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Sub-THz repetition rate frequency comb generated by dissipative parametric instability in passive driven fiber resonators

Pan Wang, Jiangyong He, Xiaosheng Xiao, Zhi Wang, and Yange Liu

DOI: 10.1364/PRJ.442615 Received 10 Sep 2021; Accepted 29 Nov 2021; Posted 30 Nov 2021  View: PDF

Abstract: Ultra-high repetition rate frequency combs generation exhibits great potential in applications of optical waveform synthesis, direct comb spectroscopy, and high capacity telecommunications. Here, we present the theoretical investigations of dissipative parametric instability mechanism in passive driven fiber resonators with a wide range of cavity dispersion regimes. In this novel concept of modulation instability, coherent frequency combs are demonstrated numerically with rates up to sub-THz. Floquet stability analysis based on Ikeda map is utilized to understand the physical origin of the dissipative parametric instability. Comparison with the well-known Benjamin-Feir instability and Turing instability is performed, revealing the intrinsic distinction in the family of modulation instabilities. Our investigations might benefit the development of ultra-high repetition rate frequency comb generation, providing an alternative method to the micro-resonators.

Integrable high-efficiency generation of three-photon entangled states by a single incident photon

Yunning Lu, Zeyang Liao, fuli li, and Xue-Hua Wang

DOI: 10.1364/PRJ.443245 Received 28 Sep 2021; Accepted 27 Nov 2021; Posted 30 Nov 2021  View: PDF

Abstract: Generation of multi-photon entangled states with high efficiency in integrated photonic quantum system is still a big challenge. The usual three-photon generation efficiency based on third-order nonlinear effect is extremely low. Here, we propose a scheme to generate three-photon correlated states which are entangled states in frequency space and bound states in real space with high efficiency. This method relies on two crucial processes. On the one hand, by employing a Sagnac interferometer, an incident photon can be transformed into a symmetric superposition of the clockwise and counterclockwise modes of the Sagnac loop which can then be perfectly absorbed by the emitter. On the other hand, the coupling strengths of the two transition paths of the emitter to the Sagnac loop are set to be equal under which the absorbed photon can be emitted completely from the cascaded transition path due to the quantum interference. By adjusting the coupling strengths between the three transition paths of the emitter and the waveguide modes, we can control the spectral entanglement and the spatial separation between the three photons. Our proposal can be used to generate three-photon entangled states on demand, and the efficiency can be higher than 90% with some practical parameters, which can find important applications in integrated quantum information processing.

Topologically Protecting Squeezed Light on a Photonic Chip

Ruo-Jing Ren, Yong-Heng Lu, Ze-Kun Jiang, Jun Gao, Wen-Hao Zhou, Yao Wang, Zhi-Qiang Jiao, Xiao-Wei Wang, Alexander Solntsev, and Xianmin Jin

DOI: 10.1364/PRJ.445728 Received 12 Oct 2021; Accepted 27 Nov 2021; Posted 30 Nov 2021  View: PDF

Abstract: Squeezed light is a critical resource in quantum sensing and information processing. Due to the inherently weak optical nonlinearity and limited interaction volume, considerable pump power is typically needed to obtain efficient interactions to generate squeezed light in bulk crystals. Integrated photonics offers an elegant way to increase the nonlinearity by confining light strictly inside the waveguide. For the construction of large-scale quantum systems performing many-photon operations, it is essential to integrate various functional modules on a chip. However, fabrication imperfections and transmission crosstalk may add unwanted diffraction and coupling to other photonic elements, reducing the quality of squeezing. Here, by introducing the topological phase, we experimentally demonstrate the topologically protected nonlinear process of four-wave mixing enabling the generation of squeezed light on a silica chip. We measure the cross-correlations at different evolution distances for various topological sites and verify the non-classical features with high fidelity. The squeezing parameters are measured to certify the protection of cavity-free, strongly squeezed states. The demonstration of topological protection for squeezed light on a chip brings new opportunities for quantum integrated photonics, opening novel approaches for the design of advanced multi-photon circuits.

Quantum non-demolition measurement based on an SU(1,1)-SU(2)-concatenated atom-light hybrid interferometer

Gao-Feng Jiao, Keye Zhang, Liqing Chen, Chunhua Yuan, and Weiping Zhang

DOI: 10.1364/PRJ.445858 Received 14 Oct 2021; Accepted 27 Nov 2021; Posted 30 Nov 2021  View: PDF

Abstract: Quantum non-demolition (QND) measurement is an important tool in the field of quantum information processing and quantum optics. The atom-light hybrid interferometer is of great interest due to its combination of atomic spin wave and optical wave, which can be utilized for photon number QND measure ment via the AC-Stark effect. In this paper, we present an SU(1,1)-SU(2)-concatenated atom-light hybrid interferometer, and theoretically study the QND measurement of photon number. Compared to the traditional SU(2) interferometer, the signal-to-noise ratio (SNR) in a balanced case is improved by a gain factor of the nonlinear Raman process (NRP) in this proposed interferometer. Furthermore, the condition of high-quality of QND measurement is analyzed.In the presence of losses, the measurement quality is reduced. We can adjust the gain parameter of the NRP in readout stage to reduce the impact due to losses. Moreover, this scheme is a multiarm interferometer, which has the potential of multiparameter estimation with many important applications in the detection of vector fields, quantum imaging and so on.

Synchronization of an optical frequency comb and a microwave oscillator with 53 zs/Hz1/2 resolution and 10-20-level stability

Changmin Ahn, Yongjin Na, Minji Hyun, Jinho Bae, and Jungwon Kim

DOI: 10.1364/PRJ.443316 Received 21 Sep 2021; Accepted 26 Nov 2021; Posted 30 Nov 2021  View: PDF

Abstract: Precise and stable synchronization between an optical frequency comb (femtosecond mode-locked laser oscillator or microresonator-based comb) and a microwave oscillator is important for various fields including telecommunication, radio astronomy, metrology, and ultrafast X-ray and electron science. Timing detection and synchronization using electro-optic sampling with an interferometer has been actively used for low-noise microwave generation, long-distance timing transfer, comb stabilization, time-of-flight sensing, and laser-microwave synchronization for ultrafast science facilities. Despite its outstanding performance, there has been a discrepancy in synchronization performance of more than 10 dB between the projected shot-noise-limited noise floor and the measured residual noise floor. In this work, we demonstrate the shot noise-limited performance of an electro-optic timing detector-based comb-microwave synchronization, which enabled unprecedented residual phase noise floor of –174.5 dBc/Hz at 8-GHz carrier frequency (i.e., 53 zs/Hz1/2 timing noise floor), integrated rms timing jitter of 88 as [1 Hz-1 MHz], rms timing drift of 405 as over 12 h, and frequency instability of 2×10-20 over 10,000 s averaging time. We identified that bandpass filtering of the microwave signal and optical pulse repetition-rate multiplication are critical for achieving this performance.

Influence of light pattern thickness on the manipulation of dielectric microparticles by optoelectronic tweezers

Shuailong Zhang, Mohammed Elsayed, Ran Peng, Yujie Chen, Yanfeng Zhang, Steven Neale, and Aaron Wheeler

DOI: 10.1364/PRJ.437528 Received 14 Jul 2021; Accepted 23 Nov 2021; Posted 24 Nov 2021  View: PDF

Abstract: Optoelectronic tweezers (OET) is a useful optical micromanipulation technology that has been demonstrated for various applications in electrical engineering and most notably cell selection for biomedical engineering. In this work, we studied the use of light patterns with different shapes and thicknesses to manipulate dielectric microparticles with OET. It was demonstrated that the maximum velocities of the microparticles increase to a peak and then gradually decrease as the light pattern's thickness increases. Numerical simulations were run to clarify the underlying physical mechanisms and it was found that the observed phenomenon is due to the co-influence of horizontal and vertical DEP forces related to the light pattern's thickness. Further experiments were run on light patterns with different shapes and objects of different sizes and structures. The experimental results indicate that the physical mechanism elucidated in this research is a universal one that applies to different light pattern shapes and different objects, which is useful for enabling users to optimize OET settings for future micromanipulation applications.

