Non-volatile holographic storage in doubly-doped photorefractive material

DETAILED DESCRIPTION The holographic devices and techniques in the present disclosure are based on a special kind of photorefractive materials that are doped to include first and second types of photorefractive centers of different energy levels.
Each type of photorefractive centers can exist in two valance states so that centers in one valance state can absorb certain radiation energy to produce charge carriers to the common band (e.
g.
, the conduction band for electrons) and centers in another valance state can recombine with a charge carrier to function as a trap.
The energy gap between the energy levels of the two different types of photorefractive centers is large enough so that a monochromatic light in resonance with a transition from the energy level of one type of photorefractive centers to the common band is not resonant with the transition from the energy level of another type of photorefractive centers to the common band.
The doping densities of the two different types of centers are preferably low so that two types of centers essentially do not interact with each other by a non-radiative coupling.
In addition, the gaps between energy levels of both types of centers and the common band are much greater than the thermal energy k.
sub.
B T associated with the operating temperature T to minimize the probability of thermal excitation of charge carriers.
Under these conditions, the population distribution of charge carriers in the photorefractive centers can be primarily controlled by the excitations by the external radiation and by the recombination process of the excited charge carriers with the centers.
These features of such doubly-doped photorefractive materials can be used to implement a special type of two-step holographic recording and to achieve non-volatile holographic storage.
One example of such doubly-doped photorefractive materials is a lithium niobate crystal (LiNbO.
sub.
3) doped with manganese ions (Mn.
sup.
2+ and Mn.
sup.
3+) and iron ions (Fe.
sup.
2+ and Fe.
sup.
3+).
FIG