Heating and rapid cooling of bulk glass after ... - OSA Publishing

Heating and rapid cooling of bulk glass after photoexcitation by a focused femtosecond laser pulse M. Sakakura 1*, M. Terazima2, Y. Shimotsuma1, K. Miura3, and K. Hirao3 1

Innovative Collaboration Center of Kyoto University, Kyotodaigaku-Katsura Nishikyo-ku, Kyoto 615-8520 JAPAN 2 Graduate School of Science, Kyoto University, Kitashirakawa-Oiwake, Sakyo-ku, Kyoto 606-8502 JAPAN 3 Graduate School of Engineering, Kyoto University, Kyotodaigaku-Katsura Nishikyo-ku, Kyoto 615-8510 JAPAN *Corresponding author: [email protected]

Abstract: To investigate the energy dissipation process after focusing a femtosecond laser pulse inside a zinc borosilicate glass, the time-dependent lens effect in the laser focal region was observed by a transient lens (TrL) method. We found that the TrL signal after 100 ns can be explained clearly by thermal diffusion. By fitting the observed signal, we obtained the phase change due to temperature increase, the initial diameter of the heated volume and the thermal diffusivity. On the basis of the results, the temperature increase and the cooling rate were estimated to be about 1800 K and 1.7X108 Ks-1, respectively. We have also observed the signal change on a 100 ns scale, which can not be explained by the thermal diffusion model. This change was attributed to the relaxation of the heated material. ©2007 Optical Society of America OCIS codes: (140.3390) Laser materials processing; (320.3980) Microsecond phenomena; (350.5340) Photothermal effects; (350.6830) Thermal lensing

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Received 13 Sep 2007; revised 8 Nov 2007; accepted 27 Nov 2007; published 3 Dec 2007

10 December 2007 / Vol. 15, No. 25 / OPTICS EXPRESS 16800

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M. Sakakura, and M. Terazima, “Initial temporal and spatial changes of the refractive index induced by focused femtosecond pulsed laser irradiation inside a glass,” Phys. Rev. B 71, 024113 (2005). M. Sakakura, M. Terazima. Y. Shimotsuma, K. Miura, and K. Hirao, “Observation of pressure wave generated by focusing a femtosecond laser pulse inside a glass,” Opt. Express 15, 5674-5686 (2007). J.F. Power, “Pulsed mode thermal lens effect detection in the near field via thermally induced probe beam spatial phase modulation: a theory,” Appl. Opt. 29, 52-63 (1990). S. J. Sheldon, L. V. Knight, and J. M. Thorne, “Laser-induced thermal lens effect - a new theoretical model,” Appl. Opt. 21, 1663-1669 (1982). T. Yagi, and M. Susa, “Temperature dependence of the refractive index of Al2O3-Na2O-SiO2 melts: role of electronic polarizability of oxygen controlled by network structure,” Meta. Mat. Trans. B 34B, 549-554 (2003). Glass Technical Data of Corning 0211 http://www.eriesci.com/custom/cor0211-tech.aspx G. Ghosh, “Model for the thermo-optic coefficients of some standard optical glasses,” J. Non-Cryst. Sol. 189, 191-196 (1995). A. K. Varshneya, Fundamentals of Inorganic Glasses (Academic Press, 1994), Chap. 12. J. Yu, P-.F. Paradis, T. Ishikawa, and S. Yoda, “Microstructure and dielectric constant of BaTiO3 synthesized by roller quenching,” Jpn. J. Appl. Phys. 43, 8135 (2004). R. Bruckner, “Properties and structure of vitreous silica. I,” J. Non-Cryst. Sol. 5, 123-175 (1970). J. W. Chan, T. R. Huster, S. H. Risbud and D. M. Krol, “Modification of the fused silica glass network associated with waveguide fabrication using femtosecond laser pulses,” Appl. Phys. A 76, 367-372 (2003).

1. Introduction Optical waveguides can be fabricated inside glasses by focusing femtosecond (fs) laser pulses [1-7]. One of the important factors in the production of the optical waveguide is the repetition rate of the laser irradiation, because the laser irradiation with high repetition rate (>200 kHz) causes heat accumulation in the laser focal volume [2-7]. For example, it has been reported that the heat accumulation effects result in a waveguide having an effective region larger than the photoexcited volume and create smooth structures without any cracks. Although several researchers have investigated the heat accumulation effects in the production of the waveguides [7-9], the thermal diffusion dynamics after photoexcitation have never been elucidated by the time-resolved technique. By the observation of the thermal diffusion, the initial heated volume and the thermal diffusion rate can be determined. The volume of the heat accumulation during the fs laser machining is also determined by the initial heated volume and the thermal diffusion rate (i.e. cooling rate); therefore, it is important to elucidate the heating process and the thermal diffusion by a time-resolved method. The transient grating [10-12] and transient lens (TrL) [13-17] methods have been utilized to observe thermalization and thermal diffusion dynamics in various laser-induced reactions so far. In the TrL method, the refractive index lens in the sample is observed by detecting a spatial deformation of a probe beam passing through the photoexcited volume, which is called a ‘lens effect’. As a temperature change in the material causes a refractive index change (Δn), the temperature distribution change due to the thermal diffusion can be detected by the TrL method. In this article, we report a time-resolved observation of the thermalization and the thermal diffusion process after a single shot irradiation of a fs laser pulse inside a glass by the TrL method. 2. Experimental methods Experimental setup for the TrL method is shown schematically in Fig. 1(a). Femtosecond laser pulses from (Femtolite C-10-SP; IMRA) were amplified by a regenerative amplifier (IFRIT; Cyber Laser, 780 nm, 220 fs) with a repetition rate of 100 Hz. A portion of this laser light was detected by a photodiode and the frequency of this signal was electrically reduced to 3 Hz by a frequency divider. By using a mechanical shutter triggered by this 3 Hz-signal, laser pulses was selected from the 100 Hz-pulse train with a repetition rate of 3 Hz. The opening time of the shutter was synchronized to the pulse train by a delay generator (DG535; Stanford Research Systems). The selection of one pulse from the pulse train was confirmed by monitoring the selected pulse with a photodiode and oscilloscope. The repetition rate of 3 Hz #87515 - $15.00 USD

