Showing papers in "Optical Materials Express in 2021"

Recent progress in topological waveguides and nanocavities in a semiconductor photonic crystal platform [Invited]

Abstract: Topological photonics provides a novel route for designing and realizing optical devices with unprecedented functionalities. Topological edge states, which are supported at the boundary of two photonic systems with different band topologies, enable robust light transport immune to structural imperfections and/or sharp bends in waveguides. Furthermore, the topological edge states are expected to revolutionize cavity-based optical devices such as lasers. Optical devices with built-in topological protection with a small footprint are fascinating as on-chip optical devices for low-loss and functional photonic integrated circuits. Semiconductor photonic crystals are promising platforms enabling the miniaturization of topological optical devices. Herein, we review the recent realizations of semiconductor topological photonic crystals. In particular, we discuss topological waveguides in valley photonic crystals, which have received increasing attention because of their simple realization. In addition, we provide recent demonstrations of topological nanocavities, which are another key component of topological nanophotonics. Progress in semiconductor topological photonic crystals will propel the use of topological photonic devices in various applications as well as deepen the understanding of topological photonic phenomena at the wavelength scale.

Bismuth doped fibre amplifier operating in E- and S- optical bands

Abstract: Bismuth-doped fibre amplifiers offer an attractive solution for expanding the bandwidth of fibre-optic telecommunication systems beyond the current C-band (1530-1565 nm). We report a bismuth-doped fibre amplifier in the spectral range from 1370 to 1490 nm, with a maximum gain exceeding 31 dB, and a noise figure as low as 4.75 dB. The developed system is studied for forward, backward, and bi-directional pumping schemes and three different signal power levels. The forward pumping scheme demonstrates the best performance in terms of the achieved noise figure. The developed amplifier can be potentially used as an in-line amplifier with >20dB gain in the spectral band from 1405 to 1460 nm.

Microcavity polaritons for topological photonics [Invited]

Abstract: Microcavity polaritons are light-matter quasiparticles that arise from the strong coupling between excitons and photons confined in a semiconductor microcavity. They are typically studied at visible or near visible wavelengths. They combine the properties of confined electromagnetic fields, including a sizeable spin-orbit coupling, and the sensitivity to external magnetic fields and particle interactions inherited from their partly matter nature. These features make polaritons an excellent platform to study topological phases in photonics in one and two-dimensional lattices, whose band properties can be directly accessed using standard optical tools. In this review, we describe the main properties of microcavity polaritons and the main observations in the field of topological photonics, which include, among others, lasing in topological edge states, the implementation of a polariton Chern insulator under an external magnetic field, and the direct measurement of fundamental quantities, such as the quantum geometric tensor and winding numbers in one- and two-dimensional lattices. Polariton interactions open exciting perspectives for the study of nonlinear topological phases.

Twistronics for photons: opinion

Abstract: A pair of stacked two-dimensional heterostructures suitably rotated with respect to each other support exotic electronic properties with interesting implications for nanoelectronics and quantum technologies. A similar paradigm can be extended to light, offering a great promise for emerging low-dimensional nanophotonic heterostructures. In this Opinion article, we discuss emerging photonic responses enabled by twisting and stacking suitably tailored nanostructures. We discuss how the multi-physics interactions of light with matter in twisted bilayers can tailor their photonic response and engineer light dispersion in extreme ways. We conclude by providing an outlook on this emerging field of research and its potential for classical and quantum light manipulation at the nanoscale.

