Flexible design of chiroptical response of planar chiral metamaterials using deep learning.

Optical chirality is highly demanded for biochemical sensing, spectral detection, and advanced imaging, however, conventional design schemes for chiral metamaterials require highly computational cost due to the trial-and-error strategy, and it is crucial to accelerate the design process particularly in comparably simple planar chiral metamaterials. Herein, we construct a bidirectional deep learning (BDL) network consists of spectra predicting network (SPN) and design predicting network (DPN) to accelerate the prediction of spectra and inverse design of chiroptical response of planar chiral metamaterials. It is shown that the proposed BDL network can accelerate the design process and exhibit high prediction accuracy. The average process of prediction only takes ∼15 ms, which is 1 in 40000 compared to finite-difference time-domain (FDTD). The mean-square error (MSE) loss of forward and inverse prediction reaches 0.0085 after 100 epochs. Over 95.2% of training samples have MSE ≤ 0.0042 and MSE ≤ 0.0044 for SPN and DPN, respectively; indicating that the BDL network is robust in the inverse deign without underfitting or overfitting for both SPN and DPN. Our founding shows great potentials in accelerating the on-demand design of planar chiral metamaterials.

Unidirectional self-imaging in multiple shifted photonic crystal interfaces.

In this study, we investigate the unidirectional self-imaging phenomenon in the shifted photonic crystal (PC) heterostructure. A spin-locked topological edge state, which originates from the mismatch of the Wannier center positions, can propagate along the shifted PC interface without backscattering. When the neighboring shifted PC interfaces are close enough, the coupling between the edge states happens, and coupled edge states (CES) can be found. Based on the finite element method (FEM) simulation, the spin-locked multimode interference (MMI) and self-imaging phenomenon of CES, including paired and symmetrical interference, are achieved in multiple shifted PC interfaces. To illustrate the application of the frequency splitters, the T-shaped and double cross-shaped structures with backscattering immunity and spin-locked characteristics are proposed. Our work provides an alternative way toward the design of a topological splitter by utilizing the photonic frequency and spin degrees of freedom at the same time.

Topological transport in heterostructure of valley photonic crystals.

We propose a heterogeneous structure, which are composed of two valley photonic crystals (VPCs) with opposite valley Chern numbers and air channel. With the increasing width of the air channel, valley-locked waveguide modes are found in topological bandgap by analyzing energy bands. Finite element method (FEM) simulation results show that the fundamental and high order modes are valley-locked, propagating unidirectionally under the excitation of chiral source, and possess higher flux compared to the valley-locked topological edge state in the domain wall. Besides, the immunity to backscattering in bend and couplers, and the robustness to random disorders are discussed in detail. We also investigate the one-way multimode interference (MMI) effect based on valley-locked waveguide modes, and design topological beam splitters. Our study provides a novel idea for topological transport with high flux, and more freedom to design valley-locked waveguide devices, including bends, couplers and splitters.

Simulated annealing algorithm with neural network for designing topological photonic crystals.

In this work, we utilize simulated annealing algorithm with neural network, to achieve rapid design of topological photonic crystals. We firstly train a high-accuracy neural network that predicts the band structure of hexagonal lattice photonic crystals. Subsequently, we embed the neural network into the simulated annealing algorithm, and choose the on-demand evaluation functions for optimizing topological band gaps. As examples, designing from the Dirac crystal of hexagonal lattice, two types of valley photonic crystals with the relative bandwidth of bandgap 26.8% and 47.6%, and one type of pseudospin photonic crystal with the relative bandwidth of bandgap 28.8% are obtained. In a further way, domain walls composed of valley photonic crystals (pseudospin photonic crystals) are also proposed, and full-wave simulations are conducted to verify the valley-locked (pseudospin-locked) edge states unidirectionally propagates under the excitation of circularly polarized source. Our proposed method demonstrates the efficiency and flexibility of neural network with simulated annealing algorithm in designing topological photonic crystals.

Phase-change metasurfaces for dynamic control of chiral quasi-bound states in the continuum.

Chiral quasi-bound states in the continuum (QBIC) offer novel mechanisms to achieve intrinsic chiroptical responses. However, current studies on chiral QBIC metasurfaces are restricted to the excitation of intrinsic chirality and fail to dynamically control its circular dichroism (CD) responses. Herein, we construct a phase-change metasurface based on paired Ge2Sb2Te5 (GST) bars to demonstrate the dynamic control of the CD responses of chiral QBIC. The modified coupled mode theory (CMT) is proposed to evaluate the intrinsic chirality, and the predicted results are in good agreement with the finite-difference time-domain (FDTD) results. The maximal intrinsic chirality is associated with the spin-selected dipole mode, i.e., the coupled magnetic dipole (MD) QBIC mode for the left-handed circularly polarized (LCP) light and the decoupled electric dipole (ED) QBIC mode for the right-handed circularly polarized (RCP) light. By varying the volume fraction of GST, the location of chiral BIC can be tuned linearly, and the corresponding chiral response can be switched.

