Asymmetric Optical Transmission Research Articles

Multiband and bidirectional multiplexing asymmetric optical transmission empowered by nanograting-coupled defective multilayer photonic crystal

Asymmetric optical transmission (AOT) has been an enduring hot topic of interest in various fields, including optical communication, information processing, and so on. Particularly, the development of reciprocal micro-nanostructures achieving AOT further facilitates and accelerates the miniaturization and integration of traditional optical components. However, most of these optical components merely consider a single AOT band and transmission in a specified direction, limiting the development of their versatile functions. In this paper, we theoretically propose an all-dielectric metamaterial consisting of a nanograting and a defective multilayer photonic crystal, exhibiting multi-band and bidirectional multiplexing AOT. More specifically, the proposed metamaterial demonstrates both narrowband and wideband AOT for incidence from the nanograting to the photonic crystal, and a completely different narrowband AOT for the opposite incidence, namely, from the photonic crystal to the nanograting. These distinctive AOT spectral features are achieved by matching the diffraction effect of the nanograting with the special energy band of the defective multilayer photonic crystal. Remarkably, the device exhibits a transmittance difference of up to 0.974 and a contrast ratio of up to 0.997 (transmittance ratio of up to 673), with a transmission bandwidth of 62.7 nm for incident light with a wavelength of 624 nm illuminating from the nanograting to the defective multilayer photonic crystal. Furthermore, the bandwidth and number of transmission bands can be flexibly tuned by changing the polarization angle of the incident light, showcasing its excellent polarization multiplexing characteristics. The designed metamaterial provides an effective strategy for the realization of versatile AOT devices and is conducive to expanding the application scenarios of AOT devices.

Broadband and highly efficient asymmetric optical transmission through periodic Si cylinder arrays on the dielectric substrates

Asymmetric optical transmission is widely used in various fields due to the manipulation of light direction. This work theoretically investigates highly efficient and polarization-independent asymmetric optical transmission in the broadband wavelength through the periodic Si cylinder arrays on the dielectric substrates. The simulated results show that the maximum asymmetric optical transmission can reach 0.894, with high forward transmission of 0.898 and nearly zero backward transmission of 0.004 at the wavelength of 540 nm. The structure also exhibits excellent light-trapping characteristics with low energy loss and anti-reflection. The formation of asymmetric optical transmission comes from the diffraction which contributes differently to the transmission under forward and backward illuminations. Furthermore, asymmetric optical transmission in the structure is insensitive to the refractive index of the dielectric substrate and the height of the Si cylinder, and the wavelength position of the maximum asymmetric optical transmission is only sensitive to the height of the Si cylinder. This work proposes a multifunctional nanostructure and provides guidelines for designing practical devices that utilize asymmetric optical transmission.

2D asymmetric diffraction grating controlled by vortex light in double-Λ-type atomic system

A two-dimensional (2D) asymmetric diffraction grating controlled by vortex light in a double-Λ-type atomic system is studied. Such an atomic system is driven by a weak traveling-wave probe field and a signal field, a position-dependent strong standing-wave (SW) control field, and a Laguerre–Gaussian (LG) vortex field. Due to the asymmetric properties of the LG vortex field, the probe photons can be asymmetrically diffracted into four different domains after passing through the atomic media. The Diffraction patterns and intensities of the 2D asymmetric diffraction grating can be manipulated by the detuning of the probe field, the interaction length, and the intensity of SW control field. In addition, the relative phase and the azimuth parameter which is closely related to the vortex light also can be used to regulate the asymmetric diffraction grating effectively. This work may provide useful reference for optical information processing, especially for the design of optical beam dividers with desired intensities and novel quantum devices requiring asymmetric optical transmission.

