the potential application of the proposed method in the biomedical fields such as oligonucleotide drug delivery area and the Human Proteome Project.

Lasers differ from other light sources in that they are coherent, and their coherence makes them indispensable to both fundamental research and practical application. In optomechanical cavities, photon and phonon lasing is facilitated by the ability of photons and phonons to interact intensively and excite one another coherently. The lasing linewidths of both phonons and photons are critical for practical application. This study investigates the lasing linewidths of photons and phonons from the underlying dynamics in an optomechanical cavity. We find that the linewidths can be accounted for by two distinct physical mechanisms in two regimes, namely the normal regime and the reversed regime, where the intrinsic optical decay rate is either larger or smaller than the intrinsic mechanical decay rate. In the normal regime, an ultra-narrow spectral linewidth of 5.4 kHz for phonon lasing at 6.22 GHz can be achieved regardless of the linewidth of the pump light, while these results are counterintuitively unattainable for photon lasing in the reversed regime. These results pave the way towards harnessing the coherence of both photons and phonons in silicon photonic devices and reshaping their spectra, potentially opening up new technologies in sensing, metrology, spectroscopy, and signal processing, as well as in applications requiring sources that offer an ultra-high degree of coherence.

ABSTRACT: Spectral imaging extends the concept of traditional color cameras to capture images across multiple spectral channels and has broad application prospects. Conventional spectral cameras based on scanning methods suffer from the drawbacks of low acquisition speed and large volume. On-chip computational spectral imaging based on metasurface filters provides a promising scheme for portable applications, but endures long computation time due to point-by-point iterative spectral reconstruction and mosaic effect in the reconstructed spectral images. In this study, on-chip rapid spectral imaging was demonstrated, which eliminated the mosaic effect in the spectral image by deep-learning-based spectral data cube reconstruction. The experimental results show that 4 orders of magnitude faster than the iterative spectral reconstruction were achieved, and the fidelity of the spectral reconstruction for the standard color plate was over 99% for a standard color board. In particular, video-rate spectral imaging was demonstrated for moving objects and outdoor driving scenes with good performance for recognizing metamerism, where the concolorous sky and white cars can be distinguished via their spectra, showing great potential for autonomous driving and other practical applications in the field of intelligent perception.

Abstract Quantum key distribution (QKD) would play an important role in future information technologies due to its theoretically proven security based on the laws of quantum mechanics. How to realize QKDs among multiple users in an effective and simple way is crucial for its real applications in communication networks. In this work, we propose and demonstrate a fully connected QKD network without trusted node for a large number of users. Using flexible wavelength division multiplexing/demultiplexing and space division multiplexing, entanglement resources generated by a broadband energy-time entangled quantum light source are distributed to 40 users. Any two users share a part of entanglement resources, by which QKD is established between them. As a result, it realizes a fully connected network with 40 users and 780 QKD links. The performance of this network architecture is also discussed theoretically, showing its potential on developing quantum communication networks with large user numbers owing to its simplicity, scalability, and high efficiency.

Social media platforms have become the most prominent medium for spreading hate speech, primarily through hateful textual content. An extensive dataset containing emoticons, emojis, hashtags, slang, and contractions is required to detect hate speech on social media based on current trends. Therefore, our dataset is curated from various sources like Kaggle, GitHub, and other websites. This dataset contains hate speech sentences in English and is confined into two classes, one representing hateful content and the other representing non-hateful content. It has 451,709 sentences in total. 371,452 of these are hate speech, and 80,250 are non-hate speech. An augmented balanced dataset with 726,120 samples is also generated to create a custom vocabulary of 145,046 words. The total number of contractions considered in the dataset is 6403. The total number of bad words usually used in hateful content is 377. The text in each sentence of the final dataset, which is utilized for training and cross-validation, is limited to 180 words. The generated contractions dataset can be used for any projects in the area of NLP for data preprocessing. The augmented dataset can help to reduce the number of out-of-vocabulary words, and the hate speech dataset can be used as a classifier to detect hate or no hate on social media platforms.

