Nataliya V. Polikarpova

Acousto-optic investigation of propagation and reflection of acoustic waves in paratellurite crystal

The paper presents results on acousto-optic investigation of unusual acoustic phenomena taking place in crystals possessing strong anisotropy of elastic properties. Advantages of the applied method of analysis are demonstrated by the example of the commonly used acousto-optic material tellurium dioxide. The major goal of the research consists of experimental verification of theoretical conclusions related to peculiar cases of acoustic propagation and reflection recently observed in the crystalline material. In particular, the case of glancing incidence and the following reflection of elastic energy from a free boundary separating the paratellurite crystal and the vacuum is examined in the paper. It is shown in the acousto-optic experiment that, in the case of glancing incidence, energy flow of a reflected acoustic wave may propagate practically in a reverse direction with respect to an incident wave. It is also proved that strong elastic anisotropy of the crystal is responsible for the unusual propagation and reflection of the acoustic waves. The research confirms the conclusion that the examined acoustic effects may be useful in development of new acousto-optic devices.

Reflection of plane elastic waves in tetragonal crystals with strong anisotropy

Propagation and reflection of plane elastic waves in the acousto-optic crystals tellurium dioxide, rutile, barium titanate, and mercury halides are examined in the paper. The reflection from a free and flat boundary separating the crystals and the vacuum is investigated in the (001) planes in the case of glancing acoustic incidence on the boundary. The analysis shows that two bulk elastic waves may be reflected from the crystal surface. The energy flow of one of the reflected waves in paratellurite and in the mercury compounds propagates in a quasi-back-direction with respect to the incident energy flow. It is proved that energy flows of the incident and reflected elastic waves are separated by a narrow angle of only a few degrees. It is also found that the relative intensity of the unusually reflected waves is close to a unit in a wide variety of crystal cuts. General conclusions related to acoustic propagation and reflection in crystals have been made based on the examined phenomena in the materials.

Nearly backward reflection of bulk acoustic waves at grazing incidence in a TeO2 crystal

Reflection of bulk acoustic waves in a TeO2 acoustooptic single crystal is studied for the case of a grazing incidence on the free crystal-vacuum boundary. The propagation and reflection of elastic waves is considered in the XOY plane of the material cut out in the form of a rectangular prism. An extraordinary case of reflection at the grazing incidence, when the energy flow of one of the two reflected waves in the crystal is directed opposite to that of the incident wave, is studied. It is shown that the transformation of the incident elastic energy into the energy of the backward-reflected wave can occur with an efficiency close to 100% and can be observed in a wide range of crystal cut angles. An abrupt change of the reflection coefficients in the vicinity of the critical angle is predicted.

Acousto-optic study of the unusual cases of reflection of bulk elastic waves in a paratellurite crystal

The propagation and reflection of bulk ultrasonic waves in a paratellurite crystal have been investigated by the acousto-optic method. Simultaneous excitation of two acoustic waves by one piezoelectric transducer has been observed in a cell of complex, specially chosen configuration. Maximally efficient conversion of the energy of a quasi-longitudinal wave into a single reflected quasi-shear wave with a walkoff angle 72° is implemented for the reflection from a free crystal boundary. The wave beam compression by a factor of 7 is observed for this reflection. A reflection (close to backward) of waves with an angle of spatial separation of the incident and reflected beams equal to 8° is also implemented. The acousto-optic figure of merit values of the medium are calculated for arbitrary propagation and polarization directions of the interacting light and ultrasonic waves. Possible applications of the effects studied in acousto-optics and acoustoelectronics are indicated.

Intermediate regime of light diffraction in media with strong elastic anisotropy

A quantitative method of evaluation of phase mismatches and calculation of intensities of diffracted light is described in the paper. The analysis is carried out for the case of the acousto-optic interaction taking place in an acoustically anisotropic medium at Bragg angle of light incidence. An example of the calculation is provided for the particular regime of light diffraction in the single crystal of TeO2 possessing the walkoff angle between directions of acoustic phase and group velocities as large as 74 degrees.

Intensity of reflected acoustic waves in acousto-optic crystal tellurium dioxide

ΩPropagation and reflection of plane elastic waves in the acousto-optic crystal tellurium dioxide is examined in the presentation. The reflection of the waves from a free and flat boundary separating the paratellurite crystal and the vacuum is investigated in the (001) and the (110) planes of TeO2 in the case of glancing and inclined acoustic incidence on the boundary. The analysis shows that as much two elastic waves may be reflected from the crystal surface. Energy flow of one of the reflected waves propagates in a quasi-back direction with respect to the incident energy flow unusually reflected wave may be close to a unit in a wide variety of crystal cuts. Possible applications of the examined phenomena in acousto-optic devices are discussed in the presentation.

The anisotropy of elastic waves in a tellurium crystal

For acoustic waves propagating in an acoustooptic tellurium crystal, the dependence of their polarization on the propagation direction with respect to the crystal axes is discussed. The characteristic features of waves propagating in the crystal are considered; these features manifest themselves in an excess of the phase velocity of shear acoustic modes over the velocity of longitudinal modes. The change in the wave type from quasi-longitudinal to quasi-transverse as a result of the variation in the propagation direction of ultrasound is investigated. It is shown that such a behavior of bulk acoustic waves is caused by the specific relation between the elastic moduli, which differs from the corresponding relations observed in other acoustooptic materials.

Glancing incidence and back reflection of elastic waves in tetragonal crystals

Propagation and reflection of plane acoustic waves in crystals of similar tetragonal symmetry but different grade of elastic anisotropy is examined. The reflection of the waves from a homogeneous and flat boundary separating a crystal and the vacuum in the case of glancing acoustic incidence is investigated. As much as two elastic waves may be reflected from the crystal surface. It is predicted that in the materials with a strong anisotropy of elastic properties, the energy flow of one of the reflected waves propagates in a quasi-back direction with respect to the incident energy flow. It is found that the energy flows of the incident and the back reflected waves are separated in space by the angle as narrow as a few degrees. Analysis proves that the relative intensity of the unusually reflected wave may be close to a unit. A decrease in the grade of the anisotropy in a crystal results in vanishing of the backward reflected wave so that its energy fraction is diminishing down to zero. A regular trend is predicted that the effect of the unusual reflection increases with the growth of the grade of the elastic anisotropy.

Development of acousto-optic devices based on transformation of acoustic waves

One of practical methods for generating shear waves in paratellurite crystals has been investigated. Slow shear waves are excited during transformation of longitudinal modes into shear ones, which occurs during acoustic reflection from a crystal-vacuum free interface.

Semicollinear diffraction of light by ultrasound in a medium with strong elastic anisotropy

A new regime of diffraction of light by ultrasound in which the direction of the acoustic energy flux does not coincide with the phase velocity of ultrasound is theoretically analyzed and experimentally studied. In the diffraction regime under study, the incident optical beam intersects the ultrasonic column, but the optical beam of the +1 diffraction order is directed strictly along the ultrasonic column. It is demonstrated that this regime of the acousto-optic interaction exhibits the properties of both transverse and collinear diffraction regimes and cannot be correctly analyzed in the framework of the existing theory of the diffraction of light by ultrasound. The theoretical and experimental results show that the diffraction efficiency and the frequency and angular interaction ranges significantly depend on both aperture of the optical beam and width of the ultrasonic column.