X-ray dichroism in biaxial gyrotropic media: Differential absorption and fluorescence excitation spectra

The differential absorption and the differential change in the polarization state of an X-ray beam propagating inside a gyrotropic crystal are described using a Müller matrix, the 16 elements of which are related to the anisotropic components of the multipolar polarizability tensors at the absorbing site. Analytical expressions are given up to third order for X-ray linear and circular dichroism, X-ray optical rotation and X-ray circular polarimetry in transmission. The same formalism is extended to discuss Fluorescence detected dichroism spectra with or without polarization analysis of the fluorescence. Fluorescence detected dichroism is strictly proportional to dichroism measured in the transmission geometry only for uniaxial crystals. In biaxial crystals, the tiny effects of X-ray gyrotropy are swamped by large linear dichroism signals due to the imperfect polarization transfer function of Bragg monochromators. Second order effects should also be taken into consideration. Our general formulation of linear and circular dichroism includes terms of odd parity with respect to the action of the time reversal operator: such terms cannot contribute to natural dichroism but can be activated by a magnetic field. The terms responsible for X-ray magnetic circular dichroism are well known but non-reciprocal X-ray gyrotropy effects are also predicted in magnetic crystals of appropriate symmetry. Received 3 March 1999     

All-silicon chiral metasurfaces and wavefront shaping assisted by interference

Chiral metasurfaces have different electromagnetic responses with circularly polarized lights, showing as circular dichroism and optical activity. Here, a novel kind of all-silicon chiral metasurface is proposed by introducing destructive interference between achiral meta-atoms. The maximum value of circular dichroism spectra can reach 0.49. By adding an antireflective layer at the side of the silicon substrate, the maximum circular dichroism reaches 0.54. What is more, the bandwidth of circular dichroism greater than 0.4 reaches 0.15 THz. Two samples are fabricated to verify the feasibility of this scheme, and the experimental results are in good agreement with the simulations. In addition, the proposed scheme can also be used to generate various interesting functions, such as beam control and vortex generator. This flexible and efficient implementation solution of chiral metasurface can bring new ideas to the development of chiral devices in the future.     

A universal ellipsometer

This paper describes the design of an instrument which can be used for studies of optical rotation, circular dichroism and conventional ellipsometry. The main features are that it is automatically recording and, unlike other instruments, allows the azimuth and ellipticity to be measured simultaneously on a twin channel recorder. The recorder has a common chart drive which is synchronised to a scanning monochromator that covers a wide wavelength range.     

Beam alignment with the axis of a rotation stage for laser fabrication of microcircular structures

An optical technique to align laser beam with the axis of a rotating stage is proposed for laser fabrication of circular microstructures. The laser beam is first aligned parallel to the rotation axis and subsequently adjusted to coincide with the axis. An optical arrangement consisting of two quadrant photodiodes for the x- and y-directions and a specially designed beam splitter is utilized for the alignment. Mathematical modeling of the alignment system is carried out to estimate alignment errors caused by misalignment of mirror surfaces in the beam path. It is shown that parallelness of the laser beam to the rotation axis is a key factor to reduce alignment errors. The proposed method was applied to a microstereolithography system and proved that the laser beam can be aligned within a distance of about 25 μm from the rotation axis.     

Two photon magnetooptic effects

The theory of two photon magnetooptic effects such as polarization rotation and circular dichroism is developed. An effect which has not been previously discussed, the optical analog of the Hanle effect, is proposed.     

Astigmatic telescopic transformation of a high-order optical vortex

Properties of an optical vortex light beam formed after the astigmatic telescopic transformation of a circular Laguerre-Gaussian mode are considered both theoretically and experimentally. The beam evolution is found to be in conformity with the general notions on the high-order optical vortex symmetry breakdown. Upon propagation, the asymmetric beam shows a sort of rotation of its transverse profile in accord with the energy circulation in the original circular mode; this process is described on the base of the beam intensity moments and the vortex and asymmetry components of its orbital angular momentum. An l-charged optical vortex converts into |l| secondary first-order vortices positioned on a straight line crossing the beam axis. Orientation of this straight line in the beam cross section and spatial separation of the secondary vortex cores depend on the propagation distance. Morphology (orientation and anisotropy) of all the secondary vortices is the same and depends on the propagation distance; the anisotropy can be characterized by the vortex component of the beam angular momentum. At certain distance, relative separation of secondary vortices with respect to the beam transverse size reaches its maximum that corresponds to the minimum anisotropy of the vortices. The results can be useful in the context of current research of the optical vortex arrays.     

