Near-Field Thermal and Vacuum Fluctuations at an Interface: Experimental Attempts for a Precise Determination of Surface Resonances
Résumé
Quantum and e.m. Near-Field (NF) fluctuations are at the origin of Casimir-type interaction, and of applications playing with NF thermal exchanges. This makes it essential to evaluate experimentally the features of surface resonances, which fully govern the range of extreme NF. Accurate measurements reveal delicate, as measurements on nanostructures are often affected by defects in the nano-realization, instead of being test beds of theory. High-quality sapphire windows should offer the possibility of an accurate evaluation of surface modes, determined in planar geometry by resonances of the complex response S(ω) = [ε(ω) -1] / [ε(ω) + 1] –with ε the (complex) relative dielectric permittivity. Evaluation was performed with 3 competing methods: (i) Through optical NF measurements of Casimir-Polder (CP) interaction at a Cs(7P)/sapphire interface, we extrapolated the temperature-dependent surface resonance, in a situation reverse to the one previously exploited to make CP repulsive and to induce surface quenching. (ii) Through Far-Field (FF) thermal emissivity, we remeasured ε(ω,T) for sapphire: disagreements found between the two methods were tentatively explained by inaccuracy when extrapolating ε(ω) to very remote wings of resonances, in order to determine S(ω) resonances. As a problem, the extrapolation requires a multi-parameter fitting for emissivity, or equivalently a Kramers-Kronig approach, while the spectral features of emissivity are very unspecific close to surface resonances. (iii) Lastly, ellipsometry, which is an alternate FF technique directly yielding the complex optical index nc [with ε(ω)= nc²], fully agrees with FF emissivity. We will comment on possible origins of the observed discrepancies between NF and FF measurements. Also, the expected NF thermal transfer Cs(7P)→Cs(6D), induced by surface emission of hot sapphire, has remained unobserved.
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