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Multiphoton excitation allows reversible photorefractive recording without the disadvantage of erasure during readout. The light intensity required for two-photon storage can be drastically reduced if two-step excitation via long†lived real intermediate states is used. Experimental results for LiNbO3:Cr3+ and LiTaO3:Cr3+ demonstrate the advantages of the two-step process. The energy and intensity requirements for optical two-step recording are within reach of presently available repetitively pulsed lasers.
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Linde et al. (1976) studied this question.
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