Ph. Roussignol

Interferometric correlation spectroscopy in single quantum dots

We report high-resolution spectroscopy by interferometric correlation measurements on the photoluminescence signal of a single quantum dot. We demonstrate that the insertion of a Michelson interferometer in the detection path gives a compact and flexible setup for linewidth measurements. We have used this technique to study self-assembled InAs/GaAs quantum dots. We observe linewidth variations from one quantum dot to another, and we bring evidence of environment effects on the broadening processes.

Third-order optical nonlinearities of carbon nanotubes in the femtosecond regime

Femtosecond pump–probe experiments have been carried out on an ensemble of single-wall carbon nanotubes deposited on a glass substrate. Measurements of transient changes of transmission and reflection provide an estimate of the real and imaginary parts of the second-order hyperpolarizability of carbon nanotubes. These values are compared with previous measurements and are discussed in the light of a simple model of the optical nonlinearities near the optical band-gap.

Time-resolved spin-polarization spectroscopy in GaAs/AlGaAs quantum wells

Abstract We present a study of spin relaxation of excitons in a set of nominally undoped GaAs/AlGaAs quantum well (QW) structures by using time-resolved photoluminescence techniques. The initial polarization and the spin relaxation time have been measured varying the excitation energy between the heavy- and light-hole exciton of each well, at a temperature of 4 K. Typical spin relaxation times in the 50–120 ps range are found when increasing the well width from 25 to 90 A; however no dependence of the spin relaxation time on the excitation wavelength has been observed.

Direct measurement of exciton diffusion in quantum wells

Abstract We have measured the diffusion of excitons in GaAs quantum wells by using spatial and time-resolved photoluminescence (PL) spectroscopy at liquid helium temperature. A displacement of the detected region (O 1.5 μm) with respect to the laser spot allows us to monitor the lateral change of the PL time-dependence. With increasing displacement the maximum of the PL-intensity shifts systematically to later times. For a theoretical description an in-plane diffusion model is applied, with the diffusion constant D as the only fit parameter. We obtain a continuous increase of D from 15 cm2s−1 to 80 cm2s−1 for displacements from 0 to 4.2 μm. A measurement with only spatial resolution leads to a diffusion constant of 20 cm2s−1.

Electron and hole tunneling times in GaAs/AlGaAs asymmetric double quantum well heterostructures

Abstract We present CW and time-resolved photoluminescence measurements on GaAs/AlGaAs asymmetric double quantum well heterostructures. Several samples (eight) have been considered in order to study the influence, on the carrier tunneling process, of both barrier thickness and energy separation between the levels in the different wells. A comparison between photoluminescence decay time of excitonic recombinations in the narrow well and wide well, under resonant and non resonant excitation, allows us to determine both the electron and hole tunneling times. Evidence of non resonant and LO phonon-assisted electron tunneling, resonant and near-resonant hole tunneling is found and discussed.

Ultrafast pump–probe measurements in single wall carbon nanotubes

Abstract We present a time-resolved experimental study of carrier dynamics in single wall carbon nanotubes (SWCNTs). This study is performed by means of pump–probe experiments in either a degenerate or a two-color configuration with a resonant excitation of semi-conductor SWCNTs. Both pump and probe are in resonance with the interband transitions of the semiconductor nanotubes (at 0.8 and 1.47 eV ). We observe photobleaching with a typical recovery time of 1 ps . This non-linear response is governed by the carrier recombination at the band edge of semiconductor nanotubes.

Strong reduction of exciton-phonon coupling in high crystalline quality single-wall carbon nanotubes: a new insight into broadening mechanisms and exciton localization

Carbon nanotubes are quantum sources whose emission can be tuned at telecommunication wavelengths by choosing the diameter appropriately. Most applications require the smallest possible linewidth. Therefore, the study of the underlying dephasing mechanisms is of utmost interest. Here, we report on the low-temperature photoluminescence of high crystalline quality individual single-wall carbon nanotubes synthesized by laser ablation (L-SWNTs) and emitting at telecom-munication wavelengths. A thorough statistical analysis of their emission spectra reveals a typical linewidth one order of magnitude narrower than that of most samples reported in the literature. The narrowing of the PL line of L-SWNTs is due to a weaker effective exciton-phonon coupling subsequent to a weaker localization of the exciton. These results suggest that exciton localization in SWNTs not only arises from interfacial effects, but that the intrinsic crystalline quality of the SWNT plays an important role. Photoluminescence (PL) emission in semiconducting carbon nanotubes arises from exciton recombination [1–3] and has been extensively studied in view of possible applications in opto-electronics, bio-imaging or photovoltaics [4–7]. Observation of photon antibunching in the near infrared [8, 9] suggests that SWNTs are also promising single-photon sources for the implementation of quantum information protocols. Interestingly, the PL emission energy (i.e. the excitonic recombination energy) strongly depends on the tube diameter and can be easily tuned in the telecommunication bands at 0.83 eV (1.5µm) by choosing SWNTs with a diameter of about 1-1.2 nm [10]. SWNTs could therefore make up a very versatile light source for quantum optics. Several studies suggested that the optical properties of SWNTs at low temperature are best described in terms of localized excitons (zero-dimensional confinement), leading to a quantum dot like behavior [11, 12]. Nevertheless, the nature of the traps responsible for this exciton localization is not elucidated yet. In order to address the issue of exciton localization, we studied carbon nanotubes produced by high-temperature synthesis methods such as electric arc or laser ablation methods, which are known for their higher crystalline quality, with a lower density of defects [13–17].

Vertical stacks of small InAs/GaAs self-assembled dots: resonant and non-resonant excitation

Abstract We have performed photoluminescence experiments in samples containing self-assembled quantum dots with different spacer layer thicknesses. A strong filtering effect produced by the GaAs spacer layer on the dots size being stacked is observed for spacers thinner than 10 nm . This effect produces a blue shift of the emission band from stacked dots and a simultaneous line width narrowing. At the same time, given the existence of a broad dot size distribution in the first layer, bigger dots can evolve towards InAs cylinder-like structures, whose emission occurs at appreciably lower energies as compared to the emission band associated to dot stacks (with some GaAs separation).

Carrier dynamics in shallow GaAs/AlGaAs quantum wells

Abstract We report a comprehensive study of carrier relaxation, recombination and escape mechanisms in a set of nine high-quality GaAs/Al x Ga 1− x As shallow quantum wells (SQW), under various conditions of applied electric field, temperature and excitation energy, by means of time-integrated and time-resolved photoluminescence. Our experimental findings are analyzed theoretically and bring a better understanding of SQW properties as well as conventional QWs. In a biased SQW at low temperature, it is shown that photo-carrier escape via direct tunneling results in a strong quenching of the luminescence at fields one order of magnitude smaller than what prevails in conventional QWs. Apart from the field-activated escape process, we demonstrate the existence of thermally activated escape dynamics due to the low effective barrier height in SQWs. Time-resolved photoluminescence at low temperature reveals both a major increase in the relaxation times and radiative recombination times in SQWs, in good agreement with our theoretical model.

Hole spin relaxation in a n-doped quantum well structure

Abstract The dependence of the hole spin relaxation on the electron density is studied in a n-modulation doped 75 A GaAs/ AlGaAs quantum well by means of cw and time-resolved photoluminescence techniques. A slow hole spin relaxation time has been measured (~ 1 ns) and the polarization has been found to be strongly dependent on the in-plane wavevector of the photocreated holes. Calculations are presented which support the experimental findings.