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The transport properties of a graphene multiquantum well system are investigated numerically using transfer-matrix method. There are transmission gaps for electrons and holes in the transmission spectra at tilted incidence. In the transmission gaps, a few resonant tunneling peaks appear, defined as transmission windows, which are related with the bound states in the quantum wells. Unlike conventional semiconductor nanostructures, the location and the width of the transmission windows are sensitive not only to the quantum well width but also the incident angle. The number of the quantum wells determines the fine structure of the transmission windows. The anisotropic property is affected in the following way: the increase in well width makes the nonzero-transmission incident angle range decrease, and the interference effect is enhanced as the well number increases. Tiny oscillation of the conductance and fine structures in the middle energy range are due to the resonant tunneling induced by the multiquantum well structure. These oscillating features may be helpful in explaining the oscillatory characteristics in experiment. [ABSTRACT FROM AUTHOR]

When epitaxially grown on silicon carbide, a single layer graphene will exhibit a finite energy bandgap like a conventional semiconductor, and its energy dispersion is no longer linear in momentum space in the low energy regime. In this paper, we present a quantitative analysis on the effect of the SiC substrate in the optical absorption of π-electrons in graphene. We calculated the absorption matrix element and the optical absorption in the near infrared even to the visible region by taking into account the SiC substrate effect. It has been found that the substrate effect can significantly enhance the optical absorption in graphene in the near-infrared region, even by up to 90%. It may be helpful to eliminate the previous discrepancy of optical transmission between the theoretical results and the experimental results in the near-infrared to the visible region. [ABSTRACT FROM AUTHOR]

We have calculated the fifth-order nonlinear optical response at experimentally relevant field strengths within the model of massless Dirac fermions by coupling the massless Dirac fermions to the time-dependent electric field quantum mechanically. It demonstrates that the fifth-order nonlinear optical response plays an important role in the contribution to the optical conductivity of pristine single-layer graphene in the low frequency part of the terahertz regime. The nonlinear effect can enhance the optical activity of single-layer graphene in the terahertz regime and significantly decreases the transmittance of graphene in the regime of frequencies from 0.1 to 0.5 THz. These properties of graphene may be used for photonic and optoelectronic device in the terahertz regime. [ABSTRACT FROM AUTHOR]

The impact of the bandgap induced by transversal constriction on the sub-Poissonian properties of graphene armchair-edge nanoribbons (GANRs) has been investigated in a theoretical perspective. For a typical GANR with a bandgap, the minimal conductivity at the Dirac point becomes more suppressed than that of the gapless case 4e2/πh, and the Fano factor becomes more enhanced than the originally predicted value 1/3. The amplitudes of conductivity suppression and Fano factor enhancement will grow large as the nanoribbon width decreases. And the variance of Fano factor is qualitatively consistent with the reported experimental data. The carriers of GANRs with gaps behave like counterparticles in a semiconductor, and the transition from the sub-Poissonian to a Poissonian process takes place gradually with the reduction of the nanoribbon width. For the low aspect ratio (the sample width over its length) limit, the shot noise property at the Dirac point is no longer sensitive to the boundary edges. For the high limit, it requires a larger aspect ratio for the minimal conductivity and maximal Fano factor to achieve stationary values than that of the gapless case. [ABSTRACT FROM AUTHOR]

We show that the optical responses of a graphene pn junction is dominated by nonlinear intraband and interband processes. At the experimentally relevant electric field intensity, nonlinear conductance is an order of magnitude larger than the linear conductance. Furthermore, the total conductance is negative in the terahertz to far infrared regime. The negative conductance provides a unique mechanism for photon generation in graphene and could be used for developing coherent terahertz radiation sources. [ABSTRACT FROM AUTHOR]

We demonstrate that within the model of massless Dirac fermions, graphene has a strong nonlinear optical response in the terahertz regime. It is found that the nonlinear contribution significantly alters both the single frequency and frequency tripled optical response at experimentally relevant field strengths. The optical activity of single layer graphene is significantly enhanced by nonlinear effects, and the frequency tripled response opens the gateway to photonic and optoelectronic device applications. [ABSTRACT FROM AUTHOR]