Circular Polarization States Research Articles

Raman optical activity instrument for studies of biopolymer structure and dynamics

The latest version of a multi-channel Raman optical activity (ROA) instrument implementing incident circular polarization (ICP) modulation within a in a backscattering geometry and optimized for measurements on biopolymers in aqueous solution is described. It is based on a fast single-stage, imaging (stigmatic) monochromator equipped with a high optical density holographic notch filter, a holographic transmission grating and a thinned back-illuminated thermoelectrically cooled charge-coupled device (CCD) detector with a high quantum efficiency. A large-aperture longitudinal electro-optic modulator (Pockels cell) is employed to switch between orthogonal circular polarization states in the incident laser radiation. A thick Lyot depolarizer is used for depolarization of the backscattered Raman radiation. This backscattering ICP CCD ROA design is currently realized with two prototype instruments dedicated to studies of the structure and dynamics of biopolymers in aqueous solution. Backscattered ICP ROA spectra of the following samples are presented as typical examples of the excellent performance characteristics: poly(L-glutamic acid) in α-helical and disordered conformations; bovine α-lactalbumin in native and acid molten globule states; human immunoglobulin; calf thymus DNA and both magnesium-bound and magnesium-free phenylalanine-specific transfer RNA; and filamentous bacterial viruses Pf1 and M13. Copyright © 1999 John Wiley & Sons, Ltd.

Failure of a proposed superluminal scheme

We consider a “superluminal quantum Morse telegraph”, recently proposed by Garuccio, involving a polarization-correlated photon pair and a Michelson interferometer in which one of the mirrors is replaced by a phase-conjugating mirror (PCM). Superluminal information transfer in this scheme is precluded by the impossibility of distinguishing between unpolarized photons prepared by mixing linear polarization states or by mixing circular polarization states.

Polarization discrimination for active imaging in scattering media

The results of a Monte Carlo computer simulation of active-illumination imaging of surfaces immersed in a scattering media are presented. The simulation is based on scattering by spherical or spheroidal Rayleigh particles and rigorously accounts for the polarization effects of the scattering process. Illumination with linear or circular polarization states and detection in the original and orthogonal polarization states are investigated. The object surfaces are modelled as diffuse scatterers which either preserve or randomize the polarization of the reflected light. The simulations clearly indicate that, in some situations, polarization discrimination can be effective in extending the depth of visibility. The effectiveness of this approach depends largely on the polarization properties of the surface to be imaged rather than the properties of the intervening scattering medium or the imaging geometry.

Backscatter linear and circular polarization analysis of roughened aluminum

A study of cross-polarized and copolarized intensities backscattered from roughened aluminum surfaces is presented for both linear and circular incident polarization states. The angular variation of measured Mueller matrices is shown to contain only diagonal elements, as predicted by the reciprocity theorem. The ratio of cross-depolarized to copolarized scattered intensities is significantly larger for circular than for linear input polarization states. In the linear case the ratio saturates beyond 50 degrees , whereas in the circular case the ratio continues to increase monotonically with angle. A phenomenological model for copolarization and cross-polarization intensities is shown to predict the observed behavior of both linear and circular input polarization states up to incident angles of 70 degrees .

HN22 sheet polarizer, an inexpensive infrared retarder

The popular sheet polarizer, Polaroid HN22, has been measured to be a nearly half-wave retarder in the 3.6-5.4-microm spectral band with a transmittance of approximately 20%. Tuning of the retardance value between 60 degrees and 260 degrees has been demonstrated by tilting of the HN22 sheet with respect to the incident beam. The material's availability, relatively large aperture, large field of view, and low cost make it an excellent candidate for use as an infrared retarder for systems operating in this wave band. Thus HN22 may be employed as an inexpensive half-wave linear retarder and used for rotating the plane of polarization as well as for conversion between circular polarization states.

