Terahertz-frequency intraband absorption in semiconductor quantum dot molecules

Abstract

We have studied THz absorption of samples containing two layers of self-aligned, self-assembled InAs quantum dots separated by a thin GaAs barrier. The electronic population of the vertically-coupled dots is controlled by an applied bias between a metal gate and a doped layer beneath the dots. Under flat band conditions, an absorption peak is observed near 10 meV (2.4 THz). The absorption is attributed to the intersublevel transition between the quantum mechanically split bonding and antibonding levels in the quantum dot molecules. This absorption can be bleached under high excitation intensity delivered by a free-electron laser. The saturation intensity is found to be of order 1 W cm-2. A lower limit for the relaxation time T1 of the order of 30 ps is deduced from the saturation intensity.