Abstract

We have previously reported¹ the demonstration of a GaP nonlinear optical limiter for 532 nm pulses that provides a clamped output fluence that is considered "eye-safe." Optical excitation at this wavelength couples states across the indirect band gap, resulting in the linear generation of large free-carrier densities. These carriers subsequently provide both nonlinear absorption and refraction. Here, we report the measurement of these nonlinearities using picosecond pump-probe techniques. Time-resolved absorption measurements indicate not only the anticipated effects of free-carrier absorption associated with linearly generated electron-hole pairs, but also clear evidence of two-photon absorption. Analysis of these data yields both the free-carrier cross section and the two-photon absorption coefficient. Time-resolved measurements of the pump-induced defocusing of a weak probe beam allow us to determine the index change per carrier pair and to isolate this process from lensing associated with the electronic Kerr effect.

© 1992 Optical Society of America