The hybrid integration of superconducting nanowire single-photon detectors (SNSPDs) on various substrates and photonic structures has great potential on developing complicated photonic devices based on single-photon detections, such as photon-counting reconstructive spectrometers for the spectral sensing of single-photon level faint light. In this paper, we introduce the cascaded absorption effect of SNSPDs to develop a photon-counting reconstructive spectrometer. The device includes a Rowland grating as the spatial dispersion element and a tailored cascaded SNSPD array in the focusing region of the grating. The spectral responses of the SNSPDs could be flexibly modulated by their coiled patterns and the cascaded absorption in the array, which are used as the bases for spectral reconstruction. A prototype device was designed and fabricated to demonstrate the principle of the scheme. The experiment results showed the feasibility of the spectral response modulations by the coiled pattern design and the cascaded absorption effect of the SNSPD array. It supports the spectral measurement and reconstruction in the wavelength range of 1,495 to 1,515 nm, with a spectral resolution of 0.4 nm. The proposed scheme achieves the bases for spectral reconstruction only by the design of SNSPDs and without the spectral modulation effects of additional photonic structures. It provides an interesting and promising way to develop devices with high photon utilization.

Diode end-pumped Nd,La:CaNb2O6 self-Raman laser with acousto-optic Q-switching was successfully demonstrated for the first Stokes wave generation at 1,174 nm. A 1.0 at.% Nd3+ and 1.0 at.% La3+-doped CaNb2O6 crystal in dimensions 3 × 3 × 14.3 mm3 was used as the self-Raman laser crystal. Doping 1 at.% La3+ ions into this crystal could subdue the fluorescence quenching caused by cross-relaxation between Nd3+ ions and finally improve the laser output performance. Under the incident pump power of 9.9 W, the first Stokes wave at 1,174 nm with a maximum output power up to 928 mW was obtained, with the diode to Stokes conversion efficiency of about 9.4%. The results show that the Nd,La:CaNb2O6 is also a promising self-Raman crystal for efficient fundamental and Raman laser operation.

In this paper, we realize an adjustable biphoton frequency comb and investigate its spatial quantum beating. Vector spontaneous four wave mixing processes in a piece of dispersion shifted fiber are produced by non-degenerate continuous wave pump lights with orthogonal polarizations to generate broadband biphoton states, and then a pulse shaper is used to realize the biphoton frequency comb with high-dimensional frequency-bin entanglement. The property of the biphoton frequency comb is indicated by the shape of the fringes of spatial quantum beating, which is also comb-like with a series of dips. The variations of the beating fringes show the changes of frequency bins in the comb by the setting of the pulse shaper. It can be expected that the adjustable biphoton frequency comb has great potential on high-dimensional quantum information processing and quantum communication based on the frequency degree of freedom.

Quantum key distribution (QKD) is a crucial technology for information security in the future. Developing simple and efficient ways to establish QKD among multiple users is important to extend the applications of QKD in communication networks. Herein, we proposed a scheme of symmetric dispersive optics QKD and demonstrated an entanglement-based quantum network based on it. In the experiment, a broadband entangled photon pair source was shared by end users via wavelength and space division multiplexing. The wide spectrum of generated entangled photon pairs was divided into 16 combinations of frequency-conjugate channels. Photon pairs in each channel combination supported a fully connected subnet with eight users by a passive beam splitter. Eventually, it showed that an entanglement-based QKD network over 100 users could be supported by one entangled photon pair source in this architecture. It has great potential on applications of local quantum networks with large user number.

Multimode optomechanics exhibiting several intriguing phenomena, such as coherent wavelength conversion, optomechanical synchronization, and mechanical entanglements, has garnered considerable research interest for realizing a new generation of information processing devices and exploring macroscopic quantum effect. In this study, we proposed and designed a hetero-optomechanical crystal (OMC) zipper cavity comprising double OMC nanobeams as a versatile platform for multimode optomechanics. Herein, the heterostructure and breathing modes with high mechanical frequency ensured the operation of the zipper cavity at the deep-sideband-resolved regime and the mechanical coherence. Consequently, the mechanical breathing mode at 5.741 GHz and optical odd mode with an intrinsic optical Q factor of 3.93 × 105 were experimentally demonstrated with an optomechanical coupling rate g0 = 0.73 MHz between them, which is comparable to state-of-the-art properties of the reported OMC. In addition, the hetero-zipper cavity structure exhibited adequate degrees of freedom for designing multiple mechanical and optical modes. Thus, the proposed cavity will provide a playground for studying multimode optomechanics in both the classical and quantum regimes.

