Uniaxial Gyrotropic Crystals Research Articles

Peculiarities of the acousto-optical interactions of Bessel light beams in crystals

The noncollinear acousto-optical filtration of diffracted Bessel light beams in uniaxial paratellurite crystals has been investigated. With the use of the method of overlap integrals it is shown that independently on the order of Bessel light beams in the conditions of transverse phase-matching of diffracted waves in the range of optical spectrum of 0.4-1.1 nm in paratellurite crystals the bandwidth of transmission of ∼0.52 nm is reached due to detuning from Bragg’s synchronism only. It is shown that in uniaxial gyrotropic crystals polarization-independent diffraction of Bessel light beams is possible, i.e. Bragg diffraction efficiency is independent of the polarization state of the incident beam. It has been stated that the physical reason of such diffraction is simultaneous realization of two processes of anisotropic scattering under which the Bragg synchronism conditions are realized for the orthogonally polarized elliptical Bessel beams.

Double Bragg Diffraction of Bessel Light Beams on Ultrasound in Uniaxial Gyrotropic Crystals

Noncollinear double acousto-optical diffraction of Bessel light beams on ultrasound in uniaxial gyrotropic crystals is studied theoretically and experimentally. It is found that when a Bessel light beam propagates along the optic axis of a crystal, the diffracted light beams of the ±1 order are symmetric relative to the incident beam and have roughly the same intensity. Double Bragg diffraction is also observed when a Bessel light beam is incident at the Bragg angle and near the optic axis, but the intensities of the +1 and +2 order diffracted beams are significantly different.

Influence of light-induced gratings on acousto-optic interaction of Bessel light beams in uniaxial gyrotropic crystals

The influence of light-induced gratings, caused by cubic optical nonlinearity, on the Bragg diffraction of Bessel light beams by ultrasound in uniaxial gyrotropic crystals was studied theoretically. It was shown that, when Bessel light beams are used, the threshold for a substantial manifestation of optical nonlinearity during Bragg diffraction is significantly reduced. It was established that the efficiency of the acousto-optic interaction increases with increasing light intensity in the anti-Stokes regime of diffraction and decreases monotonically in the Stokes regime. Satisfactory qualitative agreement between the theoretical and experimental dependences of the diffraction efficiency on the light and ultrasound intensities was demonstrated using a uniaxial gyrotropic quartz crystal.

Ultrasonic diffraction of Bessel light beams in uniaxial gyrotropic crystals

Specific features of the acousto-optic diffraction of Bessel light beams propagating in the vicinity of the optical axis of a uniaxial gyrotropic crystal have been investigated. The dependences of the diffraction efficiency on the acousto-optic interaction length, ultrasound power, and polarization state of the incident Bessel light beam have been analyzed using the coupled-wave equations and the overlap-integral method. It is shown that polarization-independent diffraction of Bessel light beams is observed in paratellurite crystals, when the Bragg diffraction efficiency is independent of the polarization state of the incident beam. The physical reason for this diffraction has been established (both theoretically and experimentally) to be simultaneous implementation of two processes of anisotropic scattering, at which the Bragg synchronism conditions are satisfied for orthogonal polarized Bessel beams with elliptical polarization.

Polarization-Independent Acousto-Optical Modulation of Bessel Light Beams

The acousto-optical diffraction of Bessel light beams (BLB) propagating near the optical axis of uniaxial gyrotropic crystals is examined. It is shown that polarization-independent modulation of Bessel beams is possible in paratellurite crystals; that is, the Bragg diffraction efficiency is independent of the polarization state of the incident beam. The physical reason for this kind of modulation is found to be the simultaneous occurrence of two anisotropic diffraction processes, such that the conditions for Bragg synchrony are satisfied for orthogonally polarized elliptical Bessel beams. The coupled wave equations and an overlap integral method are used to study the dependence of the diffraction efficiency on the length of the acousto-optical interaction, the ultrasonic power, and the polarization state of the incident Bessel beam.

Polarization features of the 2D contouring of an optical image under double Bragg diffraction conditions

The polarization features concerning the formation of a 2D image contour in the first diffraction order, which arise from the double Bragg diffraction occurring in the uniaxial gyrotropic crystal, are investigated. It is demonstrated that changes in sound power make it possible to create 2D image contours in different polarization states. Experiments based on the double diffraction in the TeO2 crystal have confirmed the basic theoretical assumptions.