Wafer-level Hermetically Sealed Silicon Photonic MEMS

Gaehun Jo, Pierre Edinger, Simon Bleiker, Xiaojing Wang, Alain Takabayashi, Hamed Sattari, Niels Quack, Moises Jezzini, Peter Verheyen, Iman Zand, Umar Khan, Wim Bogaerts, Goran Stemme, Kristinn Gylfason, and Frank Niklaus

DOI: 10.1364/PRJ.441215 Received 25 Aug 2021; Accepted 23 Nov 2021; Posted 30 Nov 2021  View: PDF

Abstract: The emerging fields of silicon (Si) photonic micro-electromechanical systems (MEMS) and optomechanics enable a wide range of novel high-performance photonic devices with ultra-low power consumption, such as integrated optical MEMS phase shifters, tunable couplers, switches, and optomechanical resonators. In contrast to conventional SiO2-clad Si photonics, photonic MEMS and optomechanics have suspended and movable parts that need to be protected from environmental influence and contamination during operation. Wafer-level hermetic sealing can be a cost-efficient solution, but Si photonic MEMS that are hermetically sealed inside cavities with optical and electrical feedthroughs have not been demonstrated to date. Here, we demonstrate wafer-level vacuum sealing of Si photonic MEMS inside cavities with ultra-thin caps featuring optical and electrical feedthroughs that connect the photonic MEMS on the inside to optical grating couplers and electrical bond pads on the outside. We used Si photonic MEMS devices built on foundry wafers from the iSiPP50G Si photonics platform of IMEC, Belgium. Vacuum confinement inside the sealed cavities was confirmed by an observed increase of the cut-off frequency of the electro-mechanical response of the encapsulated photonic MEMS phase shifters, due to reduction of air damping. The sealing caps are extremely thin, have a small footprint, and are compatible with subsequent flip-chip bonding onto interposers or printed circuit boards (PCBs). Thus, our approach for sealing of integrated Si photonic MEMS clears a significant hurdle for their application in high-performance Si photonic circuits.

Mid-infrared active metasurface based on Si/VO₂ hybrid metaatoms

Tongtong Kang, Boyu Fan, Jun Qin, Weihao Yang, Shuang Xia, Zheng Peng, Bo Liu, Sui Peng, Xiao Liang, Tingting Tang, Longjiang Deng, Yi Luo, Hanbin Wang, Qiang Zhou, and Lei Bi

DOI: 10.1364/PRJ.445571 Received 14 Oct 2021; Accepted 22 Nov 2021; Posted 24 Nov 2021  View: PDF

Abstract: Active metasurfaces, whose optical properties can be tuned by an external stimulus have attracted great research interest recently. Introduction of VO₂ phase change material in all dielectric metasurfaces has been demonstrated to modulate the resonance wavelength and amplitude in the visible to near infrared wavelength range. In this study, we report a mid-infrared active metasurface based on Si/VO₂ hybrid metaatoms. By incorporating VO₂ thin films in different locations of Si/VO₂ all-dielectric nanodisks, we demonstrate different modulation amplitude of the electric or magnetic resonance scattering cross-sections, leading to drastically different transmission spectrum upon VO₂ insulator to metal phase transition (IMT). The physical mechanism is originated from the field profiles of the resonance modes, which interacts with VO₂ differently depending on its locations. Based on this mechanism, we experimentally demonstrated a large modulation of the transmittance from 82% to 28% at 4.6 μm wavelength. Our work demonstrates a promising potential of VO₂ based active all dielectric metasurface for mid-infrared photonic applications such as infrared camouflage, chemical/biomedical sensing, optical neuromorphic computing and multispectral imaging.

Dynamic full-field reflective index distribution measurements using total internal reflection terahertz digital holography

Dayong Wang, Duoxuan Ma, Kunlun Li, Yaya Zhang, Jie Zhao, Yunxin Wang, and Lu Rong

DOI: 10.1364/PRJ.442388 Received 09 Sep 2021; Accepted 21 Nov 2021; Posted 23 Nov 2021  View: PDF

Abstract: Massive usage scenarios prompt the prosperity of terahertz refractive index (RI) measurement methods. In this study, we propose total internal reflection terahertz digital holography as a technique to measure the terahertz full-field dynamical RI distribution, especially for liquid samples, for the first time. A RI measurement model is established based on an attenuated total reflection prism with a pitching angle. A non-contact numerical approach is developed to estimate the angle, which solves the vertical orientation adjustment issue of the visually opaque prism irradiated by the invisible terahertz beam. Full-field RI measurements are conducted using the droplets of solid-state soy wax and drilled water, and the results are compared with THz time-domain spectroscopy. The evaporation of a droplet of 75% alcohol solution is recorded and the variation of the RI distribution at the interface is quantitatively visualized with a temporal resolution of 10 Hz. The proposed method greatly expands the sample range for terahertz RI measurements and provides unprecedented insights in investigating spontaneous and dynamic terahertz phenomena.

Near-optimal intense and powerful terahertz source by optical rectification in lithium niobate crystal

Léo Guiramand, Joel Edouard NKECK, xavier ropagnol, Tsuneyuki Ozaki, and Francois Blanchard

DOI: 10.1364/PRJ.428418 Received 20 Apr 2021; Accepted 21 Nov 2021; Posted 23 Nov 2021  View: PDF

Abstract: Using an affordable ytterbium laser with sub-mJ of energy combined with a novel pulse compression technique, we demonstrate an extremely competitive state-of-the-art terahertz (THz) source with 53 mW of average power and 310 kV/cm at focus from the tilted-pulse front pumping scheme in lithium niobate at room temperature. Key points of this demonstration include the use of a pump pulse duration of 280 fs in combination with an echelon mirror. Our results present unmatched combined characteristics and are highly competitive with the existing THz sources pumped at the mJ range. This demonstration is a step towards the democratization of access to intense and powerful THz pulses.

Metasurface doublet-integrated bidirectional grating antenna enabling enhanced wavelength-tuned beam steering

Woo-Bin Lee, Chul-Soon Im, Changyi Zhou, Bishal Bhandari, Duk-Yong Choi, and Sang-Shin Lee

DOI: 10.1364/PRJ.433024 Received 18 Jun 2021; Accepted 20 Nov 2021; Posted 23 Nov 2021  View: PDF

Abstract: We propose and demonstrate an optical-phased-array-based bidirectional grating antenna (BDGA) in silicon nitride waveguides. The BDGA is integrated with a miniaturized all-dielectric metasurface doublet (MD) formed on a glass substrate. The BDGA device, which takes advantage of alternately feeding light to its ports in opposite directions, is presumed to effectively provide a doubled wavelength-tuned steering efficiency compared to its unidirectional counterpart. The MD, which is based on vertically cascaded convex and concave metalenses comprising circular hydrogenated amorphous silicon nano-pillars, is meticulously placed atop the BDGA chip to accept and deflect a beam emanating from the emission area, thereby boosting the beam steering performance. The manufactured BDGA could achieve an enhanced beam steering efficiency of 0.148°/nm as well as a stable spectral emission response in the wavelength range of 1530–1600 nm. By deploying a fabricated MD atop the silicon photonic BDGA chip, the steering efficiency was confirmed to be boosted by a factor of ~3.1, reaching 0.461°/nm, as intended.

Compact logic operator utilizing single-layer metasurface

Zihan Zhao, Yue Wang, XUMIN DING, Haoyu Li, Kuang Zhang, Jia-Hui Fu, Shah Nawaz Burokur, and Qun Wu

DOI: 10.1364/PRJ.439036 Received 28 Jul 2021; Accepted 20 Nov 2021; Posted 23 Nov 2021  View: PDF

Abstract: In this paper, we design and demonstrate a compact logic operator based on a single-layer metasurface at microwave frequency. By mapping the nodes in the trained Fully Connected Neural Network (FCNN) to the specific unit cells with phase control function of the metasurface, logic operator with only one hidden layer is physically realized. When the incident wave illuminates specific operating regions of the metasurface, corresponding unit cells are activated and can scatter the incident wave to two designated zones containing logical information in the output layer. The proposed metasurface logic operator is experimentally verified to achieve three basic logic operations (NOT, OR, and AND) under different input signals. Our design shows great application potentials in compact optical systems, low-power consumption information transmission, and ultrafast wave-based fully signal processing.

Multi-particle resonant optical sorting using topological photonic structure

Bojian Shi, Yongyin Cao, Tongtong Zhu, Hang Li, Yanxia Zhang, rui feng, Fangkui Sun, and Weiqiang Ding

DOI: 10.1364/PRJ.441644 Received 26 Aug 2021; Accepted 18 Nov 2021; Posted 18 Nov 2021  View: PDF

Abstract: Resonance between light and object is highly desired in optical manipulation because the optical forces reach maximum values in this case. However, in traditional waveguide structures, the resonant interaction also greatly perturbs the incident field and weaken or completely destroy the manipulation on the subsequent particles. In order to avoid this dilemma, we propose to perform optical manipulation in a topological photonic structure. Due to the topological protection, the light mode can almost keep its original form when an object is already being manipulated. Therefore, resonant optical sorting can be achieved in a multiple and high throughput manner. The mechanism and results presented here pave the way for efficient on-chip optical sorting for biophysical and biochemical analysis.

