Semiconductor Nonlinearities For Optical Limiting

Passive optical limiting results from irradiance-dependent nonlinear optical processes in materials. We review the mechanisms responsible for passive limiting using semiconductors.1,2These devices utilize two-photon absorption (2PA) along with photogenerated carrier defocusing within the material to limit the output fluence and irradiance. We previously ignored the contribution from the bound electronic nonlinear refactive index n2.1,2Recent measurements by us how that n2contributes significantly to the observed self-defocusing.3It has previously been assumed that the electronic Kerr induced refraction (n2) was positive. Using our newly developed Z-scan technique we observe (see Fig. 1) that n2changes sign from positive to negative at a wavelength somewhere between the 2PA and fundamental absorption edges.