THz intervalence band antipolaritons

THz polaritons and antipolaritons have strong potential for device applications and are a challenging field of fundamental studies. In this paper, we start from a numerically exact nonequilibrium many body solutions and adjust it to a simplified nonlinear dielectric constant approach to the optical susceptibility. The resulting expression is inserted in the wave equation to describe the coupling of TE-polarized THz radiation with an intervalence band transition in GaAs/Al0.3Ga0.7As multiple quantum wells embedded in microcavities. The energy dispersions relations leading to THz polaritons are investigated. Here we focus on the impact of dephasing and scattering processes for different structures and excitation conditions in an inverted medium leading to antipolaritons

Tunable quantum dots in bilayer graphene

We demonstrate theoretically that quantum dots in bilayers of graphene can be realized. A position-dependent doping breaks the equivalence between the upper and lower layer and lifts the degeneracy of the positive and negative momentum states of the dot. Numerical results show the simultaneous presence of electron and hole confined states for certain doping profiles and a remarkable angular momentum dependence of the quantum dot spectrum which is in sharp contrast with that for conventional semiconductor quantum dots. We predict that the optical spectrum will consist of a series of non-equidistant peaks.Comment: 5 pages, to appear in Nano Letter

Simulation of Transport and Gain in Quantum Cascade Lasers

Quantum cascade lasers can be modeled within a hierarchy of different approaches: Standard rate equations for the electron densities in the levels, semiclassical Boltzmann equation for the microscopic distribution functions, and quantum kinetics including the coherent evolution between the states. Here we present a quantum transport approach based on nonequilibrium Green functions. This allows for quantitative simulations of the transport and optical gain of the device. The division of the current density in two terms shows that semiclassical transitions are likely to dominate the transport for the prototype device of Sirtori et al. but not for a recent THz-laser with only a few layers per period. The many particle effects are extremely dependent on the design of the heterostructure, and for the case considered here, inclusion of electron-electron interaction at the Hartree Fock level, provides a sizable change in absorption but imparts only a minor shift of the gain peak.Comment: 12 pages, 5 figures included, to appear in in "Advances in Solid State Physics", ed. by B. Kramer (Springer 2003

Landau levels and oscillator strength in a biased bilayer of graphene

We obtain analytical expressions for the eigenstates and the Landau level spectrum of biased graphene bilayers in a magnetic field. The calculations are performed in the context of a four-band continuum model and generalize previous approximate results. Solutions are presented for the spectrum as a function of interlayer coupling, the potential difference between the layers and the magnetic field. The explicit expressions allow us to calculate the oscillator strength and the selection rules for electric dipole transitions between the Landau states. Some transitions are significantly shifted in energy relative to those in an unbiased bialyer and exhibit a very different magnetic field dependence.Comment: To appear in Phys. Rev.

Snake states in graphene quantum dots in the presence of a p-n junction

We investigate the magnetic interface states of graphene quantum dots that contain p-n junctions. Within a tight-binding approach, we consider rectangular quantum dots in the presence of a perpendicular magnetic field containing p-n, as well as p-n-p and n-p-n junctions. The results show the interplay between the edge states associated with the zigzag terminations of the sample and the snake states that arise at the p-n junction, due to the overlap between electron and hole states at the potential interface. Remarkable localized states are found at the crossing of the p-n junction with the zigzag edge having a dumb-bell shaped electron distribution. The results are presented as function of the junction parameters and the applied magnetic flux.Comment: 13 pages, 23 figures, to be appeared in Phys. Rev.

Confined states and direction-dependent transmission in graphene quantum wells

We report the existence of confined massless fermion states in a graphene quantum well (QW) by means of analytical and numerical calculations. These states show an unusual quasi-linear dependence on the momentum parallel to the QW: their number depends on the wavevector and is constrained by electron-hole conversion in the barrier regions. An essential difference with non-relativistic electron states is a mixing between free and confined states at the edges of the free-particle continua, demonstrated by the direction-dependent resonant transmission across a potential well.Comment: Submitted to PR

Simplified model for the energy levels of quantum rings in single layer and bilayer graphene

Within a minimal model, we present analytical expressions for the eigenstates and eigenvalues of carriers confined in quantum rings in monolayer and bilayer graphene. The calculations were performed in the context of the continuum model, by solving the Dirac equation for a zero width ring geometry, i.e. by freezing out the carrier radial motion. We include the effect of an external magnetic field and show the appearance of Aharonov-Bohm oscillations and of a non-zero gap in the spectrum. Our minimal model gives insight in the energy spectrum of graphene-based quantum rings and models different aspects of finite width rings.Comment: To appear in Phys. Rev.