Helicon plasma deposition of a TiO2/SiO2 multilayer optical filter with graded refractive index profiles

Thirty one layer ${\mathrm{TiO}}_{\mathrm{2}}{\mathrm{/SiO}}_{\mathrm{2}}$ optical filters with graded refractive index profiles were fabricated by helicon plasma sputtering at room temperature. Multilayer films were deposited on glass (BK7) and Si (100) substrates simultaneously and sequentially. The measured transmittance spectrum exhibited a reflectance of 99.8% at a central wavelength of 730 nm and high transmittance over the wavelength region outside of the reflected band as a result of the suppression of the sidelobes. The experimental transmittance spectrum corresponded almost completely with that calculated based on the optical multilayer film theory and using the measured refractive indices of ${\mathrm{TiO}}_{\mathrm{2}},$ ${\mathrm{SiO}}_{\mathrm{2}}$ and ${\mathrm{TiO}}_{\mathrm{2}}{\mathrm{–SiO}}_{\mathrm{2}}$ composite films. Transmission electron microscopic observations confirmed the expected microstructure of the filter.