Athanasios Laliotis

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.

Three-dimensional confinement of vapor in nanostructures for sub-Doppler optical resolution

We confine a Cs thermal vapor in the interstitial regions of a glass opal. We perform linear reflection spectroscopy on a cell whose window is covered with a thin film (10 or 20 layers) of ∼1000 nm (or 400 nm) diameter glass spheres and observe sub-Doppler structures in the optical spectrum for a large range of oblique incidences. This original feature associated with the inner (3-dimensional) confinement of the vapor in the interstitial regions of the opal evokes a Dicke narrowing. We finally consider possible micron-size references for optical frequency clocks based on weak, hard to saturate, molecular lines.

Rb optical resonance inside a random porous medium.

We studied resonant laser interaction with Rb atoms confined to the interstitial cavities of a random porous glass. Due to diffusive light propagation, the effect of atomic absorption on the light scattered by the sample is almost entirely compensated by atomic fluorescence at low atomic densities. For higher densities, radiation trapping increases the probability of nonradiative decay via atom-wall collisions. A simple connection of the fluorescence/absorption yield to the sample porosity is given.

Casimir-Polder effect with thermally excited surfaces

We take a closer look at the fundamental Casimir-Polder interaction between quantum particles and dispersive dielectric surfaces with surface polariton or plasmon resonances. Linear response theory shows that in the near field, van der Waals, regime the free energy shift of a particle contains a thermal component that depends exclusively on the population/excitation of the evanescent surface polariton/plasmon modes. Our work makes evident the link between particle surface interaction and near field thermal emission and demonstrates how this can be used to engineer Casimir-Polder forces. We also examine how the exotic effects of surface waves are washed out as the distance from the surface increases. In the case of molecules or excited state atoms, far field approximations result in a classical dipole-dipole interaction which depends on the surface reflectivity and the mean number of photons at the frequency of the atomic/molecular transition. Finally we present numerical results for the CP interaction between Cs atoms and various dielectric surfaces with a single polariton resonance and discuss the implications of temperature and retardation effects for specific spectroscopic experiments. The Casimir-Polder (CP) interaction between a polar-isable quantum object (atom or molecule) and a surface arises from quantum fluctuations in vacuum. Its an excellent candidate for fundamental tests of cavity quantum electrodynamics and crucial for any experiments attempting to measure non-Newtonian gravity interactions [1, 2]. CP forces are also relevant in physical chemistry playing an important role in the interpretation of physical phenomena such as atomic adsorption and desorption from hot surfaces or even surface chemistry and cataly-sis. The continuous urge for miniaturisation has led to integrated devices, such as atom and molecule chips [3– 6], used for a variety of applications and more recently tapered nano-fibers were used to trap atoms at distances as small as 200 nm away from the surface [7–9], where atom-surface forces become exceedingly relevant. Novel trapping schemes that exploit the complexity of the van der Waals (vdW) potential of excited atoms have also been proposed [10]. The most basic description of the CP effect is that of a classical dipole interacting with its surface induced image. This approach is mostly valid in the vdW (z

Optics of an opal modeled with a stratified effective index and the effect of the interface

Reflection and transmission for an artificial opal are described through a model of stratified medium based upon a 1D variation of an effective index. The model is notably applicable to a Langmuir–Blodgett-type disordered opal. Light scattering is accounted for by a phenomenological absorption. The interface region between the opal and the substrate, or the vacuum, induces a periodicity break in the photonic crystal arrangement, which exhibits prominent influence on the reflection, notably away from the Bragg reflection peak. Experimental results are compared with our model. The model is extendable to inverse opals, stacked cylinders, or irradiation by evanescent waves.

