  Characterization of the vacuum ultraviolet (VUV) reflectance of silicon
photomultipliers (SiPMs) is important for large-scale SiPM-based photodetector
systems. We report the angular dependence of the specular reflectance in a
vacuum of SiPMs manufactured by Fondazionc Bruno Kessler (FBK) and Hamamatsu
Photonics K.K. (HPK) over wavelengths ranging from 120 nm to 280 nm. Refractive
index and extinction coefficient of the thin silicon-dioxide film deposited on
the surface of the FBK SiPMs are derived from reflectance data of a FBK silicon
wafer with the same deposited oxide film as SiPMs. The diffuse reflectance of
SiPMs is also measured at 193 nm. We use the VUV spectral dependence of the
optical constants to predict the reflectance of the FBK silicon wafer and FBK
SiPMs in liquid xenon.