Dynamical polarization states above the instability threshold for circularly polarized laser emission

We show that in a laser with a transversally isotropic resonator operating on a J = 1 → J = 0 transition the instability of Circularly Polarized states leads to Linearly Polarized states with a rotating direction of polarization. This transition occurs through intermediate states which are Elliptically Polarized with rotating azimuth. For the range of pump values for which the intermediate Rotating Elliptically Polarized states occur, the total output intensity is constant, and all the energy is pumped into the coherence between the m z = 1 and m z = −1 state.

Fiber Bragg grating as an optical filter tuned by a magnetic field

We describe a novel tunable optical filter for use in optical-frequency-domain multiplexed communication systems. The shift in the Bragg condition of a fiber Bragg grating as a result of magnetically induced circular birefringence is calculated with coupled-mode theory on the basis of circular states of polarization, and the values obtained for silica and terbium-doped optical fibers are compared.

Theory of oblique-incidence phase retarders

Phenomenological and mathematical analyses of oblique incidence in total-internal-reflection phase retarders and the procedure for constructing extremely achromatic quarter-wave retarders are presented. It is shown that the retardance can assume the same value twice at two definite spectral lines. As an example, a quarter-wave phase retarder is described that introduces a retardance of exactly 90 degrees at two He-Ne laser lines of 632.8 and 1150 nm with a maximum deviation of 0.007 degrees for other wavelengths between these two lines. This application is advantageous in conjunction with two-line and multiline lasers where the radiation is usually pumped in a state of circular polarization.

Recording and Readout of a Cholesteric Liquid Crystal Based Multilayer Disk

A novel approach to optical storage based on layering of cholesteric liquid crystal (CLC) media is being investigated. CLC media have been chosen due to their unique property of selective reflection, whereby a CLC film reflects light of a characteristic wavelength and polarization handedness with near-zero loss while transmitting all other light. This new technology has the potential of enabling terabyte storage on a single optical disk. A two-layer disk with 5 µm data marks was recorded and crosstalk-free readout was demonstrated by selecting the correct circular polarization state of a 514 nm laser.

Chirality-dependent two-photon absorption probabilities and circular dichroic line strengths: theory, calculation and measurement

Theory and calculations relevant to the measurement, analysis, and interpretation of chirality-dependent two-photon absorption (TPA) properties of molecules and crystals are presented and discussed. The theory development focuses on TPA processes in which excitation is with a single, one-color beam of either left - or right -circularly polarized light, and it gives expressions that show the dependence of TPA line strengths on the circular polarization state of the exciting radiation. Three types of molecule-photon interaction mechanisms are represented in the theory, each of which makes a distinctive contribution to molecular transition polarizabilities and two-photon transition amplitudes. Two of these mechanisms assume either purely electric-dipole (μμ) or purely magnetic-dipole (mm) interaction processes in the two-photon absorption events, whereas the third mechanism assumes a combination of electric- and magnetic-dipole (μm) interaction processes. The molecular transition polarizabilities associated with μm interaction processes carry information about both the degree and sense of structural handedness in the absorbing systems and they vanish in the absence of any structural chirality. Differences between the left- versus right-circularly polarized TPA line strengths of a chiral system reflect interferences between transition amplitudes associated with μm and μμ (and/or mm) interaction processes. In this study, a model is developed for calculating the circularly polarized TPA line strengths of atomic-like chromophores in a chiral molecular or crystalline environment, and a parametrized form of this model is used to perform calculations on Gd 3+ ions in single-crystal enantiomorphs of Na 3 [Gd(C 4 H 4 O 5 ) 3 ] · 2NaC1O 4 · 6H 2 O. The results obtained from these calculations are compared to results obtained from fluorescence-detected two-photon circular dichroism (FD-TPCD) measurements reported for Na 3 [Gd(C 4 H 4 O 5 ) 3 ] · 2NaC10 4 · 6H 2 O.

