Temperature and polarization dependent photoluminescence studies of WO$_{3}$ and WO$_{3-x }$single crystals

WO$_{3}$ is an important material not only due to its interesting electronic properties but also for applications in electrochromics and energy storage. The mechanism behind the electrochromic effect has been debated for several decades.\footnote{Satyen K. Deb, Solar energy materials and solar cells \textbf{92}, 245 (2008), and the references therein.} We have studied two WO$_{3}$ single crystals, a transparent and a doped WO$_{3-x}$. A photoluminescence center around 865 nm is observed after sub-band gap excitation at 405 nm with relatively higher intensity in the crystal containing oxygen vacancies. The center appears as a broad transition of 35 nm FWHM and does not follow the band gap energy with temperature. However polarization dependent studies reveal at least two polarization dependent component of the center.