8.1.1 Uses of absorbing films
Every effort is made to minimize, if not eliminate, absorption in many of the multilayers described previously. Â§18.104.22.168 demonstrates, for example, that the performance of the multilayer reflector is degraded by absorption. There are numerous instances in which the absorption of metals and other layers is a welcomed effect. A few examples are:
â¢ Bandpass filter. The absorption of the metallic films in a bandpass filter provides superb off-band attenuation (see Fig. 1-93).
â¢ Absorbing coating. In many instances, the goal is a highly absorbing coating that manifests a small radiant reflectance.
â¢ Selective absorber. This coating is highly absorptive at certain wavelengths and highly reflective at others.
Quite often the absorption of a bandpass filter is not undesirable. For example, if the bandpass filter rejects radiation by reflecting it, the reflected flux is liable to scatter from baffles in the optical system and thence leak through the bandpass filter, especially at high incidence angles. An absorbing coating (such as the type mentioned in Â§8.5) is coated on each baffle that reduces its reflectances to small values. The performance of the coating is usually superior to the black lacquer often applied to visual optics. The IR reflectance of such coatings is quite small. Another advantage is that they do not outgas in a vacuum, as does the organic lacquer. Selective absorbers are used extensively in photothermal collection systems.
8.1.2 Generic absorbing materials
In this chapter there are many references to a âmetal.â This âmetalâ may, indeed, be a metallic layer of aluminum, silver or titanium. There are other possibilities: (1) The âmetalâ might be an oxide semiconductor such as indium oxide, tin oxide or cadmium stannate. (2) The âmetalâ could be a semiconductor, such as germanium, that is absorbing because the photon energy exceeds its band gap. (3) The âmetalâ might be a âdielectricâ coating material that manifests absorption in the near IR because of its reststrahlen bands [Turner 65, Martin 66]. Kagarise  used a bismuth (a semi-metal) to coat Ã©talon plates in the IR.
8.1.3 Absorption controlled by the standing wave electric field
Suppose that an absorbing layer is immersed in a multilayer stack. The amount of absorption that it produces is determined by its position in the standing wave electric field of the stack. This phenomenon may be understood in terms of the standing wave field in a microwave waveguide. As depicted in Fig. 8-1, a cavity is created by inserting two iris diaphragm reflectors into a waveguide. The spacing between the irises is adjusted so that there is second-order interference in the cavity. The upper panels of Fig. 8-1 show schematically (as a solid black curve) the standing wave electric field that exists in the cavity.