Non-linear optical phenomena, like self focusing, phase conjugation and four-wave mixing (FWM) have direct applications in coherent control of optical communications. InN is a semiconductor particularly interesting for telecommunications, since it has direct band-gap with a room-temperature energy of 0.65-0.7 eV, and predicted high optical nonlinear coefficients. These nonlinear properties can be enhanced using low dimensional structures, like InN/InGaN multiple quantum wells (MQWs). At the same time, these structures offer the possibility of tuning the resonance wavelength, making them very attractive for all-optical devices in communications applications, like optically controlled delay lines and wavelength converters.

In this work, we have measured the non-linear absorption of InN/In_xGa_1-xN MQWs with 0.9>x>0.7 grown by plasma-assisted molecular beam epitaxy on GaN-template substrates. The structural quality of the samples were confirmed from high-resolution X-ray diffraction data, which reveal well-defined multiple satellite peaks for all samples, confirming the existence of the MQW structure with abrupt interfaces. Low temperature (5 K) photoluminescence measurements show a redshift of the emission energy when increasing the Ga content in the barriers. This behaviour is explained in terms of an increase of the piezoelectric fields present in the well. The Z-scan method is used to estimate the non-linear optical absorption coefficient, a₂, of the samples at 1.5 µm. The nonlinear behaviour of the MQWs shows a dependence on the Ga content in the QW barrier. Saturable absorption, with a₂~-8.9x10³ cm/GW, is obtained for samples with low Ga content. On the other hand, Two Photon Absorption (TPA) mechanism is observed for samples with high Ga contents, obtaining values of a₂~3.6x10³ cm/GW. The observed TPA is attributed to photoexcitation of photogenerated carriers from the confined level in the QWs.

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