Аннотация:The electronic structure and the relaxation rates for thin layers are not the same as for the bulk samples due to the quantum-size effect (QSE)1. The QSE results in quantitative (renormalization of bands) and qualitative (splitting of 3D bands into series of 2D subbands) changes of the electronic structure. Similar effects have been found in superlattices as well as in quantum wires and quantum dots2. For today, optical spectroscopy methods have been proved to be a powerful tool in experimental studying of the QSE in wide-band semiconductors. However, in narrow-band semiconductors, one needs to use lasers of the middle- or far-IR ranges. This problem can be solved by means of nonlinear spectroscopy techniques based on two-photon excitation of electronic transitions, for example, biharmonic pumping (BP)3. Using BP, one can obtain detailed information about spectra of states of electron and phonon subsystems, constants of electron-electron (e-e) and electron-phonon (e-p) interactions, relaxation times, etc. In BP, due to e-e and e-p interactions, two electromagnetic waves with different frequencies ω1, 2 and wave vectors k 1, 2 result in electron and phonon excitations at frequency Δ=ω1-ω2 with wave vector Δk=k 1-k 2. Scattering of the same waves on such a transient grating leads to generation of two new waves with frequencies ω4=2ω1, 2-ω2, 1 and wave vectors k 4=2k 1, 2-k 2, 1 It is this process — the selfdiffraction — that enables one to investigate low-frequency transitions using tunable lasers in the visible range.