LIGO Document G1000746-v1

Developing the TiO2/SiO2 mixed films for low optical loss dielectric mirror --- A retrospect

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Low optical loss, scattering plus absorption, for a multi-layer dielectric mirror is crucial in high performance mirror application. Scattering loss often comes from the grain boundary of the poly-crystalline film structure and surface roughness of the interfaces. Absorption loss often comes from the insufficient oxidation during the deposition process of the oxide film. Ion beam sputtering that produces dense amorphous film with higher refractive index is widely accepted as the ideal deposition technique to produce low loss thin film. Post-deposition thermal annealing is a common practice to enhance the oxidation and hence reduce the optical absorption of the film. However, high temperature annealing tends to crystallize the amorphous structure of the film and hence increases the scattering loss. TiO2 has the highest refractive index in the visible range among many potential oxide materials to be used for the high index layer material in the multi-layer dielectric mirror. Reducing the optical loss of the TiO2 film is therefore important. We found that adding SiO2 into TiO2 can inhibit the crystallization of the TiO2 film during the annealing process such that higher annealing temperature can be used to fully oxidize the TiO2 films without crystallization. The dielectric mirror that employing the TiO2-SiO2 mixed film thus has very low optical loss. We shall present the deposition process, structural and optical characteristics of the mixed films and the multi-layer mirror that we have pursued in the past. Discussions about possibility of applying the mixed film to the laser interferometer for gravitational wave detection are welcome.
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