We have calculated two-dimensional plasmon energy spectra in HgTe/CdHgTe quantum wells with normal, gapless, and inverted energy spectra with different electron concentrations, taking into account spatial dispersion of electron polarizability and plasmon interaction with the optical phonons. The spectra of the absorption coefficients of two-dimensional plasmons are found. It is shown that an increase of electron concentration in a quantum well leads to a decrease in the plasmon absorption coefficient. We have calculated the probabilities to recombine via the plasmon emission for nonequilibrium holes. The threshold concentrations of the nonequilibrium holes, above which the plasmon amplification is possible, have been calculated for various electron concentrations. It is shown that the presence of equilibrium electrons can significantly reduce the threshold hole concentration required for amplification of plasmon in the terahertz wavelength region. The dependencies of threshold hole concentration on electron concentration for different quantum wells are discussed. Gain spectra of the two-dimension plasmon are calculated.
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