Incident circularly polarized Raman optical activity spectrometer based on circularity conversion method

A simple incident circular polarization Raman optical activity (ICP ROA) spectrometer was constructed by applying the method of circularity conversion. The circular polarization of the incident laser light was modulated between right and left by the insertion of a half‐wave plate and not by using a Pockels cell which is usually used in ICP ROA instruments. On the basis of the concept of the virtual enantiomer (Hug, W. Applied Spectroscopy, 2003, 57, 1), circularity converters were inserted in the optical train, which could effectively compensate the systematic offset. The new instrument successfully attained photon shot‐noise‐limited conditions for all bands except for the very strongly polarized Raman band. The ROA spectra of some standard chiral samples were measured to demonstrate the performance of the spectrometer. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

A novel Raman optical activity instrument operating in the deep ultraviolet spectral region

Raman optical activity (ROA) has been exclusively observed in the visible (VIS) and near‐infrared (NIR) spectral regions to date. During the last few years, we have designed, constructed and tested the first ROA instrument, operating in the deep‐ultraviolet (DUV) spectral region employing 244‐nm excitation. This novel DUV ROA instrument is based on a backscattering geometry and incident circular polarization modulation (ICP); it makes use of a fast DUV imaging lens‐based spectrograph and specially designed DUV grade polarization optics. The performance of this instrument has been evaluated by analysing measured non‐resonant DUV ROA spectra of non‐absorbing enantiomeric liquid samples and by comparing these with corresponding ROA spectra recorded in the visible spectral region. Copyright © 2015 John Wiley & Sons, Ltd.     

Switchable vortex beam polarization state terahertz multi-layer metasurface

We propose a switchable vortex beam polarization state terahertz multi-layer metasurface, which consists of three-layer elliptical metal crosses, four-layer dielectrics, and two-layer hollow metal circles, which are alternately superimposed. Under the normal incidence of left-handed circularly polarized (LCP) wave and the right-handed circularly polarized (RCP) waves, the proposed structure realizes three independent control functions, i.e., focused and vortex beam, vortex beam with different topological charges, and polarization states switching, and azimuth switching of two vortex beams with different polarization states. The results show that the proposed metasurface provides a new idea for investigating the multifunctional terahertz wave modulation devices.     

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.     

La_(1-x)Ca_xMnO_3薄膜的Raman光谱

测量了１３Ｋ到室温下外延生长的Ｌａ１－ｘＣａｘＭｎＯ３薄膜的Ｒａｍａｎ光谱，发现当冷却样品穿过磁有序温度时，纯的和掺Ｃａ２＋的样品的一些Ｒａｍａｎ模式发生红移，而散射强度在转变点附近形成极大值。为了检查散射张量的对称性，进行了左旋和右旋入射光偏振实验，发现当温度降低时，两种入射偏振下散射强度差明显增大，这表明散射过程中有磁激发涉入。这些事实提出了这类材料中单声子－单磁振子散射机制的可能     

分振幅型全Stokes同时偏振成像系统波片相位延迟误差分析

分振幅型全Stokes同时偏振成像仪具有实时性好、空间分辨率高、精度高等优点,有很高的应用价值.分振幅型全Stokes同时偏振成像系统利用偏振分束器、1/2波片和1/4波片将入射光Stokes矢量调制在4幅图像中,可解析入射光Stokes矢量. 1/2波片和1/4波片的相位延迟误差对Stokes矢量测量精度有着不可忽略的影响.建立了包含上述两种误差的Stokes矢量测量误差方程,分析了1/2波片和1/4波片相位延迟耦合误差对自然光、0°/45°线偏光、左旋圆偏光等典型基态入射光的Stokes矢量测量误差的影响,推导了任意偏振态的Stokes矢量测量误差的表征方法.在邦加球球面和球内选取不同偏振度的Stokes矢量作为入射光进行仿真.结果表明, Stokes矢量测量误差和偏振度测量误差均随着入射光偏振度的增大而增大.选取入射光偏振度为1时的偏振测量精度评估系统.为满足2%的偏振测量精度, 1/2波片相位延迟误差应在±1.6°内, 1/4波片相位延迟误差应在±0.5°内.这对提高系统的偏振测量精度具有重要意义,为系统设计和研制提供了重要的理论指导.     