Ultra-sensitive Dirac-point-based Biosensing on Terahertz Metasurfaces Comprising Patterned Graphene and Perovskites

Xin Yan, Tengteng Li, Guo-Hong Ma, Ju Gao, Tongling WANG, Haiyun Yao, Yang Maosheng, Lanju Liang, Jing Li, Jie Li, deiquan wei, Meng Wang, Yunxia Ye, XIAOXIAN SONG, HAITING ZHANG, Chao Ma, Yunpeng Ren, Xudong Ren, and Jian-Quan Yao

DOI: 10.1364/PRJ.444225 Received 24 Sep 2021; Accepted 18 Nov 2021; Posted 18 Nov 2021  View: PDF

Abstract: Biosensors are a focus of research on terahertz metasurfaces. However, reports of ultra-sensitive biosensors based on Dirac points are rare. Here, a new terahertz metasurface is proposed that consists of patterned graphene and perovskite. This serves as an ultra-sensitive Dirac point-based biosensor for qualitative detection of sericin. Theoretically, sericin may make graphene n-doped and drive the Fermi level to shift from the valence band to the Dirac point, causing a dramatic decrease in conductivity. Correspondingly, the dielectric environment on the metasurface undergoes significant change, which is suited for ultra-sensitive biosensing. In addition, metal halide perovskites, which are up-to-date optoelectronic materials, have a positive effect on the phase during terahertz wave transmission. Thus, this sensor was used to successfully detect sericin with a detection limit of 780 pg/mL, achieved by changing the amplitude and phase. The detection limit of this sensor is as much as five orders of magnitude lower than that of sensors in published works. These results show that the Dirac point-based biosensor is a promising platform for a wide range of ultra-sensitive and qualitative detection in biosensing and biological sciences.

12Emission spectroscopy of NaYF₄:Eu nanorods optically trapped by Fresnel lens fibers

Aashutosh Kumar, Asa Asadollahbaik, Jeongmo Kim, Khalid Lahlil, Simon Thiele, Alois Herkommer, Sile Nic Chormaic, Jong-wook Kim, Thierry Gacoin, Harald Giessen, and Jochen Fick

DOI: 10.1364/PRJ.434645 Received 21 Jun 2021; Accepted 18 Nov 2021; Posted 22 Nov 2021  View: PDF

Abstract: NaYF₄:Eu nanorods with a high aspect ratio are elaborated and optically trapped using a dual fiber optical tweezers in a counter-propagating geometry. High trapping efficiency is observed using converging beams, emitted from diffractive Fresnel lenses directly 3D printed onto cleaved fiber facets. Stable nanorod trapping and alignment are reported for a fiber-to-fiberdistance of 200 μm and light powers down to 10 mW. Trapping of nanorod clusters containing one to three nanorods and the coupling of nanorod motion in both axial and transverse directions are considered and discussed. The Europium emission is studied by polarization-resolved spectroscopy with particular emphasis on the magnetic and electric dipole transitions. Therespective σ and π orientation of the different emission lines is determined. The angle with respect to the nanorod axis of the corresponding magnetic and electric dipoles are calculated. Mono-exponential emission decay with decay times of 4 - 5 ms is reported. It is shown that the nanorod orientation can be determined by purely spectroscopic means.

Lensless Fourier terahertz digital holography for real-time full-field phase imaging

Yaya Zhang, Jie Zhao, Dayong Wang, Yunxin Wang, and Lu Rong

DOI: 10.1364/PRJ.435769 Received 01 Jul 2021; Accepted 18 Nov 2021; Posted 22 Nov 2021  View: PDF

Abstract: With the development of continuous-wave terahertz (THz) sources and array detectors, the pursuit of high-fidelity real-time imaging is receiving significant attention within the THz community. Here, we report a real-time full-field THz phase imaging approach based on lensless Fourier THz digital holography (LF-TDH). A triangular interferometric layout is proposed based on an oblique illumination of 2.52 THz radiation. A spherical reference beam is generated by a reflective parabolic mirror with minor propagation loss. The point source of the reference beam and the object are kept in the same plane and equidistant from the recording plane. The complex-valued images are reconstructed using a single inverse Fourier transform of the hologram without complex calculation of the diffraction propagation. The experimental result for a Siemens star validates the lateral resolution of 346.19 µm in the diagonal direction. Sub-pixel image registration and image stitching algorithms are applied to enlarge the size of the reconstructed images. The dehydration process of an aquatic plant leaf (Hottonia inflata) is monitored for the first time at the THz band. Quantitative changes in water content and morphology are measured with a time interval of 0.6 s and a total time of 5 min from a series of reconstructed amplitude and phase images, respectively. The proposed method has the potential to become a powerful tool to investigate spontaneous phenomena at the THz band.

Scan-free microfabrication of axially tunable helices

He Cheng, Pooria Golvari, Chun Xia, Mingman Sun, Meng Zhang, Stephen Kuebler, and Xiaoming Yu

DOI: 10.1364/PRJ.439592 Received 04 Aug 2021; Accepted 18 Nov 2021; Posted 22 Nov 2021  View: PDF

Abstract: Helical structures exhibit novel optical and mechanical properties and are commonly used in different fields such as metamaterials and microfluidics. A few methods exist for fabricating helical microstructures, but none of them has the throughput or flexibility required for patterning large surface area with tunable pitch. In this paper, we report method for fabricating helical structures with adjustable form over large area based on multiphoton polymerization (MPP) using single exposure, three-dimensionally structured, self-accelerating, axially tunable light-fields. The light-fields are generated as a superposition of high-order Bessel modes and have a closed-form expression relating the design of the phase mask to the rotation rate of the beam. The method is used to fabricated helices with different pitches and handedness in the material SU8. Compared to point-by-point scanning, the method reported here can be used to reduce fabrication time by three ordersof magnitude, paving the way for adopting MPP in many industrial applications.

Compact low-birefringence polarization beam splitter using vertical-dual-slot waveguides in silicon carbide integrated platforms

Xiaodong Shi, JingJing Zhang, Weichen Fan, Yaoqin Lu, Nianhua Peng, Karsten Rottwitt, and Haiyan Ou

DOI: 10.1364/PRJ.443543 Received 20 Sep 2021; Accepted 17 Nov 2021; Posted 18 Nov 2021  View: PDF

Abstract: The polarization beam splitter is a key component for polarization manipulation in photonic integrated circuits, but it is challenging to design for low-refractive-index optical materials, due to the low birefringence of the waveguides. We propose a novel compact vertical-dual-slot waveguides based coupling scheme for silicon carbide, enabling efficient low-birefringence polarization splitting by extensively modulating the TM mode distribution. We numerically and experimentally demonstrate the device in the 4H-silicon carbide-on-insulator integrated platform, with a small footprint of 2.2 ×15 μm². The device, easy to fabricate via single lithography process as other components on the chip, exhibits low insertion loss of <0.71 dB and <0.51 dB for the TE and TM polarized light, respectively, and high polarization extinction ratio of >13 dB, over 80 nm wavelength range.

Frequency up-conversion rotational Doppler effect

Haoxu Guo, Xiaodong Qiu, song qiu, Ling Hong, Fei Lin, yuan ren, and Lixiang Chen

DOI: 10.1364/PRJ.441785 Received 31 Aug 2021; Accepted 16 Nov 2021; Posted 16 Nov 2021  View: PDF

Abstract: We demonstrated an efficient scheme of measuring the angular velocity of a rotating object with the detection light working at the infrared regime. Our method benefits from the combination of second-harmonic generation (SHG) and rotational Doppler effect, i.e., frequency up-conversion rotational cDoppler effect. In our experiment, we use one infrared light as the fundamental wave (FW) to probe the rotating objects while prepare the other FW to carry the desired superpositions of orbital angular momentum (OAM). Then these two FWs are mixed collinearly in a potassium titanyl phosphate (KTP) crystal via type-II phase matching, which produces the visible second-harmonic light wave. The experimental results show that both the angular velocity and geometric symmetry of rotating objects can be identified from the detected frequency shift signals at the photon-count level. Our scheme will find potential applications in infrared monitoring.

Lock-in incoherent differential phase contrast imaging

Chiara Bonati, Damien Loterie, Timothé Laforest, and Christophe Moser

DOI: 10.1364/PRJ.445896 Received 14 Oct 2021; Accepted 15 Nov 2021; Posted 15 Nov 2021  View: PDF

Abstract: We introduce a lock-in method to increase the phase contrast in incoherent Differential Phase Contrast (DPC) imaging. This method improves the phase sensitivity by the analog removal of the background. The use of a smart pixel detector with in-pixel signal demodulation, paired with synchronized switching illumination, provides the basis of a bit efficient approach to emulate a lock-in DPC. The experiments show an increased sensitivity by a factor of up to 8, as expected from theory, and a reduction of collected data by a factor of 70, for equivalent standard DPC measurements; single-shot sensitivity of 0.7 mrad at a frame rate of 1 400 fps is demonstrated. This new approach may open the way for the use of incoherent phase microscopy in biological applications where extreme phase sensitivity and millisecond response time is required.

