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Characterization and mechanism of glass microwelding by double-pulse ultrafast laser irradiation

Sizhu Wu, Dong Wu, Jian Xu, Yasutaka Hanada, Ryo Suganuma, Haiyu Wang, Testuya Makimura, Koji Sugioka, and Katsumi Midorikawa

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Abstract

We investigated the physical mechanism of high-efficiency glass microwelding by double-pulse ultrafast laser irradiation by measuring the dependences of the size of the heat-affected zone and the bonding strength on the delay time between the two pulses for delay time up to 80 ns. The size of the heat-affected zone increases rapidly when the delay time is increased from 0 to 12.5 ps. It then decreases dramatically when the delay time is further increased to 30 ps. It has a small peak around 100 ps. For delay time up to 40 ns, the size of the heat-affected zone exceeds that for a delay time of 0 ps, whereas for delay time over 60 ps, it becomes smaller than that for a delay time of 0 ps. The bonding strength exhibits the same tendency. The underlying physical mechanism is discussed in terms of initial electron excitation by the first pulse and subsequent excitation by the second pulse: specifically, the first pulse induces multiphoton ionization or tunneling ionization, while the second pulse induces electron heating or avalanche ionization or the second pulse is absorbed by the localized state. Transient absorption of glass induced by the ultrafast laser pulse was analyzed by an ultrafast pump–probe technique. We found that the optimum pulse energy ratio is unity. These results provide new insights into high-efficiency ultrafast laser microwelding of glass and suggest new possibilities for further development of other ultrafast laser processing techniques.

©2012 Optical Society of America

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Figures (9)
Fig. 1
Fig. 1 (a) Schematic illustration of experimental setup used for microwelding photosensitive glass substrates by irradiation of double-pulse train. (b) Irradiation scheme for evaluation of the heat-affected zone. (c) Definition of the heat-affected zone based on optical microscopy observation.

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Fig. 2
Fig. 2 (a) Schematics of microwelding of two glass substrates by focused femtosecond laser. (b) Scanning scheme in x-y plane. (c) Schematic diagram of tensile tester.

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Fig. 3
Fig. 3 Optical microscope images of laser irradiated regions for delay time ranging from 0 to 40 ns and different irradiation times ranging from 0.1 to 10 s.

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Fig. 4
Fig. 4 (a) Dependence of size of the heat-affected zone on delay time. The first and second pulses both have a duration of 0.2 s and a pulse energy of 0.775 μJ. The red circle and blue triangle indicate the sizes of the heat-affected zones produced by a conventional single-pulse train (pulse energy: 1.55 μJ) for p- and s-polarized beams, respectively. (b) Optical microscopy images for delay times of 0 ps, 60 ns, and 80 ns (pulse energy: 1.35 μJ).

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Fig. 5
Fig. 5 (a) Cross-sectional images of laser-irradiated regions for delay times between 0 and 30 ps and exposure times between 0.1 and 10 s. (b) Dependence of the vertical length of the cross-section of heat-affected zone on exposure time for different delay time.

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Fig. 6
Fig. 6 Dependence of bonding strength on delay time. The bonding strength exhibits the same tendency as the size of the heat-affected zone. It increases rapidly when the delay time is increased from 0 to 12.5 ps, but it decreases dramatically above 12.5 ps. It almost saturates between 30 ps and 2 ns with a small peak at 100 ps and it decreases gradually in the range 1–2 ns.

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Fig. 7
Fig. 7 Dependence of bonding strength on ratio of energies of first and second pulses. The optimal energy ratio is unity.

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Fig. 8
Fig. 8 A possible physical mechanism of glass welding by double-pulse irradiation. It involves electron excitation and relaxation processes including multiphoton ionization or tunneling ionization, avalanche ionization or electron heating, and the electron trapping at the localized state.

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Fig. 9
Fig. 9 Transient absorption of second pulse for various delay times.. The transmittance decreases from 0 to 15 ps, and then increases from 15ps. It has a small dip at 100 ps.

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