Metal–Graphene Hybrid Chiral Metamaterials for Tunable Circular Dichroism

In the field of optoelectronics, circular dichroism (CD) has caused great research interest because it is widely used in imaging and biosensing. A new method for dynamically controlling terahertz (THz) CD in metamaterials is proposed. By introducing chirality and graphene to metamaterials, a pair of chiral structures with completely opposite responses to left-handed circularly polarized (LCP) waves and right-handed circularly polarized (RCP) waves are designed. The influencing factors of CD are explored, including the gap of the structure, the linewidth of graphene, and the Fermi level of graphene. The largest CD (ΔR) is 77%. The CD can be actively modulated in a modulation range of 39–77% and the modulation depth is up to 38%. In addition, two-channel and four-channel chiral metasurfaces for near-field imaging are designed in this way. Good imaging effects and on (“1”) or off (“0”) effects of the multichannel metasurface are demonstrated. This work provides new ideas for the design of tunable metasurfaces and promotes the application of metasurfaces in THz dynamic imaging.     

On chemiluminescent emission from an infiltrated chiral sculptured thin film

The theory describing the far-field emission from a dipole source embedded inside a chiral sculptured thin film (CSTF), based on a spectral Green function formalism, was further developed to allow for infiltration of the void regions of the CSTF by a fluid. In doing so, the extended Bruggeman homogenization formalism — which accommodates constituent particles that are small compared to wavelength but not vanishingly small — was used to estimate the relative permittivity parameters of the infiltrated CSTF. For a numerical example, we found that left circularly polarized (LCP) light was preferentially emitted through one face of the CSTF while right circularly polarized (RCP) light was preferentially emitted through the opposite face, at wavelengths within the Bragg regime. The centre wavelength for the preferential emission of LCP/RCP light was red shifted as the refractive index of the infiltrating fluid increased from unity, and this red shift was accentuated when the size of the constituent particles in our homogenization model was increased. Also, the bandwidth of the preferential LCP/RCP emission regime decreased as the refractive index of the infiltrating fluid increased from unity.     

Optical Chirality from Dark‐Field Illumination of Planar Plasmonic Nanostructures

Dark‐field illumination is shown to make planar chiral nanoparticle arrangements exhibit circular dichroism in extinction, analogous to true chiral scatterers. Single oligomers, consisting rotationally symmetric arrangements of gold nanorods, are experimentally observed to exhibit circular dichrosim at their maximum scattering with strong agreement to numerical simulation. A dipole model is developed to show that this effect is caused by a difference in the projection of a nanorod onto the handed orientation of electric fields created by a circularly polarized dark‐field normally incident on a glass‐air interface. Owing to this geometric origin, the wavelength of the peak chiral response is experimentally shown to shift depending on the separation between nanoparticles. All presented oligomers have physical dimensions less than the operating wavelength, and the applicable extension to closely packed planar arrays of oligomers is demonstrated to amplify the magnitude of circular dichroism. This realization of strong chirality in these oligomers demonstrates a new path to engineer optical chirality from planar devices using dark‐field illumination.     

双层开口环的圆二色性数值研究

基于琼斯矩阵的推导提出一种金属手性纳米结构-双层开口环阵列结构，其开口方向扭转一定角度以破坏对称性，并应用有限元方法研究圆二色性（CD）特性.研究表明：在左旋圆偏振光与右旋圆偏振光的激发下，该结构表现出巨大差异使得CD产生，其CD值达到0.34.结构的电流分布表明，在短波长处，上下层圆环的等效偶极子耦合可形成反键模式，在长波长处，上下层圆环的等效偶极子耦合可形成成键模式，研究结构参数对CD的影响.理解圆二色的物理机制，并提供一种设计手性结构的方法.     

Circularly polarized fluorescence from light-emitting microcavities with sculptured-thin-film chiral reflectors

A light-emitting microcavity with sculptured-thin-film (STF) chiral reflectors is fabricated to produce narrowband circularly polarized (CP) light. The device is composed of a layer of fluorescent molecules (Alq3) sandwiched between two STF chiral reflectors to form a resonant microcavity with CP-selectivity, which emits narrowband left CP (LCP) and right CP (RCP) light upon photoexcitation. Our results also indicate the possibility of developing STF reflector-based diode laser devices delivering pure and controllable circular polarization.     

