Rugate Thin Films
We have described how interference effects rely on discrete films of materials with different properties. An alternative design technique exists, where the refractive index can vary continuously throughout the thin film structure in a periodic way. Such a structure is called a rugate. A ‘rugate’ is an inhomogeneous transparent medium with a periodically varying refractive index. This powerful and flexible technique can extend the design possibilities to thin film coatings with properties not readily available with conventional thin film designs. One such area is the design and manufacture of relatively narrowband reflectors or ‘notches’.
For a single continuous film with a periodically varying refractive index, the Fourier transform of the index profile gives the spectral response of the structure. A sinusoidal index profile produces a single reflection ‘notch’ within the transmission band.
Although a single notch is produced, ripples occur near the shoulders. This can be virtually eliminated by progressively attenuating the modulation amplitude, matching to the substrate on one side and air on the other.
The refractive index profile can be manufactured by depositing two thin film materials simultaneously, varying the proportion of high index to low index material continuously. Alternatively, the refractive index profile can be broken up into discrete steps and then digitised into a two material structure using very many thin layers. This is a manufacturing strategy that is very powerful.