Efficient generation of the second harmonic of CO 2 laser pulses was achieved in GaSe single crystals at repetition frequencies up to 100 Hz. The phase-matching angles were measured for GaSe at pump wavelengths of 9.3, 9.6,10.3, and 10.6/zm. The threshold power densities for surface optical damage and for the formation of a plasma jet on the surface of GaSe were determined. The efficiency of second harmonic generation in a GaSe crystal 0.65 cm thick was up to 9%. A comparison was made of GaSe and ZnGeP 2 crystals as materials suitable for the generation of the second harmonic of CO 2 laser radiation.
The energy spectrum of charge carriers in thin films differs from that in bulk specimens. This distinction shows itself in the quantum size effects. F o r the first time quantum size effects were found in thin B i films /l/.The authors of /1/ observed oscillations of the kinetic coefficients in dependence on the film thickness. These oscillations were due to sharp changes in the density of states when the energy of quantized levels was equal to the Fermi energy of charge carriers.It is well-known that the density of states is governed by the dispersion relation. For example, in the case of an ellipsoidal non-parabolic spectrum /2/ the density of electron states (per unit surface) in Bi films has the following form /3/:Here axis 1 is directed along the binary axis, axis 2 along the elongation of the constant energy surface, axis 3 along the trigonal axis (we neglected the tilt angle 'p%6 of the axis 3 t o the trigonal axis), g, = 2 is the spin degeneracy, gv = 3 is the valley degeneracy, n = 1, 2 . . . is the size 0 quantdm number, a = 1/E (E is the band g g m, side function, d is the film thickness, and EnWe must note that formula (1) is correct for the case when theJfilm a plane is normal to the trigonal axis. In the present note we restrict ourselves to this case only for simplicity.As it is seen from (1) the density of states is a step-like function of energy.The expression of N(E) for an ellipsoidal parabolic spectrum may be 1) prospekt Narimanova, 370143 Baku, USSR.
The theory of the electron Raman scattering (ERS) in a size-quantized film within the framework of the Abrikosov-Cohen model for the energy spectrum of Bi is developed. The wavefunctions and spectrum of electrons in the film are obtained. lt is shown that the zero boundary conditions must be imposed on the total wavefunction instead of on the envelope. The boundary conditions for the envelope have a complicated form and are similar to those found by Volkov and Pinsker. It is shown that unlike the standard model of the energy spectrum of carriers the scheme of an intraband ERS process with an intermediate state in thevalence band is realizedwithin the frameworkofthe Abrikosov-Cohenspectrum. On the basisofthescheme described, the frequencydependenceofthe ERsdifferentialcross section is calculated. The cross section has a non-zero value for any An (An is the change in the electron quantum number due to the ERS process) at arbitrary orientation of the polarization vectors of incident and scattered radiation with respect to the film surface.
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