Optical Response Induced by Intersubband Transitions in Quantum Wells: The Role of Multiple Reflections

Abstract

The selection rule for intersubband transitions in GaAs/AlGaAs quantum wells involves a component of the incident electric field along the growth direction (z) [7]. Therefore, the total internal reflection geometry is very often used to enhance the coupling of the infrared radiation with intersubband excitations in the single quantum well (SQW) [2] and multiple quantum well (MQW) [7] structures. Practically, in all papers using this coupling technique modification of the MQW intersubband absorption spectra induced by interference of the light reflected by particular quantum wells and by the semiconductor-air interface is not considered. In another words, the authors assume that absorption results from double pass of the incident wave through the volume of the MQW (see Fig. 1). The above ‘traveling wave approximation’ leads to the following expression for the absorptance of the MQW system: A MQW ≃ 1 - exp(-2N A SQW ) where N is the number of the quantum wells in the system and A SQW (≪ 1) is the absorptance of the SQW (e.g. Refs [7],[6]). In a few papers authors try to include the standing wave effect (connected with the light reflection by semiconductor-air interface) using a perturbation approach [13],[5]. It is based on the assumption that the electromagnetic field distribution in the structure is practically the same as that which exists in the absence of the intersubband transitions [2].