The penetration depth and pseudomomentum (wave vector) of evanescent electromagnetic waves are measured at a planar dielectric surface by means of laser spectroscopy using the Cs ${\mathrm{D}}_2$ line with a wavelength of 852.1 nm. The same experimental setup is employed in all the experiments. The first result shows a sharp, asymmetric absorption profile, which is explained in terms of the thin penetration depth, and the second shows an actual transfer of the pseudomomentum of the surface electromagnetic mode to atoms via a resonance interaction. The measured penetration depth and pseudomomentum agree with the natural interpretation of the complex wave number characterizing evanescent waves at a planar dielectric-vapor interface. Direct excitation of evanescent waves by excited Cs atoms has also been observed as atomic fluorescence emitted into the dielectric material at the angle of total internal reflection. The results are in good agreement with the theoretical predictions and exhibit the characteristics of excitation transfer or tunneling from Cs atoms into the surface mode.

DOI:https://doi.org/10.1103/PhysRevA.55.2406