Multifunctional metasurface for broadband absorption, linear and circular polarization conversions

Abstract: With the development of metasurfaces and the improvement of manufacturing technology, it is important and imperative to design novel metasurfaces that can flexibly manipulate terahertz (THz) waves. In this paper, a multifunctional metasurface (MFMS) based on graphene and photosensitive silicon (Si) is proposed, which integrates three functions: broadband absorption, broadband linear and circular polarization conversions in THz band. For absorption mode, the MFMS can absorb above 90% energy in the frequency band of 1.74-3.52 THz with the relative bandwidth of 67.6%. For both linear-linear and circular-circular polarization conversion modes, the relative bandwidth with over 90% polarization conversion rates (PCRs) reaches 49.3% in the frequency band of 1.54-2.55 THz. The working mechanism of the MFMS is analyzed by the surface current distributions, and its properties of the absorption and polarization conversion under oblique incident angles are investigated, respectively. The proposed metasurface has promising prospects in terahertz devices such as modulators, smart switches and other terahertz devices.

Highly sensitive terahertz metamaterial biosensor for bovine serum albumin (BSA) detection

Abstract: Terahertz (THz) metamaterials are widely used in biosensor devices due to their unique superiority, and the demand for new high sensitivity biosensors based on THz metamaterials is increasing. This paper presents a polarization-insensitive terahertz metamaterial sensor used for BSA detection. Simulation reveals that the peak of transmission spectrum shifts obviously when the sensor is covered with analytes of different refractive index and thickness. After the sensor is covered with 10 μm thick non-destructive analytes, its sensitivity is as high as 135 GHz/RIU. Experiments show that the lowest detectable concentration of BSA solutions by this sensor is 0.1 mg/mL, the peak red shift of the transmission spectrum reaches 137 GHz when the concentration is 17.6 mg/mL, and the frequency shift percentage is 16.4%. This study provides a highly sensitive solution for biosensor detection in the pharmaceutical and food fields.

Dual-band electromagnetically induced transparency (EIT) terahertz metamaterial sensor

Abstract: We propose a dual-band terahertz metamaterial sensor (MS), which exhibits the low loss and high quality (Q) factor of electromagnetically induced transparency (EIT) effects at the frequencies of 0.89 THz and 1.56 THz simultaneously. The physical natures of EIT effects are analyzed by using numerical simulations and a “two particle” model. Further, THz sensing is performed based on the shifts of two EIT resonances when the analyte is coated at the metamaterial surface. The sensitivities of the sensor are investigated with respect to different thicknesses, cover areas and refractive indexes of the coated analyte film. Results show that the first EIT resonance is suitable for sensing the analyte with the refractive index from 1.5 to 2, while the second EIT resonance is more suitable for sensing the refractive index of the analyte from 1 to 1.5. The sensitivity is 280.8 GHz/RIU, the average Q value is 14.3, and the figure of merit (FOM) value is 4 for the first EIT resonance. Meanwhile, the sensitivity is 201.6 GHz/RIU, the average Q value is 56.9, and the FOM value is 11.5 for the second EIT resonance. Such a metamaterial sensor with high refractive index sensitivity and dual-band would have great potentials for promoting the developments of multi-band/broadband terahertz sensing and detection technology.

Efficient design of a dielectric metasurface with transfer learning and genetic algorithm

Abstract: Machine learning has been widely adopted in various disciplines as they offer low-computational cost solutions to complex problems. Recently, deep learning-enabled methods for metasurface design have received increasing attention in the field of subwavelength electromagnetics. However, the previous metasurface design methods based on deep learning usually use huge datasets or complex networks to make deep neural networks achieve high prediction accuracy which results in more time for dataset establishment and network training. Here, we propose an expeditious and accurate scheme for designing phase-modulating dielectric metasurface through employing the transfer learning technology and genetic algorithm. The performance of the neural network is improved distinctly by migrating knowledge between real part and imaginary part spectrum-prediction tasks. Furthermore, the target meta-atoms can be optimized readily without increasing a large dataset through transfer learning. Finally, we design two deflectors and two metalenses as a proof-of-concept demonstration to validate the ability of our proposed approach. The scheme provides an efficient and promising design method for phase-modulating metasurface.

