Time-resolved carrier dynamics in single-wall carbon nanotubes is investigated by means of two-color pump-probe experiments. The recombination dynamics is monitored by probing the transient photobleaching observed on the interband transitions of the semiconducting tubes. This dynamics takes place on a 1 ps time scale which is 1 order of magnitude slower than in graphite. Transient photoinduced absorption is observed for nonresonant probing and is interpreted as a global redshift of the pi-plasmon resonance. We show that the opening of the band gap in semiconducting carbon nanotubes determines the nonlinear response dynamics over the whole visible and near-infrared spectrum.

Motional narrowing refers to the striking phenomenon where the resonance line of a system coupled to a reservoir becomes narrower when 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 since 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.PACS numbers: 78.67. Hc, 78.55.Cr, In the seminal work on motional narrowing by Bloembergen et al., relaxation effects in nuclear magnetic resonance were beautifully explained by taking into account the influence of the thermal motion of the magnetic nuclei upon the spin-spin interaction [1]. The general treatment of relaxation processes for a system interacting with a reservoir was later formulated by Kubo in a stochastic theory that assumes random perturbations of the system by a fluctuating environment [2]. Depending on the relative magnitude of the spectral modulation amplitude and the inverse of the modulation correlation time, the relaxation dynamics is either in the slow modulation limit, where the optical line-shape reflects directly the statistical distribution of the different system energies, or in the fast modulation limit where the fluctuation is smoothed out and the line-shape is motionally narrowed into a Lorentzian profile. The relevance of motional narrowing for the description of relaxation phenomena has spread throughout many different fields, such as spin relaxation in semiconductors [3], vibrational dephasing in molecular physics [4], or phase noise in optical pumping [5].The optical spectrum of a material system with localized, zero-dimensional electronic states provides a generic example of the influence of a fluctuating environment on the coherence relaxation dynamics. In that case, the perturbing interactions induce a stochastic shift over time of the optical spectrum, resulting in the so-called spectral diffusion effect, which was observed for rare-earth ions [6], molecules [7], or semiconductor quantum dots [8,9]. In this latter system, impurities, defects or localized charges in the vicinity of a quantum dot induce micro-electric fields that shift the quantum dot emission line through the quantum confined Stark effect. The fluctuation of the quantum dot environment thus randomize the emission energy over a spectral range Σ on a characteristic time scale τ c . Spectral dif...

Optical parametric oscillation is a nonlinear process that enables coherent generation of 'signal' and 'idler' waves, shifted in frequency from the pump wave. Efficient parametric conversion is the paradigm for the generation of twin or entangled photons for quantum optics applications such as quantum cryptography, or for the generation of new frequencies in spectral domains not accessible by existing devices. Rapid development in the field of quantum information requires monolithic, alignment-free sources that enable efficient coupling into optical fibres and possibly electrical injection. During the past decade, much effort has been devoted to the development of integrated devices for quantum information and to the realization of all-semiconductor parametric oscillators. Nevertheless, at present optical parametric oscillators typically rely on nonlinear crystals placed into complex external cavities, and pumped by powerful external lasers. Long interaction lengths are typically required and the phase mismatch between the parametric waves propagating at different velocities results in poor parametric conversion efficiencies. Here we report the demonstration of parametric oscillation in a monolithic semiconductor triple microcavity with signal, pump and idler waves propagating along the vertical direction of the nanostructure. Alternatively, signal and idler beams can also be collected at finite angles, allowing the generation of entangled photon pairs. The pump threshold intensity is low enough to envisage the realization of an all-semiconductor electrically pumped micro-parametric oscillator.

We present an experimental and theoretical study of the existence of acoustic phonon sidebands in the emission line of single self-assembled InAs/GaAs quantum dots. Temperature-dependent photoluminescence measurements reveal a deviation from a Lorentzian profile with the appearance of lateral sidebands. We obtain an excellent agreement with calculations done in the framework of the Huang-Rhys formalism. We conclude that the only relevant parameter for the observation of acoustic phonon sidebands is the linewidth of the central zero-phonon line. At high temperature, the quasi-Lorentzian quantum dot line appears to be fully determined by the acoustic phonon sidebands

We study the excitonic recombination dynamics in an ensemble of (9,4) semiconducting single-wall carbon nanotubes by high-sensitivity time-resolved photoluminescence experiments. Measurements from cryogenic to room temperature allow us to identify two main contributions to the recombination dynamics. The initial fast decay is temperature independent and is attributed to the presence of small residual bundles that create external nonradiative relaxation channels. The slow component shows a strong temperature dependence and is dominated by nonradiative processes down to 40 K. We propose a quantitative phenomenological modeling of the variations of the integrated photoluminescence intensity over the whole temperature range. We show that the luminescence properties of carbon nanotubes at room temperature are not affected by the dark/bright excitonic state coupling.