Nonlinear generation of light in random structures

It is widely accepted that quadratic nonlinear processes, such as parametric generation or amplification, require the use of materials with a high degree of ordering. In some occasions, such ordering is found at a nanoscale, and in other cases, the order is at a micron scale. When such ordering is not intrinsic to the material, one may introduce a periodical distribution within the nonlinear material to, for instance, compensate the phase mismatch. In that event, the final material would be, in general, composed of two types of domains, distributed periodically across the entire material, one with a given nonlinear coefficient, and the other with the same coefficient with opposite sign [1]. In principle, one would expect that small deviations from the adequate period, or some dispersion in the size of the domains, would lead to a cancellation of the coherent nonlinear process of three wave mixing. However, very recently, it was observed that with polycrystalline samples fabricated with a random orientation of Zinc Selenide (ZnSe) crystalline domains, when the average size of the domains was close to one coherence length (l_c), difference frequency generation grew linearly with the total length of the sample [2]. Similar observations were reported some years ago from SBN needlelike crystalline domains [3] and with the use of rotationally twinned crystals of ZnSe [4, 5]. In all these observations, the efficiency of the process seemed to be strongly linked to an average size of the domain close to the optimal value for quasi-phase matching with periodical inverted domains.