Mid-infrared circular-polarization-sensitive photodetector based on a chiral metasurface with a photothermoelectric effect.

Photothermoelectric conversion in chiral metasurfaces with thermoelectric material provides an effective way to achieve circular polarization recognition. In this paper, we propose a circular-polarization-sensitive photodetector in a mid-infrared region, which is mainly composed of an asymmetric silicon grating, a film of gold (Au), and the thermoelectric B i 2 T e 3 layer. The asymmetric silicon grating with the Au layer achieves high circular dichroism absorption due to a lack of mirror symmetry, which results in a different temperature increasing on the surface of the B i 2 T e 3 layer under right-handed circularly polarized (RCP) and left-handed circularly polarized (LCP) excitation. Then the chiral Seebeck voltage and output power density are obtained, thanks to the thermoelectric effect of B i 2 T e 3. All the works are based on the finite element method, and the simulation results are conducted by the Wave Optics module of COMSOL, which is coupled with the Heat Transfer module and Thermoelectric module of COMSOL. When the incident flux is 1.0W/c m 2, the output power density under RCP (LCP) light reaches 0.96m W/c m 2 (0.01m W/c m 2) at a resonant wavelength, which achieves a high capability of detecting circular polarization. Besides, the proposed structure shows a faster response time than that of other plasmonic photodetectors. Our design provides a novel, to the best of our knowledge, method for chiral imaging, chiral molecular detection, and so on.

Active control of resonant asymmetric transmission based on topological edge states in paired photonic crystals with a Ge2Sb2Te5 film.

For many high-precision applications such as filtering, sensing, and photodetection, active control of resonant responses of metasurfaces is crucial. Herein, we demonstrate that active control of resonant asymmetric transmission can be realized based on the topological edge state (TES) of an ultra-thin G e 2 S b 2 T e 5 (GST) film in a photonic crystal grating (PCG). The PCG is composed of two pairs of one-dimensional photonic crystals (PCs) separated by a GST film. The phase change of the GST film re-distributes the field distributions of the PCG; thus active control of narrowband asymmetric transmission can be achieved due to the switch of the on-off state of the TES. According to multipole decompositions, the appearance and disappearance of the synergistically reduced dipole modes are responsible for the high-contrast asymmetric transmission of the PCG. In addition, the asymmetric transmission performances are robust to the variation of structural parameters, and good unidirectional transmission performances with a high peak transmission and high contrast ratio can be balanced, as the layer number of the two PCs is set as four. By changing the crystallization fraction of GST, the peak transmission and peak contrast ratio of asymmetric transmission can be flexibly tuned with the resonance locations kept almost the same.

Coupled topological edge states in one-dimensional all-dielectric heterostructures.

We theoretically propose a coupled-topological-edge-state waveguide (CTESW), which is composed of stacked binary one-dimensional (1D) photonic crystals with opposite topological properties. The CTESW modes originate from the coupling between a sequence of topological edge states (TESs), which can be verified by the coupled mode theory (CMT). Based on finite element method (FEM), the tunable multiple transmission peaks due to CTESW modes are obtained, and the optical properties of the system can be modulated by the geometric parameters. Besides, the CTESW modes can also be tuned by changing incident angle from 0° to 60° under TE and TM polarization. Moreover, considering the relationship between channel spacing and the frequency spectrum utilization, a dense wavelength division multiplex (DWDM) filter with 50 GHz channel spacing based on CTESW is designed in communication band.

Lithography-free tunable absorber at visible region via one-dimensional photonic crystals consisting of an α-MoO3 layer.

Flexible control of light absorption within the lithography-free nanostructure is crucial for many polarization-dependent optical devices. Herein, we demonstrated that the lithography-free tunable absorber (LTA) can be realized by using two one-dimensional (1D) photonic crystals (PCs) consisting of an α-MoO3 layer at visible region. The two 1D PCs have different bulk band properties, and the topological interface state-induced light absorption enhancement of α-MoO3 can be realized as the α-MoO3 thin film is inserted at the interface between the two 1D PCs. The resonant cavity model is proposed to evaluate the anisotropic absorption performances of the LTA, and the results are in good agreement with those of the transfer matrix method (TMM). The absorption efficiency of the LTA can be tailored by the number of the period of the two PCs, and the larger peak absorption is the direct consequence of the larger field enhancement factor (FEF) within the α-MoO3 layer. In addition, near-perfect absorption can be achieved as the LTA is operated at the over-coupled resonance. By varying the polarization angle, the absorption channels can be selected and the reflection response can be effectively modulated due to the excellent in-plane anisotropy of α-MoO3.

Multimode interference in topological photonic heterostructure.

