V. Ya. Aleshkin

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.

Feasibility of terahertz lasing in optically pumped epitaxial multiple graphene layer structures

A multiple graphene layer (MGL) structure with a stack of GLs and a highly conducting bottom GL on SiC substrate pumped by optical radiation is considered as an active region of terahertz and far infrared lasers with external metal mirrors. The dynamic conductivity of the MGL structure is calculated as a function of the signal frequency, the number of GLs, and the optical pumping intensity. The utilization of optically pumped MGL structures might provide the achievement of lasing with the frequencies of about 1 THz at room temperature due to a high efficiency of pumping.

Cyclotron resonance and interband optical transitions in HgTe/CdTe(0 1 3) quantum well heterostructures

Cyclotron resonance spectra of 2D electrons in HgTe/CdxHg1−xTe (0 1 3) quantum well (QW) heterostructures with inverted band structure have been thoroughly studied in quasiclassical magnetic fields versus the electron concentration varied using the persistent photoconductivity effect. The cyclotron mass is shown to increase with QW width in contrast to QWs with normal band structure. The measured values of cyclotron mass are shown to be systematically less than those calculated using the 8 × 8 Kane model with conventional set of HgTe and CdTe material parameters. In quantizing pulsed magnetic fields (Landau level filling factor less than unity) up to 45 T, both intraband (CR) and interband magnetoabsorption have been studied at radiation wavelengths 14.8 and 11.4 µm for the first time. The results obtained are compared with the allowed transition energies between Landau levels in the valence and conduction bands calculated within the same model, the calculated energies being again systematically less (by 3–14%) than the observed optical transition energies.

Injection terahertz laser using the resonant inter-layer radiative transitions in double-graphene-layer structure

We propose and substantiate the concept of terahertz (THz) laser enabled by the resonant electron radiative transitions between graphene layers (GLs) in double-GL structures. We estimate the THz gain for TM-mode exhibiting very low Drude absorption in GLs and show that the gain can exceed the losses in metal-metal waveguides at the low end of the THz range. The spectrum of the emitted photons can be tuned by the applied voltage. A weak temperature dependence of the THz gain promotes an effective operation at room temperature.

Self-organization of germanium nanoislands obtained in silicon by molecular-beam epitaxy

Nanometer germanium islands in epitaxial layers of silicon are obtained by molecular-beam epitaxy. The dimensions and shapes of the islands are determined in an atomic-force microscope. The photoluminescence spectra are found to contain lines that can be interpreted as quasidirect optical transitions in the islands. It is concluded on the basis of optical and microprobe measurements and theoretical calculations of the energies of electronic states that silicon is dissolved in the germanium islands. Values of the germanium and silicon contents in the solid solution are presented.

Study of lifetimes and photoconductivity relaxation in heterostructures with HgxCd1 − xTe/CdyHg1 − yTe quantum wells

Carrier lifetimes in the continuum of the quantum well of a HgxCd1 − xTe/CdyHg1 − yTe hetero-structure were studied by terahertz pump-probe spectroscopy. It is found that the relaxation duration of the transmission signal is ∼65 ps and is independent of the pump power. Such rapid relaxation in these structures is most likely determined by the interaction of holes with acoustic phonons due to a high density of states in the valence band and a larger effective mass compared with electrons. By the obtained data, the times of the interband nonradiative recombination of holes are determined. In this publication, we report the results of numerical calculation of the energy spectrum of the model structure, in which the possibility of obtaining population inversion at specified concentrations of nonequilibrium carriers is analyzed.

Difference mode generation in injection lasers

The nonlinear generation of a difference mode in an injection laser is considered. A new design based on the InGaP/GaAs/InGaAs heterostructure is suggested in order to generate two laser modes with a wavelength of about 1 µm and a difference mode at a wavelength of about 10 µm. In lasers with a 100-µm-wide waveguide, the power output of the difference mode can be as high as ∼10 mW at ∼10 W in the short-wavelength modes.

Voltage-tunable terahertz and infrared photodetectors based on double-graphene-layer structures

We propose and theoretically substantiate the concept of terahertz and infrared photodetectors using the resonant radiative transitions between graphene layers (GLs) in double-GL structures. The calculated absorption spectrum and the spectral characteristics of the photodetector responsivity exhibit sharp resonant maxima at the photon energies in a wide range. The resonant maxima can be tuned by the applied voltage. We compare the photodetector responsivity with that of the GL p-i-n photodiodes and quantum-well infrared photodetectors. Weak temperature dependences of the photocurrent and dark current enable the effective operation of the proposed photodetector at room temperature.

Terahertz laser based on optically pumped graphene: Model and feasibility of realization

We consider a terahertz laser based on optically pumped graphene layer and bilayer as the active media and suggested waveguide structure. Using the developed model, we calculate the spectral dependences the dynamic conductivity of the optically pumped graphene layer and bilayer associated with the interband and intraband transitions, estimate the pumping optical power required for lasing, and demonstrate the feasibility of realization of such a laser.

Terahertz spectroscopy of quantum-well narrow-bandgap HgTe/CdTe-based heterostructures

The energy spectra of quantum-well narrow-bandgap Hg1 − yCdyTe/CdxHg1 − xTe heterostructures have been studied. The dependences of the effective cyclotron mass on the density (in classical magnetic fields) and the transition energy (in quantizing fields) have been obtained from the cyclotron resonance measurements. These dependences confirm the near-linear dispersion law for the electrons with small mass at the band bottom (the minimum cyclotron mass measured is 0.003 m0). The interband photoconductivity of the CdHgTe-based structures with the long-wavelength photoresponse edge lower than 6 meV has been demonstrated.