Making an omnidirectional reflector

The effect of having a finite number of layers on the design of omnidirecti onal reflectors was investigated. It was shown that the structure should be finished with a low-index layer having a thickness larger than a quarter-w ave to increase reflectivity, whereas layers below may remain of quarter-wa ve optical thickness at normal incidence angle. This general trend has been used for designing and realizing two a-Si-SiO2 (amorphous silicon and sili con dioxide) omnidirectional reflectors in the near-infrared range on a sil icon and a silica substrate, respectively. Owing to the decrease of absorpt ion of recrystallized silicon as compared with a-Si in the visible range, t he transmissivity of the structure realized on silica substrate was dramati cally increased in the visible range upon annealing, whereas the high refle ctivity and the omnidirectional effect were maintained in the near-infrared range. (C) 2001 Optical Society of America.