Opus musicum

The rotation of the plane of linearly polarized light by an optically-active medium arises from a difference between the refractive indices of the medium for left- and right-circularly polarized light. The circular birefringence is related to a circular dichroism, which is exhibited by all molecules which are not superposable on their mirror image and exist as laevo- and dextro-rotatory isomers. The circular dichroism of a dissymmetric molecule in a particular absorption region originates from an electronic transition with collinear electric and magnetic dipole moments, corresponding to the displacement of a molecular valency electron through a helical path by the absorption of radiation. The right- or left-handed form of the helical path depends upon the molecular structure, and the absolute stereochemical configuration of dissymmetric molecules may be determined by comparing their circular dichroism spectra with the calculated rotational strengths. In addition the polarization direction of an electric moment and the magnitude of a magnetic moment of an electronic transition in a dissymmetric molecule may be derived from circular dichroism measurements.     

Modelling of transmission through a chiral slab using fractional curl operator

Transmission of electromagnetic plane wave through a slab of reciprocal chiral medium has been modelled using fractional curl operator. It is noted that when order of the fractional curl operator becomes zero, the equivalent situation may correspond to absence of the chiral slab. Variation of the order of fractional curl operator may explore situations which may be regarded as intermediate step of a situation dealing with no chiral slab and a situation dealing with a chiral slab. It is noted that real order of the fractional curl operator may model the optical rotation while complex order of the fractional curl operator may model optical rotation as well as circular dichroism.     

Optical properties of a planar metamaterial with chiral symmetry breaking

The optical properties of a planar metamaterial with gammadion-shaped chiral symmetry breaking holes array have been investigated both theoretically and experimentally. The results indicate that the introduction of the chiral symmetry breaking causes the split of the transmission peak and exerts large influence on the optical rotation and circular dichroism. Our metamaterials might have potential applications in future design of plasmonic devices.     

On the possibility of determining anthracycline antibiotics in aqueous solutions using optical analytical system (biosensor)

We propose a version of the theory describing the circular dichroism spectra of cholesteric liquidcrystal dispersion particles of double-stranded DNA. The basis of the theory is the concept of absorption of electromagnetic waves by large molecular systems. The effect of physical parameters of dispersion particles on their circular dichroism is theoretically determined. It is experimentally demonstrated that circular dichroism can be used as a convenient tool for creating an optical analytical system for the determination of biologically active compounds that interact with DNA molecules.     

Shaping the beam profile of a partially coherent beam by a phase aperture

By use of a tensor method, an analytical formula for a partially coherent Gaussian Schell-model (GSM) beam truncated by a circular phase aperture propagating through a paraxial ABCD optical system is derived. The propagation properties of a GSM beam truncated by a circular phase aperture in free space are studied numerically. It is found that the circular phase aperture can be used to shape the beam profile of a GSM beam and generate partially coherent dark hollow or flat-topped beam, which is useful in many applications, e.g., optical trapping, free-space optical communication, and material thermal processing. The propagation factor of a GSM beam truncated by a circular phase aperture is also analyzed.     

非线性磁光效应用于测量磁光阱四极磁场的方案

为了研究磁光阱冷原子团所在区域的磁场大小,从而得出磁场零点附近磁场的微弱变化及其分布。提出利用右旋圆偏振作为探测光场穿过冷原子,根据左右旋圆偏振光场引起的跃迁几率的不同,导致穿过冷原子团零点前后探测场跃迁几率的变化,用来计算零点附近由冷原子团引起的非线性磁光效应,通过这一效应推导出旋转角随磁场大小的变化,从而获得了磁光阱四极磁场零点附近数量级达到10^-13T的磁场值。利用这一效应,同时在理论上获得了不同于以往理论及实验的双峰色散曲线。     

圆二色性对旋光现象的影响

分析旋光活性物质的圆二色性对旋光现象的影响,给出了一种测量介质圆二色性的方法。     

A phase sensitive optical rotation measurement in a scattered chiral medium using a Zeeman laser

A novel circular polarized optical heterodyne interferometer using a Zeeman laser to measure optical rotation both in nonscattered and scattered chiral medium is proposed. A pair of correlated orthogonal circular polarized light waves of different temporal frequency propagating in the chiral medium at different speed is studied. This results in phase retardation between circular polarized light waves of which the phase difference is proportional to the optical rotation angle of a linear polarized light in a chiral medium. In the mean time, two orthogonal circular polarized light waves can be treated as a circular polarized photon pair that is able to reduce the scattering effect in a scattered chiral medium. Then the optical rotation angle can be measured in the scattering medium. In addition, a common-path configuration with respect to circular polarized light waves immune the background noise. This further improves the sensitivity on optical rotation measurement based on phase difference detection.     