High efficiency and ultra-wideband polarization converter based on an L-shaped metasurface

Abstract: An ultra-wideband and efficient single layer polarization converting metasurface based on an L-shaped resonator is presented. The metasurface is based on an F4B dielectric substrate with relative permittivity of 2.65 and a loss tangent of 0.002. The size of the unit cell is 0.132λo × 0.132λo and the thickness of the metasurface is 0.05λo, where λo is the largest wavelength (corresponding to the lower frequency) in the operation band of interest. The proposed structure effectively transforms the linearly or circularly polarized incident wave to its orthogonal equivalent, which is justified by both simulated and measured results where the polarization conversion ratio (PCR) is greater than 90% in the frequency range from 8.6 GHz to 22 GHz with a fractional bandwidth of 88%. The polarization transformation process is illustrated in depth by the surface current distribution. Simulation results reveal that ultra-wideband is achieved because of strong electric and magnetic dipole resonances on the upper and the lower layer of the metasurface. Furthermore, the bandwidth and central frequency can be efficiently adjusted over a wide spectrum by changing the geometric aspects of the unit cell, thereby retaining high transformation proficiency. The designed converter can be used in applications such as antenna design, radar invisibility, imaging, microwave communications, and remote sensing.

Generalized hybrid anapole modes in all-dielectric ellipsoid particles[Invited]

Abstract: Numerous exciting optical effects in all-dielectric high-refractive-index structures are associated with so-called toroidal electrodynamics. Among these effects are anapoles, nonradiated states caused by interference phenomena, e.g. between electric dipole and toroidal dipole modes. For a spherical particle it is possible to reach simultaneous destructive interference for electric, magnetic, and corresponding toroidal dipole modes (so-called hybrid anapole mode), by varying the refractive index and/or particle size. However, there are no sufficient degrees of freedom within spherical geometry to extend the hybrid anapole mode effect to higher multipoles. Due to the optical theorem, it is also impossible to create the ideal anapole with destructive interference for all multipoles under plane wave illumination. In principle, it is possible to suppress radiation losses for the finite number of multipoles only by constructing the nanoantenna with complex geometry. Our approach of the hybrid anapole state excitation, we demonstrate in ellipsoidal all-dielectric particle providing cancellation of both electric and magnetic scattering up to quadrupole modes. This effect is achieved due to the optimised geometry of the ellipsoidal particle. Moreover, we provide classification of novel anapoles arising due to interference between moments and their mean- square radii (MSR) of electric, magnetic and toroidal family and introduce generalized anapoles for high order interaction between moments. Our concept is useful for the design of light controlling devices, reflectionless metasurfaces, high Q-factor opened resonators and nonscattering particle development.

Topology in quasicrystals [Invited]

Abstract: Topological phases of matter have sparked an immense amount of activity in recent decades. Topological materials are classified by topological invariants that act as a non-local order parameter for any symmetry and condition. As a result, they exhibit quantized bulk and boundary observable phenomena, motivating various applications that are robust to local disorder and imperfections. In this review, we explore such a topological classification for quasiperiodic systems, and detail recent experimental activity using photonic metamaterials.

Near-complete violation of Kirchhoff’s law of thermal radiation in ultrathin magnetic Weyl semimetal films

Abstract: The ability to break Kirchhoff’s law is of fundamental importance in thermal radiation. Various nonreciprocal emitters have been proposed to break the balance between absorption and emission. However, the thicknesses of the nonreciprocal materials are usually larger than 1/10 times of the wavelength. Besides, the previous proposed nonreciprocal emitters are complex, thus they can hardly be fabricated in experiment to verify the Kirchhoff’s law for nonreciprocal materials. In this paper, we investigate the nonreciprocal thermal radiation of the magnetic Weyl semimetal (MWSM) film atop of the metal substrate. It is found that the strong nonreciprocal radiation at the wavelength of 9.15 µm can be achieved when the thickness of the MWSM film is 100 nm. The enhanced nonreciprocity is attributed to the Fabry-Perot resonances. The results indicate that the MWSM film is the promising candidate to engineer the ultrathin and simple nonreciprocal thermal emitters. What is perhaps most intriguing here is that the proposed structure can be more easily fabricated in experiment to verify the Kirchhoff’s law for nonreciprocal materials.

