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2022
Volume: 10 Issue 4
32 Article(s)

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Special Issue on NEXT-GENERATION SILICON PHOTONICS
Polarization-independent multimode interference coupler with anisotropy-engineered bricked metamaterial
Carlos Pérez-Armenta, Alejandro Ortega-Moñux, José Manuel Luque-González, Robert Halir, Pedro J. Reyes-Iglesias, Jens Schmid, Pavel Cheben, Íñigo Molina-Fernández, and J. Gonzalo Wangüemert-Pérez
Many applications, including optical multiplexing, switching, and detection, call for low-cost and broadband photonic devices with polarization-independent operation. While the silicon-on-insulator platform is well positioned to fulfill most of these requirements, its strong birefringence hinders the development of polarization-agnostic devices. Here we leverage the recently proposed bricked metamaterial topology to design, for the first time, to our knowledge, a polarization-independent 2×2 multimode interference coupler using standard 220 nm silicon thickness. Our device can be fabricated with a single etch step and is optimized for the O-band, covering a wavelength range of 160 nm with excess loss, polarization-dependent loss, and imbalance below 1 dB and phase errors of less than 5°, as demonstrated with full three-dimensional finite-difference time-domain simulations.
Photonics Research
  • Publication Date: Mar. 11, 2022
  • Vol. 10 Issue 4 A57 (2022)
Research Articles
Correction
Holography, Gratings, and Diffraction
Quasi-omnibearing retro-reflective metagrating protected by reciprocity
Yuxiang Jia, Jiafu Wang, Yajuan Han, Ruichao Zhu, Zhongtao Zhang, Jie Yang, Yueyu Meng, Yongfeng Li, and Shaobo Qu
Reciprocity is ubiquitous in antennas for receiving and radiating electromagnetic (EM) waves, i.e., if an antenna has good receiving performance at a given direction, it also has good radiation performance in that direction. Inspired by this, we propose a method of designing a quasi-ominibearing retro-reflective metagrating (RRMG) protected by the reciprocity of antennas. Based on the second-order mode around 15.0 GHz of a short-circuited structured patch antenna (SPA), incident transverse magnetic waves can be received, channeled into the coaxial lines, reflected by the shortened end, and finally re-radiated into free space with a reversed wave vector. RRMGs are contrived consisting of this identical SPA, with a grating constant allowing ±2nd-, ±1st-, and zeroth-order diffractions. Oblique incidence, plus the tilted nulls of the re-radiation pattern, can eliminate -1st, zeroth, +1st, and +2nd orders, and only the -2nd order is left to achieve retro-reflections. Prototypes were fabricated and measured. Simulated and measured results show that the RRMGs maintain only -2nd-order diffraction for incident angles 32.2°≤θi90.0° in four quadrants, and that RRMGs can achieve quasi-omnibearing retro-reflections for θi=50.0°. The use of higher-order diffraction brings more degrees of freedom in manipulating EM waves, and this strategy can be readily extended to millimeter waves, THz wave, or even optical regimes.
Photonics Research
  • Publication Date: Mar. 04, 2022
  • Vol. 10 Issue 4 843 (2022)
Full-Stokes polarization transformations and time sequence metasurface holographic display | Spotlight on Optics
Shifei Zhang, Lingling Huang, Guangzhou Geng, Junjie Li, Xiaowei Li, and Yongtian Wang
With the development of micro/nano fabrication technology, metasurface holography has emerged as a revolutionary technology for the manipulation of light with excellent performance. However, for applications of full-Stokes polarization encryption and time sequence holographic display, multiplexing strategies of metasurfaces with large bandwidths and simple operations still need to be developed. As one of the most popular schemes of multiplexing, polarization multiplexed metasurfaces have shown flexible recording abilities for both free-space beam and surface waves. Here, by using a dielectric metasurface equipped with double phase holograms, we have achieved flexible polarization multiplexed transformations from one full-Stokes space to another. The vectorial hologram is optimized by a hybrid genetic algorithm and digitalized with subwavelength modulated units. Based on a quantitative map and remarkable information capacity, time sequence holographic display and complex optical encryption are experimentally demonstrated by changing input/output polarization channels in real time. We believe our method will facilitate applications in smart compact devices of dynamic display, dynamic optical manipulation, optical encryption, anticounterfeiting, etc.
Photonics Research
  • Publication Date: Mar. 25, 2022
  • Vol. 10 Issue 4 1031 (2022)
Instrumentation and Measurements
Regenerated polymer optical fiber Bragg gratings with thermal treatment for high temperature measurements
Dinusha Serandi Gunawardena, Xin Cheng, Jingxian Cui, Geraldi Edbert, Linyue Lu, Yuk Ting Ho, and Hwa-Yaw Tam
We report for the first time, to the best of our knowledge, regenerated polymer optical fiber Bragg gratings (RPOFBGs) in ZEONEX-based polymer optical fibers (POFs). The regeneration temperature can be adjusted using a heat treatment process on the POF before FBG inscription, enabling a scalable improvement of the thermal stability of the RPOFBGs. Thermal sustainability of the RPOFBGs at high temperature conditions was investigated for their prolonged use in diverse environments. Furthermore, these RPOFBGs can withstand strain levels up to 2.8% while maintaining a good linearity, even at temperature of 110°C. The RPOFBGs are capable of short-term operation at elevated temperatures of up to 132°C, which is the standard temperature for steam sterilization with at least a 4 min exposure period. The distinction in the morphologies of the two grades of ZEONEX (E48R and 480R, ZEON Corp.) used to fabricate the optical fiber together with the characteristics of UV irradiated and regenerated gratings is explained using micro-Raman spectroscopy. Collectively, these findings provide new heights for long-term operation of POF Bragg gratings (POFBGs) at elevated temperature environments and would be applicable to a wide range of disciplines.
