Quasi-Permanent All Optical Encoding of Noncentrosymmetry

To avoid the possible painting over of the encoded interference pattern, it is essential for the writing waves to be mutually coherent. This condition is satisfied, for example, in a KDP crystal, where the wave at 2co is produced through frequency doubling of the fundamental wave. 

Readout of the with a third beam (o(M) at fundamental frequency produces a nondisappearing second harmonic polarization, with the complex 

The entire seeding and readout scheme can be viewed as a si x-wave mixing (SWM) process. Such a description can be generalized to a parametric conversion process, where writing waves are the sum of three fields of cOi, 0)2 and CO3 frequencies, satisfying co = co, + 

As can be inferred from Equation 11.1, for a transparent medium at frequency CO and taking into account the losses by absorption with coefficient a at frequency 2co, the light-induced second-order susceptibility can be written as 

ALEKSANDRA APOSTOLUK.CtUNE FIORINI.AND JEAN-MICHEL NUNZl 

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