  Terahertz photoconductivity of $100~\mu$m and $20~\mu$m 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.
