Anisotropy, birefringence, and optical phase retardation related to intersubband transitions in multiple quantum well structures

It is well known that the selection rules for intersubband transitions in quantum well structures require that the infrared light be polarized parallel to the growth direction. As a result, the induced intersubband susceptibility tensor becomes highly anisotropic and the crystal becomes birefringent. We have studied the effect of induced birefringence at the midinfrared range of the spectrum using a number of experimental techniques, including Fourier transform infrared absorption spectroscopy and optical phase retardation measurements using a tunable CO₂ laser and a cross polarizer setup. We have observed that linearly polarized light becomes almost circularly polarized due to optical phase retardation between the ordinary and the extraordinary directions over a short optical path length of the order of 20 μm near the resonance (but not at the resonance). The real and the imaginary parts of the induced extraordinary refractive index were measured and have been found to be of the same order of magnitude. We also show that a solution of the Fresnel equation, modified to take into account both the imaginary and the real parts of the susceptibility tensor, is in good agreement with our experimental results.