SPACE-CHARGE FIELDS IN PHOTOREFRACTIVE CRYSTALS IN THE PRESENCE OF DIRECT-CURRENT AND ALTERNATING-CURRENT FIELDS - A NEW RESONANCE

It is shown theoretically that the space charge field generated by two -wave mixing in a photorefractive crystal may have a resonant response when a de field and a sinusoidal ac field are simultaneously applied. Resonance occurs when the frequency of the ac held is equal to the ei genfrequency of the space charge wave determined by the de voltage. An alytical expressions are derived for the magnitude of the space charge wave and the optimum ratio of the ac to de field is determined. The e nhancement of the space charge field is shown to be considerably highe r than that obtained by a nonresonant method utilizing a sinusoidal ac field alone. Under optimum conditions the maximum space charge held a vailable is equal to mE(q)/2 root 2, where m is the modulation of the interference pattern and E(q) is the saturation field. (C) 1996 Americ an Institute of Physics.