Frequency Reconfigurable and Multifunctional Metastructure Regulated by Nematic Liquid Crystal: Broadband Circular to Linear Polarization Converter

With the tunability of the nematic liquid crystal (NLC), a broadband frequency reconfigurable and versatile metastructure (MS) is proposed and theoretically investigated in this paper, combining circular-to-linear (CTL) polarization conversion (PC) and circular-to-circular (CTC) PC simultaneously. The MS is composed of two via-coupled modules, which can respond differently to the incident waves. Each module is connected utilizing a metal via a column, thus exceedingly enhancing the energy transmission and reducing the loss when transmitting. When the applied bias voltage (V_bias) is 0 V, the NCL molecules follow the initial orientation. The MS converts the incident right circular polarized (RCP) waves into linear polarized (LP) waves within 8.11–9.95 gigahertz (GHz) with a relative bandwidth of 20.38% and achieves the PC of left circular polarized (LCP) into RCP waves. As the V_bias reaches 20 V, the original operating interval reconfigures and shifts overall toward a lower frequency. The bandwidth of CTL is 7.66–9.02 GHz, and the CTC PC is broadened to 20.20%. Meanwhile, the structure justification is verified, and the inducing mechanism of PC is expounded. Possessing the merits of versatile collaborative processing and wide operating bands, such an MS is promising to be a polarization-controlled application candidate and enrich multifunctional designs.     

Polarization conversions of diffractive wave plates based on orthogonal circular-polarization holography

A polarization holographic grating, which integrates the functions of a grating and a wave plate and is called a diffractive wave plate, is recorded by two beams(left and right circularly polarized) of a 532 nm laser in an azo polymer with a liquid-crystal structure. The polarization conversion characteristics of the diffractive wave plates are investigated with a detecting light of 650 nm by metering the polarization state of first-order diffracted light.It is confirmed that the diffractive wave plates convert the incident linear polarization into circular polarization for a linearly polarized probe laser and reverse the sense of rotation of the circular polarization when the detecting light is circularly polarized light.     

Observation of resonance electronic and non‐resonance‐enhanced vibrational natural Raman optical activity

Natural resonance electronic Raman optical activity (ROA) is observed for the first time. Coincidently, the first example of vibrational ROA enhanced by low‐lying electronic transition is reported. These new phenomena were measured using the rare‐earth complex Eu(tfc)₃ (+)‐tris[3‐trifluoroacetyl‐D ‐camphorato]europium(III), where electronic resonance occurs between the 532‐nm laser excitation and the ⁷F₁ → ⁵D₁ transition of the Eu³⁺ metal center. Electronic Raman spectra involve the Raman transitions terminating on the low‐lying electronic states of Eu(tfc)₃. The observed vibrational ROA spectra are enhanced relative to typical ROA spectra by the proximity of vibrational states of Eu(tfc)₃ to its low‐lying electronic states with significant magnetic‐dipole character, whereas the parent vibrational Raman spectra do not appear to be resonance‐enhanced since the 532‐nm vibrational Raman spectrum has similar relative intensities to the corresponding Raman spectrum measured with 1064‐nm laser excitation. Copyright © 2010 John Wiley & Sons, Ltd.     

磁化分层等离子体中电磁波传播特性研究

采用混合矩阵法,分析了磁化分层等离子鞘套对斜入射电磁波传播特性的影响,分别计算了不同入射角以及外加磁场下电磁波透射系数和极化特性的变化。以GPS导航信号右旋圆极化波(RCP)为例,研究了磁场、电子密度对电磁波右旋圆极化特性的影响。结果表明,外加磁场能够使右旋圆极化波在等离子体中的阻带向高频方向移动,此外,外加磁场能在一定程度上改善斜入射是圆极化波的极化特性,有利于GPS信号接收。     

Measurement and calculation of the Q‐branch spectrum of nitrogen using inverse Raman spectroscopy and cw Raman‐induced polarization spectroscopy

