Physically, what happens in FTIR is this: the electric field encounters a glass-air boundary and undergoes Total Internal Reflection. However, the electric field on the air side of the boundary cannot go to zero infinitely quickly. Therefore the electric field on the air side of the boundary goes to zero in a very short, but non-zero distance. If another piece of glass is placed sufficiently close to the first, the evanescent field in air couples into the second glass block and light continues to propagate.

Now normally, the reflection coefficient that a ray sees is dependent purely on the refractive index on either side of the boundary, and the angle and polarization of the ray. In FTIR however, we need to account for three materials, not two: we need both glass indices and the gap material. This is done in ZEMAX by adding a coating between the two glass objects.

To do this, we alter our coating.dat file, adding a new coating as follows:

COAT AIR-100nm
AIR .100 1

This coating is an airgap with a thickness of 100nm.  Add these lines to the coating.dat file, and then save the file.  It’s also a good idea to save the file with a different filename (such as coating2.dat), because each time you install a new version of ZEMAX, the coating.dat file will get overwritten. 

We now need to reload the coatings file in ZEMAX (Tools | Coating | Reload Coating File) so that ZEMAX knows about this new addition to the coating file.

Now we can add an airgap coating to the second slab.  Note that coating the first slab will have no effect – you must coat the second slab in order to have the airgap.  The reason is that ZEMAX overwrites objects as it reads through the list of objects in the order they appear in the NSC Editor.  If you were to coat the top slab with a small airgap, and then the next object in the list is the bottom slab in direct contact (as we have here) then the bottom slab “overwrites” the airgap.  If you really desire, for whatever reason, to coat the top slab with the airgap, then you would have to place the bottom slab before the top slab in the NSC Editor.

Right click (or double-click) Object 2 in the NSC Editor.  Click on the Coat/Scatter tab, and then click the drop-down box next to “Coating:” and select the AIR-100NM coating.  Note that the default Face to coat is “0, Side Faces,” which is want we want in this case.  When coating an object, always be careful to look at which Face is selected in this window.

Note that the layout plot shows both transmitted and reflected rays (make sure "Split Rays" is select in the layout plot's settings):

Clear the detectors and trace rays again.  You’ll find that only 20% of the light is internally-reflected now, and 80% of the light is “drained” across the airgap into the bottom slab.

Experimentation with the light wavelength, airgap thickness, and angle of incidence will show that ZEMAX gives extremely accurate results, providing the user with a powerful model for FTIR.