Photonics Research
  • Publication Date: Mar. 25, 2022
  • Vol. 10 Issue 4 1011 (2022)
Integrated Optics
Octave-spanning microcomb generation in 4H-silicon-carbide-on-insulator photonics platform | Editors' Pick
Lutong Cai, Jingwei Li, Ruixuan Wang, and Qing Li
Silicon carbide has recently emerged as a promising photonics material due to its unique properties, including possessing strong second- and third-order nonlinear coefficients and hosting various color centers that can be utilized for a wealth of quantum applications. Here, we report the design and demonstration of octave-spanning microcombs in a 4H-silicon-carbide-on-insulator platform for the first time, to our knowledge. Such broadband operation is enabled by optimized nanofabrication achieving >1 million intrinsic quality factors in a 36-μm-radius microring resonator, and careful dispersion engineering by investigating the dispersion properties of different mode families. For example, for the fundamental transverse-electric mode whose dispersion can be tailored by simply varying the microring waveguide width, we realized a microcomb spectrum covering the wavelength range from 1100 nm to 2400 nm with an on-chip power near 120 mW. While the observed comb state is verified to be chaotic and not soliton, attaining such a large bandwidth is a crucial step towards realizing f-2f self-referencing. In addition, we also observed a coherent soliton-crystal state for the fundamental transverse-magnetic mode, which exhibits stronger dispersion than the fundamental transverse-electric mode and hence a narrower bandwidth.
Photonics Research
  • Publication Date: Mar. 04, 2022
  • Vol. 10 Issue 4 870 (2022)
Radio-frequency line-by-line Fourier synthesis based on optical soliton microcombs | Editors' Pick
Beichen Wang, Zijiao Yang, Shuman Sun, and Xu Yi
Radio-frequency (RF) waveform synthesis has broad applications in ultrawide-bandwidth wireless communications, radar systems, and electronic testing. Photonic-based approaches offer key advantages in bandwidth and phase noise thanks to the ultrahigh optical carrier frequency. In this work, we demonstrate Fourier synthesis arbitrary waveform generation (AWG) with integrated optical microresonator solitons. The RF temporal waveform is synthesized through line-by-line amplitude and phase shaping of an optical soliton microcomb, which is down-converted to the RF domain through dual-comb optical coherent sampling. A variety of RF waveforms with tunable repetition cycles are shown in our demonstration. Our approach provides not only the possibility of precise Fourier synthesis at microwave and millimeter-wave frequencies, but also a viable path to fully integrated photonic-based RF AWG on a chip.
Photonics Research
  • Publication Date: Mar. 11, 2022
  • Vol. 10 Issue 4 932 (2022)
Terahertz topological photonic waveguide switch for on-chip communication
Xudong Liu, Jialiang Huang, Hao Chen, Zhengfang Qian, Jingwen Ma, Xiankai Sun, Shuting Fan, and Yiwen Sun
Photonics Research
  • Publication Date: Mar. 29, 2022
  • Vol. 10 Issue 4 1090 (2022)
Interferometric method to estimate the eigenvalues of a non-Hermitian two-level optical system
Stefano Biasi, Riccardo Franchi, Filippo Mione, and Lorenzo Pavesi
Non-Hermitian physics has found a fertile ground in optics. Recently, the study of mode coalescence, i.e., exceptional points, has led to the discovery of intriguing and counterintuitive phenomena. Degeneracies are typically modeled through the coupled mode theory to determine the behavior of eigenstates and eigenvalues. However, the complex nature of the eigenvalues makes their characterization from the response spectrum difficult. Here, we demonstrate that a coherent interferometric excitation allows estimation of both the real and imaginary parts of the eigenvalues. We study the clockwise and counter-clockwise modes in optical microresonators both in the case of Hermitian and non-Hermitian intermodal coupling. We show the conditions by which a resonant doublet, due to the dissipative coupling of counter-propagating modes caused by surface roughness backscattering, merges to a single Lorentzian. This permits us to estimate the optimal quality factor of the microresonator in the absence of modal coupling caused by backscattering. Furthermore, we demonstrate that a taiji microresonator working at an exceptional point shows a degeneracy splitting only in one propagation direction and not in the other. This follows from the strongly non-Hermitian intermodal coupling caused by the inner S-shaped waveguide.