Quantum Dots assisted and NIR-II Emissive In Vivo Two-photon microscopy

Huwei Ni, Yalun Wang, Tao Tang, Wenbin Yu, Dongyu Li, Mubin He, Runze Chen, Mingxi Zhang, and Jun Qian

DOI: 10.1364/PRJ.441471 Received 25 Aug 2021; Accepted 15 Nov 2021; Posted 16 Nov 2021  View: PDF

Abstract: With the advantages of high resolution and deep penetration depth, two-photon excited NIR-II (900-1880 nm) fluorescence (2PF) microscopic bioimaging is very promising. However, due to the lack of imaging system and suitable probes, few such works were demonstrated utilizing NIR-II excitation and NIR-II fluorescence simultaneously. Herein, we used aqueously dispersible PbS/CdS quantum dots with bright NIR-II fluorescence as the contrast agents. Under the excitation of a 1550 nm femtosecond (fs) laser, they emitted bright 2PF in NIR-II region. Moreover, a 2PF lifetime imaging microscopic (2PFLIM) system was implemented, and in vivo 2PFLIM images of mouse brain blood vessels were obtained for the first time. To improve the imaging speed, a in vivo two-photon fluorescence microscopy (2PFM) system based on an InGaAs camera were implemented, and in vivo 2PFM images of QDs-stained mouse brain blood vessels were obtained.

Single-scan polarization-resolved degenerate four-wave mixing spectroscopy using vector optical field

jiaqi yuan, xue cheng, Xing Wang, tengfei jiao, and Zhaoyu Ren

DOI: 10.1364/PRJ.423799 Received 03 Mar 2021; Accepted 14 Nov 2021; Posted 15 Nov 2021  View: PDF

Abstract: We report on a new method to achieve single-scan polarization-resolved DFWM spectroscopy in Rb atomic medium using vector optical field, in which two pump beams are kept linearly polarized and a vector beam is employed as the probe beam. As the polarization and intensity of DFWM signal is closely dependent on the polarization state of the probe beam, a vector probe beam with space-variant states of polarization is able to generate DFWM signal with space-variant states of polarization and intensity across the DFWM image. Accordingly, the polarization-resolved spectroscopy can be retrieved from a single DFWM image. To our best knowledge, it is the first time that single-scan polarization-resolved spectroscopy detection is realized experimentally with a vector beam. This work provides a simple but efficient single-scan polarization-resolved spectroscopic method, which would be of special meaning for the samples of poor light stability and the fast processes.

Miniature, highly sensitive MOSCAP ring modulators in co-optimized electronic-photonic CMOS

Hayk Gevorgyan, Anatoly Khilo, Mark Wade, Vladimir Stojanovic, and Milos Popovic

DOI: 10.1364/PRJ.438047 Received 29 Jul 2021; Accepted 13 Nov 2021; Posted 15 Nov 2021  View: PDF

Abstract: Convergence of high-performance silicon photonics and electronics, monolithically integrated in state-of-the-art CMOS platforms, is the holy grail for enabling the ultimate efficiencies, performance and scaling of electronic-photonic systems-on-chip. It requires the emergence of platforms that combine state-of-the-art RF transistors with optimized silicon photonics, and a generation of photonic device technology with ultra low energies, increased operating spectrum, and the elimination of power-hungry thermal tuning. In this paper, in a co-optimized monolithic electronics-photonics platform (GlobalFoundries 45CLO), we turn the MOSFET transistor’s basic structure into a novel, highly efficient MOS capacitor ring modulator. It has the smallest ring cavity (1.5 μm radius), largest corresponding spur-free free spectral range (FSR=8.5 THz), and record 30 GHz/V shift efficiency in the O band, among silicon modulators demonstrated to date. With 1 Vpp RF drive, we show an open optical eye while electro-optically tuning the modulator to track over 400 pm (69 GHz) change in the laser wavelength (using 2.5 V DC range). A 90 GHz maximum electro-optic resonance shift is demonstrated with under 40 nW of power, providing a strong non-thermal tuning mechanism in a CMOS photonics platform. The modulator has a separately optimized body layer but shares the gate device layer with 45 nm transistors, while meeting all CMOS manufacturability design rules. This type of convergent evolution of electronics and photonics may be the future of platforms for high-performance systems on chip.

Comb-based photonic neural population for parallel and nonlinear processing

Bowen Ma, Junfeng Zhang, and Weiwen Zou

DOI: 10.1364/PRJ.437798 Received 13 Jul 2021; Accepted 12 Nov 2021; Posted 15 Nov 2021  View: PDF

Abstract: It is believed that neural information representation and processing relies on the neural population instead of a single neuron. In neuromorphic photonics, photonic neurons in the form of nonlinear responses have been extensively studied in single devices and temporal nodes. However, to construct a photonic neural population (PNP), the process of scaling up and massive interconnections remain challenging considering the physical complexity and response latency. Here, we propose a comb-based PNP interconnected by carrier coupling with superior scalability. Two unique properties of neural population are theoretically and experimentally demonstrated in the comb-based PNP, including nonlinear response curves and population activities coding. A classification task of three input patterns with dual radio-frequency (RF) tones is successfully implemented in a real-time manner, which manifests the comb-based PNP can make effective use of the ultra-broad bandwidth of photonics for parallel and nonlinear processing.

2 μm optical frequency comb generation via optical parametric oscillation from a lithium niobate optical superlattice box resonator

Xiaohan Wang, Kunpeng Jia, Mengwen Chen, HANSHAN CHENG, Xin Ni, Jian Guo, Yihao Li, Huaying Liu, Liyun Hao, Jian Ning, Gang Zhao, Xinjie Lv, Shu-Wei Huang, Zhenda Xie, and Shining Zhu

DOI: 10.1364/PRJ.432076 Received 20 May 2021; Accepted 12 Nov 2021; Posted 30 Nov 2021  View: PDF

Abstract: Optical parametric oscillators (OPOs) can down-convert the pump laser to longer wavelengths with octave separation via χ(2), which is widely used for laser wavelength extension including mid-infrared (MIR) generation. Such process can be integrated in microresonators, being compact and low in threshold. In this work, we show that the χ(2) micro-OPO can also be used for optical frequency comb generation around 2096 nm and enters the boundary of MIR range. A new geometry called optical superlattice box resonator is developed for this realization with near-material-limited quality factor of 4.0 × 10⁷. Only a continuous-wave near-infrared pump laser is required, with OPO threshold of 80 mW and output power up to 340 mW. Consistently revival temporal profile is measured at a detectable repetition frequency of 1.426 GHz, and narrow beatnote linewidth of less than 10 Hz shows high comb coherence. These results are in good agreement with our simulation for a stable comb generation. Such OPO-based comb source is useful for carbon dioxide sensing or the mine prospect applications, and can be generalized to longer MIR wavelengths for general gas spectroscopy.

Electrically injected GeSn lasers with peak wavelength up to 2.7 µm

Yiyin Zhou, Solomon Ojo, CHEN-WEI WU, Yuanhao Miao, Huong Tran, Joshua Grant, Grey Abernathy, Sylvester Amoah, Jake Bass, Gregory Salamo, Wei Du, Guo-En Chang, Jifeng Liu, Joe Margetis, John Tolle, Yong-Hang Zhang, Gregory Sun, Richard Soref, Baohua Li, and Shuiqing Yu

DOI: 10.1364/PRJ.443144 Received 14 Sep 2021; Accepted 09 Nov 2021; Posted 11 Nov 2021  View: PDF

Abstract: GeSn lasers enable the monolithic integration of lasers on the Si platform using all-group-IV direct-bandgap material. The GeSn laser study recently moved from optically pumping into electrical injection. In this work, we present explorative investigations of GeSn heterostructure laser diodes with various layer thicknesses and material compositions. Cap layer material was studied by using Si0.03Ge0.89Sn0.08 and Ge0.95Sn0.05, and cap layer total thickness was also compared. The 190-nm-SiGeSn-cap device had threshold of 0.6 kA/cm2 at 10 K and a maximum operating temperature (Tmax) of 100 K, compared to 1.4 kA/cm2 and 50 K from 150-nm-SiGeSn-cap device, respectively. Furthermore, the 220-nm-GeSn-cap device had 10 K threshold at 2.4 kA/cm2 and Tmax at 90 K, i.e., higher threshold and lower maximal operation temperature compared to SiGeSn cap layer, indicating that enhanced electron confinement using SiGeSn can reduce the threshold considerably. The study of the active region material showed that device gain region using Ge0.87Sn0.13 had a higher threshold and lower Tmax, compared to Ge0.89Sn0.11. The performance was affected by the metal absorption, free carrier absorption, and possibly defect density level. The maximum peak wavelength was measured as 2682 nm at 90 K by using Ge0.87Sn0.13 in gain regions. The investigations provide directions to the future GeSn laser diode designs towards the full integration of group-IV photonics on Si platform.

