Tunable inter-Landau-level lasing in resonant tunneling multiple quantum well structures

A mechanism of frequency tunable terahertz emission based on inter-Landau level transition in cascade quantum well structures under a condition of sequential resonant tunneling was proposed and theoretically proved. he carrier distribution over Landau levels in resonant tunneling multiple quantum well structures was studied in situation when the upper subband is pumped by resonant tunneling. The Landau level populations were numerically calculated, taking into account the electron-electron scattering as well as single-electron (phonon and interface roughness ) scattering. The contributions of various scattering mechanisms to electron relaxation were outlined and the electron-electron scattering was shown to be the most important one in determining the carrier distribution over the Landau levels. The population inversion between 1-st Landau level of the lowest subband and 0-th Landau level of one of the upper subbands was demonstrated to exist in a wide range of applied magnetic field strength [1,2], thus allowing a continuous tuning of the emission frequency. It was shown that the selection rule forbidding the transition of interest in a magnetic field directed perpendicularly to the structure layers can be effectively suppressed in tilted magnetic field if asymmetric potential along the structure axis is simultaneously applied [3]. A periodic cascade quantum well structure with period consisting of two strongly coupled wells with different widths was considered as one of the possible solutions. For such a structure an optical gain was calculated as a function of the perpendicular component of the tilted magnetic field (Fig.1). Fig.1. Calculated optical gain for (2,0)→(1,1) inter-Landau level transition in periodic double quantum well GaAs/Al0.3Ga0.7As structure (well width 25 and 11 nm, barrier width 2 nm, 100 periods, donor density 8·10 cm ). The work was supported by Russian Basic Research Foundation (project No. 12-02-00564) and MISIS (grant No. 3400022).