Fresnel diffraction at [formula omitted] of the Talbot distance of an anisotropic grating

We show that an anisotropic grating with two alternate linear perpendicular states of polarization is transformed by free propagation, at 1 4 of the Talbot distance, into another grating with circular polarization states. Experimental results that verify our proposal are included.

Polarization stability of circularly polarized laser pulses propagating in non-birefringent single-mode optical fibers

The polarization state of circularly and linearly polarized picosecond laser pulses propagating through nonbirefringent single-mode optical fibers have been studied. It was found that circularly polarized laser pulses were less sensitive to the intensity than that of linearly polarized laser pulses. A model using self-phase modulation (SPM) and cross-phase modulation (XPM) has been proposed to explain the measured preservation of the circular polarization state and the depolarization of linear polarization state in optical fibers. The model is in good agreement with the experimental results. >

Instrument for natural and magnetic Raman optical activity studies in right‐angle scattering

AbstractA simple computer‐controlled instrument for natural and magnetic incident circular polarization Raman optical activity (ICP ROA) measurements in right‐angle scattering is described. The key spectral elements consist of a fast single‐stage monochromator equipped with a holographic notch filter, a highly efficient holographic diffraction grating and a thinned back‐illuminated, thermoelectrically cooled charge‐coupled device (CCD) detector. A temperature‐stabilized longitudinal electro‐optic modulator is employed to switch between orthogonal circular polarization states in the incident laser radiation. A polaroid film together with a mica half‐wave retardation plate are used to select particular linear polarization states in the Raman scattered radiation with the same instrumental response. High‐quality depolarized and polarized Stokes and depolarized anti‐Stokes ICP ROA spectra of (R)‐(−) and (S)‐(+)‐phenyloxirane and depolarized magnetic resonance ICP ROA spectra of copper(II) tetrabromide are presented as typical examples of the excellent performane characteristics.

High-resolution magneto-optic measurements with a Sagnac interferometer (invited)

A technique for measuring the Faraday effect and the magneto-optic Kerr effect has been developed. In a Sagnac interferometer, two optical beams follow identical paths in opposite directions. Effects which break time-reversal symmetry, such as magneto-optic effects, will cause destructive interference between the two beams. By measuring the phase shift between circular polarization states reflected from a magnetized sample, the polar magneto-optic Kerr effect is measured to an accuracy of 3 μrad, with a spatial resolution of 2 μm. The interferometric technique provides a number of advantages over conventional polarizer methods, including insensitivity to linear birefringence, the ability to completely determine the magnetization vector in a region, and the ability to sensitively measure magneto-optic effects without an external field. It is also shown that this device has great potential if incorporated into a near-field optical device. Some of the considerations for the design of a near-field Sagnac magneto-optic sensor are introduced and the advantages of the device are discussed. Some preliminary experiments are shown.

Magneto-optic measurements with a sagnac interferometer

Abstract We have developed a novel technique for measuring the Faraday effect and the magneto-optic Kerr effect. In a Sagnac interferometer, two optical beams follow identical paths in opposite directions. Effects which break time-reversal symmetry, like magneto-optic effects, will cause destructive interference between the two beams. By measuring the phase shift between circular polarization states reflected from a magnetized sample, we measure the polar magneto-optic Kerr effect to an accuracy of 2 μrad, with a spatial resolution of 2 μm. The interferometric technique provides a number of advantages over conventional polarizer methods, including insensitivity to linear birefringence, the ability to completely determine the magnetization vector in a region, and the ability to sensitively measure magneto-optic effects without an external field.

