The questions of the existence and stability of multi-excitonic states, in which the one-dimensional character of carbon nanotubes implies strong Coulomb interactions, are currently the subject of intense debates. These states and their coupling with excitonic states play a fundamental role within the exploitation of excitonic properties. Recently, experimental studies reported the observation of the trion (exciton-carrier bound state). In order to study the biexciton, we have performed nonlinear spectral-hole burning experiments at a low temperature, bringing the first evidence for biexcitonic induced-absorption.

We present a review on the excitonic homogeneous linewidth, with an emphasis on original experiments by means of nonlinear spectral hole-burning. We address three specific aspects of the excitonic homogeneous broadening: (i) the variation of the homogeneous linewidth with the sonication energy, revealing the influence of extrinsic processes such as exciton-defect interactions, (ii) the collision-induced broadening due to exciton-exciton interactions, investigated by power-dependent measurements, and (iii) the phonon-induced dephasing due to exciton-phonon interactions, deduced from temperature-dependent experiments. We discuss the specific properties of one-dimensional excitons in carbon nanotubes in the context of the homogeneous broadening.

We report the observation of the biexciton in semiconducting single-wall carbon nanotubes by means of nonlinear optical spectroscopy. Our measurements 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 χ(3) 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.

GaN/AlN quantum dots are semiconductor nanostructures where the huge conduction band offset of 1.75 eV leads to intraband transitions at exceptionnaly short wavelengths compared to III-As semiconductor materials. For specific quantum dot dimensions, it was shown that the absorption spectrum covers the optical communication spectral range. The ultrafast relaxation dynamics observed for these materials motivates the development of opto-electronic devices, and especially ultrafast switches. However, a detailed insight into the microscopic processes governing the relaxation dynamics is still missing although of fundamental importance for the understanding and the optimization of the devices operation. In particular, there is no study of the homogeneous broadening which drives the saturation properties of GaN/AlN quantum dots-devices. In this paper, we present homogeneous linewidth measurements by means of the nonlinear optical technique of spectral hole-burning. Spectrally-resolved pump-probe experiments with cw-lasers allow us to characterize the coherence relaxation dynamics down to 5K by resolving the homogeneous linewidth in an ensemble of GaN/AlN quantum dots. The differential transmission measured at 5K for a TM-polarized pump shows a pronounced bleaching with an amplitude of 2x10-4 on a background signal of 5x10-5. According to the selection rules of the s-pz intraband transition in GaN/AlN QDs, we attribute the photoinduced increase of the probe transmission to the partial saturation of the quantum dots by the pump laser, and we deduce a homogeneous linewidth of 15 meV at 5K. We find that the differential transmission signal scales like the square-root of the incident pump power, thus revealing the importance of electron-electron scattering in the population relaxation dynamics. As a matter of fact, we find that the population relaxation from the pz excited state is predominantly governed by Auger-type processes whereas optical phonon emission only plays a minor role, even if it may contribute to secondary processes during the carrier cascade to the fundamental s state via intermediate confined states.

The one-dimensionality of carriers in SWNTs results in strong Coulomb interactions, the non-perturbative binding of an electron-hole pair into an exciton being the first striking signature. Besides the excitonic nature of the optical resonances in SWNTs, several time-resolved experiments have shown the evidence that the strong Coulomb correlations also affect the relaxation dynamics in SWNTs. In particular, photoluminescence and pump-probe measurements using ultrafast excitation pulses have revealed that the population relaxation dynamics is driven by an efficient exciton-exciton annihilation (EEA) process, even at low exciton density. This Auger process is one type of exciton-exciton interaction where one exciton recombines while the second one is promotted to a high-energy state. Since EEA is known to play a key role in the population relaxation of Frenkel excitons in organic materials, especially J-aggregates and conjugated polymers, the high efficiency of EEA in SWNTs suggests that excitons in SWNTs are mostly of Frenkel type. In the following, we will show that the EEA process does not account for the excitonic collision-induced broadening measured in SWNTs. Power-dependent measurements of the excitonic homogeneous linewidth demonstrate the predominance of another type of exciton-exciton interaction, namely elastic exciton-exciton scattering (EES). This latter process redistributes the population within exciton bands and contributes to pure dephasing of the excitonic transition. EES is widely observed for Wannier excitons in inorganic bulk semiconductors or quantum wells but not for Frenkel excitons in organic materials, leading to a refined picture of excitons in SWNTs in between Wannier and Frenkel excitons.

Single photon emission is one of the most striking optical properties of semiconductor quantum dots (QDs), and the corresponding photon anti-bunching effect was observed by non-resonant and resonant photoluminescence experiments in single QDs [1,2,3,4]. Other experiments demonstrated the possibility to use single QDs for the generation of indistinguishable single photons [3]. An important general issue is to reach the so-called fundamental radiative limit. Several strategies have been followed in that sense such as the shortening of population lifetime induced by the Purcell effect [5] or the reduction of pure dephasing by resonant excitation [2,3,4]. Up to now, all the measurements reported in the literature focus on the incoherent photoluminescence signal for achieving single photon emission. In this regime, the linewidth of the photoluminescence spectrum is always limited to the radiative limit. In this paper, we report on the resonant emission (RE) of the exciton in single InAs/GaAs QDs embedded in a planar microcavity, in an orthogonal excitation-detection geometry [4]. We present an original type of single photon source based on the coherent laser light scattering by a single QD [6]. 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. We present high-resolution measurements by Fourier transform spectroscopy of the RE where the data are fitted to the first-order correlation function g(1)(τ) of the RE of a two-level system. The inverse Fourier transform of the g(1)(τ) function allows a direct comparison of the different RE emission profiles. We show that, 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 an ultra-coherent character [6]. 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. [1] P. Michler et al., Science 290, 2282 (2000). [2] A. Muller et al., Phys. Rev. Lett. 99, 187402 (2007). [3] S. Ates et al., Phys. Rev. Lett. 103, 167402 (2009). [4] H. S. Nguyen et al., Phys. Rev. Lett. 108, 057401 (2012). [5] C. Santori et al., Nature 419, 594 (2002). [6] H. S. Nguyen et al., Appl. Phys. Lett. 99, 261904 (2011).

Single photon emission is one of the most striking optical properties of semiconductor quantum dots (QDs), and the corresponding photon anti-bunching effect was observed by non-resonant and resonant photoluminescence experiments in single QDs [1,2]. Up to now, all the measurements reported in the literature focus on the incoherent photoluminescence signal for achieving single photon emission. In this regime, the linewidth of the photoluminescence spectrum is limited to the so-called radiative limit. In this paper, we report on the resonant emission (RE) of the exciton in single InAs/GaAs QDs embedded in a planar microcavity [2]. We present an original type of single photon source based on the coherent laser light scattering by a single QD [3]. Besides the antibunching effect showing the nonclassical nature of the emitted light, we observe that the QD emission spectrum is determined by the spectrum of the resonant excitation laser. Figure (a,b,c) shows high-resolution measurements by Fourier transform spectroscopy of the RE where the data are fitted to the first-order correlation function g(1)(τ) of the RE of a two-level system. The inverse Fourier transform of the g(1)(τ) function allows a direct comparison of the different RE emission profiles in figure (d,e,f). By reducing the resonant excitation power, the 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 an ultra-coherent character [3]. References [1] P. Michler et al, Science 290, 2282 (2000). [2] H. S. Nguyen et al, Phys. Rev. Lett. (accepted, in press). See arXiv 1109.4304. [3] H. S. Nguyen et al, Appl. Phys. Lett. 99, 261904 (2011).