Photonics Research
  • Publication Date: Mar. 29, 2022
  • Vol. 10 Issue 4 1134 (2022)
Nanophotonics and Photonic Crystals
Abnormal optical response of PAMAM dendrimer-based silver nanocomposite metamaterials
Xianfeng Wu, Zhenchun Li, Yuan Zhao, Chaoshun Yang, Wei Zhao, and Xiaopeng Zhao
Optical metamaterials present opportunities and challenges for manipulation of light. However, metamaterials with visible and near infrared responses are still particularly challenging to fabricate due to the complex preparation process and high loss. Here, a visible light poly(amidoamine) (PAMAM)-Ag metamaterial is prepared with the assistance of fifth-generation PAMAM (5G PAMAM), based on the dendritic structure. The large area of metamaterials, where Ag nanoparticles are spherical with diameters of ∼9 nm and distributed in a multilevel netlike sphere, results in broadband resonance. The negative Goos–Hänchen shift and anomalous spin Hall effect of light generated by 5G PAMAM-Ag in visible broadband are observed, and a strong slab focusing effect at 750–1050 nm is demonstrated. In addition, the simulation shows possible application of the dendritic structure in topological photonics. The results offer advances in the preparation of large-scale visible light metamaterials, showing the potential for subwavelength super-resolution imaging and quantum optical information fields.
Photonics Research
  • Publication Date: Mar. 16, 2022
  • Vol. 10 Issue 4 965 (2022)
Theories of indirect chiral coupling and proposal of Fabry–Perot resonance as a flexible chiral-coupling interface
Pingzhun Ma, Junda Zhu, Ying Zhong, and Haitao Liu
The chiral coupling of an emitter to waveguide mode, i.e., the propagation direction of the excited waveguide mode is locked to the transverse spin (T-spin) of a circularly polarized emitter, has exhibited unprecedented applications in nanophotonics and quantum information processing. This chiral coupling can be largely enhanced in terms of unidirectivity, efficiency, and spontaneous emission rate by introducing resonant modes as coupling interfaces. However, this indirect chiral coupling still undergoes limitations in flexibility and miniaturization, and the underlying physical mechanisms are to be clarified. Here, we present an intuitive and rigorous approach for analyzing the direct/indirect chiral coupling, and thereout, derive some general relations between the chiral-coupling directionality and the T-spin of the field or emitter. Based on the theories, we propose an indirect chiral-coupling system on the platform of surface plasmon polariton (SPP), with a nanocavity supporting Fabry–Perot (FP) resonance of dual SPP modes serving as a novel coupling interface. The FP resonance provides flexible design freedoms which can modulate the chirality of the T-spin (and the resultant chiral-coupling directionality) to flip or disappear. A unidirectivity up to 99.9% along with a high coupling efficiency and enhancement of spontaneous emission rate is achieved. Two first-principles-based SPP models for the reciprocal and original problems are built up to verify the decisive role of the FP resonance in achieving the chiral coupling. The proposed theories and novel chiral-coupling interface will be beneficial to the design of more compact and flexible chiral-coupling systems for diverse applications.
Photonics Research
  • Publication Date: Mar. 29, 2022
  • Vol. 10 Issue 4 1071 (2022)
Nonlinear Optics
Ultrabroadband nonlinear Raman–Nath diffraction against femtosecond pulse laser
Lihong Hong, Baoqin Chen, Chenyang Hu, and Zhi-Yuan Li
Nonlinear Raman–Nath diffraction (NRND) offers an effective way to realize multiple noncollinear parametric processes based on the partially satisfied transverse phase-matching conditions in quadratic nonlinear media. Here, the realization of ultrabroadband NRND (UB-NRND) driven by a high-peak-power ultrashort femtosecond pump laser in two types of nonlinear crystals is reported: periodically poled lithium niobate (PPLN) and chirped PPLN (CPPLN). Multi-order ultrabroadband Raman–Nath second-harmonic (SH) signal outputs along fixed diffraction angles are simultaneously observed. This distinguished transversely phase-matched supercontinuum phenomenon is attributed to the synergic action of natural broad bandwidth of an ultrashort femtosecond pump laser and the third-order nonlinear effect induced spectral broadening, in combination with the principal ultrabroadband noncollinear second-harmonic generation processes. The NRND process with multiple quasi-phase matching (QPM) interactions from CPPLN leads to the SH output covering a wide range of wavelengths between 389 and 997 nm and exhibiting an energy conversion efficiency several orders of magnitude higher than previous studies. This UB-NRND scheme would bring better techniques and tools for applications ranging from ultrashort pulse characterization and nondestructive identification of domain structures to accurate parameter monitoring of second- and third-order nonlinear susceptibilities within solid-state nonlinear microstructured materials.
Photonics Research
  • Publication Date: Mar. 11, 2022
  • Vol. 10 Issue 4 905 (2022)
Imaging lattice switching with Talbot effect in reconfigurable non-Hermitian photonic graphene
Zhaoyang Zhang, Yuan Feng, Shaohuan Ning, G. Malpuech, D. D. Solnyshkov, Zhongfeng Xu, Yanpeng Zhang, and Min Xiao
By taking advantage of the optical induction method, a non-Hermitian photonic graphene lattice is efficiently established inside an atomic vapor cell under the condition of electromagnetically induced transparency. This non-Hermitian structure is accomplished by simultaneously modulating both the real and imaginary components of the refractive index into honeycomb profiles. The transmitted probe field can either exhibit a hexagonal or honeycomb intensity profile when the degree of non-Hermiticity is effectively controlled by the ratio between imaginary and real indices. The experimental realization of such an instantaneously tunable complex honeycomb potential sets a new platform for future experimental exploration of non-Hermitian topological photonics. Also, we demonstrate the Talbot effect of the transmitted probe patterns. Such a self-imaging effect based on a non-Hermitian structure provides a promising route to potentially improve the related applications, such as an all-optical-controllable Talbot–Lau interferometer.
