(a* is the light absorption coefficient).
In [10], the photovoltaic coefficient in lithium niobate crystals of the order K = (2-3) * 10—9A * cm* (W) -1 was determined.
This paper reviews and discusses photovoltaic, optical (photorefractive) and sound memory in lithium niobate crystals.
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.
Lithium niobate is widely used in holography and storage devices due to its excellent ferroelectric and piezoelectric properties. Just as magnetic materials «remember» the magnetic field, ferroelectrics can «remember» the electric field under certain conditions.
1. OPTICAL MEMORY IN LITHIUM NIOBATE CRYSTALS
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.
2. SOUND MEMORY IN LITHIUM NIOBATE CRYSTALS
Slightly yellowish single crystals of lithium niobate LiNbO3 have been used in technology for quite a long time. This is a surprisingly "talented" material: ferroelectric (its permittivity depends on the electric field strength, temperature and pre-polarization). The crystal contains special microscopic regions – ferroelectric domains that differ in the direction of polarization. The dimensions of the domains are 10-7-10-5 m, or 0.1—10 microns. By acting with an electric field, the domains can be moved around the crystal, in a strong field, the direction of polarization of all domains can be made the same (the crystal becomes monodomain). When the temperature rises to a certain value, the ability to polarize and form domains disappears. Lithium niobate has a very high temperature (Curie point) of 1210°C. Polarization occurs as a result of the mismatch of the "centers of gravity" of positive and negative charges in the crystal, a small and consistent displacement of ions from the position at which the charges completely compensate each other.
Physicists from the University of Mississippi M. McPherson, I. Ostrovsky and M. Breazil. studying the passage of short pulses of ultrasound through a thin plate of lithium niobate (LiNbO3), a new physical effect of "sound memory" in crystals was discovered [11].
Unexpectedly, scientists discovered that another ultrasonic signal with the same frequency and phase is emitted by the crystal seventy milliseconds after the passage of the main pulse. The study showed that the volume of the "echo" depends on the temperature of the crystal and the frequency of ultrasound. The effect is maximal at 26 megahertz and disappears at temperatures above 75 degrees Celsius, but it was reproduced at lower temperatures.
The acoustic quirk of lithium niobate may be related to its very unusual and extremely useful electrical properties: when compressed, it creates an electric field. Electric fields change the trajectory of light passing through it. Therefore, the substance is used in fiber-optic communication media and in holographic memory.
Each lithium niobate crystal consists of patches of so-called ferroelectric domains. Brizil suspects that the frequency of the delayed echo generated by the crystal is related to the size of these domains, which determine the suitability of the material for various purposes.
Just as magnetic materials "remember" the magnetic field, ferroelectrics can "remember" the electric field under certain conditions. This feature is widely used in the manufacture of electromagnetic detectors and other devices.
According to scientists, it is closely related to the properties of domains (regions with the same electrical polarization) inside the crystal and is explained by the formation and subsequent relaxation of electric charges near the boundaries of the domains.
The effect has not yet found a reliable theoretical explanation and needs to be rechecked, but it is already clear that it can be successfully used to control the quality of lithium niobate plates.
It appears that lithium niobate stores sound energy temporarily. How this happens is not yet clear, but researchers and we note that the sound wave compresses the substance through which it passes. This creates electric fields in the crystal, which moves the electrically charged atoms that the crystal contains. When the incoming sound from the outside stops, the ions return back, but not all in the same direction – the movement is divided by domains that define the boundaries at which the direction changes.
According to the law of conservation of energy, when the ions return, they release the received energy in the form of a delayed acoustic wave. This makes every domain sound. A stronger echo is associated with the resonance frequency of the domains, which depends on their size.
It is assumed that the magnitude of the echo depends on the concentration of domains and that the effect can be used to determine the quality of crystals. What the actual nature of the effect is remains to be seen.
Maybe the stones will really talk?
CONCLUTION
This paper reviews and discusses photovoltaic, optical (photorefractive) and sound memory in lithium niobate crystals. Coefficients in lithium niobate crystals of the order K = (2-3) × 10—9A ×cm× (W) -1.
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 in LiNbO3:Fe crystals of the order (103-105) V/cm is responsible for optical memory.
Lithium niobate is widely used in holography and storage devices due to its excellent ferroelectric and piezoelectric properties. Just as magnetic materials «remember» the magnetic field, LiNbO3 ferroelectric can «remember» the electric field under certain conditions.
LITERATURE
1. Ryvkin. S. M. Photoelectric phenomena