Bandgap Engineering for the Control of Second Order Non-linearities in GaAs/AlGaAs Asymmetric Multiple Quantum Well Waveguides

Second order non-linear optical effects in quasi-phase-matched structures have been the subject of much research activity in recent years. In particular, cascaded χ(2):χ(2) interactions can give rise to non-linear phase shifts which may be employed in an all-optical switch device [1]. Here we are concerned with a novel quasi-phase-matching scheme which relies on the fact that an asymmetric MQW waveguide has associated with it significant second-order susceptibility tensor components which are not present for a symmetric QW structure [2]. By periodically intermixing an asymmetric MQW waveguide along its length therefore, these second-order non-linearities can, in theory, be modulated to achieve domain-disordered quasi-phase-matching. In this paper we shall present experimental evidence to demonstrate the feasibility of realising such a device in GaAs/AlGaAs for operation at 1.55µm. Un-phase-matched second-harmonic conversion efficiencies have been observed for disordered and non-disordered waveguides which suggest that the nonlinearity associated with asymmetric QWs is significantly reduced by intermixing. Experimental results also indicate that the spatial resolution of the impurity-free vacancy diffusion (IFVD) QW intermixing process [3] used throughout this work is sufficient for first-order quasi-phase-matching of the second harmonic generation.