Chiral mirror and optical resonator designs for circularly polarized light: suppression of cross-polarized reflectances and transmittances

A left-handed chiral sculptured thin film (STF) that reflects strongly at the wavelength of the circular Bragg resonance tends to partially convert the handedness of incident LCP (left-circularly-polarized) light to RCP (right-circularly-polarized). We show that the cross-polarized component of the reflected RCP beam can be eliminated by interference with an additional RCP beam that is reflected at the interface of an isotropic cover and an AR (antireflecting) layer. For best results the refractive index and thickness of the AR layer need to accommodate a phase change on reflection that occurs at the chiral film. Effective suppression of the reflectances R_RR, R_RL, R_LR and the transmittances T_RL, T_LR can be achieved by sandwiching the chiral reflector between such amplitude and phase-matched AR coatings. Co-polarized chiral reflectors of this type may form efficient handed optical resonators. For LCP light the optical properties of such a handed resonator are formally the same as the properties of the isotropic passive or active Fabry–Perot resonators, but the handed resonator is transparent to RCP light.     

Terahertz radiation driven chiral edge currents in graphene

We observe photocurrents induced in single-layer graphene samples by illumination of the graphene edges with circularly polarized terahertz radiation at normal incidence. The photocurrent flows along the sample edges and forms a vortex. Its winding direction reverses by switching the light helicity from left to right handed. We demonstrate that the photocurrent stems from the sample edges, which reduce the spatial symmetry and result in an asymmetric scattering of carriers driven by the radiation electric field. The developed theory based on Boltzmann's kinetic equation is in a good agreement with the experiment. We show that the edge photocurrents can be applied for determination of the conductivity type and the momentum scattering time of the charge carriers in the graphene edge vicinity.     

基于FPGA的四频陀螺信号解调系统的设计与仿真

为了简化四频差动激光陀螺的信号读出系统,采用了一种基于FPGA实现的数字电路解调方案取代传统的合光棱镜中的1／4波片和偏振片来分离左、右旋陀螺信号。简述了四频差动激光陀螺的信号读出原理及解调电路方案,设计了主要由A／D转换器和FPGA构成的四频陀螺信号读出系统。通过Quartus Ⅱ等EDA工具在软件上对系统予以了实现,并针对其主要功能模块进行了仿真验证。仿真结果表明,将四频陀螺信号转化为数字信号后,利用FPGA实现信号解调并最终达到分离左、右旋陀螺信号的方案是可行的,该方案将有助于全数字化激光陀螺的研制。     

Broken enantiomeric symmetry for electromagnetic waves interacting with planar chiral nanostructures

Simulations of Maxwell’s equations for electromagnetic waves interacting with planar chiral structures are shown to depend on the polarization state of the exciting light field. These results illustrate generic features of light interaction with planar chiral structures and imply broken enantiomeric symmetry for excitation with circularly polarized light. PACS 78.67.-n; 11.30.-j     

Bispectral Circular Dichroic Coding Metasurfaces

The frequency-dependent circular dichroism is proposed to extend the wave manipulating capability of coding metasurfaces. As a proof of concept, the bispectral circular dichroic coding metasurfaces (CDCMs) are realized using circular dichroism resonators (CDRs) implemented via introducing loss resistors into the circular polarization conversion resonators. The CDRs are distinguished into left-handed CDRs and right-handed CDRs. Left-handed CDRs absorb left-handed circularly polarized (LCP) wave and convert right-handed circularly polarized (RCP) wave into LCP wave. Conversely, they are defined as right-handed CDRs. Two bispectral CDRs are designed with the left-handed (right-handed) and right-handed (left-handed) working bands in 7–8.5 and 22.2–22.5 GHz, respectively. And then, 1 bit bispectral CDCM with 0101…/1010… coding sequence is designed. Simulated results indicate that the designed CDCM strongly absorb the incident LCP (RCP) waves in the frequency region 7–8.5 GHz (22.2–22.5 GHz), but the incident RCP (LCP) waves are anomalously reflected into four beams of the LCP (RCP) waves with high efficiency. The measured results agree well with the simulations. The results in this work may provide an effective solution for the inverse and helicity-dependent manipulation of the electromagnetic waves in two distinct frequency regions.     

