Probing the non-Planckian spectrum of thermal radiation in a micron-sized cavity with a spin-polarized atomic beam

TL;DR

This paper demonstrates that micron-sized metallic cavities contain non-Planckian, non-resonant thermal radiation with TE polarization, which can be measured via atomic hyperfine transitions, challenging traditional expectations about cavity radiation.

Contribution

It reveals the presence of non-Planckian thermal radiation in small metallic cavities and proposes a method to measure it using atomic hyperfine transitions, addressing a longstanding puzzle in dispersion forces.

Findings

Micron-sized metallic cavities host non-resonant, non-Planckian radiation.

The radiation density exceeds black-body predictions at room temperature.

Measurement of this radiation can clarify the thermal Casimir force controversy.

Abstract

It is commonly thought that thermal photons with transverse electric polarization cannot exist in a planar metallic cavity whose size $a$ is smaller than the thermal wavelength $\lambda_T$, due to absence of modes with $\lambda < 2a$. Computations based on a realistic model of the mirrors contradict this expectation, and show that a micron-sized metallic cavity is filled with non-resonant radiation having transverse electric polarization, following a non-Planckian spectrum, whose average density at room temperature is orders of magnitudes larger than that of a black-body. We show that the spectrum of this radiation can be measured by observing the transition rates between hyperfine ground-state sub-levels $1S_{1/2}(F,m_F) \rightarrow 1S_{1/2}(F',m'_F)$ of D atoms passing in the gap between the mirrors of a Au cavity. Such a measurement would also shed light on a puzzle in the field of dispersion forces, regarding the sign and magnitude of the thermal Casimir force. Recent experiments with Au surfaces led to contradictory results, whose interpretation is much controversial.