Slope-assisted Raman distributed optical fiber sensing

Jian Li, Xinxin Zhou, Yang Xu, Lijun Qiao, Jianzhong Zhang, and Mingjiang Zhang

DOI: 10.1364/PRJ.442352 Received 02 Sep 2021; Accepted 09 Nov 2021; Posted 11 Nov 2021  View: PDF

Abstract: Raman distributed fiber sensing is required to achieve an accurately temperature measurement in a micro-scale area. However, the principle of Optical Time Domain Reflection (OTDR) leads to that the temperature detection result is much smaller than its true temperature value, when the detection area is smaller than the spatial resolution of the system. It causes the effective temperature signal to be submerged in the collected average data, which imposes significant challenges in the development of more performing optical sensors. Here, we propose a novel optical scheme of Raman distributed optical fiber sensing for improving the capability of temperature monitoring in a micro-scale region. In this work, the pulse transmission feature in the temperature variation area of the sensing fiber and superposition characteristics of Raman OTDR signal are first theoretically analyzed and demonstrated by numerical simulation. And the equations of superimposed Raman anti-Stokes scattered signals at different stages are presented. It provides a theoretical basis for the positioning and physical quantity demodulation of whole optical fiber systems based on the OTDR principle, such as BOTDR, BOTDA, ROTDR, Rayleigh-OTDR system, etc. Furthermore, a slope-assisted sensing principle and scheme are proposed and experimental demonstrated in Raman distributed optical fiber sensing. The falling edge of the superimposed Raman OTDR curve is defined as the slope-assisted detection area in FUT. The temperature variation information along the fiber-line can be demodulated by using the calculated slope-assisted coefficients. The experiment indicates that the slope-assisted scheme can realize an accurately measurement in a centimeter-level region, even if the spatial resolution of the system is meter-level. To the best our knowledge, this is the first experimental demonstration of Raman distributed optical fiber sensing in a centimeter-level measurement region.

Silicon mode-loop Mach-Zehnder modulator with L-shaped PN junction for 0.37-V∙cm VπL high efficiency modulation

Jiacheng Liu, Gangqiang Zhou, Jiangbing Du, Weihong Shen, Linjie Zhou, and Zuyuan He

DOI: 10.1364/PRJ.442699 Received 15 Sep 2021; Accepted 08 Nov 2021; Posted 11 Nov 2021  View: PDF

Abstract: Optical signaling without a high voltage driver for electric-optic modulation is highly demanded for reduction of the power consumption, packaging complexity and cost. In this work, we proposed and experimentally demonstrated a silicon mode-loop Mach-Zehnder modulator (ML-MZM) with record-high modulation efficiency. We use a mode loop structure to recycle light twice in the phase shifter. With an L-shaped PN junction, comparably large overlap between the PN junction and optical modes of both TE0 and TE1 was achieved, so as to lower the driving voltage or to decrease the photonic device size. Proof-of-concept high efficiency modulation with low VπL of 0.37 V∙cm was obtained. Sub-voltage Vπ can be realized with millimeters length phase shifter by this scheme, which makes the realization of CMOS-compatible driverless modulation highly possible. 40 Gb/s signaling with a bit error rate below the 7% forward-error-correction (FEC) threshold was then demonstrated with the fabricated ML-MZM, indicating great potential for high speed optical communication.

Silicon substrate-integrated hollow waveguide for miniaturized optical gas sensing

Shaonan Zheng, Hong Cai, Linfang Xu, Nanxi Li, Zhong hua Gu, Yao Zhang, Weiguo Chen, Yanyan Zhou, Qingxin Zhang, and Lennon Lee

DOI: 10.1364/PRJ.439434 Received 04 Aug 2021; Accepted 07 Nov 2021; Posted 11 Nov 2021  View: PDF

Abstract: Gas sensor has a wide variety of applications. Among various existing gas sensing technologies, optical gas sensor has outstanding advantages. The developments of internet of things and consumer electronics put stringent requirements on miniaturized gas sensing technology. Here, we demonstrate chip-scale silicon substrate-integrated hollow waveguide (Si-iHWG) to serve as optical channel and gas cell in an optical gas sensor. It is fabricated through silicon wafer etching and wafer bonding. The Si-iHWG chip is further assembled with off-chip light source and detector to build a fully functional compact nondispersive infrared (NDIR) CO2 sensor. The chip size is 10×9 mm2 and the dimension of the sensor excluding microcontroller board is 50×25×16 mm3. Gas testing results show the sensor has a sensitivity of 17.4 ppm at 400-ppm concentration. This chip solution with compactness, versatility, robustness, and low cost provides a cost-effective platform for miniaturized optical sensing applications ranging from air quality monitoring to consumer electronics.

Experimental observation of multiple edge and corner states in photonic slabs heterostructure

mingxing li, yueke wang, Tian Sang, Hongchen Chu, Yun Lai, and Guofeng Yang

DOI: 10.1364/PRJ.440640 Received 17 Aug 2021; Accepted 06 Nov 2021; Posted 11 Nov 2021  View: PDF

Abstract: Photonic topological insulator has become an important research field with a wide range of applications. Especially the higher-order topological insulator, which possesses the gapped edge states and corner or hinge states in the gap, provides a new scheme for the control of light in a hierarchy of dimensions. In this paper, we propose a heterostructure composed of ordinary-topological-ordinary (OTO) photonic crystal slabs, two coupled edge states (CESs) are generated due to the coupling between the topological edge states of the ordinary-topological interfaces, which opens up an effective way for high-capacity photonic transport. In addition, we obtain a new band gap between the CESs, and the two kinds of coupled corner states (CCSs) appear in the OTO bend structure. Besides, the topological corner state is also found, which originates from the non-trivial bulk polarization. Compared with the previous topological photonic crystal based on C-4 lattice, CESs, CCSs and the topological corner state are all directly observed in experiment by using near-field scanning technique, which makes the manipulation of the electromagnetic wave more flexibly. We also verify that the three corner states are all robust to defects. Our work opens up a new way for guiding and trapping the light flow, and provides a useful case for the coupling of topological photonic states.

Ultrahigh Detectivity, High Speed and Low Dark Current AlGaN Solar-Blind Heterojunction Field-Effect Phototransistors Realized Using Dual-Float-Photogating Effect

Xinjia Qiu, Kai Wang, and Hao Jiang

DOI: 10.1364/PRJ.444444 Received 28 Sep 2021; Accepted 04 Nov 2021; Posted 04 Nov 2021  View: PDF

Abstract: High detectivity is essential for solar-blind deep ultraviolet (DUV) light detection because the DUV signal is extremely weak in most applications. In this work, we report ultrahigh-detectivity AlGaN-based solar-blind heterojunction-field-effect phototransistors fabricated utilizing dual-float-photogating effect. The p+-Al0.4GaN layer and Al0.4GaN absorber layer deposited on the Al0.6GaN barrier serve as top pin-junction photogate, while the thin Al0.4GaN channel layer with a strong polarization field inside acts as virtual back photogate. Due to the effective depletion of the two-dimensional electron gas at the Al0.6Ga0.4N/Al0.4Ga0.6N heterointerface by the top photogate, the dark current was suppressed below 2 pA in the bias range of 0 to 10 V. A high photo-to-dark current ratio over 10^8 and an optical gain of 7.5×10^4 were demonstrated at a bias of 5 V. Theoretical analysis indicates that the optical gain can be attributed to the joint action of the floating top and back photogates on the channel current. As a result, a record high flicker-noise (Johnson and shot noise) limited specific detectivity of 2.84×10^15 (2.91×10^17) cmHz^0.5W^-1 was obtained. Furthermore, high response speed at the μs level was also shown in the devices. This work provides a promising and feasible approach for high-sensitivity DUV detection.

Efficient, high-CRI white LEDs by combining traditional phosphors with cadmium-free InP/ZnSe red quantum dots

Youri Meuret, Hannes Van Avermaet, Leila Mingabudinova, Bega Karadza, and Zeger Hens

DOI: 10.1364/PRJ.428843 Received 23 Apr 2021; Accepted 03 Nov 2021; Posted 04 Nov 2021  View: PDF

Abstract: Quantum dots offer an interesting alternative for traditional phosphors in on-chip LED configurations. Earlier studies showed that the spectral efficiency of white LEDs with high CRI values could be considerably improved by replacing red-emitting nitride phosphors with narrow-band QDs. However, the red QDs in these studies were cadmium-based, which is a restricted element in the EU under the RoHS directive. The use of InP-based QDs, the most promising Cd-free alternative, is often presented as an inferior solution because of the broader linewidth of these QDs. However, while narrow emission lines are the key to display applications that require a large color gamut, the spectral efficiency penalty of this broader emission is limited for lighting applications. Here, we report efficient, high-CRI white LEDs with an on-chip color convertor coating based on red InP/ZnSe QDs and traditional green/yellow powder phosphors. Using InP/ZnSe QDs with a quantum yield of nearly 80% and a full width at half maximum of 45 nm, we demonstrate high spectral efficiency for white LEDs with very high CRI values. One of the best experimental results in terms of both luminous efficacy and color rendering performance is a white LED with an efficacy of 132 lm/W, and color rendering indices of Ra≈90, R9≈50 for CCT≈4000 K. These experimental results are critically compared with theoretical benchmark values for white LEDs with on-chip down-conversion from both phosphors and red Cd-based QDs. The various loss mechanisms in the investigated white LEDs are quantified with an accurate simulation model, and the main impediments to an even higher efficacy are identified as the blue LED wall-plug efficiency and light recycling in the LED package.

