Impact of heat treatment on NBOHC luminescence of OH-containing and H2-impregnated fused silica for deep-ultraviolet applications

Abstract

OH-containing and H2-impregnated fused silica is the optical materials of choice for deep-ultraviolet (DUV) applications. Intrinsic and UV laser irradiation induced non-bridging oxygen hole center (NBOHC) defects play a central role in the optical properties of fused silica in the DUV spectra range. Understanding the NBOHC generation mechanism is of great importance to the performance improvement. In this paper, NBOHC-related 650 nm photoluminescence (PL) band is investigated in details by heat-treating the fused silica at different temperature and exciting the PL band at different wavelength. From the 650 nm PL characteristics of un-treated and heat-treated fused silica samples excited at 193 nm, 266 nm, 355 nm, and 532 nm, it is found that the 650 nm PL band of un-treated fused silica is excited at 193 nm and 266 nm mainly via a two-photon process, and cannot be excited with 355 nm and 532 nm. The un-treated fused silica also presented a reversible time delay (annealing) in 650 nm PL intensity following an abrupt change of excitation fluence at 193 nm, due to the recombination of NBOHCs with impregnated H2. On the other hand, for the heat-treated fused silica, the 650 nm PL band is excited by a combination of one- and two-photon processes at 193 nm and 266 nm with the one-photon process dominating, and by the one-photon excitation process at 355 nm and 532 nm. The quantum yield of 650 nm PL band decreases with the decreasing excitation photon energy. From these experimental results it is confirmed with no doubt that in un-treated fused silica NBOHCs are produced by 193 nm and 266 nm laser irradiation induced photolysis of an Si‒OH group via a two-photon process, with a minimum photolysis energy between 7.0 eV and 7.9 eV. In heat-treated fused silica NBOHCs are produced by heat treatment induced breakage of Si‒OH bonds. Heat treatment also removed totally the impregnated H2 in fused silica, that makes the heat-treatment produced NBOHCs become stable and show one-photon excited 650 nm PL band when excited at 193 nm, 266 nm, 355 nm, and 532 nm. In contrary to the un-treated fused silica, the heat-treated fused silica presented no reversible time delay in 650 nm PL intensity following the abrupt change of excitation fluence at 193 nm, due to the total removal of H2 in fused silica and the absence of recombination of NBOHCs with H2.