Plasmon absorption reduction in multiple quantum well structures.

The damping of two-dimensional plasmons in structures with several quantum wells due to absorption by free carriers is studied theoretically. Both gate structures and structures without a gate are considered. It is shown by the example of structures with GaAs quantum wells that an increase in the number of quantum wells while maintaining the electron concentration in each of them leads to a decrease in the damping coefficient of two-dimensional plasmons. The physical reasons for the decrease in the absorption of plasmons are discussed. It is shown that an increase in the number of quantum wells should lead to a decrease in the decay of plasmons in systems with a finite gate width as well.

Terahertz plasmons in doped HgTe quantum well heterostructures: dispersion, losses, and amplification.

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.

Threshold energies of Auger recombination in HgTe/CdHgTe quantum well heterostructures with 30-70 meV bandgap.

We calculate the threshold energies of Auger recombination in the HgTe/CdHgTe quantum well heterostructures with the bandgap in the 30-70 meV range. It is shown that there is a maximum in the temperature dependence of the threshold energy for Auger process involving two electrons and a hole. For Auger process involving two holes and an electron, in which the hole is in the second valence subband in the final state, the threshold energy decreases down to zero and then increases steeply as temperature grows. The results of calculation can be considered as guidelines for designing the long-wavelength laser structures.

Landau level spectroscopy of valence bands in HgTe quantum wells: effects of symmetry lowering.

The Landau level spectroscopy technique has been used to explore the electronic structure of the valence band in a series of p-type HgTe/HgCdTe quantum wells with both normal and inverted ordering of bands. We find that the standard axial-symmetric 4-band Kane model, which is nowadays widely applied in physics of HgTe-based topological materials, does not fully account for the complex magneto-optical response observed in our experiments-notably, for the unexpected avoided crossings of excitations and for the appearance of transitions that are electric-dipole forbidden within this model. Nevertheless, reasonable agreement with experiments is achieved when the standard model is expanded to include effects of bulk and interface inversion asymmetries. These remove the axial symmetry, and among other, profoundly modify the shape of valence bands.

Radiative recombination in narrow gap HgTe/CdHgTe quantum well heterostructures for laser applications.

Radiative recombination is studied in CdHgTe/HgTe QWs with bandgap in the 40-140 meV range using four-band Kane model. Calculated radiative lifetimes agree well with the photoconductivity kinetics measurements. We show that the side maxima in the valence band hinder the radiative recombination at high carrier concentrations and discuss how to overcome this effect for the development of long-wavelength lasers.

Terahertz surface plasmons in optically pumped graphene structures.

We analyze the surface plasmons (SPs) propagating along optically pumped single-graphene layer (SGL) and multiple-graphene layer (MGL) structures. It is shown that at sufficiently strong optical pumping when the real part of the dynamic conductivity of SGL and MGL structures becomes negative in the terahertz (THz) range of frequencies due to the interband population inversion, the damping of the THz SPs can give way to their amplification. This effect can be used in graphene-based THz lasers and other devices. Due to the relatively small SP group velocity, the absolute value of their absorption coefficient (SP gain) can be large, substantially exceeding that of optically pumped structures with dielectric waveguides. A comparison of SGL and MGL structures shows that to maximize the SP gain the number of graphene layers should be properly chosen.