TE- and TM-polarization-resolved spectroscopy on quantum wells under normal incidence

Abstract

We present TE- and TM-polarization-resolved photocurrent measurements on quantum well pin diodes under normal incidence. Usually, optical experiments performed in such a geometry yield information only about transitions involving in-plane ( p x and p y ) components of the hole wave functions because of the in-plane (TE) polarization of the light. Information on transitions sensitive to the p z components can be obtained by focussing a radially polarized laser beam through a microscope objective with high numerical aperture ( NA = 0.9 ). With our setup, the electrical field vector at the focal tail has a significant component along the optical axis (TM-polarization!) which enables excitation of transitions sensitive to p z components also. Additionally, the existence of a degenerate (azimuthally polarized) optical mode enables switching these p z components on and off easily. Experimental evidence of these features has been achieved by exploiting the selection rules for e–hh and e–lh transitions in a quantum well structure. We present a comparison of our recorded spectra with theoretical predictions obtained from simple geometric optics assumptions. For our quantum wells the polarization effects are small because our measurement averages the intensity distribution of the whole focal plane. We plan to extend our measurements to polarization resolved single quantum dot spectroscopy. By restricting the detection region to the spatial extent of a single dot, one can exploit the almost pure TM-polarization on the optical axis for obtaining high contrast between heavy- and light-hole exciton absorption.