Nonlinear intersubband THz absorption in asymmetric quantum wells

Abstract

The THz nonlinear absorption of asymmetric double quantum wells is calculated. The dependence of the absorption on different parameters of the quantum wells is studied. In particular, different values of the barrier and wells width are considered. Also, a DC field in the direction of growth of the wells is introduced in the calculations and the nonlinear absorption is obtained for different DC and AC field intensities. The nonlinear absorption is obtained by using a perturbation calculation of the nonlinear susceptibility up to the third order within a density matrix approach. The absorption spectra are studied near the anticrossing of the first two excited sub-bands. Special attention is given to the nonlinear peaks due to the intensities of both fields. In particular some interesting negative nonlinear peaks are found and their dependence on the intensities of the fields is discussed. IntroductionThe terahertz band is of great importance due to the rich physical and chemical processes that occur in that region and the lack of sources for applications. Quantum well semiconductors are model systems for the study of the light-matter interaction. West and Eglash have observed large oscillator strengths associated with intersubband transitions [1]. Later other authors predicted that asymmetric quantum well would display giant second-order susceptibilities [2] and it was supported by measurements [3]. In addition large third order susceptibilities in asymmetric wells were observed [4]. More recently Adriano A. Batista et. al. presented a complete study of non linear dynamics in a triple well obtaining a region of optical bistability [5]. In this work we report a systematic study of the role of the geometric interaction (barrier width) between two 2D diluted electron gases confined in an asymmetric double quantum well separated by a barrier (see Fig. 1), which plays the role of coupling the two wells. We calculate the transition energies, linear and nonlinear absorption coefficients for these devices. Then we add a second interaction to the system by applying a external static electric field, which can be varied many orders of magnitude because an exact calculation method is used [6]. Furthermore, we study the dependence of the absorptions spectra on the intensity of the electromagnetic radiation in order to obtain and discuss the nonlinear steady state absorption.