Nonlinear frequency conversion in isotropic semiconductor waveguides
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This thesis describes an experimental investigation of optical frequency conversion in isotropic semiconductor waveguides by use of several phase-matching approaches. Efficient, type I second harmonic generation of femtosecond pulses is reported in birefringently-phase-matched GaAs/Alox waveguides pumped at 2.01 µm. Practical second harmonic average powers of up to ~ 650 µW are obtained, for an average launched pump power of ~ 5 mW. This corresponds to a waveguide conversion efficiency of ~ 20 % and a normalized conversion efficiency of greater than 1000 % W⁻¹cm⁻². Pump depletion of more than 80 % is recorded. Second harmonic generation by type I, third order quasi-phase-matching in a GaAs- AlAs superlattice waveguide is reported for fundamental wavelengths from ~1480 to 1520 nm. Quasi-phase-matching is achieved through modulation of the nonlinear coefficient χ[sub](zxy)⁽²⁾, which is realised by periodically tuning the superlattice bandgap. An average output power of ~25 nW is obtained for a launched pump power of <2.3 mW. Type I second harmonic generation by use of first order quasi-phase-matching in a GaAs/AlAs symmetric superlattice waveguide is also reported, with femtosecond fundamental pulses at 1.55 µm. A periodic spatial modulation of the bulk-like second- order susceptibility χ[sub](zxy)⁽²⁾ is realized using quantum well intermixing by As⁺ ion implantation. A practical second harmonic average power of ~1.5 µW is detected, for a coupled pump power of ~11 mW. Second harmonic generation through modal-phase-matching in GaAs/AlGaAs semiconductor waveguides is reported. Using femtosecond pulses, both type I and type II second harmonic conversion is demonstrated for fundamental wavelengths near 1.55 µm. An average second harmonic power of ~10.3 µW is collected at the waveguide output for a coupled pump power of <20 mW. For a complete characterisation, the optical loss is measured in these nonlinear GaAs- based waveguides over the spectral range 1.3-2.1 µm in the infrared, by deploying a femtosecond scattering technique. Typical losses of ~5-10 dB/cm are measured for the best of the waveguides, while a systematic intensity and wavelength dependent study revealed the contribution of Rayleigh scattering and two photon absorption in the overall transmission loss.
Thesis, PhD Doctor of Philosophy
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