Spectrally Sharp Near-Field Thermal Emission: Revealing Some Disagreements between a Casimir-Polder Sensor and Predictions from Far-Field Emittance.

Near-field thermal emission largely exceeds blackbody radiation, owing to spectrally sharp emission in surface polaritons. We turn the Casimir-Polder interaction between Cs(7P_{1/2}) and a sapphire interface into a sensor sharply filtering, at 24.687 THz, the near-field sapphire emission at ∼24.5 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.

Surface response around a sharply resonant surface polariton mode is simply a Lorentzian.

At the planar interface between a material and vacuum, the complex surface response S(ω)=[ε(ω)-1]/[ε(ω)+1], with ε(ω) being the relative complex dielectric permittivity of the material, exhibits resonances typical of the surface polariton modes, when ε(ω)∼-1. We show that for a moderately sharp resonance, S(ω) is satisfactorily described with a mere (complex) Lorentzian, independent of the details affecting the various bulk resonances describing ε(ω). Remarkably, this implies a quantitative correlation between the resonant behaviors of ℜe[S(ω)] and ℑm[S(ω)], respectively, associated to the dispersive and dissipative effects in the surface near-field. We show that this "strong resonance" approximation easily applies and discuss its limits, based on published data for sapphire, CaF2, and BaF2. An extension to interfaces between two media or to a non-planar interface is briefly considered.

Linear Probing of Molecules at Micrometric Distances from a Surface with Sub-Doppler Frequency Resolution.

We report on precision spectroscopy of subwavelength confined molecular gases. This was obtained by rovibrational selective reflection of NH_{3} and SF_{6} gases using a quantum cascade laser at λ≈10.6 µm. Our technique probes molecules at micrometric distances (≈λ/2π) 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 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.