Optics of Thin Films

the branch of optics concerned with the passage of light through one or, in succession, more than one nonabsorbing layer of a substance when the thickness of such layers is comparable to a wavelength of light. Specific to the optics of thin films is the great role played by interference between the light waves that are partially reflected at the upper and lower boundaries of the layers. As a result of this interference there occurs an amplification or attenuation of the transmitted or reflected light. This effect depends on a number of factors, including the path difference of the light rays introduced by the optical thicknesses of the layers, the light’s wavelength or set of wavelengths, and the light’s angle of incidence.

Thin films can be formed on a solid substrate of glass, quartz, or other optical medium through the thermal vaporization and deposition of a substance on the surface of the substrate, through chemical deposition, through cathode sputtering, or through a chemical reaction of the material of the substrate with the chosen substance. The substances used to obtain such films include the oxides Al₂O₃ (1.59), SiO₂ (1.46), and TiO₂ (2.2–2.6), the fluorides MgF₂ (1.38), CaF₂ (1.24), and LiF (1.35), the sulfides ZnS (2.35) and CdS (2.6), and the semiconductors Si (3.5) and Ge (4.0). (The numbers in parentheses are the indexes of refraction.)

A very important practical application of the optics of thin films is the reduction of the reflectivity of the surfaces of such optical parts as lenses and plates. Mirrors with a high reflection coefficient are made by building up a multilayer coating that consists of a large number (13–17 or more) of alternating layers with high and low index of refraction n. This high reflection coefficient is usually for a relatively narrow spectral region, but the region can lie in the ultraviolet or infrared range and need not be in the visible range. The reflection coefficient of such mirrors is between 50 and 99.5 percent and depends both on the wavelength and on the incident angle of the radiation. Multilayer coatings make possible the division of the incident light into transmitted and reflected components with practically no absorption losses. Efficient beam-splitters (semi-transparent mirrors) are based on this principle.

Systems that consist of alternating layers with high and low n are used also as interference polarizers. Such polarizers reflect the light component that is polarized perpendicular to the plane of incidence of the light, that is, the plane containing the direction of the light ray and the normal to the surface, and they transmit the parallel polarized light component. For multilayer polarizers the degree of polarization in the transmitted light can be as high as 99 percent.

The optics of thin films made possible the building of the optical interference filters that have come to be widely used. The passband of such filters can be very narrow. Existing multilayer light filters can separate wavelength intervals of 0.1–0.15 nanometer (nm) from a spectral region that is 500 nm wide. Thin dielectric layers are used to protect metal mirrors from corrosion and to correct lens and mirror aberrations. The optics of thin films is the basis for many high resolution optical devices, measuring instruments, and spectral instruments. The light-sensitive layers of photocathodes and bolometers consist mostly of thinfilm coatings; the efficiency of such coatings depends mainly on their optical properties.

The optics of thin films finds broad application in lasers and light amplifiers—for instance, in the construction of Fabry-Perot interferometers. Other applications include interference microscopy and the construction of dichroic mirrors for color television. Newton’s rings, fringes of equal inclination, and fringes of equal thickness are effects associated with the optics of thin films.