In this Letter, topological photonic heterostructures, which are composed of finite-size photonic crystals with different topological phases, are proposed. The coupled topological edge states (CTESs), which originate from the coupling between topological edge states, are found. By using the finite element method, the multimode interference effect of CTESs is predicted and investigated. Paired and symmetrical interferences are discussed, and the respective imaging positions are calculated. In addition, the multimode interference effect is topologically protected when introducing disorders. As examples of application, frequency and power splitters of topological edge states based on the multimode interference effect are designed and demonstrated numerically. Our findings pave a new, to the best of our knowledge, way of designing topological photonic integrated circuit applications such as filters, couplers, multiplexers, and so on.

Ultrabroadband and ultrathin absorber based on an encapsulated T-shaped metasurface.

Ultrabroadband absorbers are vital for applications such as solar energy harvesting and integrated optoelectronic devices. Herein, we design, fabricate and characterize a novel ultrabroadband and ultrathin absorber based on the encapsulated T-shaped metasurface (ETM). The ETM consists of a 20 nm Cr film and a Cr substrate sandwiched by the T-shaped polymethyl methacrylate (PMMA) arrays. The Cr film provides a robust absorptive surface with improved impedance matching, and ultrabroadband absorption can be achieved via the excitation of the localized surface plasmon (LSP) of this ultrathin film. The average absorption of simulated and experimental results of the ETM in the visible range of 400-800 nm for the TM (TE) polarization are 96.4% (96.3%) and 90.6% (89.4%), respectively. Three-dimensional (3D) power dissipation density distributions of the proposed structure have been investigated, which indicates that the synergistic absorption effect of different parts of the T-shaped ultrathin Cr film contributes to the major absorption enhancement. The absorption of the ETM is very robust to the changes of geometrical parameters and the symmetry of the structure, and it can be maintained almost the same even if T-shaped profiles are changed to L-shaped profiles. Moreover, the absorption performance of the ETM exhibits polarization-insensitive and wide-angle features, which has advantages for many potential applications.

Tunable reflective dual-band line-to-circular polarization convertor with opposite handedness based on graphene metasurfaces.

In this letter, we propose a dual-band tunable reflective linear-to-circular (LTC) polarization converter, which is composed of a graphene sheet etched with an I-shaped carved-hollow array. In the mid-infrared region, two LTC bands with opposite handedness are simultaneously realized due to the excitation of the three graphene surface plasmon (GSP) modes. The band of line-to-right-circular-polarization (LTRCP) ranges from 9.87 to 11.03THz with ellipticity χ <-0.95, and from 9.69 to 11.36 THz with an axial ratio of less than 3 dB; the band of line-to-left-circular-polarization (LTLCP) ranges from 13.16 to 14.43THz with χ >0.95, and from 12.79 to 14.61 THz with an axial ratio of less than 3 dB. The tunable responses of the reflective polarizer with Fermi energy (Ef) and electron scattering time (τ) are discussed, and especially the perfect LTLCP can be changed to LTRCP with increasing Ef. Also, the influences of geometric parameters, incident angle, and polarization angle on the performances of the dual-band LTC are also investigated, and it is found that our polarizer converter shows angle insensitivity. All simulation results are conducted by the finite element method. Our design enriches the research of tunable LTC polarizers and has potential applications in integrated terahertz systems.

Dual-frequency unidirectional reflectionless propagation in a non-Hermitian graphene plasmonic waveguide-cavity coupling system.

We theoretically investigate a controllable dual-frequency unidirectional reflectionlessness at exceptional points by applying external voltage in a graphene plasmonic waveguide system. The system consists of a graphene waveguide and two end-coupled resonators. COMSOL simulation results show that the reflection of edge fundamental graphene surface plasmon polaritons mode for forward (backward) incidence is near to zero at frequency 24.418 THz (20.865 THz), while that for backward (forward) incidence is 24.71% (22.945%), respectively. In addition, the non-Hermitian scattering matrix is proposed to verify the existence of double exceptional points, and the tunable unidirectional reflectionless phenomenon is also achieved by changing the Fermi level (Ef) of graphene.

Dynamically tunable directional subwavelength beam propagation based on photonic spin Hall effect in graphene-based hyperbolic metamaterials.

Photonic spin Hall effect (PSHE) of type II hyperbolic metamaterials is achieved due to near filed interference, which provides a way to decide the propagation direction of subwavelength beam. In this paper, we propose graphene-based hyperbolic metamaterials (GHMMs), which is composed of the alternating graphene/SiO2 multilayer. The numerical results show that when a dipole emitter is placed at the boundary of the GHMMs, the subwavelength beam with λ/40 full-with half maximum can be excited and propagates along the left or right channel, which is dependent on polarization handedness. In addition, we further demonstrate that the unidirectional propagation angle can be dynamically tuned by changing the external electric field bias applied to graphene.