Single-pixel imaging using physics enhanced deep learning

Guohai Situ, Fei Wang, Chenglong Wang, Shensheng Han, and chenjin Deng

DOI: 10.1364/PRJ.440123 Received 16 Aug 2021; Accepted 03 Nov 2021; Posted 04 Nov 2021  View: PDF

Abstract: Single-pixel imaging~(SPI) systems synergistically combine structured illuminations and single point intensity measurements to computationally form an image, making it suitable for imaging in low light and in a wide waveband that one could not with traditional imaging methods. However, SPI usually requires multiple samplings for a high-resolution image, imposing a practical limit for high quality imaging in real-time. Here we propose a physics enhanced deep learning approach for SPI. By blending a physics informed layer and a physics fine tune layer, our approach opens up new possibility for SPI in terms of both robustness and fidelity. We demonstrate that the proposed method outperforms some other widespread SPI algorithms. This new framework may pave a way for its practical applications in remote sensing, microscopy and spectral imaging among many others. Besides, by using a deep neural network~(DNN), the proposed method establishes a bridge between data-driven and model-driven algorithms, allowing one to impose high level prior information for widespread computational imaging systems.

Slide-free Histological Imaging by Microscopy with Ultraviolet Surface Excitation using Speckle Illumination

Hei Man Wong, Yan Zhang, Zhenghui Chen, Lei Kang, and Terence T. W. Wong

DOI: 10.1364/PRJ.440935 Received 19 Aug 2021; Accepted 02 Nov 2021; Posted 04 Nov 2021  View: PDF

Abstract: Microscopy with ultraviolet surface excitation (MUSE) is a promising slide-free imaging technique to improve the time-consuming histopathology workflow. However, since the penetration depth of the excitation light is tissue-dependent, the image contrast could be significantly degraded when the depth of field of the imaging system is shallower than that of the penetration depth. High-resolution cellular imaging normally comes with a shallow depth of field, which also restricts the tolerance of surface roughness in biological specimens. Here, we propose the incorporation of MUSE with speckle illumination (termed MUSES), which can achieve sharp imaging on thick and rough specimens. Our experimental results demonstrate the potential of MUSES in providing histological images with ~1 µm spatial resolution and improved contrast, within 10 minutes for a field of view of 1.7 mm x 1.2 mm. With the extended depth of field feature, MUSES also relieves the constraint of tissue flatness. Furthermore, with a color transformation assisted by deep learning, a virtually stained histological image can be generated without manual tuning, improving the applicability of MUSES in clinical settings.

Single-particle trapping and dynamic manipulation with holographic optical surface-wave tweezers

xie xi, xianyou wang, Changjun Min, Hai Ma, Zhangyu Zhou, Yunqi Yuan, Yuquan Zhang, jing bu, and Xiaocong Yuan

DOI: 10.1364/PRJ.444341 Received 28 Sep 2021; Accepted 02 Nov 2021; Posted 04 Nov 2021  View: PDF

Abstract: Optical surface waves have widely been used in optical tweezers system for trapping particles sized from nanoscale to microscale, with specific importance and needs in applications of super-resolved detection and imaging if a single particle can be trapped and manipulated accurately. However, it is difficult to achieve such a trapping with high precision in conventional optical surface-wave tweezers. Here, we propose and experimentally demonstrate a new method to accurately trap and dynamically manipulate a single particle or a desired number of particles in a holographic optical surface-wave tweezers. By tailoring the optical potential wells formed by surface waves, we achieved trapping of the targeted single particle while pushing all surrounding particles away, and further dynamically controlled the particle by a holographic tweezers beam. We further prove that different particle samples, including gold particles and biological cells, can be applied in our system. This method can be used for different-type optical surface-wave tweezers, with significant potential applications in single-particle spectroscopy, particle sorting, nano assembly and others.

Long range dynamic displacement: Precision PGC with sub-nanometer resolution in LWSM interferometer

Yisi Dong, Peng-Cheng Hu, Haijin Fu, Hongxing Yang, Ruitao Yang, and Jiubin Tan

DOI: 10.1364/PRJ.442057 Received 31 Aug 2021; Accepted 02 Nov 2021; Posted 02 Nov 2021  View: PDF

Abstract: We propose a precision phase-generated-carrier (PGC) demodulation method with sub-nanometer resolution that avoids nonlinear errors in a laser wavelength sinusoidal modulation (LWSM) fiber-optic interferometer for long range dynamic displacement sensing. Using orthogonal detection and an AC-DC component extraction scheme, the PGC carrier phase delay (CPD) and laser intensity modulation phase delay (LIMPD) can be obtained simultaneously to eliminate the nonlinear error from accompanied optical intensity modulation (AOIM) and CPD. Further, to realize long range displacement sensing, PGC phase modulation depth (PMD), determined by the laser wavelength modulation amplitude and the working distance of the interferometer, is required to maintain an optimal value during the measurement, including initial position and dynamic movement. By combining frequency sweeping interference and modified PGC-Arctan demodulation to measure real-time work distance, adaptive PMD technology is realized based on the proportion control. We construct a fiber-optic Michelson and SIOS commercial interferometer for comparison and perform experiments to verify the feasibility of the proposed method. Experimental results demonstrate that an interferometer with sub-nanometer resolution and nanometer precision over a large range of 400mm can be realized.

Performance improvement approaches for optical fiber SPR sensors and their sensing applications

Jianying Jing, Kun Liu, Junfeng Jiang, Tianhua Xu, Wang Shuang, Jinying Ma, Zhao Zhang, Wenlin Zhang, and T. Liu

DOI: 10.1364/PRJ.439861 Received 22 Sep 2021; Accepted 01 Nov 2021; Posted 02 Nov 2021  View: PDF

Abstract: Optical fiber surface plasmon resonance (SPR) sensors point towards promising application potential in the fields of biomarker detection, food allergen screening and environmental monitoring due to their unique advantages. This review outlines approaches in improving the fiber SPR sensing performance, e.g. sensitivity, detection accuracy, reliability, cross-sensitivity, selectivity, convenience and efficiency, and corresponding sensing applications. The sensing principle of SPR sensors especially the performance indicators and their influencing factors have been introduced. Current technologies for improving the fiber SPR performance and their application scenarios are then reviewed from the aspects of fiber substrate, intrinsic layer (metal layer) and surface nanomaterial modification. Reasonable design of the substrate can strengthen the evanescent electromagnetic field and realize the multi-parameter sensing, and can introduce the in-situ sensing self-compensation, which allows corrections for errors induced by temperature fluctuation, non-specific binding and external disturbances. The change of the intrinsic layer can adjust the column number, the penetration depth and the propagation distance of surface plasmon polaritons. This can thereby promote the capability of sensors to detect the large-size analytes and can reduce the full width at half-maximum of SPR curves. The modification of various-dimensionality nanomaterials on the sensor surfaces can heighten the overlap integral of the electromagnetic field intensity in the analyte region and can strengthen interactions between plasmons and excitons as well as interactions between analyte molecules and metal surfaces. Moreover, future directions of fiber SPR sensors are prospected based on the important and challenging problems in the development of fiber SPR sensors.

Design and prototype of a freeform OST-HMD system with large exit pupil diameter and vision correction capability

Dewen Cheng, Jiaxi Duan, Hailong Chen, He Wang, Danyang Li, Qiwei Wang, Qichao Hou, Tong Yang, Weihong Hou, Donghua Wang, Xiaoyu Chi, Bin Jiang, and Yongtian Wang

DOI: 10.1364/PRJ.440018 Received 09 Aug 2021; Accepted 01 Nov 2021; Posted 02 Nov 2021  View: PDF

Abstract: Compact and light weight, large exit pupil diameter and distance, and little distortion for imaging and see-through optical paths are pivotal components to achieve a better, wearable experience of optical see-through head-mounted display (OST-HMD). In addition, light efficiency of the virtual image light path is an important factor for heat dissipation in HMD devices. This paper presents a new type of OST-HMD optical system that includes three wedge-shaped freeform prisms and two symmetric lenses. Based on a 0.71 in. micro-display, an OST-HMD prototype with a diagonal field of view (FOV) of 45.3°, an F-number (F/#) of 1.8, an exit pupil range of 12 mm×8 mm and an eye relief of 18mm are demonstrated. Maximum value of distortion of the final system is 0.6% and 0.4% for virtual imaging and see-through light path, respectively. The overall dimension of the optical system per eye is no larger than 30mm(width)×40mm(height)×14mm(thickness), and the weight of the optical module including lenses, holder and micro-display is 12.8g. The light efficiency of virtual image light path can reach 50%, a value exceeding that of other optical solutions.

