Abstract.We investigate the role of carrier mobility in holographic recording in LiNbO 3 crystals. Both normal holographic recording (single wavelength, single trap) and twocenter recording are considered, and the differences between the performances of the two methods are explained. We show that increasing mobility by using stoichiometric crystals or by doping with Mg does not improve sensitivity considerably, but does reduce M/# by at least one order of magnitude. 42.40.Pa; 42.40.Ht Volume holographic storage systems are tried and tested with photorefractive crystals as the recording medium [1]. In such systems, the inhomogeneous illumination created by the interference of the reference and signal beams excites charge carriers from impurity levels into the conduction or valence bands, the charge carriers migrate and they are trapped by empty impurities elsewhere. A space-charge field builds up and modulates the refractive index via the electro-optic effect.
PACS:The important material properties are sensitivity (S), dynamic range (M/#) and persistence (or non-destructive readout). For several years, lack of persistence was the major drawback of holographic storage in photorefractive crystals, and several methods (thermal fixing [2], electrical fixing [3], two-photon recording [4], frequency-difference holograms [5] and readout with wavevector spectra [6] ) were proposed to solve this problem. More recently, two-center holographic recording was proposed and demonstrated [7,8]. Two-center recording makes storage of persistent holograms possible without sacrificing the dynamic range and sensitivity considerably [9]. The sensitivity of the widely used congruently melting LiNbO 3 crystals, however, is still not very * Corresponding author.(Fax: +1-404/894-4641, E-mail: adibi@ece.gatech.edu) good. Therefore, the main challenge in holographic storage in LiNbO 3 crystals is the improvement of the sensitivity.Sensitivity is a measure of recording speed. It is defined as the initial recording slope of the square root of the diffraction efficiency normalized to recording intensity and material thickness. Recording and erasure time constants in holographic recording are approximately inversely proportional to the mobility of the carriers responsible for recording (i.e., electrons in the conduction band or holes in the valence band). Holograms can be recorded faster if we increase the mobility of carriers responsible for recording. Therefore, one idea for increasing sensitivity could be to increase the carrier mobility. In almost all holographic recording experiments in LiNbO 3 , these carriers are electrons in the conduction band. It is suggested that the mobility of these electrons (µ) can be varied by changing the stoichiometry of LiNbO 3 : congruently melting LiNbO 3 crystals (typical ratio of Li to Nb about 94%) have a lower electron mobility than perfectly stoichiometric crystals (ratio of Li to Nb equal to 1). It is also suggested that the mobility of electrons in the conduction band of LiNbO 3 can be varied by ...