Various structural changes in SiO2 introduced by one-photon excitation with undulator and two-photon excitation with excimer laser

Excitation at higher energies beyond the ultraviolet (UV) edge (similar to9 eV) in amorphous SiO2 (a-SiO2) can be achieved by two-photon process with ArF excimer laser as well as by one-photon process with undulator radiation . Photo-induced phenomena with two kinds of light sources are reported. Fre quency decrease of the Si-O stretching vibration in infrared absorption spe ctrum was observed in the a-SiO2 by one-photon process with 14.1 eV rays an d higher. Frequency decrease can be explained with transition from regular six membered ring (6 Si and 6 O in a loop) to three and four membered rings (3 (4) Si and 3 (4) O in a loop) in a-SiO2 network In case of two-photon p rocess with ArF excimer laser, ablation was observed. Threshold fluence sim ilar to1 J cm(-2) is apparently necessary to commence ablation by a single pulse. Ablation introduced with a single pulse above thr threshold fluence did not influence stoichiometry of SiO2. In contrast, for lower fluence bel ow the threshold. the ablation commenced after several pulses accompanied w ith oxygen loss and ablated thickness increased via a cumulative process. P hoto-ablation below the threshold fluence 1 J cm(-2) obeyed two-photon exci tation process, in contrast, ablation obeyed multi-photon process above the threshold fluence. (C) 2000 Elsevier Science B.V. All rights reserved.