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.