Asymmetric optical transmission through periodic metallic hemisphere arrays on the transparent substrates

Asymmetric optical transmission is useful in various fields such as electromagnetic shielding, photovoltaic devices, and optical diodes owing to its directional selectivity for light. This work theoretically investigates asymmetric optical transmission by the periodic metallic hemisphere arrays on the transparent substrates. It is found that the formation of asymmetric optical transmission can be attributed to two aspects: the difference in the driving electric field and the first-order diffraction of the arrays. The maximum asymmetric optical transmission results from the match between the resonance wavelength of the arrays and the cut-off wavelength of the first-order diffraction inside the substrates. The bandwidth of asymmetric optical transmission is determined by the cut-off wavelength of the first-order diffraction inside the substrates. In addition, the effects of the structural parameters, including the radius of the metallic hemisphere and the material of the transparent substrate, on the maximum asymmetric optical transmission and the bandwidth are studied. This work provides design guidelines for practical devices to apply asymmetric optical transmission.

Nonlinear photonic crystals for completely independent asymmetric holographic imaging.

Nonlinear photonic crystals (NPCs) are microstructures characterized by a spatially modulated second-order nonlinear coefficient that have been extensively used for the generation and beam-shaping of coherent light at new frequencies. NPCs for asymmetric optical transmission have a significant impact on novel and multifunction photonic devices. However, nonreciprocal NPCs capable of completely independent asymmetric holographic imaging for the opposite propagation directions have not been reported. Here, we propose a holographic combiner for a different independent image generation at the second-harmonic (SH) wavelength when illuminated from opposite sides of NPCs. The design of the holographic combiner is based on a 3D nonlinear detour phase holography and an orbital angular momentum (OAM) multiplexing nonlinear holography. This work achieves completely independent asymmetric holographic imaging at the SH frequency by using NPCs, which may have potential applications in classical and quantum optical devices.

Asymmetric transmission devices empowered by a cascaded structure of a dielectric metasurface-photonic crystal.

In this Letter, we theoretically propose an all-dielectric quasi-three-dimensional subwavelength structure constructed by a dielectric metasurface cascaded with a multilayer photonic crystal (PC) to achieve a high-performance asymmetric optical transmission (AOT). The desired optical control of the AOT is realized by combining the predetermined anomalous beam steering of a phase gradient metasurface with a unique bandgap as well as transmission characteristics of the multilayered stacked PC. The simulated results demonstrate that the proposed AOT device operating at the center wavelength of 633 nm with a circularly polarized state exhibits a high transmission of up to 62.4% with a contrast ratio exceeding 606. The excellent performance of AOT is achieved by making disassembled transverse magnetic and transverse electric polarized light under the same deflection angle concurrently match with respective high-efficient transmission bands in the multilayer PC. Furthermore, dependence of the performance of the proposed device on structural dimensions is also explored. Fortunately, the designed AOT structure is applicable to any linearly polarized light but is accompanied by double diffraction channels as compared to the circularly polarized light case. Owing to its planar configuration, passive operation, and compelling performance under various polarization states, the proposed strategy for achieving AOT paves a new road for realizing high-performance optical metadevices in compact optical systems.

Asymmetric wavefront shaping with nonreciprocal 3D nonlinear detour phase hologram.

Asymmetric control of light with nonlinear material is of great significance in the design of novel micro-photonic components, such as asymmetric imaging devices and nonreciprocal directional optical filters. However, the use of nonlinear photonic crystals for asymmetric optical transmission, to the best of our knowledge, is still an untouched area of research. Herein we propose the 3D nonlinear detour phase holography for realizing asymmetric SH wavefront shaping by taking advantage of the dependence of the SH phase on the propagation direction of the excitation beam. With the proposed method, the designed nonreciprocal 3D nonlinear detour phase hologram yields SH phases with opposite signs for the forward and backward transmission situations. Moreover, the quasi-phase-matching scheme and orbital angular momentum conservation in the asymmetric SH wavefront shaping process are also discussed. This study conceptually extends the 2D nonlinear detour phase holography into 3D space to build the nonreciprocal 3D nonlinear detour phase hologram for achieving SH twin-image elimination and asymmetric SH wavefront shaping, offering new possibilities for the design of nonreciprocal optical devices.

