Abstract: We present a theoretical investigation of the plasmon-polariton modes in grating coupled graphene inside a Fabry-Perot cavity. The cavity or photon modes of the device are determined by the Finite Difference Time Domain (FDTD) simulations and the corresponding plasmon-polariton modes are obtained by applying a many-body self-consistent field theory. We find that in such a device structure, the electric field strength of the incident electromagnetic (EM) field can be significantly enhanced near the edges of the grating strips. Thus, the strong coupling between the EM field and the plasmons in graphene can be achieved and the features of the plasmon-polariton oscillations in the structure can be observed. It is found that the frequencies of the plasmon-polariton modes are in the terahertz (THz) bandwidth and depend sensitively on electron density which can be tuned by applying a gate voltage. Moreover, the coupling between the cavity photons and the plasmons in graphene can be further enhanced by increasing the filling factor of the device. This work can help us to gain an in-depth understanding of the THz plasmonic properties of graphene-based structures.

Abstract: We present a theoretical study on the optoelectronic properties of ABC-stacked trilayer graphene (TLG). The optical conductance and light transmittance are evaluated through using the energy-balance equation derived from the Boltzmann equation for an air/graphene/dielectric-wafer system in the presence of linearly polarized radiation field. The results obtained from two band structure models are examined and compared. For short wavelength radiation, the universal optical conductance sigma(0) = 3e(2)/(4h) can be obtained. Importantly, there exists an optical absorption window in the radiation wavelength range 10-200 mu m, which is induced by different transition energies required for inter- and intra-band optical absorption channels. As a result, we find that the position and width of this window depend sensitively on temperature and carrier density of the system, especially the lower frequency edge. There is a small characteristic absorption peak at about 82 mu m where the largest interband transition states exist in the ABC-stacked TLG model, in contrast to the relatively smooth curves in a simplified model. These theoretical results indicate that TLG has some interesting and important physical properties which can be utilized to realize infrared or THz optoelectronic devices.

Abstract: We present a theoretical study of the optoelectronic properties of bilayer graphene. The optical conductance and transmission coefficient are calculated using the energy-balance equation derived from a Boltzmann equation for an air/graphene/dielectric-wafer system. For short wavelengths (<0.2 µm), we obtain the universal optical conductance =e2/(2). Interestingly, there exists an optical absorption window in the wavelength range 10100 µm, which is induced by different transition energies required for inter- and intra-band optical absorptions in the presence of the MossBurstein effect. As a result, the position and width of this absorption window depend sensitively on temperature, carrier density, and sample mobility of the system. These results are relevant for applications of recently developed graphene devices in advanced optoelectronics such as the infrared photodetectors.

Abstract: We present a detailed theoretical study on the optoelectronic properties of topological insulator thin film (TITFs). The k . p approach is employed to calculate the energy spectra and wave functions for both the bulk and surface states in the TITF. With these obtained results, the optical conductivities induced by different electronic transitions among the bulk and surface states are evaluated using the energy-balance equation derived from the Boltzmann equation. We find that for Bi2Se3-based TITFs, three characteristic regimes for the optical absorption can be observed. (i) In the low radiation frequency regime (photon energy (h) over bar omega < 200 meV), the free-carrier absorption takes place due to intraband electronic transitions. An optical absorption window can be observed. (ii) In the intermediate radiation frequency regime (200 < (h) over bar omega < 300 meV), the optical absorption is induced mainly by interband electronic transitions from surface states in the valance band to surface states in the conduction band and an universal value sigma(0) = e(2) / (8<(h)over bar>) for the optical conductivity can be obtained. (iii) In the high radiation frequency regime ((h) over bar omega > 300 meV), the optical absorption can be achieved via interband electronic transitions from bulk and surface states in the valance band to bulk and surface states in the conduction band. A strong absorption peak can be observed. These interesting findings indicate that optical measurements can be applied to identify the energy regimes of bulk and surface states in the TITF. (C) 2015 AIP Publishing LLC.