Sub-Doppler resonances in the backscattered light from random porous media infused with Rb vapor

We report on the observation of sub-Doppler resonances on the back-scattered light from a random porous glass medium with rubidium vapor filling its interstices. The sub-Doppler spectral lines are the consequence of saturated absorption where the incident laser beam saturates the atomic medium and the back-scattered light probes it. Some specificities of the observed spectra reflect the transient atomic evolution under confinement inside the pores. Simplicity, robustness and potential miniaturization are appealing features of this system as a spectroscopic reference.

Backward-emitted sub-Doppler fluorescence from an optically thick atomic vapor

Literature mentions only incidentally a sub-Doppler contribution in the excitation spectrum of the backward fluorescence of a dense vapor. This contribution is here investigated on Cs vapor, both on the first resonance line (894 nm) and on the weaker second resonance line (459 nm). We show that in a strongly absorbing medium, the quenching of excited atoms moving towards a window irradiated under near normal incidence reduces the fluorescence on the red side of the excitation spectrum. Atoms moving slowly towards the window produce a sub- Doppler velocity-selective contribution, whose visibility is here improved by applying a frequency-modulation technique. This sub-Doppler feature, induced by a surface quenching combined with a short absorption length for the incident irradiation, exhibits close analogies with the narrow spectra appearing with thin vapor cells. We also show that a normal incidence irradiation is essential for the sub-Doppler feature to be observed, while it should be independent of the detection geometry

Experimental observations of temperature effects in the near-field regime of the Casimir-Polder interaction

We investigate the temperature dependence of the Casimir–Polder interaction on the electrostatic limit. This unusual phenomenon relies on the coupling between a virtual atomic transition and a thermal excitation of surface polariton modes. We first focus on the scenario in which a Cs(8P3/2) atom is next to a CaF2 or BaF2 surface. Our theoretical predictions show a strong temperature dependence of the van der Waals coefficient at experimentally accessible conditions. A series of spectroscopic measurements performed in a specially designed Cs vapor cell containing a CaF2 tube is presented. Our results illustrate the sensitivity of atom–surface interaction experiments to the quality and chemical stability of the surface material and emphasize the need for using more durable materials, such as sapphire. Finally, we discuss selective reflection experiments with Cs(7D3/2) in an all-sapphire cell that clearly demonstrate a temperature-dependent van der Waals coefficient.

Selective reflection spectroscopy of a vapour at a calcium fluoride interface

Fluoride materials exhibit surface resonances located in the thermal infrared. This makes them interesting to search for a fundamental temperature dependence of the atom-surface interaction, originating in the near-field thermal emissivity of the surface. Preliminary selective reflection experiments performed on a special Cs vapour cell that includes a CaF 2 interface show a temperature dependence, yet to be analyzed.

Tailoring optical metamaterials to tune the atom-surface Casimir-Polder interaction

We tailor the atom-surface Casimir-Polder interaction of cesium atoms using near-infrared surface plasmons of a metamaterial. Metamaterials are fascinating tools that can structure not only surface plasmons and electromagnetic waves but also electromagnetic vacuum fluctuations. The possibility of shaping the quantum vacuum is a powerful concept that ultimately allows engineering the interaction between macroscopic surfaces and quantum emitters such as atoms, molecules, or quantum dots. The long-range atom-surface interaction, known as Casimir-Polder interaction, is of fundamental importance in quantum electrodynamics but also attracts a significant interest for platforms that interface atoms with nanophotonic devices. We perform a spectroscopic selective reflection measurement of the Casimir-Polder interaction between a Cs(6P3/2) atom and a nanostructured metallic planar metamaterial. We show that by engineering the near-field plasmonic resonances of the metamaterial, we can successfully tune the Casimir-Polder interaction, demonstrating both a strong enhancement and reduction with respect to its nonresonant value. We also show an enhancement of the atomic spontaneous emission rate due to its coupling with the evanescent modes of the nanostructure. Probing excited-state atoms next to nontrivial tailored surfaces is a rigorous test of quantum electrodynamics. Engineering Casimir-Polder interactions represents a significant step toward atom trapping in the extreme near field, possibly without the use of external fields.