Single-walled carbon nanotubes (SWNT) are one-dimensional nanostructures where the Coulomb interactions between charge carriers are strongly enhanced compared to systems of higher dimensionality. This results in an electron-hole bound state -the so-called exciton- with a binding energy of the order of one third of the bandgap, which controls the SWNTs optical properties. As a matter of fact, the exciton-exciton interactions are particularly efficient and drive the exciton recombination and dephasing dynamics. The investigation of excitonic complexes in semiconducting SWNTs is currently a topic of intense debate. The biexciton and the trion are expected to have a binding energy of about a hundred meVs. Whereas the trion has been recently observed, first in doped nanotubes and then by means of all-optical generation, there has been no experimental evidence for the biexciton. In this talk I will present our recent work reporting the observation of the biexciton in semiconducting single-wall carbon nanotubes using nonlinear optical spectroscopy. Our experiments consist in a spectrally resolved pump-probe technique in SWNTs embedded in a gelatine at cryogenic temperature. Our measurements of the differential transmission spectrum reveal the universal asymmetric line shape of the Fano resonance intrinsic to the biexciton transition. For nanotubes of the (9,7) chirality, we find a biexciton binding energy of 106 meV. From the calculation of the nonlinear response, we provide a quantitative interpretation of our measurements, leading to an estimation of the characteristic Fano factor q of 7±3 . This value allows us to extract the first experimental information on the biexciton stability and we obtain a biexciton annihilation rate comparable to the exciton-exciton annihilation one.

We investigate experimentally and theoretically the resonant emission of single InAs/GaAs quantum dots in a planar microcavity. Due to the presence of at least one residual charge in the quantum dots, the resonant excitation of the neutral exciton is blocked. The influence of the residual doping on the initial quantum dots charge state is analyzed, and the resonant emission quenching is interpreted as a Coulomb blockade effect. The use of an additional non-resonant laser in a specific low power regime leads to the carrier draining in quantum dots and allows an efficient optical gating of the exciton resonant emission. A detailed population evolution model, developed to describe the carrier draining and the optical gate effect, perfectly fits the experimental results in the steady state and dynamical regimes of the optical gate with a single set of parameters. We deduce that ultra-slow Auger- and phonon-assisted capture processes govern the carrier draining in quantum dots with relaxation times in the 1 - 100 microsecond range. We conclude that the optical gate acts as a very sensitive probe of the quantum dots population relaxation in an unprecedented slow-capture regime.

Depuis la première mesure directe de la réponse optique de boîtes quantiques sous excitation résonnante réalisée par Muller et al. [1], de nombreux résultats sur l'émission résonnante de boîtes quantiques uniques ont été présentés dans la littérature [2]. Toutefois, excepté des études dans le régime de faible puissance d'émission résonnante où la cohérence est imposée par le laser d'excitation résonnante [3], la limite radiative n'est jamais atteinte, même à très basse température. Ceci suggère qu'un environnement électrostatique fluctuant influençant la cohérence des boîtes quantiques existe toujours, même dans le cas d'une excitation strictement résonnante où les porteurs sont uniquement photo-créés dans la boîte quantique. Nos travaux récents sur l'extinction de l'émission résonnante des boîtes quantiques corroborent fortement cette hypothèse [4]. L'excitation résonnante de l'exciton neutre peut être en effet totalement inhibée par un effet de blocage de Coulomb à cause de la présence d'au moins une charge résiduelle dans la boîte quantique. Toutefois, ce problème est résolu grâce à l'utilisation d'un laser non résonnant supplémentaire de très faible puissance qui neutralise la boîte quantique et contrôle optiquement son émission résonnante. Le poster présente une étude complète expérimentale et théorique du contrôle optique dans les boîtes quantiques uniques sous excitation résonnante [5]. L'influence du dopage résiduel sur l'état de charge initial d'une boîte quantique, et par conséquent sur l'extinction de son émission résonnante, est analysée. Un modèle détaillé de l'évolution de la population est développé pour décrire la neutralisation de la boîte quantique par le laser non résonnant supplémentaire de faible puissance. Ce modèle, qui ajuste parfaitement les résultats expérimentaux dans les régimes permanent et dynamique avec un seul ensemble de paramètres, montre que la neutralisation de la boîte quantique est régie par des processus particuliers de capture assistée par effet Auger et par phonons. Au-delà de l'explication de l'effet du contrôle optique, la configuration originale du laser supplémentaire de faible puissance nous permet d'étudier un régime sans précédent où les processus de capture des porteurs impliqués dans la neutralisation des boîtes quantiques se révèlent être ultra-lents avec des constantes de temps de l'ordre de 1-100 μs. Par conséquent, le contrôle optique apparaît comme une sonde très sensible du dopage résiduel de l'échantillon et de la population des boîtes quantiques dans un régime d'excitation non résonnant de très faible puissance. [1] A. Muller et al., PRL 99, 187402 (2007). [2] R. Melet et al., PRB 78, 073301 (2008) ; S. Ates et al., PRL 103, 167402 (2009); A. N. Vamivakas et al., Nat. Phys. 5, 198 (2009) ; H. S. Nguyen et al., PRL 108, 057401 (2012) ; A. Reinhard et al., Nat. Phot. 6, 93 (2012). [3] H. S. Nguyen et al., APL 99, 261904 (2011) ; C. Matthiesen et al., PRL 108, 093602 (2012) ; K. Konthasinghe et al., PRB 85, 235315 (2012). [4] H. S. Nguyen et al., PRL 108, 057401 (2012). [5] H. S. Nguyen et al., PRB 87, 115305 (2013).

