The broad-area vertical-cavity surface-emitting lasers (VCSELs) can be brought into a bistable regime by the use of a frequency-selective feedback (as well as saturable absorbers), leading to the formation of cavity solitons (CSs). Such CSs are generated under electrical injection below the lasing threshold of the VCSEL, and appear in the broad area laser as bright spots. They can also be arranged into self-organized arrays, switched on and off with an external laser holding beam, and brought to high-order states [1]. The self-localization can even be obtained in space and time, and self-pulsing spatial solitons have been demonstrated in a mode-locked VCSEL [2]. Here we explore the topological properties of the high-order states of the CS, and we report the spontaneous formation of anti-vortex CSs with a hyperbolic polarization structure. Beams with a spatially non-uniform distribution of polarization are interesting due to their intrinsic beauty, novel functionalities in quantum optics and newly enabled applications [3]. They possess a circular symmetric intensity structure, most often in form of a doughnut, combined with a spatially non-uniform polarization field containing polarization singularities and are referred to as `vector vortex beams'. The engineering of such vectorial vortex beams has been demonstrated in solid-state lasers with a mesa or a meta-surface shaping the spatial pattern [4], whereas they are here spontaneously obtained in the broad-area laser. Vortex beams, including the exotic anti-vortex reported here, have been predicted within the framework of spin-flip models of VCSELs [5], and result from the interplay between the natural anisotropies of the laser and the spin-dependent gain dynamics.

Three-dimensional solitons are highly sought after objects in nonlinear science. In nonlinear photonics they correspond to light bullets, but their experimental observation as long-term stable structures remains elusive in spite of impressive advances [1]. Broad-area vertical-cavity surface-emitting lasers (VCSELs) combine a large Fresnel number with a high nonlinearity and fast time scales. Stationary spatial solitons in 2D as well as mode-locking and temporal solitons without self-localization in space are known to exist. It is intriguing to explore a combination of these two aspects to achieve self-localization of light in three dimensions [2]. Here we report on the observation of mode-locked states of spatial solitons.The experimental system consists of a VCSEL with a 200 micrometer emission aperture emitting around 980 nm. It is coupled to a self-imaging external cavity closed by a volume Bragg grating (VBG) as a frequency-selective element. The round-trip time in the external cavity is about 0.7 ns. By varying the VCSEL current, the detuning between the VCSEL and the VBG resonances is changed and single-mode operation in the external cavity and multi-mode operation can be realized. The latter may correspond to irregular dynamics or fairly regular self-pulsing which represents mode-locked states (Fig. 1). These data are in a range where the self-pulsing spatial soliton is bistable, i.e. it coexists with the non-lasing state as expected for a self-localized state. We obtain self-pulsing at the fundamental and the harmonic round-trip frequency, as well as occasionally at even higher harmonics. The peaks in the RF-spectrum are extremely narrow (<; 100 kHz, close to the resolution limit) indicating a high phase-correlation between the participating modes. The optical spectrum displays three dominant modes. The pulse width (FWHM) is about 110 ps for the harmonic mode-locking and 270 ps for the fundamental mode-locking, roughly consistent with the expectation from spectral bandwidth. The results clearly establish mode-locking of spatial solitons on 3 modes creating temporal structures shorter than the external cavity. Numerical simulations of the model introduced in [4] are in good agreement with the experimental data [3].

Beams with a spatially non-uniform distribution of polarization attracted considerable recent interest due to their intrinsic beauty, novel functionalities in quantum optics and newly enabled applications [1]. Typical realizations investigated possess a circular symmetric intensity structure, most often in form of a doughnut, combined with a spatially non-uniform polarization field containing polarization singularities and are referred to as `vector vortex beams'. Typically, considerable engineering effort is necessary to create these non-trivial polarization configurations. In contrast, we report on the spontaneous formation of vector vortex beams in a broad-area vertical-cavity surface-emitting laser (VCSEL) with frequency-selective feedback [2]. In particular, an anti-vortex state with a hyperbolic polarization structure is observed. The relation to vectorial high-order solitons is discussed.

Interaction rayonnement-matière

Introduction au formalisme de l'EXAFS

Opto-electronic properties of single-walled carbon nanotubes can be significantly modified by chromophore confinement into their hollow core. This presentation deals with quaterthiophene derivatives encapsulated into nanotubes displaying different diameter distributions. We show that the supramolecular organizations of the confined chromophores depend on the nanocontainer size. The Raman radial breathing mode frequency is monitored by both the number of confined molecules into a nanotube section and the competition between dye/dye and dye/tube wall interactions. The confinement properties lead also to an exaltation of the infrared absorption response1 in single-walled carbon nanotubes from dye molecule interactions due to a symmetry breaking, allowing us, thanks to the complementarity of DFT calculations and experimental IR investigations to study interactions between both subsystems. Significant electron transfer from the confined molecules to the nanotubes is also reported from Raman investigations. This charge transfer leads to an important enhancement of the photoluminescence intensity by a factor of nearly five depending on the tube diameter. In addition, close to the molecule resonance, the magnitude of the Raman G-band shifts is modified and the intensity loss is amplified, indicating a photo-induced electron transfer. Results are discussed in the frame of electron-phonon coupling. Thus, confinement species into nanotubes allow moving the Fermi level and consequently to monitor their opto-electronic properties.

Opto-electronic properties of single-walled carbon nanotubes can be significantly modified by chromophore confinement into their hollow core. This presentation deals with quaterthiophene derivatives encapsulated into nanotubes displaying different diameter distributions. We show that the supramolecular organizations of the confined chromophores depend on the nanocontainer size. The Raman radial breathing mode frequency is monitored by both the number of confined molecules into a nanotube section and the competition between dye/dye and dye/tube wall interactions. The confinement properties lead also to an exaltation of the infrared absorption response1 in single-walled carbon nanotubes from dye molecule interactions due to a symmetry breaking, allowing us, thanks to the complementarity of DFT calculations and experimental IR investigations to study interactions between both subsystems. Significant electron transfer from the confined molecules to the nanotubes is also reported from Raman investigations. This charge transfer leads to an important enhancement of the photoluminescence intensity by a factor of nearly five depending on the tube diameter. In addition, close to the molecule resonance, the magnitude of the Raman G-band shifts is modified and the intensity loss is amplified, indicating a photo-induced electron transfer. Results are discussed in the frame of electron-phonon coupling. Thus, confinement species into nanotubes allow moving the Fermi level and consequently to monitor their opto-electronic properties.