Raman optical activity instrument for biochemical studies

AbstractA simple and fully computer‐controlled backscattering incident circular polarization Raman optical activity (ICP ROA) instrument suitable for chioptic studies of biologically significant molecules in aqueous solution is described. It consists of a fast single‐stage monochromator equipped with a holographic edge filter, a highly efficient holographic difiaction grating and a thinned back illuminated thermoelectrically cooled charge‐coupled device detector as the key spectral elements. A longitudinal electro‐optic modulator is employed to switch between orthogonal circular polarization states in the incident laser radiation within a backscattering geometry. A thick Lyot depolarizer is used for the depolarization of the backscattered Raman radiation. High‐quality backscattering ICP ROA spectra of (1S)‐( − )‐trans‐pinane, (1R)‐( + )‐trans‐pinane, L‐alanine, L‐alanyl‐L‐alanine, lysozyme, D‐glucose, α‐D‐cyclodextrin and 2′‐deoxycytidine are presented as typical examples of the excellent performance characteristics.

Dual and incident circular polarization Raman optical activity backscattering of (—)-trans-pinane

We report the first measurement of dual circular polarization (DCP) Raman optical activity (ROA). DCP ROA was predicted recently to be a new form of ROA in which the incident and scattered light beams are synchronously modulated between right and left circular polarization states. Using a new backscattering apparatus and (—)-trans-pinane, we confirm the theoretical predictions and compare our results to the conventional incident circular polarization (ICP) form of ROA.

A New Scattered Circular Polarization Raman Optical Activity Instrument Equipped with a Charge-Coupled-Device Detector

A simple and fully computer-controlled scattered circular polarization Raman optical activity (SCP-ROA) instrument is described which consists of a fast single monochromator and a thermoelectrically cooled charge-coupled-device (CCD) detector as the key spectral elements. A conventional circular analyzer, formed by a quartz zero-order quarter-wave plate attached to a motorized 360° rotator and a fixed linear dichroic sheet polarizer, is employed for the modulation between orthogonal circular polarization states in the scattered radiation within a right-angle scattering geometry. Well-resolved depolarized SCP-ROA spectra of (1S)-(–)- α-pinene and (R)-(+)-pulegone are presented within the wave-number region of ∼550 to 1200 cm−1 as typical examples of the excellent performance of this novel multichannel ROA instrument.

An Analysis of Modulation Experiments for Raman Optical Activity

The dependence of Raman optical activity and the associated artifacts on the scattering angle is discussed in terms of the Stokes-Mueller calculus for the modulation between right and left circular polarization states in the incident laser radiation, in the Raman scattered light, and in both simultaneously. The incident beam modulation technique implemented in backward scattering is identified as the ultimate experimental strategy for measurements on chiral molecules randomly oriented in an optically isotropic phase. Both the incident and the scattered beam modulation techniques have been found to isolate the pure magnetic dipole contribution to the circular intensity differences in right-angle scattering for orientations of the analyzer in the scattered beam and of the azimuth of the linearly polarized incident radiation, respectively, at ∼ ± 35.26° from the vertical.

Accurate calibration of the four-detector photopolarimeter with imperfect polarizing optical elements

The first three columns of the instrument matrix A of the four-detector photopolarimeter (FDP) are determined by Fourier analysis of the output current vector I(P) as a function of the azimuth angle P of the incident linearly polarized light. Therefore 12 of the 16 elements of A are measured free of the imperfections of the (absent) quarter-wave retarder (QWR). The effect of angular beam deviation by the polarizer is compensated for by taking the average, (1/2) [I(P) + I(P + 180°)], of the FDP output at 180°-apart, optically equivalent, angular positions of the polarizer. The remaining fourth column of A is determined by the FDP’s response to the right- and left-handed circular polarization states. Because these states are impossible to generate with an imperfect QWR, a novel procedure is developed. In particular, the response of the FDP to the unattainable right- or left-handed circular polarization state is found by taking the average of the responses of the FDP to an elliptical near-circular state and that state rotated in azimuth by 90°. This calibration scheme is applied to measure A of our prototype FDP of four Si detectors at λ = 632.8 nm. A is determined, in external and internal reference frames, free of imperfections in the polarizing optical elements. The FDP, with its uncontaminated A matrix, is used subsequently to evaluate the imperfections of the QWR with the help of an appropriate model.