Abstract The axis tilt of light beam in optical system would introduce the dispersion of orbital angular momentum (OAM) spectrum. To deal with it, a two-step method is proposed and demonstrated. First, the tilt angle of optical axis is identified with a deduced relation between the tilt angle and the variation of OAM topological charges with different reference axes, which is obtained with the help of a charge coupled device (CCD) camera. In our experiments, the precision of measured tilt angle is about 10−4 rad with topological charges of −3~3. With the measured angle value, the additional phase delay due to axis tilt can be calculated so that the dispersion of OAM spectrum can be corrected with a simple formula while the optical axis is not aligned. The experimental results indicate that the original OAM spectrum has been successfully extracted for not only the pure state but also the superposed OAM states.

In this paper, we propose and demonstrate a simple chalcogenide glass (ChG) waveguide structure, which is composed of a low refractive-index strip on the surface of the planar ChG film. Theoretical analysis shows that it supports single quasi-TE mode transmission at 1.55 μm with high third-order nonlinearity and negligible bending loss if the bending radius is higher than 60 μm. Then, the ChG waveguide is fabricated and measured. For the waveguide sample with a ChG film of 180 nm in thickness, the measured attenuation of the quasi-TE mode is 0.4 dB/cm at 1.55 μm. Meanwhile, the extinction ratio between the two polarization modes in the waveguide sample of 10 mm in length is higher than 25 dB over a region of 80 nm at 1.55 μm. For the waveguide sample with similar length and a smaller ChG film thickness (150 nm), the extinction ratio increases to 35 dB. These results demonstrate its property of low-loss single-polarization transmission. It provides a simple way to realize high-quality ChG waveguides, which have great potential on integrated nonlinear optical devices.

We demonstrate a compact and broadband 1 × 4 optical switch based on W2 photonic crystal waveguides (PCWs). The footprint of the W2 PCW is only 17.6 μm × 8 μm. An operation bandwidth of 15 nm ± 1 nm is implemented with an extinction ratio over 15 dB in each channel. By introducing the thermal insulation trench, the tuning efficiency is enhanced at least 77% in experiment. The experimental results indicate the potential of the proposed broadband thermo-optic switch in silicon on-chip interconnects.

An eight-channel integrated optical add-drop multiplexer with cascaded parent-sub microring resonators is experimentally demonstrated on a silicon-on-insulator (SOI) substrate. Through thermal tuning, each channel can be independently switched between the adding and dropping states. The measured thermal tuning efficiency is 0.15 nm/mW. Typically, all eight channels can be multiplexed and demultiplexed with a 2.6-dB drop loss, a 0.36-nm bandwidth (> 40 GHz), and a -20-dB channel crosstalk. Moreover, the scheme to increase the number of operating channels is also discussed.

A reverse ridge/slot hybrid chalcogenide glass (As2S3) waveguide with two vertical silicon dioxide slots is proposed in this paper. The fundamental quasi-TE mode of the waveguide shows an ultraflat dispersion with three zero-dispersion wavelengths. Its dispersion is confined between -26 and +27 ps/nm/km over a bandwidth of 1370 nm (from 1770 to 3140 nm). Two slots in the waveguide introduce the dispersion tailoring effect on the quasi-TE mode, which provides more flexibility for designing the waveguide dispersion. The nonlinear coefficient and the phase mismatching of the degenerate fourwave mixing (FWM) process in this waveguide are calculated, showing that it can support broadband FWM processes in nearand middle-infrared regions.

Radially polarized orbital angular momentum (OAM) beams could be applied in optical manipulation and optical microscopy. An integrated OAM beam emitter with radially polarized radiation is proposed on shallow-ridge silicon microring with azimuthally distributed gratings etched on top of the ring waveguide. Two key structural parameters, the number of grating elements, and the dimension of each grating element are optimized by considering both the OAM order purity (Cp) and the total radiation efficiency (ηall). With elaborate design, Cp = ~98%, ηall = ~49% are achieved by simulations, and the calculated energy ratio between the far-field radially and azimuthally polarized component is more than 11 dB, which indicates that the radially polarized component is predominant in the radiated electromagnetic field.

A slot waveguide formed by high absorption active material is proposed to reduce the propagation loss for monolithic integration. The low propagation loss is attained by concentrating the optical field inside the low-index slot region without absorption. The simulation results show that the propagation loss at 1.55 μm for the slot waveguide can be as low as 1.5 dB/cm in active material with an absorption coefficient of 3000 cm-1, whereas the optical profile is set to be confined within 650 nm.

