Quantization of evanescent electromagnetic waves: Momentum of the electromagnetic field very close to a dielectric medium

The problem of the momentum of the electromagnetic field near a homogeneous dielectric plane surface is studied. In the particular case of evanescent waves, our calculations are consistent with the experimental result of S. Huard and C. Imbert which points out that, during an interaction between a moving atom and a surface wave of pulsation $\omega$, the exchanged momentum is greater than $\frac{\hslash \omega }{c}$. This work leads at first to a straightforward procedure for quantizing the momentum parallel to the diopter of the field. The authors proceed to investigate the form of the component perpendicular to the interface which is transmitted by the field to an atom during an interaction. (In the case of an evanescent wave, this problem involves the interpretation of the imaginary part of the wave vector.) A general theory shows that the presence of a medium widens and shifts the $\overrightarrow{\mathrm{k}}$ levels of the field. This result agrees with the uncertainty principle.