Photonics Research
  • Publication Date: Mar. 16, 2022
  • Vol. 10 Issue 4 958 (2022)
Optical and Photonic Materials
Strong coupling between colloidal quantum dots and a microcavity with hybrid structure at room temperature
Zhen Zhen, Si-Yue Jin, Ren Jie, Hai-Yao Liang, and Xing-Sheng Xu
The interaction between light and matter has always been the focus of quantum science, and the realization of truly strong coupling between an exciton and the optical cavity is a basis of quantum information systems. As a special semiconductor material, colloidal quantum dots have fascinating optical properties. In this study, the photoluminescence spectra of colloidal quantum dots are measured at different collection angles in microcavities based on hybrid refractive-index waveguides. The photon bound states in the continuum are found in the low–high–low refractive-index hybrid waveguides in the appropriate waveguide width region, where the photoluminescence spectra of colloidal quantum dots split into two or more peaks. The upper polaritons and lower polaritons avoid resonance crossings in the systems. The Rabi splitting energy of 96.0 meV can be obtained. The observed phenomenon of vacuum Rabi splitting at room temperature is attributed to the strong coupling between quantum dots and the bound states in the continuum.
Photonics Research
  • Publication Date: Mar. 11, 2022
  • Vol. 10 Issue 4 913 (2022)
Inorganic halide perovskites for lighting and visible light communication
Shuangyi Zhao, Qionghua Mo, Baiqian Wang, Wensi Cai, Ru Li, and Zhigang Zang
Inorganic halide perovskites (IHPs) have received substantial attention due to their unique optoelectronic properties. Among all the intriguing performance, the efficient luminescence of IHPs enables the practical application of white light-emitting diodes (WLEDs) for lighting. During the last decade, IHP-based white lighting sources with a high luminesce and a broad color gamut have been developed as strong competitors to conventional and classic WLEDs based on rare-earth phosphors and blue LED chips. Thus, it inspires us to give an overview of the emerging progress of IHP WLEDs that can function as lighting sources. Here, in this review, the generation of luminescent properties and white light in IHPs are first presented. Then, both photoluminescence and electroluminescence WLEDs with IHPs emitters, including both lead-based and lead-free IHPs, are synthetically discussed to exhibit their advantages. Furthermore, the efforts on the optical performance enhancement of IHPs in WLEDs are demonstrated and summarized. Apart from WLEDs, visible light communication based on IHPs featuring efficient luminescence is proposed to highlight their promising potential in lighting communication. Finally, some perspectives on the evolution and challenges are described, followed by an inspirational outlook on their future development.
Photonics Research
  • Publication Date: Mar. 25, 2022
  • Vol. 10 Issue 4 1039 (2022)
Optical Devices
Terahertz metalens of hyper-dispersion
Fen Zhao, Ziping Li, Sheng Li, Xuemei Dai, Yi Zhou, Xiaoyu Liao, J. C. Cao, Gaofeng Liang, Zhengguo Shang, Zhihai Zhang, Zhongquan Wen, Hua Li, and Gang Chen
Chromatic optical lenses have promising applications in three-dimensional imaging, which allows fast spectral tomography without mechanical moving parts. The scanning range of current chromatic optical lenses is limited by their dispersion ability. The recent development in metasurfaces provides ideal blocks for optical wavefront manipulation and dispersion engineering of artificial materials at sub-wavelength scales. Hyper-dispersive metalenses can be realized by utilizing dispersive meta-atoms, which have enhanced dispersion compared to regular diffractive lenses. This is critical for increasing the imaging depth of fast spectral tomography. In this work, a hyper-dispersive metalens is realized with a chromatic dispersion 1.76 times greater than that of a regular diffractive metalens in the THz frequency range of 2.40–2.61 THz by simultaneously controlling the frequency-dependent phase, group delay (GD), and GD dispersion of the metalens. This approach can also be extended to other optical spectra and improve the performance of spectral tomography.
Photonics Research
  • Publication Date: Mar. 11, 2022
  • Vol. 10 Issue 4 886 (2022)
Arbitrary large-gradient wavefront shaping: from local phase modulation to nonlocal diffraction engineering
Xipu Dong, Jierong Cheng, Yiwu Yuan, Zhenyu Xing, Fei Fan, Xianghui Wang, and Shengjiang Chang
The powerful wavefront manipulation capability of metasurfaces originates from their subwavelength or deep subwavelength elements with designable optical responses, especially phase responses. However, they usually suffer from performance degradation as the spatial phase gradient is large. To solve this issue, we propose an accurate and efficient nonlocal diffraction engineering mechanism to tailor an arbitrary large-gradient wavefront utilizing superwavelength-scale elements. The fast-varying phase profile is cut into segments according to 2π zones rather than subwavelength discretization. Each phase segment is accurately implemented by precisely tailoring the diffraction pattern of the element, where diffraction angles, efficiencies, and phases are controlled simultaneously. As proof of the concept, high numerical aperture cylindrical metalenses are designed using this method and experimentally validated at the terahertz band. The cylindrical metalens is further extended to a full-space metalens, which enables high-quality subwavelength imaging with resolved details of 0.65λ. The proposed mechanism offers an efficient way to capture the fast-varying wavefront using relatively coarse geometries with new physical insights.