Tunable asymmetric transmission across stretchable chiral metamaterial.

A stretchable chiral metamaterial with L-shaped and T-shaped Au patterns (SCMM-LT) is proposed to generate asymmetric transmission (AT) for circularly polarized waves on the polydimethylsiloxane substrate in the mid-infrared region. The peak value of AT can reach 50.02% at the resonance wavelength of 19.1 µm, owing to the enantiomerically sensitive plasmons. With stretching along the x axis and the y axis. respectively, the band of AT shifts to a longer wavelength, which proves the SCMM-LT can be a candidate as the tunable chiral metamaterial. In the future, the proposed stretchable chiral metamaterial could potentially possess high applicability for wearable electronic devices in a variety of sensor fields.

Graphene-based hyperbolic metamaterials for a tunable subwavelength dark hollow beam.

Hyperbolic metamaterials have recently been widely investigated in nanophotonics systems. Here, we propose an alternating graphene/${{\rm SiO}_2}$SiO2 multilayer structure as an anisotropic medium with hyperbolic dispersion. When in-plane and out-of-plane effective permittivity is negative and positive, respectively, the incident beam (transverse magnetic polarization wave) can be split into two subwavelength beams, and a dark hollow beam can be achieved for circularly polarized incidence. Also, the size of the dark hollow beam can be tuned by changing the Fermi level. Our method is believed to be used as a tunable optical tweezer for controlling molecules.

Graphene surface plasmon off-axis superlens based on tilted one-dimensional Si/SiO2 gratings.

Graphene surface plasmon (GSP) superlenses, induced from the negative refraction, have recently been demonstrated in various two-dimensional photonic crystal systems. However, inplane GSP superlenses have never been reported in a one-dimensional (1D) photonic crystal system. Here, we propose a graphene-Si/SiO2 system, by transferring a graphene sheet on the tilted 1D subwavelength silicon/silica gratings. By discussing the dispersion relations of the inplane GSP in this system, the GSP negative refraction is found in the mid-infrared region. When the tilted angle, working wavelength, and Fermi level are set to be 60°, 11.22 µm, and 0.2 eV, respectively, the off-axis subwavelength focusing has the best resolution, and the full width at half-maximum (FWHM) of the image is 0.0091λ (102.1 nm). Further, we investigate the effects of the Fermi level on the superlens frequency range, and the image's FWHM, the broadband, and the deep subwavelength superlens are achieved. The full-wave numerical simulations are conducted by the finite element method. Our findings can be applied to the manipulation of inplane GSP propagation and biological imaging.

Tunable multifunctional reflection polarizer based on a graphene metasurface.

Herein, we present a tunable multifunctional reflection polarizer, based on a graphene metasurface, which is composed of an array of cross double-ellipse graphene patches. A dual band of linear-to-linear (LTL) polarization conversions is achieved due to the superimposition of the two reflection components with a near 0° or 180° phase difference, in the mid-infrared region. By carefully choosing the parameters, linear-to-circular polarization conversion and broadband of LTL polarization conversion (about 0.7 THz) are also realized. Also, the tunable responses of the proposed reflection polarizer are discussed under a different Fermi energy and electron scattering time. It is believed that our proposed polarizer can be widely used for multifunctional and tunable polarization conversion.

Curving h-BN thin films can create extra phonon polariton modes.

Hexagonal boron nitride (h-BN) thin films support volume-confined phonon polariton modes within the bulk material as well as surface-confined modes at the edges of thin films. In this Letter, we theoretically investigate the phonon polaritons in curved h-BN thin films. One-dimensional guided phonon polariton modes are found, which are caused by the curved geometry and do not exist in extended flat films. These modes are guided along a specific direction with relatively low propagation losses. So far, one-dimensional guided phonon polariton modes have only been proposed in nanowire and nanoribbon structures. Our study offers another way with the advantage of keeping the h-BN film intact, which can avoid huge scattering losses due to the structural defects. These investigations may offer an easy and robust approach toward phonon-polariton-based nanophotonic circuitry.

A tunable THz absorber consisting of an elliptical graphene disk array.

Herein, we present an adjustable absorber consisting of a periodically patterned elliptical graphene disk array, which absorbs in the THz region. When a circularly polarized light beam illuminates this structure, its absorption spectrum displays two absorption peaks, which originate from the F-P resonance of the fundamental graphene edge plasmon mode along the major and minor axes of the elliptical graphene disk. The position of these two absorption peaks can be modulated by changing the Fermi level of graphene. Furthermore, both absorption bands can merge into one broadband by changing the length of the major and minor axes. The full width at half maximum (FWHM) of the broadband can reach up to 3.52 THz. In addition, by changing the incident elliptically polarized light, the peak ratio between the two absorption bands can also be tuned to convert the double-band absorption to single-band absorption.