Conoscopic Patterns for Optically Uniaxial Gyrotropic Crystals in the Vicinity of Isotropic Point

We have presented computer simulations of conoscopic patterns occurring in optically uniaxial gyrotropic crystals in the vicinity of isotropic point for a number of sets of crystal parameters and orientations. The appearance of special directions characterized by equalization of linear and circular birefringences has been revealed.

Optoacoustic interaction in uniaxial gyrotropic quartz crystals

The specific features are considered of the noncollinear optoacoustic interaction in uniaxial gyrotropic quartz crystals in an intermeadiate regime of diffraction close to the Bragg regime. A system of equations of coupled waves is presented which describes the optoacoustic interaction in uniaxial gyrotropic crystals in the Raman-Nath, intermedaite, and the Bragg regimes of diffraction. The dependences of the diffraction efficiency on the ultrasonic wave intensity, the optoacoustic interaction length, and on the incident light polarization are investigated. A multispike structure of the Bragg maximum is studied that appears in accounting for the crystal gyrotropy. A good agreement between the theoretical calculations and the known experimental results for a quartz crystal is shown.

X-ray natural circular dichroism in a uniaxial gyrotropic single crystal of LiIO3

We produce the first experimental evidence of x-ray natural circular dichroism (XNCD) in a uniaxial gyrotropic crystal of α-LiIO3 that is known to crystallize with space group P63 and to exhibit a very strong nonlinear susceptibility. For the sake of simplicity, the optical axis was set colinear to the direction of the exciting x-ray beam in order to get rid of undesirable birefringence and linear dichroism effects. The nicely structured XNCD spectra recorded at the LI, LII, and LIII edges of iodine are assigned to the electric dipole–electric quadrupole (E1.E2) interference terms which do not vanish in gyrotropic crystals but contribute to a second order polarizability. Our interpretation is consistent with a band structure calculation and is fully supported by ab initio multiple scattering simulations. The signatures recorded at the LII and LIII edges are similar and have the same sign. This is in contrast to x-ray magnetic circular dichroism (XMCD) spectra which usually exhibit opposite signs at the LII and LIII edges: while the exchange and spin-orbit interactions are the driving terms in XMCD, this is not the case in XNCD. The XNCD signal detected at the LI edge is found to be relatively more intense and has the opposite sign. These results leave very little space for a contribution of the electric dipole–magnetic dipole (E1.M1) interference terms which are usually dominant at optical wavelengths but should be barely detectable in the x-ray range.

Anomalous angular dependence of E-mode splitting in Raman spectra of ZnP 2 and CdP 2 uniaxial gyrotropic crystals caused by spatial dispersion

It was found that the dependence of the splitting of low frequency E-doublets in Raman spectra of strongly gyrotropic tetragonal ZnP 2 and CdP 2 crystals on the angle between the direction of the phonon wave vector q and the optical axis is completely different from the well known angular dependence of the LO-TO splitting of the double degenerate mode in uniaxial crystals. This splitting may be treated as entirely caused by spatial dispersion of first order in q .

Vibrational circular dichroism and vibrational optical rotatory dispersion in molecular crystals. IV. Overtones of degenerate vibrations

Two-quanta anharmonic spectra, namely overtones and combination bands with degenerate vibrations, in uniaxial gyrotropic crystals from point group of symmetry C3, D3, C4, D4, are investigated, with due account taken of the time inversion. The symmetry of two-phonon states is studied. A general model of the anharmonicity and the influence of the crystal chiral structure on intermolecular exchange of the circularly polarized degenerate vibrations is elaborated. The contribution of overtones and of the combination bands to dielectric permittivity tensor, and to the gyration tensor is studied too. The paper also deals with the IR and vibrational circular dichroism (VCD) spectra near the frequencies of the unbound and of the bound two-phonon states as well as the general features of two-phonon circularly polarized states appearing in the linear and in nonlinear optical processes in gyrotropic crystals.

Cerenkov radiation by electric and magnetic dipoles in a gyrotropic medium

The energy-loss integral is found for the Cerenkov radiation emitted in a uniaxial gyrotropic crystal by a neutral particle having electric and magnetic dipole moments when the particle is moving at a nonvanishing angle with respect to the optic axis. Particular cases are analyzed in which the integration over the variables in wave-number space can be carried out completely in analytic form.

A representation for the energy-loss integral corresponding to Cerenkov radiation by a charged particle in a uniaxial gyrotropic crystal

A new representation is found for the energy-loss integral corresponding to Cerenkov radiation of a charged particle in a uniaxial gyrotropic crystal, for the case in which the particles is moving at some nonvanishing angle with respect to the optic axis. Particular cases are analyzed in which the integration over the variables in wave-number space can be carried out completely in analytic form.