Implementation of second-order nonlinearities in semiconductor waveguides

Semiconductors are known to exhibit large second-order nonlinearities. However, efficient generation of new frequencies has not been possible due to the difficulties of phase-matching as zinc-blende semiconductors do not exhibit birefringence and typically have a large dispersion in the near infrared. One possible quasi phase-matching scheme for second harmonic generation in semiconductors is to use a refractive index grating. However, the generation efficiency of such schemes is poor and Bragg scattering can increase losses. In addition, the high symmetry of zinc-blende semiconductors only gives one non-zero χ⁽²⁾ tensor element and therefore restricts the choice of potential device geometries. By breaking the symmetry of the structure, additional (and potentially enhanced) χ⁽²⁾ tensor elements can be generated. One particularly promising method for achieving this is to grow asymmetric quantum well structures. The technique the authors propose for the near infrared is quantum well intermixing to remove the induced asymmetry. The most promising technique in GaAs/AlGaAs systems is impurity-free vacancy disordering (IFVD) due to the low optical losses. Disordering is realised by depositing an SiO₂ cap on the material and then annealing in a rapid thermal processor. Selectivity across the wafer is achieved by placing a layer of SrF₂ between the SiO₂ and the semiconductor in regions where intermixing is not required. The spatial resolution of the process is more than adequate to define gratings with a pitch ~4 μm