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Optical Filters

OPTICAL FILTERS

Already in 1998 ALD was introduced as a potential alternative technique for the deposition of optical dielectric muitilayer structures. Using ZnS and Al₂0₃ as high and low refractive index materials respectively, antireflection coatings, neutral beam splitters, high-reflection coatings and Fabry-Perot filters were prepared. The optical performance of these components was studied by comparing their transmittance and reflectance spectra with those calculated for the ideal structures.

More recently, the control of the refractive index of laminated coatings consisting of alternating stacks of nanoscale Al2O3 and TiO2 sublayers grown by atomic layer deposition has been achieved. The refractive index of the coating linearly changed from 1.870 to 2.318 as the thickness of the single TiO₂ sublayer was varied from 2.0 to 39 Å while that of the single Al₂O₃ sublayer was kept constant at 5.5 Å. The refractive index could be varied by adjusting only the number of growth cycles of each material. This approach will have potential applications to optical multilayer coatings consisting of well-controlled extremely thin layers. the control of the refractive index makes it possible to fabricate coatings with the refractive index varying in the thickness direction gradient-index coatings, which will have attractive spectral performance exceeding that of regularly stacked normal multilayer coatings and can be used in many fields of application such as optical waveguides, rugate filter, narrow band filters, and broadband antireflection coatings.