Thierry Passerat de Silans

Temperature dependence of the dielectric permittivity of CaF2, BaF2 and Al2O3: application to the prediction of a temperature-dependent van der Waals surface interaction exerted onto a neighbouring Cs(8P3/2) atom

The temperature behaviour in the range 22-500 °C of the dielectric permittivity in the infrared range is investigated for CaF(2), BaF(2) and Al(2)O(3) through reflectivity measurements. The dielectric permittivity is retrieved by fitting reflectivity spectra with a model taking into account multiphonon contributions. The results extrapolated from the measurements are applied to predict a temperature-dependent atom-surface van der Waals interaction. We specifically consider as the atom of interest Cs(8P(3/2)), the most relevant virtual couplings of which fall in the range of thermal radiation and are located in the vicinity of the reststrahlen band of fluoride materials.

Casimir–Polder interactions in the presence of thermally excited surface modes

The temperature dependence of the Casimir-Polder interaction addresses fundamental issues for understanding vacuum and thermal fluctuations. It is highly sensitive to surface waves, which, in the near field, govern the thermal emission of a hot surface. Here we use optical reflection spectroscopy to monitor the atom-surface interaction potential between a Cs*(7D3/2) atom and a hot sapphire surface at distances of ~100 nm. In our experiments, that explore a large range of temperatures (500-1,000 K), the surface is at thermal equilibrium with the vacuum. The observed increase of the interaction with temperature, by up to 50%, relies on the coupling between atomic virtual transitions in the infrared range and thermally excited surface-polariton modes. We extrapolate our findings to a broad distance range, from the isolated atom to the short distances relevant to physical chemistry. Our work also opens the prospect of controlling atom-surface interactions by engineering thermal fields.

Tunable power law in the desynchronization events of coupled chaotic electronic circuits

We study the statistics of the amplitude of the synchronization error in chaotic electronic circuits coupled through linear feedback. Depending on the coupling strength, our system exhibits three qualitatively different regimes of synchronization: weak coupling yields independent oscillations; moderate to strong coupling produces a regime of intermittent synchronization known as attractor bubbling; and stronger coupling produces complete synchronization. In the regime of moderate coupling, the probability distribution for the sizes of desynchronization events follows a power law, with an exponent that can be adjusted by changing the coupling strength. Such power-law distributions are interesting, as they appear in many complex systems. However, most of the systems with such a behavior have a fixed value for the exponent of the power law, while here we present an example of a system where the exponent of the power law is easily tuned in real time.

Measurement of the Kerr nonlinear refractive index of Cs vapor

Atomic vapors are systems well suited for studies of opticalnonlinearities. First of all, they are easy to saturate, whichenables the observation of nonlinear effects with low intensitycontinuous-wave laser light [1,2]. At the same time, atomicvapors are damage-free which is important, for instance, forfilamentation studies [3]. Second, as the resonances are sharpthe nonlinear parameters can be easily modified by finelytuningthefrequencynearoracrossaresonance[4].Thisallowsto play with the relative contributions of linear and nonlineareffects by changing the laser wavelength. Third, atomicsystems allow for a variety of level schemes exploring fine,hyperfine, and Zeeman levels such as two-level systems [5,6], three-level schemes [4], double- four-level schemes [7,8],five-level schemes [9], and so on. Fourth, in most experiments,when one can ignore radiation trapping and collisional effects,atomic vapors behave as locally saturable media and are thuseasy to model [10].As atomic vapors are isotropic media, the first nonlinearcontribution to the polarization is a third-order term in theelectric field (

Laser-induced atomic adsorption: A mechanism for nanofilm formation

We demonstrate and interpret a technique of laser-induced formation of thin metallic films using alkali atoms on the window of a dense-vapour cell. We show that this intriguing photo-stimulated process originates from the adsorption of Cs atoms via the neutralization of Cs+ ions by substrate electrons. The Cs+ ions are produced via two-photon absorption by excited Cs atoms very close to the surface, which enables the transfer of the laser spatial intensity profile to the film thickness. An initial decrease of the surface work function is required to guarantee Cs+ neutralization and results in a threshold in the vapour density. This understanding of the film growth mechanism may facilitate the development of new techniques of laser-controlled lithography, starting from thermal vapours.