环形静电排斥旋转驱动器微光机电系统微镜北大核心CSCD

微光机电系统(MOEMS)微镜在激光显示、光束扫描、自适应光学等领域都有重要应用。现有的MOEMS微镜驱动器一般为静电吸引型,难以克服其吸合效应造成的各种问题,同时,方形的驱动器难以与圆形的镜面形成良好匹配,填充比较低。提出了一种环形静电排斥驱动器的MOEMS微镜,该静电排斥驱动器以环形排列,并围绕在镜面的周围,达到了与圆形镜面的良好匹配。所设计的微镜口径为204μm,并利用PolyMUMPS表面工艺进行加工。模拟仿真及实验测试结果显示,该微镜具有0.62°的最大旋转角,494μs的响应时间和1.191kHz的工作带宽,适用于一般光束扫描等的应用需求。     

木榄醇手性光谱的密度泛函研究

手性光学理论研究有助于解释手性分子的旋光机理和设计新的手性药物.采用B3LYP方法,计算了木榄醇A—C的旋光度和圆二色谱.从分子结构、正则振动和电子结构方面,探索了分子手性微观起源,分析了旋光度和电子圆二色谱的溶剂效应.表明OH的引入可调节分子的手性,甲基和苯环加强了分子的手性.发生在手性骨架上的振动和电子跃迁,加强了分子的手性.溶剂效应减小旋光度、削弱电子圆二色谱. 关键词： 木榄醇 光学活性 密度泛函理论 圆二色谱     

Magnetic circular dichroism from the impurity band in III-V diluted magnetic semiconductors

The magnetic circular dichroism of III-V diluted magnetic semiconductors, calculated within a theoretical framework suitable for highly disordered materials, is shown to be dominated by optical transitions between the bulk bands and an impurity band formed from magnetic dopant states. The real-space Green's functions incorporate spatial correlations in the disordered conduction band and valence-band electronic structure, and include extended and localized states on an equal basis. Our findings reconcile unusual trends in the experimental magnetic circular dichroism in III-V diluted magnetic semiconductors with the antiferromagnetic p-d exchange interaction between a magnetic dopant spin and its host.     

The use of circular optical grating for measuring angular rotation of mirrors

This paper explores the feasible use of circular optical grating for measuring the rotation of mirrors that are commonly found in micro-systems. Both theoretical and experimental results show that distortion of the circular grating that is projected onto the mirror surface is a simple function of the angular rotation of the mirror. The circular grating may readily be generated using a standard Michelson interferometer or an LCD projector. Through manipulating the distance between the interferometer and the mirror surface, the diameter of the optical grating may be varied. Furthermore, the additional use of a converging lens enables a significant reduction in the size of the grating: with simple laboratory facility, small circular grating of about 400 μm is achieved for use on micro-systems. With the use of more sophisticated optical elements, the angular rotation of even smaller micro-mirrors may be measured.     

Chiral Lithography with Vortex Non-diffracted Laser for Orbital Angular Momentum Detection

Chiroptical response, demonstrating chiral interaction between optical vortex and chiral structure, plays an important role in variety of fields like optics and material science. However, the flexibility and efficiency of chiral structure fabrication are limited due to mask requirement and a point-by-point constructing strategy. In this paper, a novel chiral lithography method is proposed that utilizes optical vortex phase beam shaping to achieve chiral structure processing with high efficiency and flexibility. By programming topological charges of the vortex phase, chiral structures with adjustable appearance, rotation, and chirality can be produced using femtosecond laser single pulse exposure. Transmittance measurements of fabricated chiral structures array confirm a 66% helical dichroism that is predicted by simulation. Moreover, with the aid of convolutional neural networks (CNN), an accuracy of 98% in Orbital Angular Momentum (OAM) recognition can be achieved. This chiral lithography method provides an alternative for chiral structure fabrication and holds promise in the development of chiral optics, optical communications, and next-generation optical devices.