Photonics Research
  • Publication Date: Mar. 11, 2022
  • Vol. 10 Issue 4 896 (2022)
Nonlinear all-optical modulator based on non-Hermitian PT symmetry
Hongbin Ma, Dongdong Li, Nanxuan Wu, Yiyun Zhang, Hongsheng Chen, and Haoliang Qian
All-optical modulators with ultrahigh speed are in high demand due to the rapid development of optical interconnection and computation. However, due to weak photon–photon interaction, the advancement of all-optical modulators is consequently hampered by the large footprint and high power consumption. In this work, the enhanced sensitivity around an exceptional point (EP) from parity-time (PT) symmetry theory is initiatively introduced into a nonlinear all-optical modulator design. Further, a non-Hermitian all-optical modulator based on PT symmetry is proposed, which utilizes the large Kerr nonlinearity from indium tin oxide (ITO) in its epsilon-near-zero (ENZ) region. The whole system is expected to operate around EP, giving rise to the advantages of nanoscale integration and large modulation depth. This presented modulator with high efficiency and high-speed all-optical control can be commendably extended to the design methodology of various nanostructures and further prompt the development of all-optical signal processing.
Photonics Research
  • Publication Date: Mar. 25, 2022
  • Vol. 10 Issue 4 980 (2022)
Dynamic terahertz anisotropy and chirality enhancement in liquid-crystal anisotropic dielectric metasurfaces
Hui-Jun Zhao, Fei Fan, Tian-Rui Zhang, Yun-Yun Ji, and Sheng-Jiang Chang
To enhance and actively control terahertz (THz) anisotropy and chirality, we have designed and fabricated a THz composite device with a liquid crystal (LC) layer and Si anisotropic metasurface. By initial anchoring and electrically rotating the spatial orientation of the LC optical axis, the different symmetry relationships are obtained in this hybrid device. When the optical axis of LC is parallel or perpendicular to the optical axis of the Si metasurface, the anisotropy of the device will be enhanced or offset, which leads to a tunable phase-shift range of more than 180°. When there is an angle between the two optical axes, due to the destruction of the mirror symmetry in the LC-Si anisotropic medium, the highest circular dichroism of the device reaches 30 dB in the middle orientation state of the LC optical axis, and the active modulation can be realized by changing the bias electric field on the LC layer. This composite device demonstrates rich characteristics for the feasible manipulation of THz polarization conversion and chiral transmission, which can be applied in THz polarization imaging and chiral spectroscopy.
Photonics Research
  • Publication Date: Mar. 29, 2022
  • Vol. 10 Issue 4 1097 (2022)
Optoelectronics
Efficient mid-infrared single-photon detection using superconducting NbTiN nanowires with high time resolution in a Gifford-McMahon cryocooler | Spotlight on Optics
Jin Chang, Johannes W. N. Los, Ronan Gourgues, Stephan Steinhauer, S. N. Dorenbos, Silvania F. Pereira, H. Paul Urbach, Val Zwiller, and Iman Esmaeil Zadeh
Shortly after their inception, superconducting nanowire single-photon detectors (SNSPDs) became the leading quantum light detection technology. With the capability of detecting single-photons with near-unity efficiency, high time resolution, low dark count rate, and fast recovery time, SNSPDs outperform conventional single-photon detection techniques. However, detecting lower energy single photons (0.8 eV) with high efficiency and low timing jitter has remained a challenge. To achieve unity internal efficiency at mid-infrared wavelengths, previous works used amorphous superconducting materials with low energy gaps at the expense of reduced time resolution (close to a nanosecond), and by operating them in complex milliKelvin (mK) dilution refrigerators. In this work, we provide an alternative approach with SNSPDs fabricated from 5 to 9.5 nm thick NbTiN superconducting films and devices operated in conventional Gifford-McMahon cryocoolers. By optimizing the superconducting film deposition process, film thickness, and nanowire design, our fiber-coupled devices achieved >70% system detection efficiency (SDE) at 2 μm and sub-15 ps timing jitter. Furthermore, detectors from the same batch demonstrated unity internal detection efficiency at 3 μm and 80% internal efficiency at 4 μm, paving the road for an efficient mid-infrared single-photon detection technology with unparalleled time resolution and without mK cooling requirements. We also systematically studied the dark count rates (DCRs) of our detectors coupled to different types of mid-infrared optical fibers and blackbody radiation filters. This offers insight into the trade-off between bandwidth and DCRs for mid-infrared SNSPDs. To conclude, this paper significantly extends the working wavelength range for SNSPDs made from polycrystalline NbTiN to 1.5–4 μm, and we expect quantum optics experiments and applications in the mid-infrared range to benefit from this far-reaching technology.
