Abstract

An explanation for the ability to optically encode χ⁽²⁾ gratings in centrosymmetric materials has eluded experimentalists and theorists for nearly a decade. Central to the problem is the creation of a large internal dc field (~10⁶ V/m) that alternates with the periodicity necessary for quasi-phase matching. These fields are believed to couple to χ⁽³⁾ of the media to create the χ⁽²⁾ grating responsible for efficient frequency doubling. Q-switched mode-locked pulses at 1.064 μm and 532 nm were used to create χ⁽²⁾ gratings in 1 cm thick samples of a high-barium low-alkali borosilicate glass (SK5, Schott Corp.). Preparation studies revealed an IR intensity threshold dependence of the order of gigiawatts per square centimeter. With the preparing IR intensity held constant at a value sufficiently above the IR threshold, we observed an exponential dependence in the optical encoding of effective χ⁽²⁾ values versus the second harmonic intensity for writing values above 100 W/cm². As the preparing green intensity is lowered, however, a sharp resonance is seen where the amplitude of the encoded grating is over 2 orders of magnitude above the value of the extrapolated exponential fit. This behavior has been observed at both room and liquid nitrogen temperatures.

© 1992 Optical Society of America