Topological edge states in an all-dielectric terahertz photonic crystal

Abstract: We present an analysis of the robustness of topological edge states in an all-dielectric photonic crystal slab in the terahertz (THz) frequency domain. We initially design a valley photonic crystal (VPC) exhibiting a nontrivial band topology. The excitation of the topological edge states in the structure is facilitated through a zigzag domain wall constructed by interfacing two types of VPCs with distinct band topologies. The robustness of the excited edge states is probed with respect to the magnitude and the sign of the asymmetry in terms of the hole diameters in the VPC, for different domain interfaces. Our study reveals that the topological edge states in the VPC structure are achieved only when the domain walls are formed by the larger air holes (i.e., asymmetry parameter has a positive value). In the case of the domain walls formed by relatively smaller air holes (i.e., asymmetry parameter has a negative value), the topological protection of the edge states is forbidden. For positive asymmetry, we demonstrate that the topological transport of THz becomes more robust with the increasing magnitude of asymmetry in the VPC structure. A robust propagation of topological edge states and strong confinement of electromagnetic fields within the domain wall are observed for asymmetry ranging from 28% to 42% in our structure. We have adopted a generic technique and therefore, the results of our study could be achieved at other frequency regimes by scaling the size parameters of the structure appropriately. At THz frequencies, such extensive analysis on the robustness of the topological edge states could be relevant for the realization of low-loss waveguides for 6G communication and other integrated photonic devices.

Mid-infrared hollow core fiber drawn from a 3D printed chalcogenide glass preform

Abstract: We report the fabrication of a microstructured optical fiber drawn from a soft glass 3D printed preform. For this proof of concept, a chalcogenide glass that is well known for its capability to be shaped at low temperature and its mid-infrared transmission was selected: Te20As30Se50. The obtained negative curvature hollow core fiber shows several transmission bands in the 2–12 µm range that are reproduced numerically using finite element-based simulations and coupled mode theory.

Effect of pyrolysis on microstructures made of various photoresists by two-photon polymerization: comparative study

Abstract: Two-photon laser polymerization (TPP) is a state-of-the-art technology that allows for the submicron-resolution printing of freeform 3D objects to be harnessed in various applications, including physics, biology, medicine, and materials science. The TPP is based on using photosensitive polymeric materials that impose restrictions on the minimum feature size and limit the functionality of printed structures within the capabilities of polymers. One of the promising yet insufficiently studied methods for overcoming these limitations is pyrolysis–high-temperature annealing of polymer objects in an inert atmosphere. It may allow both to decrease the size of the objects and modify their chemical composition. Here, we compare the effect of pyrolysis on solid objects being tens of micrometers in size printed by TPP from three commercially available photoresists: IP-Dip, OrmoComp, and SZ2080. For the annealing temperatures of 450°C and 690°C in an argon atmosphere, we assessed the changes in size, chemical composition, and adhesion to the silicon wafer substrate. Our data may be promising for developing pyrolysis as a standard post-processing method for structures created via TPP technology.

SPR sensor based on exposed core micro-structured optical fiber for salinity detection with temperature self-compensation

Abstract: A surface plasmon resonance (SPR) sensor based on exposed core micro-structured optical fiber (EC-MOF) for temperature self-compensated salinity detection is proposed. The sensing channel is fabricated by sequentially coating indium tin oxide (ITO) and Au layers at the exposed region of the fiber core. Benefiting from the large dynamic refractive index (RI) range of ITO induced by dispersion, two separated SPR peaks with equal intensity can be excited at visible spectrum by Kretschmann configuration and near-infrared spectrum by Otto configuration. The RI sensing performance at 1.33–1.39 is investigated and optimized using finite element method, with maximum wavelength sensitivities of 2000 nm/RIU and 3000 nm/RIU, respectively. The distinct RI responses of two SPR peaks make the dual-parameter demodulation realizable, which shows great potential in multiplex or self-compensated sensing applications. The temperature self-compensated salinity sensing ability is demonstrated with high sensitivity of 4.45 nm/% and a temperature compensation coefficient of −0.12%/°C. To the best of our knowledge, this is the first time temperature self-compensation of fiber SPR sensors with a single sensing channel and the single demodulation method has been realized.

