The majority of the layers of a multilayer optical coating typically have optical thicknesses equal to one quarter of the appropriate wavelength of radiation. Replacing this constraint with the stipulation that a pair of adjacent layers should have a total optical thickness of one half of a wavelength introduces a significant new component of design flexibility while having a minimal impact upon the desired optical properties of the film. Taking a matrix approach, we derive a general expression for the reflectance of a periodic thin-film structure that is based on layers of two different materials of arbitrary thickness. This result is applied to highly reflective coatings at normal incidence and to off-normal polarizing coatings. Specific results involving HfO2/SiO2
films and TiO2/SiO2 films are displayed. We discuss how the thickness of the high-index layers may be reduced to increase damage thresholds. We also show a mirror design that is effective not only at λ = 1.06 µm but also at the frequency-doubled wavelength.
© 1997 Optical Society of America
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