Refractive index is a dimensionless quantity that describes the speed of light in a medium with respect to the speed of light in vacuum. Most materials have a refractive index > 1 which means that light travels slower in a medium than it does in vacuum. The higher the refractive index, the slower the light moves in that medium. When a researcher looks up “the refractive index” of a material in a book, it is often reported as single number, but it’s not that simple.
In reality, refractive index is a complex number comprised of a real part (n) and an imaginary part (k). The real part, as described above, describes the speed of light in the material. The imaginary part of the refractive index is the extinction coefficient in the material - a measure of how much light is being absorbed at a given wavelength. Both n and k are wavelength dependent, so they vary over the spectrum.
The refractive index of a thin film can also vary significantly from that of the bulk material reported in books. The refractive index of a thin-film depends on a myriad of process conditions including deposition rates, gas flow rates, oxidation plasma parameters, base vacuum pressure, etc. Small changes in these parameters can affect the film structure and density which in turn affect the refractive index and the final product.
Interference filters are typically made with materials that do not exhibit absorption (k) in the wavelengths of interest, so that part of the refractive index can be neglected. Interference filters work because of constructive and destructive interference between alternating layers of high refractive index (n) and low refractive index materials. The most basic interference filter is a quarter wave stack. A “quarter wave” occurs when the product of the refractive index and the physical thickness of a layer is equal to ¼ of the wavelength being observed. Thin-film designers refer to this product as the optical thickness. Our thin-film design software calculates the required physical thicknesses of each layer to achieve desired spectral performance, but we often monitor film growth using optical thickness. Needless to say, this would be impossible without having tight control over the refractive index in our layers.
Next time in the blog, find out more about how we measure refractive index and keep our processes running optimally for a consistent product.