ABSTRACT
Chapter 13, Thin Films, reviews the general multilayer thin film algorithm and covers several of the most important classes of optical thin films and their polarization properties from the perspective of the optical engineer and lens designer. All optical coatings and thin films affect the polarization state of incident light. Amplitude coefficients for the s and p-eigenpolarizations characterize the polarization properties of the coating in the same way as the Fresnel coefficients. The equations for multilayer film amplitude coefficients are complicated enough to make analytical manipulation nearly impossible. For the purposes of understanding their wavefront aberrations and polarization aberrations and estimating the effects on image formation, treating the coefficients as linear, quadratic, or cubic polynomials provides insight, facilitating interfacing coating performance with the description of aberrations in a series of orders, second order wavefront aberrations, fourth order, etc. Thus, fitting the amplitude coefficients to polynomials provides forms which are easily manipulated and dovetail with aberration theory. Abrupt phase jumps in the amplitude coefficients are another issue with the interpretation of thin film calculations, since the phases of the complex amplitude coefficients are only returned within the range of –π to π by the arctan function.
Antireflection coatings, such as quarter wave of magnesium fluoride on glass, reduce reflection loss, boost transmission, and also greatly reduce diattenuation, solving several problems at once. Enhanced reflection coatings placed over metal mirrors to protect metal surface and boost reflectance. Beam-splitter coatings can divide or combine wavefronts, and are necessary in interferometers and many other applications. Beamsplitters which divide the flux equally can be made from thin films of metal, a few nanometers thick, but such single layer coatings have large differences in their s and p-properties, yielding significant diattenuation and retardance. To make non-polarizing beams splitter coatings requires far more complex coatings. If the s and p-phase changes are not equal, right circularly polarized light will generally reflect and transmit into beams which are not circular but elliptically polarized, unless more complex, phase matching designs are used. Other beam splitter coatings, such as MacNeille polarizing beam splitter coating, are designed as polarizing beam splitters, to reflect one polarization state and transmit the orthogonal state. Polarizing beam-splitters have a limited range of angle of incidence and wavelength over which they are effective, i.e. where they exceed an extinction ratio specification such as 1000:1. It is difficult to find polarizing beam-splitting coating designs which simultaneously have high extinction ratio, broad wavelength range and a large range of angles of incidence.
Coatings with high reflectance are readily fabricated from alternating high and low index layers of quarter wave thickness and can provide much higher reflectance than metal coatings. However the spectral bandwidth and polarization properties are quite different and the polarization aberrations much larger. This high reflectivity is only obtained over a limited range of wavelengths and angles, and outside this range the reflectivity, diattenuation, and retardance can be much worse than a metal mirror.