The techniques of inverse Raman spectroscopy, Raman‐induced polarization spectroscopy (RIPS), and optical heterodyne RIPS (OHD‐RIPS) are compared by probing the Q‐branch of the nitrogen molecule. The signal is measured employing either a photomultiplier tube (low background level–RIPS) or a photodetector (high background level–IRS and OHD‐RIPS). The measurements are performed using atmospheric mixtures of N₂ Ar with concentrations varying from 0 to 79% N₂. This strategy permits estimation of detection limits using the different techniques. Pump and probe energy levels are varied independently to study signal dependence on laser irradiance. A theoretical treatment is presented on the basis of the Raman susceptibility equations, which permits the calculation of spectra for all three techniques. Calculated Q‐branch spectra are compared with the measured spectra for the interactions of a linearly polarized probe beam with a linearly or circularly polarized pump beam. The polarizer angle in the detection path for OHD‐RIPS has a dramatic effect on the shape of the spectrum. The calculated and experimental OHD‐RIPS spectra are in good agreement over the entire range of investigated polarizer angles. Detection limits using these techniques are analyzed to suggest their applicability for measuring other species of importance in combustion and plasma systems. Copyright © 2007 John Wiley & Sons, Ltd.     

Polarization‐sensitive CARS spectroscopy on free‐base porphyrins: coproporphyrin I tetramethyl ester

The application of polarization‐sensitive (PS) coherent anti‐Stokes Raman scattering (CARS) spectroscopy for the investigation of highly luminescent free‐base porphyrins under Q_x band resonance is discussed. For coproporphyrin I tetramethyl ester (CP‐I‐TME), PS CARS spectra involving resonances with the electronic Q_x absorption band as well as polarized spontaneous Raman spectra involving B band resonance are presented. A quantitative evaluation of the CP‐I‐TME spectra is performed and the results are compared to our previously presented data on free‐base octaethylporphine (OEP) and mesoporphyrin IX dimethyl ester (MP‐IX‐DME), which were obtained under identical excitation conditions. This comprehensive analysis reveals several spectral differences that can be attributed to the different β–substitution pattern of the porphyrin macrocycle. Additionally, the strong resonance enhancement of totally symmetric modes under Q_x band excitation is identified as a common property for OEP, CP‐I‐TME, and MP‐IX‐DME; this enhancement selectivity distinguishes the investigated substances from what is generally observed for metallo porphyrins. Copyright © 2008 John Wiley & Sons, Ltd.     

Compensated Polarization Scrambler for Raman Depolarization and Intensity Measurements on Microsamples

It has long been recognized ^1,2,3,4,5 in measuring relative intensities and depolarization ratios of Raman lines that prism and grating spectrometers have different transmittances for light polarized parallel (T_∥) and perpendicular (T_⊥) to the slit. Several methods have been used to give equal instrument response to parallel and perpendicular polarization. Stitt and Yost³ used a Nicol prism parallel to the high-transmittance direction of the spectrometer, combined with a rotatable mica half-wave plate to transmit either vector to the Nicol prism. This method has the advantage of having a higher spectrometer transmittance, but the mica plate gives exact half-wave retardation at only a single wavelength. A mica quarter-wave plate ^4,5 averages the transmittances of the spectrometer for the two polarization directions, but is also wavelength dependent. Virtually achromatic circular polarizers have been designed and constructed ^6,7, but these devices are too bulky and expensive for the clear apertures needed for fast spectrometers.     

Polarized Raman scattering in helimagnetic β‐MnO2

A rutile β‐MnO₂ film was grown on MgO substrate using plasma‐assisted molecular beam epitaxy (PAMBE) monitored by reflection high‐energy electron diffraction (RHEED). Polarized Raman spectra at various temperatures were obtained to investigate the influence of the helimagnetic structure on the vibrational modes of β‐MnO₂. A red shift of E_g modes indicates a gradual formation of spin angles between neighboring Mn⁴⁺ ions. The intensities of the E_g and A_1g modes with y‐polarized incidence increase remarkably below the Néel temperature. A new view as vibrational mode projection (VMP) indicates the interactions between the magnetic component of incident light and the helimagnetic structure. Copyright © 2008 John Wiley & Sons, Ltd.     

