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 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.