A. A. Dubinov

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.

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.

The gain enhancement effect of surface plasmon polaritons on terahertz stimulated emission in optically pumped monolayer graphene

Giant gain enhancement effect of surface plasmon polaritons (SPPs) on terahertz (THz) stimulated emission in optically pumped monolayer graphene has been experimentally studied. We observed the spatial distribution of the THz probe pulse intensities under linear polarization of optical pump and THz probe pulses. It was clearly observed that an intense THz probe pulse was detected only at the area where the incoming TM mode THz probe pulse being capable of exciting the SPPs. The observed gain factor ~50 is in fair agreement with theoretical calculations.

Terahertz Laser with Optically Pumped Graphene Layers and Fabri–Perot Resonator

We propose and substantiate the concept of terahertz (THz) laser based on the optically pumped graphene layers and the resonant cavity of the Fabri–Perot type. The pumping scheme which corresponds to the optical interband excitation of graphene followed by the emission of an optical phonons cascade provides the population inversion for the interband transitions in a relatively wide range of THz frequencies. We demonstrate that the THz lasing in the device under consideration at room temperatures is feasible if its structure is optimized. The frequency and output power of the generated THz radiation can be tuned by varying the distance between the mirrors.

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.

Double-graphene-layer terahertz laser: concept, characteristics, and comparison

We propose and analyze the concept of injection terahertz (THz) lasers based on double-graphene-layer (double-GL) structures utilizing the resonant radiative transitions between GLs. We calculate main characteristics of such double-GL lasers and compare them with the characteristics of the GL lasers with intra-GL interband transitions. We demonstrate that the double-GL THz lasers under consideration can operate in a wide range of THz frequencies and might exhibit advantages associated with the reduced Drude absorption, weaker temperature dependence, voltage tuning of the spectrum, and favorable injection conditions.

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.

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.