ASYMMETRY OF THE OPTICAL TRANSMISSION SPECTRA OF A PULSED DISCHARGE WITH AN EXTENDED HOLLOW CATHODE DURING AT RESONANCE INTERACTION OF SHORT POLYCHROMATIC LASER PULSES WITH EXCITED NEON ATOMS

The processes of resonant interaction of short pulses of polychromatic radiation of a dye laser with plasma of a nanosecond discharge in neon with an extended hollow cathode are experimentally studied. During the propagation of laser radiation inside the cathode cavity near the spectral absorption line of Ne I with a wavelength of 650.6 nm, the formation of asymmetric optical transmission spectra of the plasma of the dispersion type are detected, while during the propagation of laser radiation in the region between the electrodes, symmetric transmission spectra of the classical kind are formed.

Asymmetric optical transmission of a metallic grating-incorporated Fabry–Pérot cavity

In this report, we show a structure of metallic grating-incorporated Fabry–Pérot (F–P) cavity demonstrating asymmetric optical transmission (AOT) for all transmission modes. The asymmetric transmission basically arises from asymmetric coupling and decoupling of resonant surface plasmons (SPs) on both sides of the metallic grating with asymmetric media of different indices. Coupling of the SP mode with anti-resonance mode of the F–P cavity in orthogonal directions further optimizes transmission characteristics of the structure for asymmetric transmission. Particularly, a high contrast AOT is achieved by locating the SP resonance position at center of the anti-resonance band of the F–P cavity. Meanwhile, SP resonance modes induced at different interfaces/regions of the cavity structure influence the transmission properties. The AOT structure has no nanoapertures and critical requirements on profile of the grating surfaces for fabrication, and can be adapted to various optical systems.

High-Performance Asymmetric Optical Transmission Based on a Dielectric-Metal Metasurface.

Asymmetric optical transmission plays a key role in many optical systems. In this work, we propose and numerically demonstrate a dielectric–metal metasurface that can achieve high-performance asymmetric transmission for linearly polarized light in the near-infrared region. Most notably, it supports a forward transmittance peak (with a transmittance of 0.70) and a backward transmittance dip (with a transmittance of 0.07) at the same wavelength of 922 nm, which significantly enhances operation bandwidth and the contrast ratio between forward and backward transmittances. Mechanism analyses reveal that the forward transmittance peak is caused by the unidirectional excitation of surface plasmon polaritons and the first Kerker condition, whereas the backward transmittance dip is due to reflection from the metal film and a strong toroidal dipole response. Our work provides an alternative and simple way to obtain high-performance asymmetric transmission devices.

Dynamically adjustable and high-contrast asymmetric optical transmission based on bilateral compound metallic gratings

A dual-band asymmetric transmission (AT) device with high contrast ratio based on bilateral compound metallic gratings is proposed. The AT structure can be manufactured through connecting silver gratings on the outer surface and inner inverted T-shaped grating with an Ag film. Dual-band forward transmission arises from unidirectional surface plasmon polaritons (SPPs) excitation by the upper metallic grating when the modulated forward wave vector matches the wave vector of SPPs. The device can achieve a high transmittance of 0.67 and 0.82 with a contrast ratio of 0.997 and 0.99 at operation wavelengths 627 nm and 1238 nm, respectively. Since the SPPs coupling between two metallic structures can be used to suppress the forward transmission in specific frequency band, single-band AT function also can be realized. By adjusting those underlying parameters, the designed structure having AT capability at 624 nm and band-pass filter function at 1234 nm. The proposed structure is simple and feasible, and has great potential applications in the field of integrated optoelectronics.

