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 ( 9 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. Frequency decrease of the Si±O stretching vibration in infrared absorption spectrum was observed in the a-SiO2 by one-photon process with 14.1 eV rays and 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 process with ArF excimer laser, ablation was observed. Threshold ¯uence 1 J cmÿ2 is apparently necessary to commence ablation by a single pulse. Ablation introduced with a single pulse above the threshold ¯uence did not in¯uence stoichiometry of SiO2. In contrast, for lower ¯uence below the threshold, the ablation commenced after several pulses accompanied with oxygen loss and ablated thickness increased via a cumulative process. Photo-ablation below the threshold ¯uence 1 J cmÿ2 obeyed two-photon excitation process, in contrast, ablation obeyed multi-photon process above the threshold ¯uence. # 2000 Elsevier Science B.V. All rights reserved