Photonics Research
  • Publication Date: Mar. 25, 2022
  • Vol. 10 Issue 4 1063 (2022)
N-polar InGaN/GaN nanowires: overcoming the efficiency cliff of red-emitting micro-LEDs | On the Cover
A. Pandey, Y. Malhotra, P. Wang, K. Sun, X. Liu, and Z. Mi
A high efficiency, high brightness, and robust micro or sub-microscale red light emitting diode (LED) is an essential, yet missing, component of the emerging virtual reality and future ultrahigh resolution mobile displays. We report, for the first time, to our knowledge, the demonstration of an N-polar InGaN/GaN nanowire sub-microscale LED emitting in the red spectrum that can overcome the efficiency cliff of conventional red-emitting micro-LEDs. We show that the emission wavelengths of N-polar InGaN/GaN nanowires can be progressively shifted from yellow to orange and red, which is difficult to achieve for conventional InGaN quantum wells or Ga-polar nanowires. Significantly, the optical emission intensity can be enhanced by more than one order of magnitude by employing an in situ annealing process of the InGaN active region, suggesting significantly reduced defect formation. LEDs with lateral dimensions as small as ∼0.75 μm, consisting of approximately five nanowires, were fabricated and characterized, which are the smallest red-emitting LEDs ever reported, to our knowledge. A maximum external quantum efficiency ∼1.2% was measured, which is comparable to previously reported conventional quantum well micro-LEDs operating in this wavelength range, while our device sizes are nearly three to five orders of magnitude smaller in surface area.
Photonics Research
  • Publication Date: Mar. 29, 2022
  • Vol. 10 Issue 4 1107 (2022)
Physical Optics
Topological multipolar corner state in a supercell metasurface and its interplay with two-dimensional materials
Zhaojian Zhang, Junbo Yang, Te Du, and Xinpeng Jiang
Second-order topological insulators (SOTIs) have recently attracted much attention due to their capability to support lower-dimensional topological states, namely, the corner states. Here, we demonstrate that properly designed supercell metasurfaces can support photonic corner states, meanwhile further serving as an ideal platform for the implementations of topological polaritons and dynamically reconfigurable corner states by assembling two-dimensional materials. Such metasurfaces consist of an array of finite-sized SOTIs mimicking the two-dimensional Su–Schrieffer–Heeger model. We reveal that the topological transition happens in unit cells without the bandgap, and nondegenerate multipolar corner states emerge in the supercell metasurface due to the inter- and intrasupercell coupling effects. Especially since these corner states are above the light line of the metasurface, we realize the collective stimulation of the two dipolar corner states and their superposition state via far-field excitation. By stacking monolayer hexagonal boron nitride film onto the metasurface, we further achieve the topological phonon polaritons through the strong coupling between the corner state and the phonon, which is confirmed by the Rabi splitting as well as anticrossing behavior emerging in the transmission spectra. Furthermore, we reveal the robustness of the corner state and strong coupling by introducing defects into the metasurface. Finally, tunable corner state and strong coupling with on-demand control are realized by assembling monolayer graphene onto the metasurface. Our theoretical study proposes a unique hybrid-material platform for topological polaritonics and reconfigurable topological photonics, which can promote large-area topological applications in practice.
Photonics Research
  • Publication Date: Mar. 04, 2022
  • Vol. 10 Issue 4 855 (2022)
Inverse design of invisibility cloaks using the optical theorem
Brian Slovick, and Josh Hellhake
Photonics Research
  • Publication Date: Mar. 25, 2022
  • Vol. 10 Issue 4 974 (2022)
Coexisting valley and pseudo-spin topological edge states in photonic topological insulators made of distorted Kekulé lattices
Guochao Wei, Zhenzhen Liu, Licheng Wang, Jianyuan Song, and Jun-Jun Xiao
Photonic topological insulators protected by the lattice spatial symmetry (e.g., inversion and rotation symmetry) mainly support single type edge state, interpreted by either valley or pseudo-spin. Here, we demonstrate theoretically, numerically, and experimentally that a type of judiciously designed two-dimensional Kekulé photonic crystal with time reversal symmetry can possess topological valley and pseudo-spin edge states in different frequency bands. Topologically robust transportation of both the valley and pseudo-spin edge states was confirmed by measuring the transmission of straight and z-shaped interface supported edge mode and comparing with bulk modes in the microwave frequency regime. In addition, we show that due to the distinct topological origins, valley and pseudo-spin edge states can be distinguished by examining their end-scattering into the free space. Our system provides an alternative way in manipulating electromagnetic waves with additional degree-of-freedom, which has potential applications for robust and high-capacity waveguiding and multi-mode dividing.
