Determination of ordering induced birefringence in (Al)GaInP

In this paper, the first measurement of the ordering induced birefringence in GaInP and (Al_0.33Ga_0.67)InP is presented. It is found that these ordered crystals are positively birefringent far below the bandgap and become negatively birefringent at the bandgap. The method to measure the birefringence is based on the modifications introduced by ordering to the mode structure of planar waveguides. The change of the sign of birefringence at the bandgap is due to the highly anisotropic interband matrix element. The dispersion of the measured birefringence is in good agreement with six-band k·p calculations.