Double-slot photonic crystal waveguide (PCW) in InP heterostructure is fabricated by inductively coupled plasma (ICP) etching. Due to using an ultra-low pressure of 0.05 Pa, etch depths up to 3.5 μm for holes with diameter of 200 nm and 1.8 μm for slots of ∼40 nm are achieved, which indicate a record-high aspect-ratio, i.e. 45, for such narrow slots in InP heterostructure. Moreover, etching quality is evaluated based on both the transmission performance and the linewidth of micro-photoluminescence (μ-PL). In our measurement, a structure-dependent transmission-dip about 17 dB is obtained from a 17-μm-long W3 PCW, and a PL widening as small as 19 nm compared to the corresponding wafer is observed. These promising experimental results evidence the high etching quality realized in this work and confirm the feasibility of etching small-feature-size patterns by ICP technology for InP based devices in future mono-/hetero-integrated photonic circuits.

A compact thermo-optic switch based on the cutoff effect of slow-light mode of a tapered W1 photonic crystal waveguide (PCW) is demonstrated with an integrated microheater. Due to the low-group-index taper, the coupling loss of the slow-light PCW is reduced, and a high switching extinction ratio (ER) is attained. Moreover, three types of microheaters are evaluated for the power consumptions, heating transfer efficiency, and temperature uniformities, and an optimized slab microheater is utilized. As a result, low switching power of 8.9 mW and high ER of 23.5 dB are achieved experimentally, while the length of W1 PCW is only 16.8 μm.

The plasmonic enhanced absorption of thin-film organic solar cell (OSC) with silver nanoparticles has been simulated and analyzed in the two-dimensional (2-D) and three-dimensional (3-D) simulation models by considering the position of nanoparticles inside the OSC and the incident angle and polarization of the incident light. It is found that, for TM polarization incidence, obvious optical absorption enhancement is obtained in both 2-D and 3-D cases. The absorption enhancement reaches more than 200% with nanoparticles deposited at the interface of PEDOT:PSS and P3HT:PCBM layer, which is larger than that with nanoparticles inside the active layer. However, for TE polarization incidence, the optical absorption is worsened rather than enhanced with metal nanostructures in the 2-D model, which is different with the results derived in the 3-D model. The absorption enhancement characteristics are also studied at oblique incidence, and the high absorption enhancement as high as 160% can be also obtained when the incident angle is increased to 60 ° in the 3-D model. By analyzing the mode profile in different circumstances, it could be concluded that the localized surface plasmon plays a significant role on improving the light absorption enhancement of OSC.

The efficiency enhancement of thin film hetero-junction organic solar cells (OSCs) with Au-nanopaticles (Au-NPs) at the interface between the anode and active layers is studied theoretically and experimentally. It is demonstrated that the interface-Au-NPs could effectively extend the spectrum of light absorption enhancement based on the localized surface plasmon, which is revealed by the incident photon-to-current efficiency with photocurrent increased at both the long (λ0 = 600 ~ 700 nm) and short (λ0 = 400 ~ 500 nm) wavelength regions. The experiment results also indicate that the efficiency improvement of OSCs is related to the size and concentration of the Au-NPs. The optimized power conversion efficiency enhancement is about 25%.

A simple on-chip integrated structure is proposed to generate optical orbital angular momentum (OAM) beams, where only silicon waveguides and couplers are involved. The operating principle is based on mode converting and proper controlling the phase shift of second-order propagating mode so that OAM modes with charge numbers of 1 or -1 can be selectively generated with mode purity > 90%. The proposed device is very compact with footprint of

A photonic crystal nanobeam cavity with stagger holes in InP/InGaAsP/InP heterostructure is proposed for ultrafast directly modulated nano-light-emitting diodes (nanoLEDs). With stagger holes, the quality factor Q can be engineered in the range of 102 ~ 104 while keeping a small mode volume (Veff). As a result, the modulation speed of nanoLEDs can be dramatically improved by a small Veff to enhanced spontaneous emission (SpE) rate and a moderate Q to counterbalance SpE lifetime and photon lifetime of the cavity. In our simulation, the direct modulation bandwidth could be higher than 60 GHz with optimal Q value of 2150 and Veff of 2.3( λ0/2n)3.