Two-Step Processes and IR Recording in Photorefractive Crystals

Abstract

Two-step excitation processes have been used for hologram storage in photorefractive crystals. By this means the interference pattern can be formed with red or near—IR light and nondestructive readout of information is possible. Often shallow levels are involved in the holographic recording process in photorefractive crystals. The shallow levels can be populated by illumination with visible or UV pulses forming states with relatively long lifetimes, thus sensitizing the crystals for holographic recording with IR pulses. In LiNbO₃ and LiTaO₃ the most important shallow levels have been identified. They result from Nb_Li⁵⁺ and Ta_Li⁵⁺ antisite defects (Nb⁵⁺ or Ta⁵⁺ on Li⁺ site). The crystals can also be pre-illuminated with visible light from a cw argon laser or a xenon lamp and holograms can be recorded with red light from a laser diode. The sensitization process is possible for other photorefractive crystals, too. The holograms can be read nondestructively with IR light and can be erased with green light. The hologram lifetime is limited by electron tunneling or by an ionic conductivity. Lifetimes up to years can be achieved. Recording of components for telecommunication applications with IR light allows one to create reconfigurable and thus more versatile devices.