Influence of temperature and plasma parameters on the properties of PEALD HfO2

Abstract: HfO2 has promising applications in semiconductors and optics due to its high dielectric constant and high refractive index. In this work, HfO2 thin films were deposited by plasma enhanced atomic layer deposition (PEALD) using tetrakis-dimethylamino hafnium (TDMAH) and oxygen plasma. The process optimization to obtain high quality HfO2 thin films with excellent uniformity over a 200 mm diameter is thoroughly discussed. The effects of deposition temperature and plasma parameters on the structural, mechanical, and optical properties, and the chemical composition of the films were investigated. Optimized process parameters yielding a high refractive index, high density, low impurities, low OH incorporation, low absorption in the UV spectral range, and high laser-induced damage threshold (LIDT) were selected for antireflection coatings. The HfO2 thin films were incorporated into antireflection coatings designed for the fundamental wavelength at 1064 nm and its higher harmonics up to the 4th order.

Spectroscopic characterization of low-phonon Er-doped BaF2 single crystal for mid-IR lasers

Abstract: We present spectroscopic properties of the low-maximum-phonon energy host material, barium fluoride (BaF2) doped with Er3+ ions. Following optical excitation at 800 nm, Er3+:BaF2 exhibited broad mid-IR emission bands centered at ∼2.75, ∼3.5, and ∼4.5 µm, corresponding to the 4I11/2 → 4I13/2, 4F9/2 → 4I9/2 and 4I9/2 → 4I11/2 transitions, respectively. Temperature-dependent fluorescence spectra and decay times were recorded for the 4I9/2 level, which was resonantly excited at ∼800 nm. The multi-phonon decay rates of several closely spaced Er3+ transitions were derived using the well-known energy-gap law, and the host-dependent energy-gap law parameters B and α were determined to be 9.15 × 108 s−1 and 5.58 × 10−3 cm, respectively. The obtained parameters were subsequently used to describe the temperature dependence of the ∼4.5 µm mid-IR emission lifetime in a temperature range of 12 - 300 K. The stimulated emission cross-section of the 4I9/2 → 4I11/2 transition of Er3+ ion was derived for the first time among the known fluoride hosts, to the best of our knowledge, and found to be 0.11 × 10−20 cm2 at room temperature and 0.21 × 10−20 cm2 at 77 K.

Design of a transmissive metasurface antenna using deep neural networks

Abstract: This article presents design methods for a transmissive metasurface antenna composed of four layers of meta-structures based on the deep neural network (DNN). Owing to the structural complexity as well as side effects such as couplings among the adjacent meta-structures, the conventional design of metasurface unit cell strongly relies on the researcher’s intuition as well as time-consuming iterative simulations. A design method for a metasurface antenna unit cell with a size of a quarter wavelength operating at a frequency of 5.8GHz is presented. We describe two unique implementations for designing the target metasurfaces: 1) utilizing the inverse network 2) data augmentation by the forward network and a random search algorithm. With the usage of the two DNNs, the average transmittance of the unit cells is improved by about 0.024 than that of the unit cells designed by the conventional approach. This research invokes the application of DNN in designing antennas and other structures operating at radio frequency.