Abstract: We present a theoretical study of the optoelectronic properties of monolayer graphene. Including the effect of the electron-photon-phonon scattering, we employ the mass- and energy-balance equations derived from the Boltzmann equation to evaluate self-consistently the carrier densities, optical conductance and transmission coefficient in graphene in the presence of linearly polarized radiation field. We find that the photo-excited carrier density can be increased under infrared radiation and depend strongly on radiation intensity and frequency. For short wavelengths (lambda <3 mu m), the universal optical conductance sigma(0) = e(2)/4h is obtained and the light transmittance is about 0.97-0.98. Interestingly, there is an optical absorption window in the range 4-100 mu m which is induced by different transition energies required for inter- and intra-band optical absorption. The position and width of this absorption window depend sensitively on temperature and carrier density of the system. These results are relevant for applications of recently developed graphene devices in advanced optoelectronics such as the infrared photodetectors. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.

Abstract: Magneto-optical (MO) properties of bi- and tri-layer graphene are investigated utilizing terahertz time-domain spectroscopy (THz TDS) in the presence of a strong magnetic field at room-temperature. In the Faraday configuration and applying optical polarization measurements, we measure the real and imaginary parts of the longitudinal and transverse MO conductivities of different graphene samples. The obtained experimental data fits very well with the classical MO Drude formula. Thus, we are able to obtain the key sample and material parameters of bi- and tri-layer graphene, such as the electron effective mass, the electronic relaxation time and the electron density. It is found that in high magnetic fields the electronic relaxation time tau for bi- and tri-layer graphene increases with magnetic field B roughly in a form tau similar to B-2. Most importantly, we obtain the electron effective mass for bi- and tri-layer graphene at room-temperature under non-resonant conditions. This work shows how the advanced THz MO techniques can be applied for the investigation into fundamental physics properties of atomically thin 2D electronic systems.

Abstract: We present a detailed theoretical study of the optoelectronic properties of a Na3Bi based three-dimensional Dirac electronic system (3DDES). The optical conductivity is evaluated using the energy-balance equation derived from a Boltzmann equation, where the electron Hamiltonian is taken from a simplified k . p approach. We find that for short-wavelength irradiation, the optical absorption in Na3Bi is mainly due to inter-band electronic transitions. In contrast to the universal optical conductance observed for graphene, the optical conductivity for Na3Bi based 3DDES depends on the radiation frequency but not on temperature, carrier density, and electronic relaxation time. In the radiation wavelength regime of about 5 mu m, < lambda < 200 mu m, an optical absorption window is found. This is similar to what is observed in graphene. The position and width of the absorption window depend on the direction of the light polarization and sensitively on temperature, carrier density, and electronic relaxation time. Particularly, we demonstrate that the inter-band optical absorption channel can be switched on and off by applying the gate voltage. This implies that similar to graphene, Na3Bi based 3DDES can also be applied in infrared electro-optical modulators. Our theoretical findings are helpful in gaining an in-depth understanding of the basic optoelectronic properties of recently discovered 3DDESs.

Abstract: We present a theoretical study of the electrical generation of acoustic-phonon emission from graphene at room temperature. The drift velocity (v(x)) and temperature of electrons driven by dc electric field (F-x) are determined by solving self-consistently the momentum-and energy-balance equations derived from the Boltzmann equation. We find that in the presence of impurity, acoustic-and optic-phonon scattering, v(x) can be much larger than the longitudinal (v(l)) and transverse (v(t)) sound velocities in graphene even within the linear response regime. As a result, although the acoustic Cerenkov effect cannot be obviously seen in the analytical formulas, the enhanced acoustic-phonon emission can be observed with increasing F-x when v(x) > v(l) and v > v(t). The frequency of acoustic-phonon emission from graphene can be above 10 THz, which is much higher than that generated from conventional semiconductor systems. This study is pertinent to the application of graphene as hypersonic devices such as terahertz sound sources. (C) 2013 AIP Publishing LLC.

Abstract: We investigate the effect of ellipticity ratio of the polarized radiation field on optoelectronic properties of a two-dimensional (2D) semi-Dirac (SD) system. The optical conductivity is calculated within the energy balance equation approach derived from the semiclassical Boltzmann equation. We find that there exists the anisotropic optical absorption induced via both the intra-and interband electronic transition channels in the perpendicular xx and yy directions. Furthermore, we examine the effects of the ellipticity ratio, the temperature, the carrier density, and the band-gap parameter on the optical conductivity of the 2D SD system placed in transverse and vertical directions, respectively. It is shown that the ellipticity ratio, temperature, carrier density, and band-gap parameter can play the important roles in tuning the strength, peak position, and shape of the optical conductivity spectrum. The results obtained from this study indicate that the 2D SD system can be a promising anisotropic and tunable optical and optoelectronic material for applications in innovative 2D optical and optoelectronic devices, which are active in the infrared and terahertz bandwidths.

