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.

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.

It has been predicted theoretically that for distances smaller than the peak wavelength of the blackbody spectrum, radiative heat transfer can be increased by the contribution of evanescent waves [1]. These effects have already been observed but a quantitative comparison is still missing. We measure the heat flux between a sphere and a plate as a function of the sphere-plate distance. We find a good agreement between the experimental and theoretical results . Particularly, the distance dependence is well reproduced by the theory in a large range of gap widths: 50nm-2μm.

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.

Radiative heat transfer measurement at the nanoscale

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.

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.