Design of asymmetric transmission of photonic crystal heterostructure based on two-dimensional hexagonal boron nitride material

Two-dimensional (2D) hexagonal boron nitride (hBN) possesses many unique properties such as high mechanical strength and excellent chemical and thermal stability. The 2D hBN exhibits a wide bandgap in the UV region and optically-stable ultra-bright quantum emitters that make hBN a promising nanophotonic platform for quantum computing and information processing, especially in the visible wavelength range. Therefore, it is greatly important to build up different nanophotonic devices with different functionalities based on this material platform to achieve the integrated photonic chips. Among the devices, the integratable optical asymmetric transmission devices are important elements for functional quantum computing chips. Since hBN is a dielectric material, photonic crystal (PhC) structure is the most suitable in principle and allows on-chip integration with other photonic devices. In this study, we theoretically design an asymmetric transmission device based on 2D hBN PhC heterostructures in the visible wavelength range for the first time. Due to the relatively low refractive index of 2D hBN material (<i>n</i> < 2.4), we design a free-standing hBN PhC heterostructure to maximize the light trapping in the structure and minimize the propagation loss. The asymmetric transmission device is composed of two square-lattice 2D PhC structures, namely PhC 1 and PhC 2. We use the plane wave expansion method (PWM) to calculate the iso-frequency contours (EFCs) of the PhC structures to study the light propagation inside of the PhCs, which will propagate along the gradient of direction of the EFCs. We design the PhC structure in the way that the incident light beams from different angles can be self-collimated along the <i>Г-X</i> direction of the PhC 2 and coupled out. On the other hand, the backward incident light is blocked by the bandgaps of PhC 2. In this way, asymmetric optical transmission is achieved with high forward transmittance and contrast ratio. In addition, we further finely tune the structural parameters, including the lattice constant and column radius of the PhCs to optimize the performance by using the finite difference time domain (FDTD) method. The resulting 2D hBN PhC heterostructure achieves an asymmetric transmission in a wavelength range of 610–684 nm with a peak forward transmittance of 0.65 at a wavelength of 652 nm. Meanwhile, the backward transmittance is controlled to be 0.04. As a result, the contrast ratio can reach up to 0.95. The working bandwidth of the hBN PhC is 74 nm (<i>T</i><sub>F</sub> > 0.5). In addition, the designed asymmetric transmission device has a small size of 11 μm × 11 μm, thus it is suitable for on-chip integration. Our results open up possibilities for designing new nanophotonic devices based on 2D hBN material for quantum computing and information processing. The design principle can be generally used to design other photonic devices based on 2D hBN material.

Highly efficient asymmetric optical transmission based on the gradient metasurface and subwavelength grating

In this paper, an asymmetric transmission device by combining the gradient metasurface and the subwavelength grating is proposed and investigated numerically. The gradient metasurface produces a transmitted wave with a certain angle upon normal incidence. The subwavelength grating can block light under the condition of normal incidence, although it can transmit light with an angle realized by the gradient metasurface. All materials employed in the proposed device are dielectric. The asymmetric transmission device exhibits high working efficiency and possesses a high-contrast ratio up to 41 dB. Our results are beneficial in designing devices with high-performance asymmetric optical transmission.

Plasmonic analogue of geometric diodes realizing asymmetric optical transmission.

Geometric diodes represent a relatively new class of diodes used in rectennas that rely on the asymmetry of a conducting thin film. Here, we numerically investigate a plasmonic analogue of geometric diodes to realize nanoscale optical asymmetric transmission. The device operates based on spatial symmetry breaking that relies on a unique property of surface plasmon polaritons (SPPs), namely, adiabatic nanofocusing. We show that the structure can realize on-chip asymmetric electromagnetic transmission with a total dimension of ∼2µm×6µm. We demonstrate a signal contrast of 0.7 and an asymmetric optical transmission ratio of 4.77 dB. We investigate the origin of the asymmetric transmission and show that it is due mainly to asymmetric out-coupling of SPPs to far-field photons. We highlight the role of evanescent field coupling of SPPs in undermining the asymmetric transmission efficiency and show that by adjusting the plasmonic waveguide dimensions, a signal contrast of 0.94 and an asymmetric optical transmission ratio of 5.18 dB can be obtained. Our work presents a new paradigm for on-chip nanoscale asymmetric optical transmission utilizing the unique properties of SPPs.

