Manipulation of optical patterns by frequency detuning of the pump beams

We show that manipulation and selection of optical patterns is possible by detuning the frequency of one of the two waves in counterpropagating photor efractive two-wave mixing with a feedback mirror. An analytical expression for the onset of modulational instability is derived for the general case o f an arbitrary complex coupling constant, including a possible frequency sh ift of the generated sidebands. The influence of frequency detuning of the pump beams on the spatial scale of the resulting pattern and on the thresho ld value of the photorefractive coupling strength for modulational instabil ity is investigated. The spatial scale and the frequency detuning of the ge nerated sidebands were measured in an experiment using a photorefractive KN bO3 crystal. Comparison of theory and experiment show agreement for positiv e frequency detuning of the pump beams. For negative detuning we observe cl amping of the spatial scale and no frequency shift of the sidebands. Possib le explanations of this deviation between analytical results and experiment are discussed.