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The second harmonic generation efficiency was studied for the single crystalline potassium dihydrogen phosphate matrix with incorporated TiO2 nanocrystals in the field of the mode-locked Nd:YAG laser pulses. For the first time the frequency conversion efficiency gain was observed for KDP:TiO2 single crystal (SC) instead of KDP SC due to the internal self-focusing effect of the pump beam caused by the giant cubic nonlinear optical response of the embedded nanoparticles.

We investigate theoretically and experimentally the operation of a nonlinear system, feedback controlled photorefractive beam coupling. It is shown that the feedback equations used earlier (Ϯ/2 phase shift between the diffracted and transmitted beam components͒ are satisfactory only during a relatively short initial stage of development. We have generalized these equations by taking into account the inertia inherent in any real feedback loop. The inertial feedback equations ensure the permanent operation of the nonlinear system and lead to a variety of periodic and quasiperiodic regimes. We investigate the distinctive features of these regimes and transitions between them for moderately thick crystals. Good qualitative agreement between theory and experimental data is obtained for LiNbO 3 crystals.Sciences, 252650 Kiev, Ukraine. § Email address: ringhofe@uos.de FIG. 1. Scheme of an experiment on feedback controlled photorefractive two-beam coupling: PD is a photodiode, LAI denotes a lock-in amplifier and integrator, PM is a piezomounted mirror adjusting the input phase s .