Novel beam splitter for high-order harmonics with WO3/TiO2 bilayer grown on c-plane sapphire substrate by sequential surface chemical reactions

Abstract

High-intensity high-order harmonics have been investigated intensively in recent years. In the development of a beam line for the high-intensity high-order harmonics, however, utilizing a conventional beam splitter (BS) (Si or SiC) that absorbs the fundamental waves has caused serious problems such as its thermal distortion. To solve these problems, we proposed and investigated a novel BS with transparent materials that transmitted the fundamental waves and then reflected the high-order harmonics. In BS for the high-order harmonics, reflection of the fundamental waves should be minimized by entering the p-polarized fundamental waves at the Brewster's angle, which could improve the separation between the fundamental waves and the high-order harmonics at the same Brewster's angle. We have already investigated and fabricated WO 3 /TiO 2 bilayers on c-plane sapphire substrates by controlled growth with sequential surface chemical reactions (SSCR) using sequentially fast pressurized pulses of the vapor sources. Our previous experimental results revealed that WO 3 (221) and rutile TiO 2 (200) thin films could be grown epitaxially on c-plane sapphire substrates by SSCRs. Then, in this study, we proposed a WO 3 /TiO 2 bilayer grown on c-plane sapphire substrates, which could be utilized as a BS for the high-order harmonics. Reflectance characteristics were also investigated at the same Brewster's angle using monochromatized synchrotron radiation (SR) located at Ultraviolet Synchrotron Radiation Facility (UVSOR), Institute for Molecular Science, Okazaki, Japan.