Читать книгу All sciences. №4, 2023. International Scientific Journal - Ibratjon Xatamovich Aliyev - Страница 4

The effect of nonequilibrium carriers on birefringence in ferroelectric and piezoelectric crystals has been called the photorefractive effect (FR effect) in the literature and has found wide use for the registration of volumetric holograms. The FR effect is as follows. As a result of local illumination of a ferroelectric or piezoelectric crystal by intense transmitted light (focused laser beam), a reversible change in birefringence occurs in the crystal volume inside the light beam, mainly due to a change in the refractive index of the extraordinary beam ne. The magnitude of this change reaches 10-4 -10-3 for some pyroelectrics (LiNbO3 LiTa03), and its lifetime can vary widely, from milliseconds in BaTiO3 to months in LiNbO3. The hologram is recorded due to the volumetric modulation of the Dn value corresponding to the modulation of the recording beam. The recording resolution is exceptionally high, 102-104 lines/mm. [7, 9].

The main advantage of this optical memory method in comparison with photographic layers is the possibility of parallel recording, reading and erasing.

As shown by the sign, and the magnitude of the photovoltaic current depends on the symmetry of the crystal and the polarization of light. Photovoltaic current leads to the generation of abnormally large photovoltaic voltages in the same direction. Thus, during the exposure time t, a macroscopic field appears in the crystal.

(5)

Due to the linear electric effect, the field leads to the FR effect:

(6)

where rij are electro—optical coefficients. Equation (6) is written in the main coordinate system. After illumination, the field remains in the crystal for a long time due to the capture of nonequilibrium electrons and holes. This capture mechanism is responsible for optical memory.

Use in holographic recording in LiNbO3:Fe gives advantages. In this case, the recording is carried out by the photovoltaic effect (FE) corresponding to the photovoltaic current. The generated photon voltage of the order (103-105) V/cm is responsible for the optical memory in LiNbO3:Fe crystals.

Erasure can be carried out by annealing the crystal at 1700C. There are other methods of erasing.