Effect of attractor on the desynchronization events in coupled chaotic circuits

The phenomenon of attractor bubbling consists in incomplete or imperfect synchronization between two coupled chaotic oscillators, which synchronization regime is broken for brief escapes of different sizes.Here, we use as oscillators two chaotic electronic circuits coupled through unidirectional linear feedback in the regime of moderate coupling to study the occurrence of attractor bubbling in this system. For two different dynamical states, the system exhibits different chaotic attractors. Analysing temporal series, we build empirical distributions of the amplitudes of desynchronization events for different values of the coupling parameter. We observe that in the regime of attractor bubbling the distributions are characterized by a heavy tail, bearing similarity to the ones observed in complex systems with self-organized criticality. A given heavy-tailed distribution is exhibited by the two chaotic states for different coupling strengths. We explain this effect of the attractor shape on the statistics of the desynchronization as caused by a region of instability, which is more often visited for one of the attractors than it is by the other.

CONDIÇÕES PARA BIESTABILIDADE EM FREQUÊNCIA DE LASER SEMICONDUTOR SOB REALIMENTAÇÃO ÓTICA ORTOGONAL

Biestabilidade na frequencia de emissao de um laser semicondutor sujeito a realimentacao otica com polarizacao ortogonal foi observada experimentalmente por Farias et al. em 2005. Um modelo de equacoes de taxa para esse sistema dinâmico, apresentado posteriormente por Masoller et al. em 2007, que leva em conta os efeitos termicos e ganho de saturacao, preve uma variacao linear da frequencia do laser com a intensidade do campo de realimentacao. Nesse trabalho, usando o mesmo modelo, estudamos o processo de biestabilidade otica em frequencia nesses sistemas com realimentacao filtrada, determinando as condicoes espectrais do filtro de realimentacao necessarias para o aparecimento de histerese que leva a biestabilidade.

Extra sub-Doppler lines in the vicinity of the third-resonance 6S-8P transition of atomic Cs attributed to optically induced Cs dimers

We report on the observation of additional sub-Doppler lines in a saturated absorption experiment when exploring the vicinity of the 6S{sub 1/2}-8P{sub 3/2} transition of Cs ({lambda}=388 nm). These additional lines are observed only under a relatively strong irradiation of both the pump and the probe beams. Extra narrow lines are also observed in copropagating nonlinear spectroscopy, and around the lines of the V-type three-level system 8P{sub 3/2}-6S{sub 1/2}-8P{sub 1/2} ({lambda}{sub 1}=388 nm, {lambda}{sub 2}=389 nm). We attribute theses additional lines to a probing of high-lying molecular cesium, produced as a result of the optical excitation of Cs atoms, as the low Cs atom density ({<=}10{sup 12}cm{sup -3}) is unable to populate significantly the dimer states in the condition of thermal equilibrium.

Sub-Doppler extra-lines in the vicinity of the 388 nm line of Cs: A signature for dimer photoassociation in a thermal vapour ?

We report here on the observation of extra sub-Doppler resonances in the vicinity of the third resonance line of Cs (6S→8P) at 388 nm (6S<inf>1/2</inf>∓8P<inf>3/2</inf>) and 389 nm (6S<inf>1/2</inf>∓8P<inf>12</inf>), when saturated absorption spectroscopy is usually expected to provide a frequency reference in the analysis of other physical processes [1]. Our experiments show that when the pump and probe are relatively intense (typically ≥ 10 mW/mm²), there are some additional narrow lines inside the Doppler width (see fig. 1), aside from the narrow expected saturated absorption components that allow to resolve the structure of the hyperfine manifolds of Cs (8P). Because of the weak (∼10⁻³) oscillator strength of these transitions, heating up of the Cs vapour -to enhance Cs atomic density-, and focusing of the beams -in order to saturate-, are required to record a saturated absorption spectrum. However, for the various cells that we have used (length : 1–5 cm), a typical temperature of 70∓100 °C was sufficient, and we could even record signals at T ∼ 45°C, corresponding to a Cs atomic density as low as 5.10¹¹at/cm³.