High reflectance dielectric distributed Bragg reflectors for near ultra-violet planar microcavities: SiO2/HfO2 versus SiO2/SiNx

SiO2/SiNx and SiO2/HfO2 distributed Bragg reflectors for the ultra-violet (λ = 360 nm–380 nm) are compared through their structural and optical properties. The SiO2/HfO2 system exhibits a lower interface roughness, higher reflectance, larger stop band, and lower penetration depth than SiO2/SiNx. A cavity quality factor of 3700 at about 360 nm is measured on a passive SiO2/HfO2-based planar microcavity. Compared with values obtained in the literature for the near UV range, the latter is rather large. Micro-reflectance measurements have been performed on a series of passive microcavities with increasing cavity thickness to determine the residual absorption in the SiO2 and HfO2 layers. Absorption coefficients of 30 (k = 0.86 × 10−4) and 160 cm−1 (k = 4.59 × 10−4) near λ ∼ 360 nm have been extracted for SiO2 and HfO2, respectively. Transfer-matrix simulations taking into account the residual absorption show that microcavity quality factors up to 8000 can be expected at 360–380 nm with this material system. Su...