Multi-dimensional manipulation of optical field withmetasurfaces and its optimization based on machine learning

Metasurfaces, as two-dimensional artificial subwavelength nanostructures, have shown novel optical phenomena and abilities of flexible and multi-dimensional optical field manipulation with a more integrated platform. Relative to the metasurfaces with single-dimensional manipulation, the metasurfaces with multi-dimensional manipulation of optical fields show significant advantages in various practical application areas, such as optical holograms, sub-diffraction imaging, generating vector optical fields, and so on. However, the design of metasurfaces with multi-dimensional manipulation of optical fields is more complex in most cases. Optimization based on machine learning can effectively lower the difficulty of the design and achieve more accurate multi-dimensional manipulation, which has attracted great interest in recent years. In this review, we firstly classified and discussed the recent advances of two-dimensional manipulation of optical fields with metasurfaces. Then, we further introduced the multi-dimensional manipulation method of optical fields based on machine learning. We first provide a following classification and review recent works related to two-dimensional manipulation of optical fields with metasurfaces. (1) Manipulating amplitude and phase of optical fields simultaneously. In scalar optics, the transmission of complete optical field information requires both amplitude and phase modulations, as the wave equation implies. Thus, the metasurfaces with both amplitude and phase manipulation can provide full optical field information and better performance in various areas, such as optical holograms, energy-tailorable metasurfaces, generating Airy beams and bidirectional perfect absorbers. (2) Manipulating phase and polarization of optical fields simultaneously. In vectorial optics, many intriguing phenomena occur which seem impossible in scalar optics. However, in conventional optics, complex optical systems including polarizers and curved mirrors are often required to achieve simultaneous manipulation of phase and polarization. Metasurfaces with both phase and polarization manipulation can be utilized to achieve various novel functionalities, such as generating vector beams, giant optical activity without chiral structures, chiral holograms, arbitrary spin-to-orbital angular momentum conversion of light and anisotropic coding metasurfaces, with an integrated optical system. (3) Manipulating amplitude and polarization of optical fields simultaneously. Utilizing the abundant interlayer effects provided by metasurfaces, one can tailor their reflection, transmission, and absorption properties of electromagnetic waves in different polarization states. Metasurfaces with both amplitude and polarization manipulation can be utilized to achieve various novel phenomena which are difficult to be realized in conventional optics, such as asymmetric transmission and giant circular dichroism. (4) Manipulating frequency and amplitude of optical fields simultaneously. In linear optical region, the unique frequency-dependent absorption and scattering properties of metasurfaces in the visible frequencies are utilized to print color at the nanoscale, which is known as structural colors. In nonlinear optical region, the exponential relationship between the intensities of the nonlinear optical signal and the fundamental frequency signal, many optical phenomena can be further amplified, such as circular dichroism. (5) Manipulating frequency and phase of optical fields simultaneously. In linear optical region, achromatic meta-lenses can be realized by utilizing the dispersion characteristics of different nano antennas to compensate for phase dispersion at different frequencies. In nonlinear optical region, metasurfaces with efficient phase modulation of nonlinear waves have received great attention in recent years for their great value in the realization of many novel nonlinear functionalities, such as multiplexed holograms, multiplexed coding meta-devices, and terahertz generators. (6) Multi-dimensional manipulation of on-chip optical field. With the integration with the waveguides, metasurfaces can also realize multi-dimensional manipulation of on-chip optical field. Metasurfaces consisting of phased nanoantennas can be used to control guided waves via strong optical scattering at subwavelength intervals. Different on-chip meta-devices have been demonstrated based on this design principle, such mode converters, spin-selective and wavelength-selective demultiplexing devices, polarization emitters, waveguide devices supporting asymmetric optical power transmission and phase-matching-free second harmonic generator. Then, we introduce the new multi-dimensional manipulation method of optical fields based on machine learning. The manipulation method based on machine learning can quickly obtain the optimal structural parameter solution in a huge parameter space, which improves the efficiency of parameter optimization exponentially. At the same time, machine learning can be used to design complex discrete structures, which breaks the period limitation of conventional devices. The optical field manipulation with metasurfaces based on machine learning has great application value in metalens, holographic imaging, coding metasurface and so on.

