Phase shifts suffered by evanescent electromagnetic waves traversing an air gap between two glass prisms have been measured. Because the evanescent waves propagate parallel to the glass-to-air interface, they should be able to penetrate in a direction normal to the interface, without change of phase, although the amplitude is expected to decrease with depth of penetration. This has been confirmed by the experiment, which showed that the phase shifts are substantially independent of the width of the air gap.
C. K. CARNIGLIA and L. MANDEL, "Phase-Shift Measurement of Evanescent Electromagnetic Waves," J. Opt. Soc. Am. 61, 1035-1043 (1971)