Tunable distributed Bragg reflectors with wide-angle and broadband high-reflectivity using nanoporous/dense titanium dioxide film stacks for visible wavelength applications

Highly-tolerant distributed Bragg reflectors (DBRs) based on the same materials consisting of nanoporous/dense titanium dioxide (TiO2) film pair structures with wide-angle and broadband highly-reflective properties at visible wavelengths are reported. For a high refractive index contrast, the two dense and nanoporous TiO2 film stacks are alternatingly deposited on silicon (Si) substrates by a oblique angle deposition (OAD) method at two vapor flux angles (theta(alpha)) of 0 and 80 degrees for high and low refractive indices, respectively. For the TiO2 DBRs at a center wavelength (lambda(c)) of 540 nm, the maximum level in reflectance (R) band is increased with increasing the number of pairs, exhibiting high R values of > 90% for 5 pairs, and the normalized stop bandwidth (Delta lambda/lambda(c)) of similar to 17.8% is obtained. At lambda(c) = 540 nm, the patterned TiO2 DBR with 5 pairs shows an uniform relative reflectivity over a whole surface of 3 inch-sized Si wafer and a large-scalable fabrication capability with any features. The angle-dependent reflectance characteristics of TiO2 DBR at lambda(c) = 540 nm are also studied at incident angles (theta(inc)) of 20-70 degrees for p-, s-, and non-polarized lights in the wavelength region of 350-750 nm, yielding high R values of > 70.4% at theta(inc) values of 20-70 degrees for non-polarized light. By adjusting the lambda(c)/4 thicknesses of nanoporous and dense films, for lambda(c) = 450, 540, and 680 nm, tunable broadband TiO2 DBRs with high R values of > 90% at wavelengths of 400-800 nm are realized. (C)2014 Optical Society of America