Single-walled carbon nanotubes (SWNT) are one-dimensional nanostructures where the Coulomb interactions between charge carriers are strongly enhanced compared to systems of higher dimensionality. This results in an electron-hole bound state -the so-called exciton- with a binding energy of the order of one third of the bandgap, which controls the SWNT's optical properties. As a matter of fact, the exciton-exciton interactions are particularly efficient and drive the exciton recombination and dephasing dynamics. The investigation of excitonic complexes in semiconducting SWNTs is currently a topic of intense debate. The biexciton and the trion are expected to have a binding energy of about a hundred meVs. Whereas the trion has been recently observed, first in doped nanotubes and then by means of all-optical generation, there is no experimental evidence for the biexciton. Here we present the first observation of the biexciton in semiconducting single-wall carbon nanotubes using nonlinear optical spectroscopy. Our experiments consist in a spectrally resolved pump-probe technique in SWNTs embedded in a gelatine at cryogenic temperature. Our measurements of the differential transmission spectrum reveal the universal asymmetric line shape of the Fano resonance intrinsic to the biexciton transition.

Single-walled carbon nanotubes (SWNT) are one-dimensional nanostructures where the Coulomb interactions between charge carriers are strongly enhanced compared to systems of higher dimensionality. This results in an electron-hole bound state -the so-called exciton- with a binding energy of the order of one third of the bandgap, which controls the SWNTs optical properties. As a matter of fact, the exciton-exciton interactions are particularly efficient and drive the exciton recombination and dephasing dynamics [1-3]. The investigation of excitonic complexes in semiconducting SWNTs is currently a topic of intense debate. The biexciton and the trion are expected to have a binding energy of about a hundred meVs. Whereas the trion has been recently observed, first in doped nanotubes [4] and then by means of all-optical generation [5], there has been no experimental evidence for the biexciton. In this talk I will present our recent work reporting the observation of the biexciton in semiconducting single-wall carbon nanotubes using nonlinear optical spectroscopy [6]. Our experiments consist in a spectrally resolved pump-probe technique in SWNTs embedded in a gelatine at cryogenic temperature. Our measurements of the differential transmission spectrum reveal the universal asymmetric line shape of the Fano resonance intrinsic to the biexciton transition. For nanotubes of the (9,7) chirality, we find a biexciton binding energy of 106 meV. From the calculation of the nonlinear response, we provide a quantitative interpretation of our measurements, leading to an estimation of the characteristic Fano factor q of 7±3 . This value allows us to extract the first experimental information on the biexciton stability and we obtain a biexciton annihilation rate comparable to the exciton-exciton annihilation one.

For some conditions, we show that light propagation in perfectly periodic structures can be chaotic. Then we show that light motion is then better described by a diffusion process. We also show that the mean-square distance increase faster than linearly with time characterizing a superdiffusion process.

Semiconductor quantum dots (QDs) are commonly considered as artificial atoms due to the 3D confinement of the electrons. These nanostructures are very promising for the realization of integrated devices such as single photon sources for quantum cryptography applications [1,2]. However, contrarily to genuine atoms, QDs are condensed matter systems and they consequently suffer from the coupling to their environment [3] which degrades the quality of the emitted photons in view of quantum cryptography applications. One way to circumvent this coupling is to excite strictly resonantly the QD at low temperature [4-10]. This difficult task has been achieved by designing an original excitation scheme to spatially decouple the excitation and detection paths. We first report that experimental studies of the resonant emission (RE) of single QDs can be strongly limited, even impossible, since most of the QDs show a very strong quenching of the RE of the fundamental transition. This issue is a broadly met experimental situation which is highly problematic for cavity quantum electrodynamics experiments in single QDs. We propose an efficient way to overcome this issue based on the use of an ultra-weak non-resonant laser that optically gates the QD RE [8] which controls the residual doping inherent to the QDs fabrication. It induces a complete recovery of the properties of an artificial atom, and the single QDs behave as quasi-ideal two-level systems. Up to now [7,9], the measurements on the QDs RE reported in the literature mainly focus on the incoherent fluorescence signal for achieving efficient single photon emission. In this regime, the spectrum linewidth is always limited to the radiative limit. We present experimental studies on the coherent laser light scattering by a single QD [7]. Besides the antibunching effect showing the non-classical nature of the emitted light, we observe that the QD emission spectrum is determined by the spectrum of the resonant excitation laser. By reducing the resonant excitation power, the so-called Mollow triplet shrinks to a single narrow Lorentzian line, while the relative intensity of this narrow peak increases and tends to overwhelm the RE spectrum. This implies that single QDs can produce single photons with a coherence time that is not limited anymore by the QD electronic properties, resulting in what we name an " ultra-coherent " character [7]. In conclusion, the resulting ultra-coherent single photon source promises high degrees of indistinguishability of the emitted photons which is a crucial requirement for quantum information applications. References [1] P. Michler et al., Science 290 (2000). [6] S. Ates et al., PRL 103 (2009). [2] C. Santori et al., Nature 419 (2002). [7] H. S. Nguyen et al., APL 99 (2011). [3] A. Berthelot et al., Nat. Phys. 2 (2006). [8] H. S. Nguyen et al., PRL 108 (2012). [4] A. Muller et al., PRL 99 (2007). [9] C. Matthiesen et al., PRL 1082 (2012). [5] R. Melet et al., PRB 78 (2008). [10] A. Reinhard et al., Nat. Phot. 6 (2012).