Photonics Research
  • Publication Date: Mar. 25, 2022
  • Vol. 10 Issue 4 999 (2022)
Quantum Optics
All-optical silicon microring spiking neuron
Jinlong Xiang, Yujia Zhang, Yaotian Zhao, Xuhan Guo, and Yikai Su
With the rapid development of artificial intelligence and machine learning, brain-inspired neuromorphic photonics has emerged as an extremely attractive computing paradigm, promising orders-of-magnitude higher computing speed and energy efficiency compared to its electronic counterparts. Tremendous efforts have been devoted to photonic hardware implementations of mimicking the nonlinear neuron-like spiking response and the linear synapse-like weighting functionality. Here, we systematically characterize the spiking dynamics of a passive silicon microring neuron. The research of self-pulsation and excitability reveals that the silicon microring can function as an all-optical class II resonate-and-fire neuron. The typical refractory period has been successfully suppressed by configuring the pump power above the perturbation power, hence allowing the microring neuron to operate with a speed up to roughly sub-gigahertz. Additionally, temporal integration and controllable inhibition regimes are experimentally demonstrated for the first time, to the best of our knowledge. Our experimental verification is obtained with a commercial CMOS platform, hence offering great potential for large-scale neuromorphic photonics integration.
Photonics Research
  • Publication Date: Mar. 11, 2022
  • Vol. 10 Issue 4 939 (2022)
Generation and modulation of non-classical light in a strongly coupled photon–emitter system
Lingxiao Shan, Juanjuan Ren, Qi Zhang, Qi Liu, Yun Ma, Qihuang Gong, and Ying Gu
Non-classical light, especially its single photon and squeezing properties, plays a fundamental role in on-chip quantum networks. The single photon property has been widely studied in photonic cavities including photonic crystals (PhCs), micropillar cavities, nanowires, and plasmonic cavities. However, the generation and modulation of squeezing light in nanophotonic cavities remain to be explored. Here, we theoretically demonstrate a strongly coupled PhC–plasmonic-emitter system enabling non-classical light generation and modulation. The hybridization of a PhC waveguide and an Ag nanoparticle forms a band-edge mode with a narrow linewidth and a strong confined field, which enables strong light–emitter interaction, further resulting in simultaneous generation of squeezing and single photon properties for on-chip applications. Non-classical light emission can be modulated with the detuning between the band-edge mode and the emitter. The emission is efficiently channeled by the PhC waveguide with a high coupling efficiency, accompanying unidirectional transmission under excitation by a circularly polarized emitter. The system provides a candidate for tunable and bifunctional on-chip non-classical light sources at the nanoscale and may offer more possibilities to build versatile quantum networks.
Photonics Research
  • Publication Date: Mar. 25, 2022
  • Vol. 10 Issue 4 989 (2022)
Memory-assisted quantum accelerometer with multi-bandwidth
Zhifei Yu, Bo Fang, Liqing Chen, Keye Zhang, Chun-Hua Yuan, and Weiping Zhang
The accelerometer plays a crucial role in inertial navigation. The performance of conventional accelerometers such as lasers is usually limited by the sensing elements and shot noise limitation (SNL). Here, we propose an advanced development of an accelerometer based on atom–light quantum correlation, which is composed of a cold atomic ensemble, light beams, and an atomic vapor cell. The cold atomic ensemble, prepared in a magneto-optical trap and free-falling in a vacuum chamber, interacts with light beams to generate atom–light quantum correlation. The atomic vapor cell is used as both a memory element storing the correlated photons emitted from cold atoms and a bandwidth controller through the control of free evolution time. Instead of using a conventional sensing element, the proposed accelerometer employs interference between quantum-correlated atoms and light to measure acceleration. Sensitivity below SNL can be achieved due to atom–light quantum correlation, even in the presence of optical loss and atomic decoherence. Sensitivity can be achieved at the ng/Hz level, based on evaluation via practical experimental conditions. The present design has a number of significant advantages over conventional accelerometers such as SNL-broken sensitivity, broad bandwidth from a few hundred Hz to near MHz, and avoidance of the technical restrictions of conventional sensing elements.
Photonics Research
  • Publication Date: Mar. 25, 2022
  • Vol. 10 Issue 4 1022 (2022)
Silicon Photonics
Exploitation of geometric and propagation phases for spin-dependent rational-multiple complete phase modulation using dielectric metasurfaces
Ata Ur Rahman Khalid, Fu Feng, Naeem Ullah, Xiaocong Yuan, and Michael Geoffrey Somekh
Metasurfaces have drawn considerable attention in manipulation of electromagnetic waves due to their exotic subwavelength footprints. Regardless of immense progress of polarization-dependent flat optics, the realization of on-device switchable complete phase multiplication is still missing from design multifunctional devices. Here, by combining geometric and propagation phases, a generalized design principle is proposed that can achieve switchable integer or fractional multiple complete phase modulation in transmitted circularly cross-polarized light by switching the handedness of incident polarization. As a proof of concept, two types of spin-dependent bifunctional wavefront manipulating devices, including switchable beam splitter/beam deflector and spin-to-orbital angular momentum converter designs are numerically realized. It is believed that the proposed single-cell spin-switchable rational-multiple complete-phase-modulation design principle based on combined propagation and geometric phases has great potential to underpin the development of meta-optics-based multifunctional operations in the field of integrated optics, imaging, and optical communication.
