Photorefractive Simulton in Lithium Niobate

The second harmonic generation process strongly depends on the phase-matching condition of the interacting waves. Very far from perfect phase-matching, the wave coupling at the input interface of the nonlinear material generates two second-harmonic pulses, one propagating at the speed given by its refractive index, and one perfectly locked in time with the pump field, and consequently propagating exactly with the same group and phase velocities. Such phenomenon was already theoretically previewed in 1962 by J. A. Armstrong et alii and more specifically finalised in 1999 by M. Mlejnek et alii . Recently W. Su et alii have experimentally confirmed the pulse twin pulse generation [3] while E. Fazio et alii measured the exact locking of the two velocities. Would it be possible to lock the fundamental and second-harmonic beam in space as well? Yes, by means of photorefractive screening solitons. Formation of bright photorefractive-photovoltaic solitons in lithium niobate was previously demonstrated by using CW laser beams as well as light pulses from a Nd-Yag laser. In such case the generated second-harmonic pulses slowly accumulate charges that modify the refractive index. Such modification became active for the fundamental beam as well, which consequently is self-confined, increasing its conversion efficiency.