Camille Ndebeka-Bandou

Free-carrier absorption in quantum cascade structures

We show that the free-carrier absorption in quantum cascade lasers (QCLs) is very small and radically different from the classical Drude result due to the orthogonality between the direction of the carrier free motion and the electric field of the laser emission. A quantum mechanical calculation of the free-carrier absorption and intersubband oblique absorption induced by interface defects, Coulombic impurities, and optical phonon absorption/emission is presented for QCLs with a double-quantum-well design. The interaction between the electrons and the optical phonons dominates at room temperature.

Relevance of intra- and inter-subband scattering on the absorption in heterostructures

We analyze the absorption lineshape for inter-subband transitions in disordered quasi two-dimensional heterostructures by an exact calculation. The intra-subband scatterings control the central peak, while the tails of the absorption line are dominated by the inter-subband scattering terms. Our numerical study quantitatively assesses the magnitude of the free carrier absorption. The accuracy of different models currently used for gain/absorption is discussed.

Terahertz Intersublevel Transitions in Single Self-Assembled InAs Quantum Dots with Variable Electron Numbers

We propose a method for performing terahertz spectroscopy on nanometer (nm)-scale systems by using metal nanogap electrodes. Intersublevel transition spectra of single self-assembled InAs quantum dots (QDs) have been measured with high signal/noise ratios by using a single electron transistor geometry that consists of a QD and nanogap metal electrodes as a terahertz detector. Photocurrent distribution with respect to the Coulomb diamonds indicates that there are two mechanisms for the photocurrent generation. When the p shell was fully occupied, we observed rather simple photocurrent spectra induced by the p → d transitions. However, when the p shell was half-filled, the photocurrent spectra exhibited a markedly different behavior, which we attribute to the fluctuation in electron configuration when the empty p state is filled back from the electrodes.

Free carrier absorption and inter-subband transitions in imperfect heterostructures

We present the results of a quantum mechanical modelling of the free carrier absorption (FCA) in semiconductor heterolayers. Elastic and inelastic scatterers are considered with emphasis on the interface defects (optical phonons) contributions to the induced photon absorption for elastic (inelastic) scatterers. Various approaches to FCA are also presented (perturbation, Greens function technique). The connection between inter-subband absorption and FCA is thoroughly discussed. The absorption lineshape and its modification by suitable doping is presented.

Importance of the localization for inter-subband scattering rates in quasi two-dimensional structures

The inter-subband scattering in quasi two-dimensional heterostructures is critically examined in the case of cascade structures with a double well design. We show that when two kinds of elastic scatterers are present, the scattering frequencies may markedly differ from the one evaluated for plane wave states for the in-plane motion at the Born approximation. We argue that this feature results from the disorder-induced spatial localization of the eigenstates in the layer plane. The scatterer location dependence of the scattering frequencies is also discussed. At low concentration the in-plane localization is found to increase the inter-subband scattering.

Disorder effects in InAs/GaSb topological insulator candidates

We report the theoretical investigation of the disorder effects on the bulk states of inverted InAs/GaSb quantum wells. As disorder sources we consider the interface roughness and donors/acceptors supplied by intentional doping. We use a

Gate-controlled terahertz single electron photovoltaic effect in self-assembled InAs quantum dots

\bm{k}\cdot\bm{p}

Free-carrier absorption in asymmetric double quantum well structures due to static scatterers in the in-plane polarization

approach combined with a numerical diagonalization of the disordered Hamiltonian to get a full insight of the disordered eigenenergies and eigenfunctions of the electronic system. While interface roughness slightly pertubs the carrier motion, we show that dopants strongly bind and localize the bulk states of the structure. Moreover, both types of scatterers strengthen the intrinsic hybridization between holes and electrons in the structure.

Quantum model of gain in phonon-polariton lasers

We have observed a terahertz (THz) induced single electron photovoltaic effect in self-assembled InAs quantum dots (QDs). We used a single electron transistor (SET) geometry that consists of a single InAs QD and nanogap electrodes coupled with a bowtie antenna. Under a weak, broadband THz radiation, a photocurrent induced by THz intersublevel transitions in the QD is generated even when no bias voltage is applied to the SET. The observed single electron photovoltaic effect is due to an energy-dependent tunneling asymmetry in the QD-SET. Moreover, the tunneling asymmetry changes not only with the shell but also with the electron number in the QD, suggesting the manybody nature of the electron wavefunctions. The THz photovoltaic effect observed in the present QD-SET system may have potential applications to nanoscale energy harvesting.

Absorption in disordered heterostructures: Contributions from intra- and inter-subband scattering and impact of localised states

We report on the computation of the free carrier absorption induced by static scatterers in cascade structures when the electromagnetic wave propagates along the growth axis. We find that a Drude-like tail exists for this polarization. The absorption is found larger than when the wave propagates in the layer plane. Also intra-subband scattering is found more efficient than inter-subband scattering. The alloy scattering is found to be particularly efficient.