Polarization of Acoustic Waves in Two-Dimensional Phononic Crystals Based on Fused Silica.

The two-dimensional square-lattice phononic crystal is one of the recently proposed acoustic metamaterials. Strong anisotropic propagation of elastic waves makes the material promising for various potential applications in acoustics and acousto-optics. This paper presents a study of the propagation of elastic waves in two-dimensional phononic crystals based on fused silica. The band structures of a phononic crystal are obtained by solving the wave equation in its variational form by the finite element method. The main phononic crystal acoustic characteristics that are of practical interest in acousto-optics are calculated based on the analysis of the dispersion relations. It is shown that the choice of the phononic crystal geometry makes it possible to control the distributions of both the inverse phase velocities and the energy walk-off angles of acoustic modes. The calculations of the acoustic modes' polarization are in a particular focus. It is demonstrated that under certain conditions, there are exactly three acoustic modes propagating in a phononic crystal, the averaged polarization vectors of which are mutually orthogonal for any directions of the acoustic wave's propagation. It is argued that the acoustic properties of phononic crystals meet the requirements of acousto-optics.

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.

Enhanced anisotropy in Paratellurite for inhomogeneous waves and its possible importance in the future development of acousto-optic devices.

The anisotropic feature of most crystals, involves a direction dependent wave velocity for each of the possible modes. Paratellurite (Tellurium dioxide) is extraordinary because, for one of the propagation modes, i.e. the quasi shear horizontal (QSH) mode, the anisotropy is exceptional. This results, on the one hand in a very strong directional dependent sound velocity and on the other hand, in a low wave velocity in certain directions, resulting in a high figure of merit for the acousto-optical interaction. In the case of inhomogeneous waves, the slowness surfaces change their shape and magnitude, for all crystals. However, for paratellurite, this effect is again extraordinary. As soon as a relatively small inhomogeneity is considered, the sound velocity for the QSH mode becomes really exceptionally anisotropic, resulting in a slowness surface that is almost spherical, covered by pins. The velocity corresponding to those 'pins', is much lower than in the case of homogeneous plane waves, which is very promising for the future development of acousto-optic cells involving an even higher figure of merit.