EVANESCENT INTENSITY OF A FOCUSED GAUSSIAN LIGHT BEAM UNDERGOING TOTAL INTERNAL REFLECTION IN A PRISM.

The evanescent electric field and intensity profile are calculated of a circular Gaussian light beam when it is focused by a lens, enters a spatially finite prism, and is totally internally reflected at a planar interface of the prism and a dielectric substance. The electric field and intensity at the interface depend on the indices of refraction of the prism, the 1/e**2 width, wavelength, and polarization of the incident beam, the focal length and exit pupil of the focusing lens, the distance from lens to prism, the shape and size of the prism, and the beam's angle of incidence. The calculations show the evanescent intensity profile to be well approximated by a Gaussian shape for many practical realizations of the lens and prism optical apparatus. The 1/e**2 width of the profile appears to depend primarily on the beam's 1/e**2 width, the ratio of the focal length of the lens to this width, and the incidence angle.