Photonic crystal heterostructure with self-collimation effect for broad-band asymmetric optical transmission

Recently, quantum computing and information processing based on photons has become one research frontier, attracting significant attentions. The optical asymmetric transmission devices (OATD), having similar function to the diode in electric circuitry, will find important applications. In particular, the OATDs based on nanophotonic structures are preferred due to their potential applications in the on-chip integration with other photonic devices. Therefore, there have been numerous applications of OATDs based on different nanostructures, including composite grating structures, metasurfaces, surface plasmon polaritons, metamaterials, photonic crystals (PhCs). However, in general, those designs show relatively low forward transmittance (< 0.5) and narrow working bandwidth (< 100 nm), and they are able to work with only one polarization state. This makes the current OATDs unsuitable for many applications. To solve this challenge, here we design a two-dimensional (2D) PhC heterostructure based on the self-collimating effect and bandgap properties. The PhC heterostructure is composed of two square lattice 2D PhCs (PhC 1 and PhC 2) on a silicon substrate with different lattice shapes and lattice constants. The PhC 1 is composed of periodically arranged silicon cylinders in air. Meanwhile, the PhC 2 is an square air hole array embedding in silicon. The two PhCs are integrated with an inclined interface with an angle of 45° with respect to the direction of incident light. The plane wave expansion method is used to calculate the band diagrams and equal frequency contours (EFCs) of the two PhCs. As the propagation directions of light waves in PhCs are determined by the gradient direction of the EFCs, we are able to control the light propagation by controlling the EFCs of PhCs. By engineering the EFCs, the PhC 2 shows strong self-collimation effect in a broad wavelength range with a central wavelength of 1550 nm for both TE and TM polarization. By self-collimating the forward incident light from different incident angles to couple to the output waveguide, we are able to significantly increase the forward transmittance to > 0.5 for both TE and TM polarized light. Meanwhile, the backward transmittance can be effectively cut off by the unique dispersion properties of the PhC heterostructures. In this way, the heterostructure is able to achieve polarization independent asymmetric transmission of light waves in a broad wavelength range. To visualize the light propagation in the PhC heterostructure, we use the finite-difference-time-domain method to calculate the electric intensity distributions of the forward and backward propagation light of both TE and TM polarization at a wavelength of 1550 nm. Strong self-collimation effect of forward propagation light and the nearly complete blockage of backward propagation light can be identified unambiguously in the intensity plots, confirming the theoretical analysis. The calculation of transmittance and contrast ratio spectra show that the asymmetric transmission wavelength bandwidth can reach 532 nm with the forward transmittance and contrast ratio being 0.693 and 0.946 at an optical communication wavelength of 1550 nm for TE polarized light. On the other hand, for the TM polarized light, the asymmetric transmission wavelength bandwidth is 128 nm, the forward transmittance and contrast ratio are 0.513 and 0.972, respectively, at 1550 nm wavelength. Thus, it is confirmed that the PhC heterostructure achieves highly efficient, broadband and polarization independent asymmetric transmission. Finally, to further improve the forward transmittance of the TE polarized light, we modulate the radius of the front row of photonic lattice of PhC 1 at the interface. It shows that the forward transmittance can be further improved to a record high value of 0.832 with a bandwidth of 562 nm for TE polarized light. Our design opens up new possibilities for designing OATDs based on PhCs, and will find broad applications, for the design can be realized by current nanofabrication techniques.

