Spectroscopy is a unique experimental tool for measuring the fundamental Casimir-Polder interaction between excited-state atoms, or other polarizable quantum objects, and a macroscopic surface. Spectroscopic measurements probe atoms at nanometric distances away from the surface where QED retardation is usually negligible and the atom-surface interaction is proportional to the inverse cube of the separation distance, otherwise known as the van der Waals regime. Here we focus on selective reflection, one of the main spectroscopic probes of Casimir-Polder interactions. We calculate selective reflection spectra using the full, distance dependent, Casimir-Polder energy shift and linewidth. We demonstrate that retardation can have significant effects, in particular for experiments with low-lying energy states. We also show that the effective probing depth of selective reflection spectroscopy depends on the transition linewidth. Our analysis allows us to calculate selective reflection spectra with composite surfaces, such as metasurfaces, dielectric stacks, or even bidimensional materials.

The propagation of light in a resonant atomic vapor can a priori be thought of as a multiple scattering process, in which each scattering event redistributes both the direction and the frequency of the photons. Particularly, the frequency redistribution may result in Lévy flights of photons, directly affecting the transport properties of light in a resonant atomic vapor and turning this propagation into a superdiffusion process. Here, we report on a Monte Carlo simulation developed to study the evolution of the spectrum of the light in a resonant thermal vapor. We observe the gradual change of the spectrum and its convergence towards a regime of complete frequency redistribution as the number of scattering events increases. We also analyze the probability density function of the step length of photons between emissions and reabsorptions in the vapor, which governs the statistics of the light diffusion. We observe two different regimes in the light transport: superdiffusion when the vapor is excited near the line center and normal diffusion for excitation far from the line center. The regime of complete frequency redistribution is not reached for excitation far from resonance even after many absorption and reemission cycles due to correlations between emitted and absorbed frequencies.

We report on precision spectroscopy of subwavelength confined molecular gases. This was obtained by rovibrational selective reflection of ${\mathrm{NH}}_3$ and ${\mathrm{SF}}_6$ gases using a quantum cascade laser at $\lambda \approx 10.6\mu \mathrm{m}$. Our technique probes molecules at micrometric distances ($\approx \lambda /2\pi$) from the window of a macroscopic cell with submegahertz resolution, allowing molecule-surface interaction spectroscopy. We exploit the linearity and high resolution of our technique to gain novel spectroscopic information on the ${\mathrm{SF}}_6$ greenhouse gas, useful for enriching molecular databases. The natural extension of our work to thin cells will allow compact frequency references and improved measurements of the Casimir-Polder interaction with molecules.

We report on the observation of laser oscillation using the magnetically assisted gain in an ensemble of coherently prepared cold cesium atoms. This gain mechanism was recently demonstrated and is based on the modification of the Zeeman populations and coherences induced by a coupling and a probe beams by the presence of a transverse magnetic field. We have also investigated the enhancement of the gain due to the presence of a resonant ring cavity having different finesses. In the case of a low-finesse cavity operating below threshold, a 14-fold gain enhancement, in comparison with the single-pass gain, was measured and, finally, the system showed laser oscillation when a higher-finesse cavity was used. The present demonstration of laser oscillation constitutes an unconventional kind of lasing without inversion which is based on the manipulation of the dark state by an external magnetic field.

Near-field thermal emission largely exceeds blackbody radiation, owing to spectrally sharp emission in surface polaritons. We turn the Casimir-Polder interaction between $\mathrm{Cs}(7P_{1/2})$ and a sapphire interface into a sensor sharply filtering, at 24.687 THz, the near-field sapphire emission at $\sim 24.5\mathrm{THz}$. The temperature evolution of the sapphire mode is demonstrated. The Cs sensor, sensitive to both dispersion and dissipation, suggests the polariton to be redshifted and sharper, as compared, up to 1100 K, to predictions from far-field sapphire emission, affected by birefringence and multiple resonances.

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.