Victor Ryzhii

The theory of quantum-dot infrared phototransistors

A novel device - the quantum-dot infrared phototransistor (QDIP) - is proposed and considered theoretically. The QDIP utilizes intersubband electron transitions from the bound states. The dark current and sensitivity are calculated using a proposed analytical model of the QDIP. It is shown that the QDIP can exhibit low dark current, high photoelectric gain and sensitivity surpassing the characteristics of other intersubband photodetectors.

Negative dynamic conductivity of graphene with optical pumping

We study the dynamic ac conductivity of a nonequilibrium two-dimensional electron-hole system in optically pumped graphene. Considering the contribution of both interband and intraband transitions, we demonstrate that at sufficiently strong pumping the population inversion in graphene can lead to the negative net ac conductivity in the terahertz range of frequencies. This effect might be used in graphene-based coherent sources of terahertz radiation.

Plasma waves in two-dimensional electron-hole system in gated graphene heterostructures

Plasma waves in the two-dimensional electron-hole system in a graphene-based heterostructure controlled by a highly conducting gate are studied theoretically. The energy spectra of two-dimensional electrons and holes are assumed to be conical (neutrinolike), i.e., corresponding to their zero effective masses. Using the developed model, we calculate the spectrum of plasma waves (spatio-temporal variations of the electron and hole densities and the self-consistent electric potential). We find that the sufficiently long plasma waves exhibit a linear (soundlike) dispersion, with the wave velocity determined by the gate layer thickness, the gate voltage, and the temperature. The plasma wave velocity in graphene heterostructures can significantly exceed the plasma wave velocity in the commonly employed semiconductor gated heterostructures. The gated graphene heterostructures can be used in different voltage tunable terahertz devices which utilize the plasma waves.

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.

Toward the creation of terahertz graphene injection laser

We study the effect of population inversion associated with the electron and hole injection in graphene p-i-n structures at the room and slightly lower temperatures. It is assumed that the recombination and energy relaxation of electrons and holes are associated primarily with the interband and intraband processes assisted by optical phonons. The dependences of the electron-hole and optical phonon effective temperatures on the applied voltage, the current-voltage characteristics, and the frequency-dependent dynamic conductivity are calculated. In particular, we demonstrate that at low and moderate voltages, the injection can lead to a pronounced cooling of the electron-hole plasma in the device i-section to the temperatures below the lattice temperature. However at higher voltages, the voltage dependences can be ambiguous exhibiting the S-shape. It is shown that the frequency-dependent dynamic conductivity can be negative in the terahertz (THz) range of frequencies at certain values of the applied voltage...

Terahertz lasers based on optically pumped multiple graphene structures with slot-line and dielectric waveguides

Terahertz (THz) lasers on optically pumped multiple-graphene-layer (MGL) structures as their active region are proposed and evaluated. The developed device model accounts for the interband and intraband transitions in the degenerate electron-hole plasma generated by optical radiation in the MGL structure and the losses in the slot or dielectric waveguide. The THz laser gain and the conditions of THz lasing are found. It is shown that the lasers under consideration can operate at frequencies ≳1 THz at room temperatures.Terahertz (THz) lasers on optically pumped multiple-graphene-layer (MGL) structures as their active region are proposed and evaluated. The developed device model accounts for the interband and intraband transitions in the degenerate electron-hole plasma generated by optical radiation in the MGL structure and the losses in the slot or dielectric waveguide. The THz laser gain and the conditions of THz lasing are found. It is shown that the lasers under consideration can operate at frequencies & 1 THz at room temperatures.

Contact and distributed effects in quantum well infrared photodetectors

We propose a simple distributed model for intersubband quantum well infrared photodetectors (QWIPs), which explicitly takes into account the injecting properties of the contacts. We show that the QWIP operation with multiple QWs involves the formation of a high‐field domain near the emitter, caused by the modulation of the bound electron density in the QWs by applied voltage and infrared radiation. The external characteristics of the QWIP (total current, differential resistance, and quasistatic capacitance) are strong functions of the voltage and radiation intensity.

Voltage and temperature dependencies of conductivity in gated graphene

The resistivity of gated graphene is studied taking into account electron and hole scattering by short- and long-range structural imperfections the characteristics of disorder were taken from the scanning tunneling microscopy data and by acoustic phonons. The calculations are based on the quasiclassical kinetic equation with the normalization condition fixed by surface charge. The gate-voltage and temperature effects on the resistance peak, which is centered at the point of intrinsic conductivity, are found to be in agreement with the transport measurements.

Unusual capacitance behavior of quantum well infrared photodetectors

We report experimental and simulation results of capacitance of quantum well infrared photodetectors (QWIPs). We found that the QWIP capacitance displays unusual behavior as a function of voltage and frequency, deviating far from the constant geometric capacitance value. At high voltages, capacitance starts with a negative value at low frequencies, increases above zero with frequency, and eventually decays to the geometric capacitance value. The magnitude of negative capacitance exceeds the geometric capacitance by more than two orders of magnitude. Negative capacitance arises when the transient current in response to a voltage step is nonmonotonic with time. Simulation shows that this effect is due to nonequilibrium transient electron injection from the emitter resulting from the properties of the injection barrier and inertia of the QW recharging processes.