Optical coatings typically consist of thin films made up of single or multiple layers of either metallic or dielectric materials. When properly designed and fabricated, these coatings can dramatically modify the reflection and transmission properties of an optical component. The properties can be controlled from the deep UV to the IR with narrowband, broadband, or multi-band response, and can be polarization sensitive. Optical coatings can be applied directly to the surface of an optical component to tailor its reflectivity, as in the case of an
optical mirror or beamsplitter. For other components, such as
lenses, the applied coatings may simply improve their overall transmission properties by reducing surface reflectivity. When optical coatings are integrated into a monolithic component for the express purpose of controlling the spectral transmission of light, the component is referred to as an optical filter.
The individual layers that make up optical coatings are typically a few tens of nanometers to a few hundred nanometers in thickness, while a single optical coating can be comprised of several hundred layers. Consequently, the techniques used to deposit these layers require a high degree of precision. Generally, the process begins with surface fabrication to minimize surface roughness and sub-surface damage. It continues with surface cleaning and preparation and is followed by deposition of high-performance thin film designs. The deposition technologies include thermal evaporation, electron-beam, ion-assisted deposition, and advanced plasma deposition. The most appropriate coating technology for the intended product design depends on the operating environment, spectral requirements, physical characteristics, application requirements, and economic targets. The optical coating process is completed with comprehensive performance testing using sophisticated metrology tools.
Metallic coatings used on optical mirrors typically consist of a single layer approximately 100 nm thick. This ensures that the broadband high reflectivity properties of the metal due to the complex index of refraction are present. In order to provide greater tuning of the reflectivity and over specific wavelengths of interest, dielectric coatings are used. These coatings (sometimes referred to as optical interference coatings) consist of alternating high refractive index (nH = 1.8 - 4.0) and low refractive index (nL = 1.3 - 1.7) dielectric layers (see Figure 1). The thickness of each layer is chosen such that the product of the thickness and the index of refraction of the layer is λ/4. A variation of the formula given by the Reflectivity Equation (see Optical Mirror Physics) can be used to estimate the maximum reflectivity of the dielectric coating which increases with a greater number of layers but is accompanied by a concomitant reduction in the spectral bandwidth. Dielectric coatings are ubiquitous and their applications are discussed below. Other optical coatings include metal-dielectric hybrid films such as those in cube beamsplitters and absorptive coatings made of organic materials such as those found in certain optical filters.