Self-consistent solutions for single,intense beam polariton with linear polarization

New modes of propagation for a single, intense beam of light have been theoretically studied in the frequency region of giant two photon absorption (GTA) to excitonic molecule (EM) states. Self-consistent solutions for left and right circularly polarized (LCP and RCP respectively) components have been obtained for the polariton dispersion equations which include the ω- and k-dependent self-energy term for exciton due to the GTA process to EM. There are two types of solutions: [I] k(LCP) = k(RCP), [II] k(LCP) ≠ k(RCP). The induced branch for the former has a half of EM mass. There are experimental observations which support the existence of such new propagation modes.     

Strong chirality in twisted bilayer α-MoO3

Chiral structures are promising in many applications, such as biological sensing and analytical chemistry, and have been extensively explored. In this paper, we theoretically investigate the chiral response of twisted bilayer α-MoO₃. Firstly, the analytical formula for the transmissivity is derived when the structure is illuminated with circularly polarized plane waves. Furthermore, the results demonstrate that the twisted bilayer α-MoO₃ can excite the strong chirality with the maximum circular dichroism (CD) of 0.89. In this case, the chirality is due to the simultaneous breaking the rotational symmetry and mirror symmetry, which originates from the relative rotation of two α-MoO₃ layers. To better understand the physical mechanism, the polarization conversion between the left-hand circular polarization (LCP) and right-hand circular polarization (RCP) waves is discussed as well. Moreover, it is found that the structure can maintain the strong chirality (CD> 0.8) when the twisted angle varies from 69° to 80°, which effectively reduces the strictness in the requirement for rotation angle. In addition, the CD can be larger than 0.85 when the incidence angle of circularly polarized plane wave is less than 40°, implying that the chirality is robust against the angle of incidence. Our work not only provides an insight into chirality induced by the twisted bilayer α-MoO₃, but also looks forward to applications in biological sensing.     

Controllable and switchable chiral near-fields in symmetric graphene metasurfaces

A strong chiral near-field plays significant roles in the detection, separation and sensing of chiral molecules. In this paper, a simple and symmetric metasurface is proposed to generate strong chiral near-fields with both circularly polarized light and linearly polarized light illuminations in the mid-infrared region. Owing to the near-field interaction between plasmonic resonant modes of two nanosheets excited by circularly polarized light, there is a strong single-handed chiral near-field in the gap between the two graphene nanosheets and the maximum enhancement of the optical chirality could reach two orders of magnitude. As expected, the intensity and the response wavelength of the chiral near-fields could be controlled by the Fermi level and geometrical parameters of the graphene nanosheets, as well as the permittivity of the substrate. Meanwhile, based on the interaction between the incident field and scattered field, the one-handed chiral near-field in the gap also could be generated by the linearly polarized light excitation. For the two cases, the handedness of the chiral near-field could be switched by the polarized direction of the incident light. These results have potential opportunities for applications in molecular detection and sensing.     

Reflected and transmitted powers from a planar isotropic chiral–uniaxial anisotropic chiral interface

In this paper, analysis and theoretical investigations are carried out to find the reflected and transmitted powers for a planar isotropic chiral–uniaxially anisotropic chiral interface with optical axis parallel to interface. The expressions of the reflected and transmitted powers for right-circularly polarized (RCP) and left-circularly polarized (LCP) incident waves are derived. Results of the reflected and transmitted powers for the RCP and LCP incident waves from materials with different isotropic chirality and uniaxial chirality values are presented. The effect of the uniaxial chirality on three other cases of dielectric constant combinations is also studied. The numerical results derived from the presented analytical expressions are found to be in good agreement with those obtained by computational techniques elsewhere.     

Scattering of electromagnetic wave by perfect electromagnetic conductor (PEMC) sphere placed in chiral media

A theoretical investigation has been carried out for electromagnetic waves scattering from a perfect electromagnetic conductor (PEMC) sphere which is placed in chiral media. The formulation of the problem is realized by expanding the incident as well as the scattered electromagnetic fields in terms of left circularly polarized (LCP) and right circularly polarized (RCP) waves. By applying the boundary conditions at the chiral-PEMC interface, eight simultaneous equations are generated, which yield the scattering coefficients associated with the Left and Right electromagnetic waves. The relative contribution of Co-polarized and Cross-polarized components of fields to the calculations of scattering cross-section is presented. The effect of admittance parameter and the effect of chirality parameter in cases of lossless, lossy permittivity and lossy permeability on Co and Cross components of scattering cross sections are observed. The results are also compared with available published literatures which are in good agreement.     

