Intersubband transitions in quantum wells as sources of Thz radiation

A suitable engineered asymmetric double quantum well can be designed as a three level system, in which intersub-band transitions between these three states give rise to Terahertz radiation. Due to the lack of sources in this region of the electromagnetic spectrum, this subject is of technological interest. We study in this work two different tunneling processes, which can be observed in this device: coherent and resonant. The latter is related to incoherent processes that can be applied for devices filling the Terahertz gap. The absorption spectra of superlattices built from asymmetric double quantum wells are calculated and a possible population inversion mechanism is proposed. On the other hand, an ultrafast optical excitation creates in an asymmetric double quantum well a macroscopically coherent superposition of the two excited conduction sub-bands. The components eigenstates of the wave packet subsequently evolve at different rate leading to quantum beats and therefore to coherent tunneling between the two wells.