High-performance asymmetric optical transmission based on coupled complementary subwavelength gratings

Asymmetric transmission (AT) devices are fundamental elements for optical computing and information processing. We here propose an AT device consisting of a pair of coupled complementary subwavelength gratings. Different from previous works, asymmetric dielectric environment is employed for unidirectional excitation of surface plasmon polaritons (SPPs) and thus asymmetric optical transmission, and near-field coupling effect inherent in the coupled complementary structure is exploited to enhance forward transmission and AT behavior, and determine operation bandwidth as well. The influence of asymmetric dielectric environment, effect of vertical and lateral couplings, interactions of electric- and magnetic-dipole moments and the realization of Kerker conditions, are investigated in depth to unearth the AT mechanism and performance. High-performance AT with large forward transmittance of 0.96 and broad bandwidth of 174 nm is achieved at wavelength 1250 nm. Our work helps people to gain a better understanding of near-filed coupling effect in coupled complementary structures, expand their application fields, and it also offers an alternate way to high-performance AT devices.

Dual-band asymmetric optical transmission of both linearly and circularly polarized waves using bilayer coupled complementary chiral metasurface.

It is highly desirable to develop asymmetric transmission (AT) devices for both linearly and circularly polarized light. However, currently existing metamaterial-based AT devices require multi-step micro-nano fabrication processes and usually realize AT responses only for linearly or circularly polarized waves, not simultaneously for both. We here propose a dual-band AT device for both linearly and circularly polarized waves in the near-infrared region by using a bilayer coupled complementary chiral metasurface, which includes a half-gammadion-shape gold (Au) structural layer and its Babinet's complimentary copy. Unlike other multilayer AT devices working at optical frequencies, it takes less micro-nano fabrication steps. Besides, with the help of chirality and the inherent near-field coupling effect between the two complementary Au layers, the maximal AT parameters for linearly and circularly polarized waves can reach up to 0.45 and 0.56, respectively. The underlying mechanisms of dual-band AT responses are also investigated in depth from the perspectives of chirality and coupling effect. Our work offers a new and simple approach to high-performance AT devices, helps to better understand near-filed coupling effect in coupled complementary metasurfaces, and also expands their application fields.

Highly efficient asymmetric optical transmission by unbalanced excitation of surface evanescent waves in a single—layer dielectric gradient metasurface

Asymmetric optical transmission has recently been demonstrated using Lorentz‒reciprocal devices with patterned photonic structures of linear materials by breaking spatial inversion symmetry. However, it is a huge challenge to fabricate such devices due to their sophisticated multilayer configurations. In this letter, we show that a simple single‒layer dielectric gradient metasurface (GM) enables high-efficiency asymmetric optical transmission under the oblique incidence. The underlying mechanism is ascribed to the unbalanced excitation of surface evanescent waves for forward and backward light. Our results provide strategies for creating compact, integrated and high performance asymmetric optical transmission devices.

Asymmetric optical transmission through silver film with a hyperbolic air hole

Asymmetric optical transmission is one of the most important functionalities in modern optical communication systems. However, it has been impeded due to the low transmission ratio of forward transmittance to the backward transmission ratio. This paper is a continuation of the paper by Peng et al. [Opt. Express22, 17546 (2014)OPEXFF1094-408710.1364/OE.22.017546]. It was found that the guided mode reflects remarkably on the bottom of a conical air hole. A guided mode reaching the bottom of a conical air hole is the major factor leading to the asymmetric optical transmission effect. It reminded us that the asymmetric optical transmission effect of a structure having a much steeper inner wall would be better. Thus, we proposed silver film with a hyperbolic air hole (HAH), of which the transmission ratio can be as high as 17.5 at a proper wavelength. Through analysis, as we expected, the mechanism is the steep inner wall of the silver film HAH really can inhibit guided-mode reflection.