Abstract

We report use of free-carrier absorption and refraction in GaP in optical limiting of visible radiation. Excitation of GaP at wavelengths between ~446–546 nm results in indirect transitions between the valence band and the conduction band minima near the χ-points. The resulting photogenerated electron–hole pairs provide a carrier density dependent source of absorption and refraction. On time scales short compared with recombination and diffusion times, such optical nonlinearities are strictly fluence dependent. Limiters utilizing GaP therefore can operate over a wide range of pulse durations. Furthermore, the incorporation of mid-gap impurity states in GaP may extend the operation of the limiter below the intrinsic indirect absorption edge, thereby providing a relatively broadband yet resonant response. The configuration discussed here is achieved by focusing a ~30-ps 532-nm pulse into a thin wafer of GaP, recollimating the beam with a second identical lens, and then transmitting the beam through an aperture to take advantage of the optically induced lensing in the sample. As the fluence at the sample is increased, the system transmission decreases, and the output fluence becomes clamped at ~0.6 μJ/cm² for fluences at the sample greater than ~100 mJ/cm². Pump-probe measurements verify that the nonlinearity depends only on the accumulated carrier density and that is recovers on a time scale long compared with a nanosecond. This result indicates that the device should function equally well for nanosecond excitation.

© 1991 Optical Society of America