Optimized 3D printing of THz waveguides with cyclic olefin copolymer

Abstract: There is a need for low-cost and easily accessible optical devices for THz applications. THz devices can be manufactured rapidly with 3D printing while using THz transparent materials. In this work, we optimized the parameters for high-resolution 3D printing of a THz transparent filament, cyclic olefin copolymer (TOPAS), in order to 3D print high quality pipe THz waveguides. We used nozzles with diameters between 0.15 mm and 0.80 mm in a wide range of temperatures and speeds. We show that for high quality TOPAS 3D printing, the most important parameters are the bed and the printing temperature. The optimized 3D printing parameters for the nozzle diameter 0.15 mm were used for THz pipe waveguides with diameters in the range of 5-9 mm and cladding thickness of 0.3-0.8 mm. Transmission measurements corroborated the results predicted by simulations for core mode frequencies in the range of 250-900 GHz.

Designs of metareflectors based on nanodisk and annular hole arrays with polarization independence, switching, and broad bandwidth characteristics

Abstract: We propose two tunable metareflectors (MRs) composed of a suspending nanodisk and an annular hole on silicon (Si) substrate with aluminum (Al) mirrors atop. They are denoted as MR-1 and MR-2 for the former and latter, respectively. The proposed MRs exhibit high-efficient cyan-magenta-yellow (CMY) color filtering, and ultrabroad tuning range characteristics. The electromagnetic energy of the resonant wavelength is confined within the suspending nanostructure and bottom Al mirror and then performed a perfect absorption. By changing the height between suspending nanostructure and the bottom Al mirror, MRs exhibit active tuning and single-/dual-resonance switching characteristics spanning the entire visible spectra range. Furthermore, the resonant wavelengths of MRs are sensitive to the surrounding ambient media, which are red-shifted and modulated from single- to dual-resonance by changing the environmental refraction index. The corresponding sensitivities are 500 nm/RIU and 360 nm/RIU for MR-1, 289 nm/RIU and 270 nm/RIU for MR-2, respectively. These results provide an effective strategy for use in high-resolution displays, high-sensitive sensors, optical switches, optical communications, and flexible virtual reality (VR)/augmented reality (AR) applications.

Radiative heat and momentum transfer from materials with broken symmetries: opinion

Abstract: Broken inversion and time reversal symmetries affect the electromagnetic wave modes supported by continuous media, which in turn governs thermal radiation and enables control of radiative heat, linear momentum, and angular momentum transfer. We identify opportunities for exploring thermal radiation in inversion symmetry- and time reversal symmetry-breaking materials and compare and contrast radiative transport phenomena in these systems, especially nonreciprocity. Application of these phenomena can lead to novel methods of thermal management, tunability, and object manipulation at short length scales.

Inverse design of ultra-narrowband selective thermal emitters designed by artificial neural networks

Abstract: The inverse design of photonic devices through the training of artificial neural networks (ANNs) has been proven as an invaluable tool for researchers to uncover interesting structures and designs that produce optical devices with enhanced performance. Here, we demonstrate the inverse design of ultra-narrowband selective thermal emitters that operate in the wavelength regime of 2-8 µm using ANNs. By training the network on a dataset of around 200,000 samples, wavelength-selective thermal emitters are designed with an average mean squared error of less than 0.006. Q-factors as high as 109.2 are achieved, proving the ultra-narrowband properties of the thermal emitters. We further investigate the physical mechanisms of the designed emitters and characterize their angular responses to verify their use as thermal emitters for practical applications such as thermophotovoltaics, IR sensing and imaging, and infrared heating.

Colloidal Ag2S/SiO2 core/shell quantum dots with IR luminescence

Abstract: This paper presents the results of studies of the luminescent properties for colloidal Ag2S quantum dots, coated with SiO2 shell, carried out by techniques of transmission electron microscopy, optical absorption and luminescence spectroscopy time correlated single photon counting, quantum yield of luminescence. Various approaches to the formation of SiO2 shell is analyzed. It is concluded that an increase in the quantum yield of Ag2S QDs luminescence in the condition of the formation of a SiO2 shell on the interfaces provides the passivation of dangling bonds and localization of charge carriers in the nucleus. It is shown that, under the considered conditions for the synthesis of Ag2S/SiO2 core/shell structures in ethylene glycol, the use of TEOS as a precursor for SiO2 shell provides the formation of a less defective shell, leading to an increase in the quantum yield of luminescence from 1.6% to 8%. On the contrary, the use of sodium metasilicate and high concentrations of MPTMS does not ensure the formation of a dense SiO2 shell of several monolayers thickness on Ag2S interfaces, coated with 2-mercaptopropionic acid.