Photonics Research
  • Publication Date: Mar. 11, 2022
  • Vol. 10 Issue 4 877 (2022)
240 Gb/s optical transmission based on an ultrafast silicon microring modulator
Yuguang Zhang, Hongguang Zhang, Junwen Zhang, Jia Liu, Lei Wang, Daigao Chen, Nan Chi, Xi Xiao, and Shaohua Yu
An ultrafast microring modulator (MRM) is fabricated and presented with Vπ·L of 0.825 V·cm. A 240 Gb/s PAM-8 signal transmission over 2 km standard single-mode fiber (SSMF) is experimentally demonstrated. PN junction doping concentration is optimized, and the overall performance of the MRM is improved. Optical peaking is introduced to further extend the EO bandwidth from 52 to 110 GHz by detuning the input wavelength. A titanium nitride heater with 0.1 nm/mW tuning efficiency is implemented above the MRM to adjust the resonant wavelength. High bit rate modulations based on the high-performance and compact MRM are carried out. By adopting off-line signal processing in the transmitter and receiver side, 120 Gb/s NRZ, 220 Gb/s PAM-4, and 240 Gb/s PAM-8 are measured with the back-to-back bit error ratio (BER) of 5.5×10-4, 1.5×10-2, and 1.4×10-2, respectively. A BER with different received optical power and 2 km SSMF transmission is also investigated. The BER for 220 Gb/s PAM-4 and 240 Gb/s PAM-8 after 2 km SSMF transmission is calculated to be 1.7×10-2 and 1.5×10-2, which meet with the threshold of soft-decision forward-error correction, respectively.
Photonics Research
  • Publication Date: Mar. 29, 2022
  • Vol. 10 Issue 4 1127 (2022)
Surface Optics and Plasmonics
Optical topological lattices of Bloch-type skyrmion and meron topologies | Editors' Pick
Qiang Zhang, Zhenwei Xie, Peng Shi, Hui Yang, Hairong He, Luping Du, and Xiaocong Yuan
Optical skyrmions, quasiparticles that are characterized by the topologically nontrivial vectorial textures of optical parameters such as the electromagnetic field, Stokes parameters, and spin angular momentum, have aroused great attention recently. New dimensions for optical information processing, transfer, and storage have become possible, and developing multiple schemes for manipulating the topological states of skyrmions, thus, is urgent. Here we propose an approach toward achieving dynamic modulation of skyrmions via changing the field symmetry and adding chirality. We demonstrate that field symmetry governs the skyrmionic transformation between skyrmions and merons, whereas material chirality modulates the twist degree of fields and spins and takes control of the Néel-type–Bloch-type skyrmionic transition. Remarkably, the enantioselective twist of skyrmions and merons results from the longitudinal spin arising from the chirality-induced splitting of the hyperboloid in the momentum space. Our investigation, therefore, acts to enrich the portfolio of optical quasiparticles. The chiral route to topological state transitions will deepen our understanding of light–matter interaction and pave the way for chiral sensing, optical tweezers, and topological phase transitions in quantum matter.
Photonics Research
  • Publication Date: Mar. 16, 2022
  • Vol. 10 Issue 4 947 (2022)
Arbitrary manipulations of focused higher-order Poincaré beams by a Fresnel zone metasurface with alternate binary geometric and propagation phases
Xiangyu Zeng, Yuqin Zhang, Manna Gu, Zijun Zhan, Ruirui Zhang, Yu Zhang, Rui Sun, Changwei He, Chunxiang Liu, and Chuanfu Cheng
The manipulation of high-quality vector beams (VBs) with metasurfaces is an important topic and has potential for classical and quantum applications. In this paper, we propose a Fresnel zone (FZ) metasurface with metallic nanoslits arranged on FZs, which sets alternate binary geometric and propagation phases to cancel the incident spin component and focus the converted spin component (CSC). The rotation designs of nanoslits transform the incident polarization state on the conventional Poincaré sphere to VBs on the higher-order Poincaré (HOP) sphere. The two orbital angular momentum states of the CSCs were manipulated, and the focused HOP beams were generated. The experimental results demonstrate the broadband generation of arbitrarily focused HOP beams of high quality under the illumination of the red (632.8 nm), green (532 nm), and blue (473 nm) light. This work will be of significance for the applications of VBs in different areas, such as precision metrology, optical micromanipulation, and quantum information.
Photonics Research
  • Publication Date: Mar. 29, 2022
  • Vol. 10 Issue 4 1117 (2022)
Ultrafast Optics
Bulk lateral shearing interferometry for spatiotemporal study of time-varying ultrashort optical vortices
Miguel López-Ripa, Íñigo J. Sola, and Benjamín Alonso
The spatiotemporal measurement of ultrashort laser beams usually involves techniques with complex set-ups or limited by instabilities that are unable to accurately retrieve the frequency-resolved wavefront. Here, we solve these drawbacks by implementing a simple, compact, and ultra-stable spatiotemporal characterization technique based on bulk lateral shearing spectral interferometry using a birefringent uniaxial crystal. We apply it to retrieve complex spatiotemporal structures by characterizing ultrafast optical vortices with constant and time-varying orbital angular momentum. This technique can operate in all the transparency range of the anisotropic elements, enabling the characterization in different spectral ranges like infrared, visible, or ultraviolet.
Photonics Research
  • Publication Date: Mar. 11, 2022
  • Vol. 10 Issue 4 922 (2022)

About the Cover

This image represents an artistic rendering of a uniform array of GaN nanowire LEDs. The wires, grown using Molecular Beam Epitaxy, have an emitting region made using InGaN and are designed to emit red light.