Abstract

The third order susceptibility is responsible for a variety of optical non-linear phenomena - like self focusing, phase conjugation and four-wave mixing-with applications in coherent control of optical communication. InN is particularly attractive due to its near-IR bandgap and predicted high nonlinear effects. Moreover, the synthesis of InN nanostructures makes possible to taylor the absorption edge in the telecomunication spectral range and enhance nonlinear parameters thanks to carrier confinement. In this work, we assess the nonlinear optical behavior of InN/InxGa(1-x)N (0.9 > x > 0.7) multiple-quantum-well (MQW) structures grown by plasma-assisted MBE on GaN-on-sapphire templates. Low-temperature (5 K) photoluminescence measurements show near-IR emission whose intensity increases with the In content in the barriers, which is explained in terms of the existence of piezoelectric fields in the structures. The nonlinear optical absorption coefficient, alpha(2), were measured at 1.55 mu m using the Z-scan method. We observe a strong dependence of the nonlinear absorption coefficient on the In content in the barriers. Saturable absorption is observed for the sample with x = 0.9, with alpha(2) similar to -9x10(3) cm/GW. For this sample, an optically controlled reduction of the speed of light by a factor S similar to 80 is obtained at 1.55 mu m. (C) 2010 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim