A. R. Wright

Strong terahertz conductance of graphene nanoribbons under a magnetic field

We demonstrate that the optical response of graphene nanoribbons in the terahertz to far-infrared regime can be significantly enhanced and tuned by an applied magnetic field. The dependence of the threshold frequency on the magnetic field is studied. The ribbons with the strongest terahertz conductance under a magnetic field are those with one-dimensional massless Dirac Fermion energy dispersion. For a given ribbon, there exists an optimal field under which the conductance resonance can occur at the lowest frequency.

Two color plasmon excitation in an electron-hole bilayer structure controlled by the spin-orbit interaction

The dispersion and intensity of coupled plasma excitation in an electron-hole bilayer with Rashba spin-orbit coupling is calculated. We propose to use the spin-orbit coupling in individual layers to tune the intensity of two plasmons. The mechanism can be used to develop a two color terahertz source with tunable intensities.

Terahertz band-gap in InAs/GaSb type-II superlattices

We demonstrate theoretically that it is possible to realize terahertz (THz) fundamental band-gap in InAs/GaSb type-II superlattices (SLs). The presence of such band-gap can result in a strong cut-off of optical absorption at THz bandwidth. This study is pertinent to the application of InAs/GaSb type-II SLs as THz photodetectors.

Gapless insulator and a band gap scaling law in semihydrogenated graphene

We demonstrate two unusual electronic properties of semihydrogenated graphene with variable sized A- or B-hydrogenated domains within the tight-binding formalism as follows: (i) a universal band gap scaling law which states that the band gap depends linearly upon the ratio of the number of A- to B-hydrogenated atoms, NA/NB, reaching zero gap at NA=NB, but independent of the domain size, and (ii) an insulating state with zero band gap at NA=NB, a rare phenomenon in nature. We confirm this gapless insulator state by the zero optical conductance at low frequencies.

The spin-orbit interaction enhanced terahertz absorption in graphene around the K point

We present a quantitative analysis on the effect of the spin-orbit interaction in the optical absorption of @p-electrons in graphene. It has been shown that the optical absorption amplitude of graphene around the K point in the Brillouin zone has a node in the two-dimensional Brillouin zone of honeycomb lattice. We calculated the k-dependent absorption matrix by taking into account the finite spin-orbit interaction in graphene. It was found that the spin-orbit interaction lifts the nodes in the absorption matrix. Furthermore, in the terahertz frequency regime, the spin-orbit interaction can significantly enhance the optical absorption in graphene, by up to 100%.

Mid-infrared absorption by short-period InAs/GaSb type II superlattices

We present a theoretical study on optical properties of short-period InAs/GaSb type-II superlattices (SLs) which can serve for mid-infrared (MIR) detection. The miniband structure of such SLs is calculated using the Kronig-Penney model. On the basis of the energy-balance equation derived from the Boltzmann equation we calculate the optical absorption coefficient. The obtained results agree with recent experimental findings. Moreover, the dependence of the MIR absorption in InAs/GaSb type-II SLs on temperature and well-widths are examined.

Stretching induced Hall current and conductance anisotropy in graphene

We evaluate the effect of stretching on the optical conductance of graphene. It is found that the low energy (Dirac regime) isotropy that leads to the “universal conductance” is lost. More significantly, due to the loss of C3 symmetry, a nonzero Hall conductance emerges for stretching along chiral directions, reaching a maximum at a stretching angle of 45°, and being as high as σ0=e2/4ℏ at van Hove singular point for bond angle changes of about 2°. Our results indicate that the optical properties of graphene can be tuned by a weak mechanical deformation.

Terahertz transmittance in bilayer graphene

We study the optical properties of bilayer graphene in the terahertz range. Within the Dirac regime, it is demonstrated that the NNN coupling term produces a significant deviation in the transmittance of unbiased, undoped bilayer graphene. By varying the electronic doping of a sample, we retrieve the low energy features of the bilayer graphene optical transmittance with next nearest neighbor coupling omitted.

Universal geometric classification of armchair honeycomb nanoribbons by their properties in a staggered sublattice potential

We demonstrate the topological band-gap dependence of armchair honeycomb nanoribbons in a staggered sublattice potential. A scaling law is presented to quantify the band gap variation with potential strength. All armchair nanoribbons are described by one of three distinct classes depending on their width, consistent with previous classifications, namely, the well known massless Dirac condition, potentially gapless, and gapless-superlattice. The ability to tune and, in all cases close, the band-gap via external probes makes our classification particularly relevant experimentally. We propose several systems in which these results should shed considerable light, which have all already been experimentally realized.

Exchange-induced band hybridization in InAs/GaSb based type II and broken-gap quantum well systems

We demonstrate theoretically that the many-body effect such as exchange interaction can cause the hybridization of the electron and hole dispersion relations in InAs/GaSb based type II and broken-gap quantum well (QW) systems. As a result, a terahertz mini-gap at the anti-crossing points of the conduction and valence bands can be induced by the inter-layer electron-hole coupling via the Coulomb interaction. It is shown that the many-body effect is another important source of the hybridization of the dispersion relations in InAs/GaSb QW systems.