  Near-field thermal emission largely exceed blackbody radiation, owing to
spectrally sharp emission in surface polaritons. We turn Casimir-Polder
interaction between Cs(7P1/2) and a sapphire interface, into a sensor sharply
filtering, at 24.687 THz, the near-field sapphire emission at ~ 24.5 THz.
Temperature evolution of sapphire mode is demonstrated. The Cs sensor,
sensitive to both dispersion and dissipation, suggests the polariton to be
red-shifted and sharper, as compared, up to 1100 K, to predictions from
far-field sapphire emission, affected by birefringence and multiple resonances.