Photonic topological transition in dimerized chain with the joint modulation of near-field and far-field couplings

Caifu Fan, xi shi, Feng Wu, Yunhui Li, Hai Tao Jiang, yong sun, and Hong Chen

DOI: 10.1364/PRJ.441278 Received 24 Aug 2021; Accepted 31 Oct 2021; Posted 02 Nov 2021  View: PDF

Abstract: Topological systems containing near-field or far-field couplings between unit cells have been widely investigated in quantum and classic systems. Their band structures are well explained with theories based on tight-binding or multiple scattering formalism. However, characteristics of the topology of the bulk bands based on the joint modulation of near-field and far-field couplings are rarely studied. Such hybrid systems are hardly realized in real systems and cannot be described by neither tight-binding nor multiple scattering theories. Here, we propose a hybrid-coupling photonic topological insulator based on a quasi-one-dimensional dimerized chain with the coexistence of near-field coupling within the unit cell and far-field coupling among all sites. Both theoretical and experimental results show that topological transition is realized by introducing the near-field coupling for given far-field coupling conditions. In addition to closing and re-opening the bandgap, the change in near-field coupling modulates the effective mass of photonics in the upper band from positive to negative, leading to an indirect bandgap, which cannot be achieved in conventional dimerized chains with either far-field or near-field coupling only.

Active stabilization of terahertz waveforms radiated from a two-color air plasma

Yonghao Mi, Kyle Johnston, Valentina Shumakova, Søren Møller, Kamalesh Jana, Chunmei Zhang, André Staudte, Shawn Sederberg, and Paul Corkum

DOI: 10.1364/PRJ.434325 Received 16 Jun 2021; Accepted 31 Oct 2021; Posted 02 Nov 2021  View: PDF

Abstract: Intense laser fields focused into ambient air can be used to generate high-bandwidth current densities in the form of plasma channels and filaments. Excitation with bi-chromatic fields enables us to adjust the amplitude and sign of these currents using the relative phase between the two light pulses. Two-color filamentation in gas targets provides a route to scaling the energy of terahertz pulses to mJ levels by driving the plasma channel with a high-energy laser source. However, the structure of plasma channels is highly susceptible to drifts in both the relative phase and other laser parameters, making control over the waveform of the radiated terahertz fields delicate. We establish a clear link between the phase-dependence of plasma currents and terahertz radiation by comparing in-situ detection of current densities and far-field detection of terahertz pulses. We show that the current measurement can be used as a feedback parameter for stabilizing the terahertz waveform. This approach provides a route to energetic terahertz pulses with exceptional waveform stability.

Mid-infrared Quasi-BIC Resonances with Sub-diffraction Slot Mode Profile in Germanium-based Coupled Guided-Mode Resonance Structures

Lal Krishna A S, Sruti Menon, Asish Prosad, and Varun Raghunathan

DOI: 10.1364/PRJ.442650 Received 10 Sep 2021; Accepted 30 Oct 2021; Posted 02 Nov 2021  View: PDF

Abstract: In this paper, we experimentally demonstrate a novel quasi bound state in continuum (BIC) resonance in the mid-infrared wavelength region with the resonant electric field confined as a slot-mode within a low refractive index medium sandwiched between high-index layers. The structures studied here comprise of coupled amorphous germanium guided-mode resonance (GMR) structures with a top one-dimensional grating layer and bottom uniform layer separated by a low-index silicon nitride layer. The sub-diffraction slot-mode profile within the silicon nitride layer with mode field confinement >30% is achieved as a solution to the electromagnetic wave propagation through the coupled GMR structure with the dominant field component being perpendicular to the layers. The quasi-BIC resonance in symmetric 1D grating structures can be observed even at normal incidence when considering realistic excitation beams with finite angular spread. The quasi-BIC resonances with a characteristic transmission peak are found to red-shift (remain almost unchanged) under classical (full-conical) mounting conditions. The quality factor of the resonances under study are found to decrease (remain almost constant) with increasing angle of incidence under classical (full-conical) mounting condition. Quality factor of ~400 is experimentally extracted at normal incidence under classical mounting condition with resonance peak at 3.41 μm wavelength. Such slot-mode GMR structures with appropriately chosen low-index intermediate layer can find applications as resonantly enhanced sensing and active photonic devices. These structures offer the added benefit of simplified light coupling geometry when compared to the conventional slot-mode waveguide structures.

Highly sensitive plasmonic nanorod hyperbolic metamaterial biosensor

Ruoqin Yan, Tao Wang, Xinzhao Yue, Huimin Wang, Yu Zhang, Peng Xu, Lu Wang, Yuandong Wang, and Jinyan Zhang

DOI: 10.1364/PRJ.444490 Received 28 Sep 2021; Accepted 29 Oct 2021; Posted 02 Nov 2021  View: PDF

Abstract: Plasmonic sensing based on nanostructures is a powerful analytical tool for ultrasensitive label-free biomolecule detection, that holds great potential in the field of clinical diagnostics and biomedical research. Here, we report the fabrication, characterization, and principle of operation of gold nanorod hyperbolic metamaterials (NHMM) along with ultrasensitive bulk refractive index and label-free biomolecular detection. By combining electron-beam lithography and nanoscale electroplating, we demonstrate the fabrication of a highly ordered, height-controllable, and vertical array of nanorods. By exciting the bulk plasmon-polariton mode in the NHMM using a prism-coupling technique and integrating the sensor in a microfluidics, we demonstrate that the bulk sensitivity and figure of merit of our device could reach 41,600 nm/RIU and 416 /RIU, respectively. The physical mechanism of this high bulk sensitivity is revealed through theoretical and experimental studies. Moreover, by bio-functionalizing the surface of the NHMM sensor, monitoring the binding of streptavidin at dilute concentrations is performed in real time. We test different concentrations of streptavidin ranging from 200 to 5 μg/ml, and the NHMM biosensor exhibits 1 nm wavelength shift for 5 μg/ml streptavidin detection. By fitting the Hill equation of the NHMM biosensor, and taking into account that the level of noise 0.05 nm as the minimum wavelength shift of the detectable limit, the limit of detection of the NHMM biosensor to streptavidin can be estimated to be 0.14 μg/ml (2.4 nM). As a direct comparison, a 0.5 nm wavelength shift for 20 μg/ml streptavidin is reported when using a conventional gold film sensor under identical experimental conditions. The developed plasmonic NHMM sensor shows tremendous potential for highly sensitive bulk solutions and biomolecule detection and provides a promising avenue for free-label biosensing applications in the future.

Enhanced optical nonlinearity in a silicon-organic hybrid slot waveguide for all-optical signal processing

Yonghua Wang, Su He, xiaoyan Gao, Piaopiao Ye, Lei Lei, Dong wenchan, Xinliang Zhang, and Ping XU

DOI: 10.1364/PRJ.439251 Received 30 Jul 2021; Accepted 26 Oct 2021; Posted 28 Oct 2021  View: PDF

Abstract: Silicon photonic integrated devices used for nonlinear optical signal processing play a key role in ultrafast switching, computing, and modern optical communications. However, current devices suffer from limited operation speeds and low conversion efficiencies due to the intrinsically low nonlinear index of silicon. In this paper, we experimentally demonstrate enhanced optical nonlinearity in a silicon-organic hybrid slot waveguide consisting of an ultra-narrow slot waveguide coated with a highly nonlinear organic material. The fabricated slot area is as narrow as 45 nm, which is, to the best of our knowledge, the narrowest slot width that has been experimentally reported in silicon slot waveguides. The nonlinear coefficient of the proposed device with a length of 3 mm is measured to be up to 1.43 × 106 W-1km-1. Based on the nanostructured device design, the conversion efficiency of degenerate four-wave mixing showed enhancements of more than 12 dB and 5 dB compared to those measured for an identical device without the organic material and a silicon strip waveguide, respectively. As a proof of concept, all-optical canonical logic units based on the prepared device with two inputs at 40 Gb/s are analyzed. The obtained logic results showed clear temporal waveforms and wide-open eye diagrams with error-free performance, illustrating that our nanostructured device has great potential for use in high speed all-optical signal processing and high-performance computing in the nodes and terminals of the optical networks.

Controllable optofluidic assembly of biological cells using an all-dielectric one-dimensional-photonic-crystal

Douguo Zhang, fengya lu, Lei Gong, Yan Kuai, Tang Xi, yifeng xiang, and Pei Wang

DOI: 10.1364/PRJ.439288 Received 06 Aug 2021; Accepted 25 Oct 2021; Posted 28 Oct 2021  View: PDF

Abstract: Opto-thermophoretic manipulation is emerging as an effective way for versatile trapping, guiding, and assembly of biological nanoparticles and cells. Here we report a new opto-thermophoretic tweezer based on an all-dielectric one-dimensional-photonic-crystal (1DPC) for reversible assembly of biological cells with a controllable center. To reveal its ability of long-range optofluidic manipulation, we demonstrated the revisable assembly of many yeast cells as well as E. coli cells that are dispersed in water solution. The 1DPC-based tweezer can also exert short-range optical gradient forces associated with focused Bloch surface waves excited on the 1DPC, which can optically trap single particle. By combing both the optical and thermophoretic manipulation, the optically trapped single polystyrene particle can work as a controllable origin of the reversible cellular assembly. Numerical simulations were performed to calculate the temperature distribution and convective flow velocity on the 1DPC, which are consistent with the experimental observations and theoretically confirm the long-range manipulations on the all-dielectric 1DPC platform. The opto-thermophoretic tweezers based on all-dielectric 1DPC endows the micromanipulation toolbox for potential applications in biomedical sciences.

