Quantum well intermixing for the control of second order non-linear effects in GaAs/AlGaAs asymmetric quantum well waveguides
QW-intermixing was accomplished for several different multiple AQW structures using impurity-free vacancy disordering (IFVD). Unphase-matched second harmonic generation (SHG) experiments were subsequently performed with waveguides fabricated from as-grown and intermixed AQW material. No conclusive evidence was found for the existence of any AQW tensor components. This is consistent with recent calculations which predict, that for the particular AQW structures studied, the AQW non-linearities are negligible. Unphase-matched SHG was, however, observed which was attributable to the large bulk GaAs/AlGaAs d14 coefficient. Furthermore, d14 was reduced by 17% on intermixing. Since the quasi-phase-matching conversion efficiency is proportional to both the square of the magnitude of the non-linearity and the square of the modulation depth, a small but significant reduction in the large d14 coefficient such as this, could lead to useful conversion efficiencies. Selective-area IFVD was achieved using hydrogen plasma processing to inhibit intermixing. Only partial suppression of the IFVD process was, however, achieved. The spatial resolution of the selective-area process was measured for two different MQW structures under different annealing conditions, and was found to be better than 2.2 m. The resolution of the IFVD process is therefore sufficiently good for the control of second order non-linear interactions in GaAs/AlGaAs MQW waveguides. Periodically-intermixed waveguides were fabricated for quasi-phase-matching. These waveguides had measured losses ranging between 3.7 dB/cm and 18 dB/cm depending on the intermixing period. Phase-matching was not, however, observed in these devices. This may have been a consequence of one of several factors including, non-ideal laser tuning characteristics, the use of inaccurate intermixing periods, and a negligible modulation depth in d14. Further work is therefore necessary to establish why phase-matching was not achieved, and to improve the selective-area intermixing process. If these difficulties can be overcome and, in addition, an AQW structure can be designed with associated significant non-linearities, then SHG conversion efficiencies of several hundred %/Wcm2 may be possible.