General method to determine optimal values of acousto-optic figure of merit in cubic crystals

Nikitin, P.A.

Автор: Nikitin P.A.
Сборник: IEEE International Ultrasonics Symposium Proceedings
Серия: (September 18 – 21, 2016, Tours, France)
Том: 1
Год издания: 2016
Издательство: Institute of Electrical and Electronics Engineers
Местоположение издательства: Piscataway, NJ, United States
Первая страница: 162
Последняя страница: 163
Аннотация: Background, Motivation and Objective Devices based on the photoelastic effect are widely used for controlling of light beams. Among the acousto-optic (AO) media, much attention is usually paid to crystals. On the one hand, the AO devices based on the crystals predominate over those based on isotropic media. On the other hand, design of such devices becomes rather complicated. A possible way to optimize the AO devices is selecting materials demonstrating high values of AO figure of merit (AOFM). There are a number of approaches to carry out the AOFM search. However, really simple methods to find all possible geometries of AO interaction having extremal values of AOFM are not known. Statement of Contribution/Methods Obviously such method can be developed only for high-symmetry cubic crystals. The approach is based on the well-known tensor relation for the AOFM. The first step is to specify acoustic mode, i.e., wave vector and polarization direction of acoustic wave and its velocity. The second step is to determine laboratory coordinate system to diagonalize permittivity tensor dε perturbed by the acoustic wave. Diagonal components of this tensor are defined by the acoustic mode and the photo-elastic constants. The last step is to find polarization of diffracted radiation (DR) that maximises the AOFM at a given incident radiation (IR) polarization. Here the polarizations are assumed linear and set by input polarisers. Results/Discussion Straightforward tensor multiplication shows that the AOFM reaches its extremal values when the polarization directions of IR and DR are parallel to one and the same eigenvector of dε. This result has a simple physical interpretation. If the statement is true then the IR becomes an eigenmode of the medium perturbed by the acoustic wave. In other case, the IR is split into "ordinary" and "extraordinary" modes with different diffraction efficiencies I1 and I2. Therefore a resulting diffraction efficiency does not exceed the maximum value of I1 and I2. It is obvious that the AOFM attains the largest value when the IR and DR are polarized along the eigenvector of dε corresponding to the maximum absolute eigenvalue of dε. The advantage of this approach is that there is no need in solution of Fresnel equation and search for a Bragg-matched geometry of AO interaction. However, it was proved that the approach could be applied only for optically isotropic crystals. The first reason for it is that the polarization directions mentioned above depend on acoustic power and refractive index in optically anisotropic crystals. However, the main reason is that Bragg matching condition does not hold for the given polarization directions of the IR and DR. Therefore the new approach allows to determine the optimal values of the AOFM in the optically isotropic crystals directly from the magnitudes of the stiffness and photo-elastic tensors. The performed calculation gave general guidelines to develop highly efficient AO devices.
Добавил в систему: Никитин Павел Алексеевич

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