Mid-infrared nonlinear optical performances of Ge-Sb-S chalcogenide glasses

Abstract: We report a systematical investigation on the mid-infrared nonlinear performances of Ge-Sb-S glasses Laser damage threshold (Ith) of Ge-Sb-S glasses was measured under femtosecond pulsed laser incidence ranging between 155-36 μm It is found that the Ith has the maximum value at stoichiometric composition Moreover, the relationship between the refractive index refractive (n0) and nonlinear refractive indices (n2) was obtained, following the semi-empirical Miller’s rule The n2 shows a nonlinear decay with the increase of wavelength The multi-photon (up to 7-photon) absorption coefficients of Ge-Sb-S glasses were characterized The composition Ge25Sb10S65 with high Ith was selected as the core of the designed fiber A compatible composition Ge25Sb8S67 was chosen as the cladding glass A 10 μm-diameter-core fiber was made via rod-in-tube method By pumping a 10-cm-long fiber at 48 μm with 170 fs (100 kHz) pulses, we achieved a supercontinuum covering the 3–8 μm spectral range It indicates that Ge-Sb-S glass family is a type of environment-friendly host materials for mid-infrared nonlinear applications

Topological phenomena demonstrated in photorefractive photonic lattices [Invited]

Abstract: Topological photonics has attracted widespread research attention in the past decade due to its fundamental interest and unique manner in controlling light propagation for advanced applications. Paradigmatic approaches have been proposed to achieve topological phases including topological insulators in a variety of photonic systems. In particular, photonic lattices composed of evanescently coupled waveguide arrays have been employed conveniently to explore and investigate topological physics. In this article, we review our recent work on the demonstration of topological phenomena in reconfigurable photonic lattices established by site-to-site cw-laser-writing or multiple-beam optical induction in photorefractive nonlinear crystals. We focus on the study of topological states realized in the celebrated one-dimensional Su-Schrieffer-Heeger lattices, including nonlinear topological edge states and gap solitons, nonlinearity-induced coupling to topological edge states, and nonlinear control of non-Hermitian topological states. In the two-dimensional case, we discuss two typical examples: universal mapping of momentum-space topological singularities through Dirac-like photonic lattices and realization of real-space nontrivial loop states in flatband photonic lattices. Our work illustrates how photorefractive materials can be employed conveniently to build up various synthetic photonic microstructures for topological studies, which may prove relevant and inspiring for the exploration of fundamental phenomena in topological systems beyond photonics.

Detecting the temperature of ethanol based on Fano resonance spectra obtained using a metal-insulator-metal waveguide with SiO2 branches

Abstract: Based on the transmission characteristics of surface plasmon polaritons (SPPs) in sub-wavelength structures, this paper proposes a metal-insulator-metal (MIM) waveguide structure composed of a main waveguide with glass (SiO2) branches (WWGB) coupled with an elliptical split-ring resonance cavity (ESRRC). WWGB has a broadband continuous transmission spectrum, while ESRRC has a narrow-band discrete transmission spectrum. The coupling and interference between the two can generate excited dual-Fano resonance, with sensitivities and figures of merits (FOM) of 800 nm/RIU, 1150 nm/RIU, and 9.88, 104.55, respectively. After adding SiO2 branches to both sides of the main waveguide, the FOM are enhanced to 28.57 and 127.78, representing increases of 189% and 22.15%, respectively. This structure can be applied as a temperature sensor. After filling the cavity of the to-be-tested material with 75% ethanol, as the temperature increases, the Fano resonance wavelength to drift, therefore, the corresponding temperature can be calculated by the Fano resonance wavelength. Experiments show that the proposed MIM waveguide has a maximum sensitivity of 1406.25 nm/RIU, an FOM of 156.25, and a temperature sensitivity of 0.45 nm/℃. Ultimately, the results demonstrate that incorporating SiO2 branches enhances the sensing characteristics of the MIM waveguide, after adding ethanol, the MIM can be applied to temperature sensors, with a high sensitivity of 1406.25 nm/RIU, thereby providing a new design strategy for producing high-performance waveguides.