Near-Infrared Electroluminescence of AlGaN Capped InGaN Quantum Dots Formed by Controlled Growth on Photoelectrochemical Etched Quantum Dot Templates

Xiongliang Wei, Syed Ahmed Al Muyeed, Haotian Xue, Elia Palmese, Renbo Song, Nelson Tansu, and Jonathan Wierer

DOI: 10.1364/PRJ.441122 Received 23 Aug 2021; Accepted 25 Oct 2021; Posted 28 Oct 2021  View: PDF

Abstract: Near-infrared electroluminescence of InGaN quantum dot (QDs) formed by controlled growth on photoelectrochemical (PEC) etched QD templates is demonstrated. The QD template consists of PEC InGaN QDs with high density and controlled sizes, an AlGaN capping layer to protect the QDs, and a GaN barrier layer to planarize the surface. Stranski-Krastanov growth on the QD template produces high In-content InGaN QDs, grown on top of the PEC QDs due to localized strain. A high Al-content Al0.9Ga0.1N capping layer prevents the collapse of the SK QDs due to intermixing or decomposition during higher temperature GaN growth. Electroluminescence shows a significant wavelength shift (800 nm to 500 nm), caused by the SK QD’s tall height, high In-content, and strong polarization fields.

Spatial cage solitons - taming light bullets

Chao Mei, Ihar Babushkin, Gunter Steinmeyer, and Tamas Nagy

DOI: 10.1364/PRJ.438610 Received 26 Jul 2021; Accepted 25 Oct 2021; Posted 02 Nov 2021  View: PDF

Abstract: Multimode nonlinear optics offers to overcome a long-standing limitation of fiber optics, tightly phase locking several spatial modes and enabling the coherent transport of a wavepacket through a multimode fiber. A similar problem is encountered in the temporal compression of multi-mJ pulses to few-cycle duration in hollow gas-filled fibers. Scaling the fiber length to up to six meters, hollow fibers have recently reached 1 TW of peak power. Despite the remarkable utility of the hollow fiber compressor and its widespread application, however, no analytical model exists to enable insight into the scaling behavior of maximum compressibility and peak power. Here we extend a recently introduced formalism for describing mode-locking to the spatially analogue scenario of locking spatial fiber modes together. Our formalism unveils the coexistence of two soliton branches for anomalous modal dispersion and indicates the formation of stable spatio-temporal light bullets that would be unstable in free space, similar to the temporal cage solitons in mode-locking theory. Our model enables deeper understanding of the physical processes behind the formation of such light bullets and predict the existence of multimode solitons in a much wider range of fiber types than previously considered possible.

Ultralow-loss compact silicon photonic waveguide spirals and delaylines

Shihan Hong, long zhang, Yi Wang, Ming Zhang, Yiwei Xie, and Daoxin Dai

DOI: 10.1364/PRJ.437726 Received 15 Jul 2021; Accepted 25 Oct 2021; Posted 26 Oct 2021  View: PDF

Abstract: Low-loss and compact optical waveguides are the key for realizing various photonic integrated circuits with long on-chip delaylines, such as tunable optical delaylines, optical coherence tomography and optical gyroscopes. In this paper, a low-loss and compact silicon photonic waveguide spiral is proposed by introducing broadened Archimedean spiral waveguides with a tapered Euler-S-bend. A 100-cm-long waveguide spiral is realized with a minimal bending radius as small as 10 μm by using a standard 220-nm-thick silicon-on-insulator (SOI) foundry process, and the measured propagation loss is as low as 0.28 dB/cm. Furthermore, the present waveguide spirals are used to realize a 10-bit tunable optical delayline, which has a footprint as small as 2.2×5.9 mm² and a dynamic range of 5120 ps with a fine resolution of 10 ps.

Anti-parity-time symmetry enabled on-chip chiral polarizer

Yanxian Wei, hailong zhou, Yuntian Chen, Yunhong Ding, Jianji Dong, and Xinliang Zhang

DOI: 10.1364/PRJ.444075 Received 23 Sep 2021; Accepted 25 Oct 2021; Posted 26 Oct 2021  View: PDF

Abstract: Encircling an exceptional point (EP) in a parity-time (PT) symmetric system has shown great potential for chiral optical devices, such as chiral mode switching for symmetric and anti-symmetric modes. However, the chiral switching for polarization states has never been reported although chiral polarization manipulation has significant applications in imaging, sensing, and communication etc. Here inspired by the anti-PT symmetry, we demonstrate an on-chip chiral polarizer by constructing polarization-coupled anti-PT symmetric system for the first time. The transmission axes of the chiral polarizer are different for forward and backward propagation. A polarization extinction ratio of over 10 dB is achieved for both propagating directions. Moreover, a telecommunication experiment is performed to demonstrate the potential applications in polarization encoding signals. It provides a novel functionality for encircling-an-EP parametric evolution and offer a new approach for on-chip chiral polarization manipulation.

Four-channel CWDM device on thin-film lithium niobate platform using an angled multimode interferometer structure

gengxin chen, ziliang ruan, zong wang, pucheng huang, Changjian Guo, Daoxin Dai, kaixuan chen, and Liu Liu

DOI: 10.1364/PRJ.438816 Received 27 Jul 2021; Accepted 25 Oct 2021; Posted 28 Oct 2021  View: PDF

Abstract: A compact and high performance coarse wavelength-division multiplexing (CWDM) device is introduced with a footprint of 2.1mm×0.02mm using an angled multimode interferometer structure based on the thin-film lithium niobate (TFLN) platform. The demonstrated device built on a 400nm thick x-cut TFLN shows ultra-low insertion losses of <0.72dB. Measured 3dB bandwidths are 12nm for all channels, and averaged crosstalks from adjacent channels are about 20dB. Its peak wavelength positions comply with the CWDM standard with a channel spacing of 20nm. The filter bandwidth of the proposed CWDM device can be tuned by adjusting the structural parameters. This first demonstration of a CWDM device would promote the future realization of multi-channel and multi-wavelength transmitter chips on TFLN.

Micro-LED backlight module by deep reinforcement learning and micro-macro-hybrid environment control agent

Che-Hsuan Huang, Yu-Tang Cheng, Yung-Chi Tsao, xinke liu, and Hao-chung Kuo

DOI: 10.1364/PRJ.441188 Received 25 Aug 2021; Accepted 23 Sep 2021; Posted 29 Sep 2021  View: PDF

Abstract: This paper proposes a micro-LED backlight module with a distributed Bragg reflector (DBR) structure to achieve excellent micro-LED backlight module quality. And use deep reinforcement learning (DRL) architecture for optical design. In the DRL architecture, to solve the computing environment problems of the two extreme structures of micro-scale and macro-scale, this paper proposes an Environment control agent and virtual-realistic workflow to ensure that the design environment parameters are highly correlated with experimental results. This paper successfully designed a micro-LED backlight module with a DBR structure by the above methods. The micro-LED backlight module with a DBR structure improves the uniformity performance by 32% compared with the micro-LED backlight module without DBR. And the design calculation time required by the DRL method only needs 17.9% of the traditional optical simulation.

Flexoelectric Effect Based Light Waveguide Liquid Crystal Display for Transparent Display

Deng-Ke Yang, Yunho Shin, Yingfei Jiang, Qian Wang, Ziyuan Zhou, and Guangkui Qin

DOI: 10.1364/PRJ.426780 Received 01 Apr 2021; Accepted 03 Jun 2021; Posted 11 Jun 2021  View: PDF

Abstract: We report a light waveguide liquid crystal display (LCD) based on the flexoelectric effect. The display consists of two parallel flat substrates with a layer of flexoelectric liquid crystal sandwiched between them. A light emitting diode (LED) is installed on the edge of the display and the produced light is coupled into the display. When no voltage is applied, the liquid crystal is uniformly aligned and is transparent. The incident light propagates through the display by total internal reflection at the interface between the substrate and air, and no light comes out of the viewing side of the display. The display appears transparent. When a voltage is applied, the liquid crystal is switched to a micron-sized poly-domain state due to flexoelectric interaction and becomes scattering. The incident light is deflected from the waveguide mode and comes out of the viewing side of the display. We achieved thin-film-transistor (TFT) active matrix compatible driving voltage by doping liquid crystal dimers with large flexoelectric coefficients. The light waveguide LCD does not use polarizers as in conventional LCDs. It has an ultrahigh transmittance near 90% in the voltage-off state. It is very suitable for transparent display which can be used for head-up display (HUD) and augmented reality (AR) display.

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