基于极化旋转超表面的圆极化天线设计

本文通过设计出一种反射型极化旋转超表面,在8—12 GHz频域内实现高效的极化旋转,并将其加载于微带缝隙天线下方构成新型的极化旋转超表面天线,利用超表面的90°极化旋转效应,成功实现了天线的圆极化辐射调制.仿真与实验结果表明:圆极化天线的中心工作频率为GHz,阻抗带宽为8.3—10 GHz.当微带缝隙天线与极化旋转超表面的间距H=4.5mm时,天线在8.3—8.8 GHz频带内实现了圆极化辐射;当mm时,天线在8.8—9.3 GHz频带内实现了圆极化辐射;当=8mm时,天线在9.3—10 GHz频带内实现了圆极化辐射.实验结果与仿真结果相符,证明了此种设计方法的有效性,也为微带缝隙天线的圆极化设计提供了一种新的途径.     

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.     

Optical properties of polarization-dependent geometric phase elements with partially polarized light

The behavior of geometric phase elements illuminated with partially polarized monochromatic beams is investigated both theoretically and experimentally. The element discussed in this paper is composed of wave plates with π-retardation and a space-variant orientation angle. We found that a beam emerging from such an element comprises two polarization orders; right-and left-handed circularly polarized states with conjugate geometric phase modification. This phase equals twice the orientation angle of the space-variant wave plate comprising the element. Apart from the two polarization orders, the emerging beam coherence polarization matrix includes a “vectorial interference matrix” which contains information concerning the correlation between the two orthogonal, circularly polarized portions of the incident beam. In this paper we measure this correlation by a simple interference experiment. In addition, we found that the equivalent mutual intensity of the emerging beam is modulated according to the geometric phase induced by the element. Other interesting phenomena concerning propagation will be discussed theoretically and demonstrated experimentally. The experiment made use of a spherical geometric phase element that was realized by use of a space-variant subwavelength grating illuminated with CO₂ laser radiation of 10.6 μm wavelength.     

The aqueous Raman optical activity spectra of 4(R)‐hydroxyproline: theory and experiment

Vibrational Raman optical activity (ROA) spectra have been measured for aqueous solutions of 4(R)‐hydroxyproline at three different pH values and are compared with theoretical results calculated for several conformations of anionic, cationic and zwitterionic 4(R)‐hydroxyproline using density functional theory (DFT) and the polarizable continuum model (PCM). The experimental ROA bands have been ascribed to the normal modes by comparison of the experimental and calculated vibrational frequencies and ROA intensities. Overall, using PCM for geometry optimization and force field calculations gives simulated Raman and ROA spectra that agree with the main features of the experimental spectra, whereas using PCM also in the calculations of optical tensors seems more problematic. Copyright © 2010 John Wiley & Sons, Ltd.     

Raman optical activity comes of age

The theory and applications of Raman optical activity (ROA), which measures vibrational optical activity by means of a small difference in the intensity of Raman scattering from chiral molecules in right- and left-circularly polarized incident light or, equivalently, a small circularly polarized component in the scattered light, are briefly reviewed. Thanks to new developments in instrumentation, ROA may be applied to a wide range of chiral molecular species. As well as providing the absolute configuration of small chiral molecules, the application of ab initio methods to the analysis of experimental ROA spectra holds great promise for the determination of the three-dimensional structure and conformational distribution in unprecedented detail. The many structure-sensitive bands in the ROA spectra of aqueous solutions of biomolecules provide detailed structural information including, in the case of proteins, the tertiary fold in addition to secondary structure elements such as helix and sheet. ROA studies of unfolded and partially folded proteins are providing new insight into the residual structure in denatured proteins and the aberrant behaviour of proteins responsible for misfolding diseases. It is even possible to measure the ROA spectra of most intact viruses, from which information about the folds of the major coat proteins and the structure of the nucleic acid core may be obtained. Hopefully this review will stimulate interest in the molecular physics aspects of the subject, and will encourage further theoretical work aimed at extracting maximum information from the plethora of structure-sensitive bands in typical ROA spectra.