What Are Optical Coatings?
Optical Coatings are typically viewed as a classification of Thin Films deposited onto an optical substrate for the purpose of modifying or altering the transmission, reflection absorption or polarization of light properties. Thin film coatings can be applied to an optical component via most forms of Physical Vapor Deposition (PVD) technologies including evaporation deposition (resistance and electron beam), ion beam sputtering, atomic layer deposition as well as through magnetron sputter deposition. All of these can offer unique processing advantages depending on the materials being coated for the specific applications involved. These various deposition technologies have been described previously in various Plasmaterials blogs.
When a ray of light strikes a given surface, some fraction of that beam is reflected back, some of it is absorbed within the material structure and some of is transmitted through the material. In certain cases, there may be any combination of all three. Optical coatings can be created to alter the outcome of these competing affects – either separately or collectively. Thin film optical coatings are typically produced by depositing thin multi layered materials onto optical components. These layers can be applied alternatively to create a stack of varying compositional constituents. Individual layers may differ in composition, thickness, structure or in the total number of sequential layers being applied. An optical coating is composed of any combination of layers of materials such as oxides, metals (including refractory and rare earth metals), dielectrics or insulators which collectively alter the performance of an optical coatings dependent on the refractive indices (λ) of the materials being deposited.
Different materials, or the combination of different materials, can be employed to alter the refractive index within spectral ranges of wavelength between the infrared (IR) – longer wavelengths and ultra violet (UV) – shorter wavelengths in the range of 400 – 700 nanometers The refractive index is a key component in multilayered coatings. These can be classified as follows:
INDEX OF REFRACTION (λ)
Low λ Intermediate λ High λ
λ < 1.6 λ = 1.6 – 2 λ >2
Thin film optical coatings are characteristically created by depositing dielectric and metallic materials in alternating layers in order to alter the interference of the wavelength of light used for a specific application. By alternating high and low index of refraction materials it is possible to induce the interference effects desired. Such coatings can generally be classified by how, and to what extent, the light is being modified. Antireflective (AR) coatings are used to suppress unwanted reflections from the interface between two different medias (such as air and glass). High reflection (HR) coatings such as mirrors are used to increase the reflectivity at a given surface. Intermediate reflective coatings transmit a specified amount of light input and reflect the rest.
In subsequent blogs we will explore different applications and performance parameters of these optical coatings. The range is broad and the technologies are varied.