Abstract: We investigate the effect of Rashba spin-orbit coupling (SOC) on the optoelectronic properties of n- and p-type monolayer MoS2. The optical conductivity is calculated within the Kubo formalism. We find that the spin-flip transitions enabled by the Rashba SOC result in a wide absorption window in the optical spectrum. Furthermore, we evaluate the effects of the polarization direction of the radiation, temperature, carrier density, and the strength of the Rashba spin-orbit parameter on the optical conductivity. We find that the position, width, and shape of the absorption peak or absorption window can be tuned by varying these parameters. This study shows that monolayer MoS2 can be a promising tunable optical and optoelectronic material that is active in the infrared to terahertz spectral range.

Abstract: We present a detailed theoretical study of the electronic and transport properties of monolayer black phosphorus (BP). This study is motivated by recent experimental activities in investigating n-type few-layer BP systems. The electron density of states, the screening length, and the low-temperature electron mobility are calculated for monolayer BP (MLBP). In particular, the electron transport mobilities along the armchair and zigzag directions are examined on the basis of the momentum-balance equation derived from a semiclassical Boltzmann equation. The anisotropic electron mobilities in MLBP along different directions are demonstrated where the electron-impurity scattering is considered. Furthermore, we compare the results obtained from two electronic band structures of MLBP and find that the simplified model can describe quite rightly the electronic and transport properties of MLBP. This study is relevant to the application of few-layer BP based electronic systems as advanced electronic devices.

Abstract: Using the tight-binding approach, we investigate the electronic properties of bilayer phosphorene (BLP) quantum dots (QDs) in the presence of perpendicular electric and magnetic fields. Since BLP consists of two coupled phosphorene layers, it is of interest to examine the layer-dependent electronic properties of BLP QDs, such as the electronic distributions over the two layers and the so-produced layer-polarization features, and to see how these properties are affected by the magnetic field and the bias potential. We find that in the absence of a bias potential only edge states are layer polarized while the bulk states are not, and the layer-polarization degree (LPD) of the unbiased edge states increases with increasing magnetic field. However, in the presence of a bias potential both the edge and bulk states are layer polarized, and the LPD of the bulk (edge) states depends strongly (weakly) on the interplay of the bias potential and the interlayer coupling. At high magnetic fields, applying a bias potential renders the bulk electrons in a BLP QD to be mainly distributed over the top or bottom layer, resulting in layer-polarized bulk Landau levels (LLs). In the presence of a large bias potential that can drive a semiconductor-to-semimetal transition in BLP, these bulk LLs exhibit different magnetic-field dependences, i.e., the zeroth LLs exhibit a linearlike dependence on the magnetic field while the other LLs exhibit a square-root-like dependence.

Abstract: The discovery of the hydrodynamic electron liquid (HEL) in graphene [D. Bandurin et al., Science 351, 1055 (2016) and J. Crossno et al., Science 351, 1058 (2016)] has marked the birth of the solid-state HEL which can be probed near room temperature in a table-top setup. Here we examine the terahertz (THz) magneto-optical (MO) properties of a graphene HEL. Considering the case where the magnetic length l(B) = root h/eB is comparable to the mean-free path l(ee) for electron-electron interaction in graphene, the MO conductivities are obtained by taking a momentum balance equation approach on the basis of the Boltzmann equation. We find that when l(B) similar to l(ee), the viscous effect in a HEL can weaken significantly the THz MO effects such as cyclotron resonance and Faraday rotation. The upper hybrid and cyclotron resonance magnetoplasmon modes omega(+/-) are also obtained through the RPA dielectric function. The magnetoplasmons of graphene HEL at large wave-vector regime are affected by the viscous effect, and results in red-shifts of the magnetoplasmon frequencies. We predict that the viscosity in graphene HEL can affect strongly the magneto-optical and magnetoplasmonic properties, which can be verified experimentally.