Robson Ferreira

Unconventional motional narrowing in the optical spectrum of a semiconductor quantum dot

Abstract‘Motional narrowing’ refers to the striking phenomenon where the resonance line of a system coupled to a reservoir becomes narrower on increasing the reservoir fluctuation. A textbook example is found in nuclear magnetic resonance, where the fluctuating local magnetic fields created by randomly oriented nuclear spins are averaged when the motion of the nuclei is thermally activated. The existence of a motional-narrowing effect in the optical response of semiconductor quantum dots remains so far unexplored. This effect may be important in this instance because the decoherence dynamics is a central issue for the implementation of quantum information processing based on quantum dots. Here we report on the experimental evidence of motional narrowing in the optical spectrum of a semiconductor quantum dot broadened by the spectral-diffusion phenomenon. Surprisingly, motional narrowing is achieved when decreasing incident power or temperature, in contrast with the standard phenomenology observed for nuclear magnetic resonance.

Fast exciton spin relaxation in single quantum dots

Exciton spin relaxation is investigated in single epitaxially grown semiconductor quantum dots in order to test the expected spin relaxation quenching in this system. We study the polarization anisotropy of the photoluminescence signal emitted by isolated quantum dots under steady-state or pulsed nonresonant excitation. We find that the longitudinal exciton spin relaxation time is strikingly short (<= 100 ps) even at low temperature. This result breaks down the picture of a frozen exciton spin in quantum dots.

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.

Giant optical anisotropy in a single InAs quantum dot in a very dilute quantum-dot ensemble

We present experimental evidence of giant optical anisotropy in single InAs quantum dots. Polarization-resolved photoluminescence spectroscopy reveals a linear polarization ratio with huge fluctuations, from one quantum dot to another, in sign and in magnitude with absolute values up to 82%. Systematic measurements on hundreds of quantum dots coming from two different laboratories demonstrate that the giant optical anisotropy is an intrinsic feature of dilute quantum-dot arrays

Unifying the Low-Temperature Photoluminescence Spectra of Carbon Nanotubes: The Role of Acoustic Phonon Confinement

At low temperature the photoluminescence of single-wall carbon nanotubes show a large variety of spectral profiles ranging from ultranarrow lines in suspended nanotubes to broad and asymmetrical line shapes that puzzle the current interpretation in terms of exciton-phonon coupling. Here, we present a complete set of photoluminescence profiles in matrix embedded nanotubes including unprecedented narrow emission lines. We demonstrate that the diversity of the low-temperature luminescence profiles in nanotubes originates in tiny modifications of their low-energy acoustic phonon modes. When low-energy modes are locally suppressed, a sharp photoluminescence line as narrow as 0.7xa0meV is restored. Furthermore, multipeak luminescence profiles with specific temperature dependence show the presence of confined phonon modes.

Unraveling of free-carrier absorption for terahertz radiation in heterostructures

The relation between free-carrier absorption and intersubband transitions in semiconductor heterostructures is resolved by comparing a sequence of structures. Our numerical and analytical results show how free-carrier absorption evolves from the intersubband transitions in the limit of infinite number of wells with vanishing barrier width. It is explicitly shown that the integral of the absorption over frequency matches the value obtained by he f-sum rule. This shows that a proper treatment of intersubband transitions is fully sufficient to simulate the entire electronic absorption in heterostructure THz devices.

Unbound states in quantum heterostructures

We report in this review on the electronic continuum states of semiconductor Quantum Wells and Quantum Dots and highlight the decisive part played by the virtual bound states in the optical properties of these structures. The two particles continuum states of Quantum Dots control the decoherence of the excited electron – hole states. The part played by Auger scattering in Quantum Dots is also discussed.

Electronic continuum states and far infrared absorption of InAs/GaAs quantum dots

The electronic continuum states of InAs/GaAs semiconductor quantum dots embedded in a GaAs/AlAs superlattice are theoretically investigated and the far infrared absorption spectra are calculated for a variety of structures and polarizations. The effect of a strong magnetic field applied parallel to the growth direction is also investigated. We predict that the flatness of the InAs/GaAs dots leads to a far infrared absorption which is almost insensitive to the magnetic field, in spite of the reorganization of the continuum into series of quasi-Landau states. We also predict that it is possible to design InAs/GaAs photoconductors which display very strong in-plane absorption.

Influence of quasibound states on the carrier capture in quantum dots

The interaction of carriers in quantum-dot quasibound states with longitudinal optical phonons is investigated. For a level separation between the quasibound state and a discrete quantum-dot state in the vicinity of the phonon energy, a strong electron–phonon coupling occurs. A mixed electron–phonon mode—polaron—is formed. The finite lifetime of the phonons is shown to give rise to another type of carrier capture into quantum dots.

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.