Absorption mechanism of the second pulse in double-pulse femtosecond laser glass microwelding

The absorption mechanism of the second pulse is experimentally and theoretically investigated for high-efficiency microwelding of photosensitive glass by double-pulse irradiation using a femtosecond laser. The transient absorption change during the second pulse irradiation for various energies induced by the first pulse is measured at different delay times. The resulting effects depend on whether the delay time is 0-30 ps (time domain I) or 30- several ns (domain II). By solving rate equations for the proposed electronic processes, the excitation and relaxation times of free electrons in time domain I are estimated to be 0.98 and 20.4 ps, respectively, whereas the relaxation times from the conduction band to a localized state and from the localized state to the valence band in domain II are 104.2 and 714.3 ps, respectively. Single-photon absorption of the second pulse by free electrons dominates in domain I, resulting in high bonding strength. In time domain II, about 46% of the second pulse is absorbed by a single photon due to the localized state, which is responsible for higher bonding strength compared with that prepared by single-pulse irradiation.