Design of Optical Thin Films Using the 'Fencepost' Method

摘要

The technique described here is called the "Fencepost" Method because the resulting plots of index of refraction versus film thickness look like thin fence posts of high index material widely spaced and standing above a ground of low index material. This general structure has been shown to have scratch-resistance benefits in antireflection coatings. It is shown here to be generally capable of designing essentially all types of thin film coatings such as: antireflection, high reflection, edge filters, blocking filters, minus filters, narrow bandpass filters, polarizing filters, etc. All of the high index layers (for example) would be constrained to some fixed thickness of perhaps l/5th of a quarter wave optical thickness and only the low index layer thicknesses would be varied in the design optimization. The low index layers could be the fixed thin layers ("Posthole" Method), but this might be less favored for scratch resistance in the visible and near infrared spectral regions. However, postholes might be advantageous in other spectral regions and also might reduce the angular sensitivity of some designs. The restraint of not varying the thicknesses of half of the layers (high index) makes some designs somewhat more difficult, but satisfactory results can still be achieved.