Phase Retardation and Induced Birefringence Related to Intersubband Transitions in Multiple Quantum Well Structures

Abstract

It is well known that the selection rules for intersubband transitions in quantum well structures require that the infrared light should 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 mid-infrared range of the spectrum using a number of experimental techniques, including FTIR absorption spectroscopy and optical phase retardation measurements using a tunable CO2 laser and a cross polarizer set-up. We have observed that a linearly polarized light become almost circularely polarized due to optical phase retardation between the ordinary and the extraordinary components of the optical field over a short 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 extra-ordinary 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.