Optical properties of a three-dimensional chiral metamaterial

A three-dimensional chiral metamaterial with four-fold rotational symmetry is designed, and its optical properties are investigated by numerical simulations. The results show that this chiral metamaterial has the following features: high polarization conversion, perfect circular dichroism, and asymmetric transmission of circularly polarized light. A comparison of the results of chiral metamaterials without and with weak coupling between the constituent nanostructures enables us to confirm that the optical properties of our proposed nanostructure are closely related to the coupling between the single nanoparticles. This means that the coupling between nanoparticles can enhance the polarization conversion, circular dichroism, and asymmetric transmission. Due to the excellent optical properties, our metamaterial might have potential applications in the development of future multi-functional optical devices.     

基于电子信号处理的四频激光陀螺信号读出研究

针对当前四频差动激光陀螺(DLG)读出系统结构复杂、易受温度影响等问题,对现有读出系统进行了改进,设计了使用电子信号处理代替光学信号处理来分离左、右旋陀螺信号的新型读出系统。通过对一个DLG同时使用新旧两套不同的读出系统进行测试,得到了相同的测试结果,表明电子信号处理和光学信号处理在分离左、右旋陀螺信号方面是等效的。在此基础上,首先使用模数转换器对DLG的光电转换信号进行采样,然后进行了数字信号处理,并研制出了全数字化读出系统。新型读出系统不使用四分之一波片和偏振片等光学元件,具有温度稳定性好、结构简单、全数字化等优点,有利于四频陀螺性能的提高和批量生产。     

Simultaneous acquisition of all four forms of circular polarization Raman optical activity: results for α‐pinene and lysozyme

A new circularly polarized (CP) Raman spectrometer is described that demonstrates simultaneous acquisition of all four forms of circular polarization Raman optical activity (ROA). The instrument is a design extension of a commercially available back scattering circular polarization (SCP) ROA spectrometer. Circular polarization of the incident beam is introduced with a quarter‐wave plate, and a half‐wave plate alternately positioned in and out of the beam controls the modulation between right circular polarization (RCP) or left circular polarization (LCP) states. Combining this modulation with the simultaneous detection of RCP and LCP scattered Raman radiation allows the measurement of incident circular polarization (ICP), SCP, in‐phase dual circular polarization(DCP_I) and out‐of‐phase DCP_II‐ROA. In addition, three different forms of backscattered Raman spectra, namely unpolarized, highly polarized, and depolarized Raman spectra, as well as a degree of circularity spectrum are obtained. The performance of the new all‐CP ROA spectrometer is evaluated with neat α‐pinene and aqueous hen lysozyme solution. Copyright © 2011 John Wiley & Sons, Ltd.     

Circularly polarized light emission from semiconductor planar chiral nanostructures

We demonstrate circularly polarized light emission from InAs quantum dots embedded in the waveguide region of a GaAs-based chiral nanostructure. The observed phenomenon originates due to a strong imbalance between left- and right-circularly polarized components of the vacuum field and results in a degree of polarization as high as 26% at room temperature. A strong circular anisotropy of the vacuum field modes inside the chiral nanostructure is visualized using numerical simulation. The results of the simulation agree well with experimental results.     

Faraday rotations,ellipticity, and circular dichroism in magneto-optical spectrum of moiré superlattices

We study the magneto-optical conductivity of a number of van der Waals heterostructures, namely, twisted bilayer graphene, AB-AB and AB-BA stacked twisted double bilayer graphene and monolayer graphene and AB-stacked bilayer graphene on hexagonal boron nitride. As the magnetic field increases, the absorption spectrum exhibits a self-similar recursive pattern reflecting the fractal nature of the energy spectrum. Whilst twisted bilayer graphene displays only weak circular dichroism, the other four structures display strong circular dichroism with monolayer graphene and AB-stacked bilayer graphene on hexagonal boron nitride being particularly pronounced owing to strong inversion symmetry breaking properties of the hexagonal boron nitride layer. As the left and right circularly polarized light interact with these structures differently, plane-polarized incident light undergoes a Faraday rotation and gains an ellipticity when transmitted. The size of the respective angles is on the order of a degree.