SUBHARMONIC INSTABILITIES IN PHOTOREFRACTIVE CRYSTALS FOR AN APPLIED ALTERNATING ELECTRIC-FIELD - THEORETICAL-ANALYSIS

The spatial subharmonic instability arising from the material equations is investigated theoretically for both square and sinusoidal applied alternating electric fields. The threshold conditions are given for the amplitude of the applied field as a function of the detuning frequency, contrast of fundamental interference pattern, and interbeam angle by the use of the temporal solutions for the amplitude of the space-charge field presented elsewhere [J. Opt. Soc. Am. B. 10, 124 (1993)] The different effects of the square wave and sinusoidal applied fields for subharmonic generation are discussed.