Tunable Lasers

Tunable radiation in the 0.43 micrometer - 0.52 micrometer spectral region was produced by frequency doubling a tunable dye laser in ZnSe. At wavelengths shorter than the bandgap of ZnSe lambda 0.46 micrometer, second-harmonic power was limited by crystal absorption, whereas for longer wavelengths the second-harmonic power was as much as 1500 times larger than that predicted for a single-domain crystal. This enhancement is predicted by a simple theory which accounts for the favorable nonlinear optical effects of rotational twins and crystal defects. Work has continued on the two photon creation of carriers in ZnSe. It is found that this carrier production method which is dominant at high power levels is very sensitive to wavelength in the 9000 A region, falling off rapidly to about 110th the value of 9200 A that it had at 9000 A. These measurements imply that the saturation in infrared difference frequency generation, which we believe to be due to absorption by these free carriers, can be substantially reduced by using longer wavelength pump lasers. A system has been built to check this theory.