Broadband circularly polarized thermal radiation from magnetic Weyl semimetals

Abstract: We numerically demonstrate that a planar slab made of magnetic Weyl semimetal (a class of topological materials) can emit high-purity circularly polarized (CP) thermal radiation over a broad mid- and long-wave infrared wavelength range for a significant portion of its emission solid angle. This effect fundamentally arises from the strong infrared gyrotropy or nonreciprocity of these materials, which primarily depends on the momentum separation between Weyl nodes in the band structure. We clarify the dependence of this effect on the underlying physical parameters and highlight that the spectral bandwidth of CP thermal emission increases with increasing momentum separation between the Weyl nodes. We also demonstrate, using the recently developed thermal discrete dipole approximation (TDDA) computational method, that finite-size bodies of magnetic Weyl semimetals can emit spectrally broadband CP thermal light, albeit over smaller portion of the emission solid angle compared to the planar slabs. Our work identifies unique fundamental and technological prospects of magnetic Weyl semimetals for engineering thermal radiation and designing efficient CP light sources.

Sulfur-rich chalcogenide claddings for athermal and high-Q silicon microring resonators

Abstract: Heterogeneous integration of materials with a negative thermo-optic coefficient is a simple and efficient way to compensate the strong detrimental thermal dependence of silicon-on-insulator devices. Yet, the list of materials that are both amenable for photonics fabrication and exhibit a negative TOC is very short and often requires sacrificing loss performance. In this work, we demonstrate that As20S80 chalcogenide glass thin-films can be used to compensate silicon thermal effects in microring resonators while retaining excellent loss figures. We present an experimental characterization of the glass thin-film and of fabricated hybrid microring resonators at telecommunication wavelengths. Nearly athermal operation is demonstrated for the TM polarization with an absolute minimum measured resonance shift of 5.25 pm K−1, corresponding to a waveguide effective index thermal dependence of 4.28×10-6 RIU/K. We show that the thermal dependence can be controlled by changing the cladding thickness and a negative thermal dependence is obtained for the TM polarization. All configurations exhibit unprecedented low loss figures with a maximum measured intrinsic quality factor exceeding 3.9 × 105, corresponding to waveguide propagation loss of 1.37 dB cm−1. A value of−4.75(75)×10-5 RIU/K is measured for the thermo-optic coefficient of As20S80 thin-films.

Simulating the performance of a high-efficiency SnS-based dual-heterojunction thin film solar cell

Abstract: This article demonstrates a novel high efficiency ZnS/SnS/MoS2 dual-heterojunction thin film solar cell. The device has been optimized with respect to the thickness, doping concentration, and defect density of each constituent layer including working temperature and back contact metal work function using SCAPS-1D simulator. The MoS2 plays a promising role to serve as a back surface field (BSF) layer with commendatory band alignment, which provides an opportunity for higher absorption of longer wavelength photons utilizing the tail-states-assisted (TSA) two-step photon upconversion approach. The insertion of MoS2 in the ZnS/SnS pristine structure offers a significant improvement of the power conversion efficiency (PCE) within the detailed-balance limit with a rise from 20.1 to 41.4% with VOC of 0.91 V, JSC of 53.4 mA/cm2 and FF of 84.9%, respectively. This result reveals MoS2 as an effective BSF for low cost, highly efficient dual-heterojunction structure for future fabrication.