A Study of Evanescent Light Fields
Created by Total Internal Reflection

It is well known that when light moves from a medium with a certain refractive index into one with a smaller index, the light is bent "away from the normal." As a consequence, at incident angles larger than a certain "critical angle" θ_c, the boundary between the two media becomes a perfect mirror that reflects all of the incident light back into the first medium. This phenomenon is called total internal reflection, or TIR. Even though the incident light is totally reflected, it creates an evanescent wave just beyond the boundary between the two media. The evanescent wave propagates parallel to the surface but does not carry any energy away from the surface. In practice it is convenient to observe TIR in a 45-45-90 glass prism. The evanescent wave is then formed in air just beyond the hypotenuse of the prism.

This project began with a careful study of Frustrated Total Internal Reflection (FTIR). FTIR occurs when the evanescent wave meets another (third) transparent medium whose index of refraction is comparable to that of the original (first) medium. Instead of attenuating to nothing, the evanescent wave then ``wakes up'' and couples into the second medium. This effect is analogous to quantum mechanical tunneling, an effect in which a particle has a small probability of passing through a potential energy barrier whose height exceeds the energy of the particle. There are many challenges to just observing FTIR at all. Due to the steep exponential fall-off of the evanescent field, the two prisms must be placed in very close proximity, just a few microns, and this gap must be precisely controlled. For the same reason the opposed prism faces must be as flat as possible, with just a fraction of a wavelength of deviation across the prism face. Last but not least, it is crucial that the opposed surfaces be microscopically clean. Consider for example that a speck of dust the size of a red blood cell could have a diameter that exceeds the observable range of the evanescent field.

Our setup used two inexpensive ($13 each) 45-45-90 BK7 glass prisms with sides 30 x 30 mm. We assembled the prism pair in a laminar flow hood after cleaning the prism faces by methods used for precision optics. We did not attempt to directly vary the gap between the two prisms but rather created a variable gap or wedge between them by inserting a 12 micron thick copper spacer along one edge. The prism pair was mounted on a rotatable platform whose orientation could be read to within ~ 0.1 degree. The turntable assembly was mounted in turn on a translation stage driven by a micrometer. P-polarized light from a 20 mW red HeNe laser was incident on the prisms. The laser beam was made accurately level and its height could be adjusted in small steps. Thus by translating the stage and shifting the laser the entire region of the the 0 - 12 micron gap could be surveyed. The weak transmitted light was recorded in a DET-210 photodetector with a parallel 100kΩ resistor to convert current to voltage. Additional gain of x100 was provided by an amplifier when needed. Great care was taken to prevent stray room and laser light from reaching the detector.

We surveyed the entire face of the prism for incident angles about 1^o past the critical angle. We readily observed the exponential decay of the evanescent field, but in some parts of the prism this was interrupted by a "bump" on the log plot of light intensity versus gap width. We concluded that the prism was flawed or dirty in this region. Away from this flaw we were able to follow the fall-off of the evanescent field for four decades. We also carefully studied the decay rate versus proximity to θ_c. Analysis of these results is still underway. Finally, we obtained considerable further information about the regularity of the gap by observations below θ_c, where the strong transmitted light displays "etalon fringes" due to multiple reflections on the two facing glass surfaces. In summary, our simple setup provided much interesting data and excellent results.

Evanescent waves have many current and exciting applications such as near-field microscopy and very sensitive detection of specific molecules. Even more applications, such as cold atom mirrors, are possible when the evanescent wave is enhanced by the creation of a surface plasmon resonance in a very thin metallic film deposited on the glass surface. With the correct adjustment of the various parameters involved the reflectivity of the prism surface can be greatly reduced. The missing energy is dissipated by the surface plasmons, which create the evanescent field beyond the metal film. We have begun experimentation with these effects using gold films ~ 60 nm thick deposited by evaporation on the face of one of our prisms.