Nanoporous TiO2-Based Distributed Bragg Reflectors for Near-Infrared Wavelength Applications.

We reported the fabrication and characteristics of distributed Bragg reflectors (DBRs), made of titanium dioxide (TiO2) dense/nanoporous film stacks, operating in the near-infrared wavelength region of 0.8-1.1 μm. To form the nanoporous TiO2 films with a low refractive index (low-n), the oblique angle deposition (OAD) technique was employed at a high incident vapor flux angle of 80 degrees. For the fabricated DBRs consisting of TiO2 dense/nanoporous (high-n/low-n) thin films, the high reflectance band was gradually increased and the stop bandwidth was narrowed with increasing the number of DBR pairs. Particularly, the TiO2 DBR with only 6 pairs exhibited a normalized stop bandwidth (Δλ/λ(c)) of -11.6% at a center wavelength (λ(c)) of 0.96 μm as well as high reflectance values of > 95% over a wide mid-infrared wavelength region of 0.9-1.01 μm (i.e., Δλ = 0.111 μm). Furthermore, the reflectance characteristics were investigated at incident light angles of 20-70 degrees for different polarized lights. For theoretical optical analyses, the reflectance calculations were also performed by a rigorous coupled-wave analysis method, showing a similar tendency to the experimentally measured data.