Abstract
Terahertz photoconductivity of 100 μm and 20 μm Hall bars fabricated from narrow AlAs quantum wells (QWs) of different widths is investigated in this paper. The photoresponse is dominated by collective magnetoplasmon excitations within the body of the Hall structure. We observed a radical change of magnetoplasma spectrum measured precisely for AlAs QWs of width ranging from 4~nm up to 15~nm. We have shown that the observed behavior is a vivid manifestation of valley transition taking place in the two-dimensional electron system. Remarkably, we show that photoresponse for AlAs QWs of width 6~nm features two resonances, indicating simultaneous occupation of strongly anisotropic X_x-y valleys and isotropic X_z valley in the QW plane. Our results pave the way to realizing valley-selective layered heterostructures, with potential application in valleytronics.