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was slow enough to avoid multiple excitations at one spot of the material by translating the sample The fs laser pulse was focused inside a glass plate (Corning 0211; zinc borosilicate glass) placed on a XYZ stage by a 20X objective lens with a numerical aperture (NA) of 0.4 (Sigma Koki; OBL-20), and a material in the laser focal region was photoexcited. To avoid multiple excitations in the same region, the glass sample was moved vertically to the laser beam axis about 2 ms after the photoexcitation. After the photoexcitation, the time-dependent refractive index distribution, i.e. a TrL, appears around the photoexcited region. To probe the TrL, a CW He-Ne laser beam (λprobe=633 nm) was passed through the TrL region and the central region of the beam was detected by a photomultiplier (HAMAMATSU; R955). The time-dependent light intensity (ISig(t)) detected by the photomultiplier was recorded by a digital oscilloscope (Textronix; TDS 784D). The TrL signal ITrL(t) was defined by ITrL(t)=ISig(t)/Iprobe(t)

(1)

where Iprobe(t) is the intensity of the probe beam at the beam center without the photoexcitation. The TrL signal reflects the lens shape created in the sample. For example, as a convex refractive index lens focuses the probe beam at far field in the upper of Fig. 1(b), the central intensity of the probe beam is larger than that without photoexcitation, i.e. ITrL>1. In order to observe the TrL signal, the focus position of the probe beam should be different from the TrL region. The distance between the focal point of the probe beam and the TrL region is called a ‘focal mismatch’, and is denoted as d (Fig. 1(b)). Here, we defined that d is positive when the probe beam focuses prior to the TrL. The focal mismatch is important for interpreting the origin of the TrL signal, because it also affects the signal decay rate [1417]. For example, the probe beam is focused at far field by a transient convex-type refractive index lens at d>0, while it is expanded at far field by the same type lens at d0, the time of the peak is delayed as d becomes larger, and the signal intensity at d=+0.07 mm is smaller than those at d=0.03 mm and 0.01 mm. At d0. This d dependence means that this TrL signal comes from the refractive index lens, not from light absorption. The TrL signals simulated by Eqs. (4) and (5) at various ds are shown in Fig. 3(b). Clearly, the shapes after 100 ns and d-dependence of the simulated TrL signals look similar to those of the observed ones. Therefore, we can conclude that the observed TrL signal after 100 ns originates from the refractive index distribution change due to the thermal diffusion. Qualitatively, the rise-decay profile of the simulated TrL signal can be explained as follows. Initially, the TrL signal is weak, because the photoexcited region is much smaller than the radius of the probe beam. Due to the thermal diffusion to the outward with time, the temperature elevated region becomes wider and comparable to that of the probed region. Hence, the TrL signal intensity increases with increasing delay time. Further thermal diffusion leads to further broadening of the temperature profile, which causes the decrease of the TrL signal. #87515 - $15.00 USD

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Fig. 4. (a) TrL signals observed at Iex=0.6 μJ/pulse (open circles) and ones simulated (red lines) based on thermal diffusion models with Δφth=5.8, wth=1.7 μm and Dth=0.75 μm2μs-1. (b) Temporal evolutions of temperature at the center of the photoexcited region.

3.2 Estimation of ΔT0, wth, and Dth The comparison between the observed and simulated TrL signals shown above gives us an idea that we can determine the parameters in Eq. (4) by fitting the observed TrL signals. We found that the observed TrL signals with Iex=0.6 μJ/pulse can be fitted well by the TrL signals simulated with parameters Δφth=5.8, wth=1.7 μm and Dth=0.75 μm2μs-1 at all ds. The fitted signals are shown in Fig. 4(a). The positive Δφth suggests that the molar electric polarizability contribution to dn/dT [18] is larger than that of the thermal expansion. In case of this experiment, the thermal expansion could be suppressed, because the heated area is very small and confined in the solid material. We estimated the maximum temperature increase, ΔT0 from the determined phase change Δφth and Eq. (6). Assuming lz=50 μm and using n0=1.5 and dn/dT =3.4X10-6 K-1 of a borosilicate glass [20], we obtained ΔT0~1790 K. This temperature increase is higher than the melting temperature of the glass. Therefore, we can speculate that the material in the laser focal region could melt after photoexcitation until heat dissipation begins. Previously, it was reported that a contribution of the thermal radiation to a thermal diffusion becomes significant at high temperature [21]. Hence, it is rather surprising to find that the observed signal can be reproduced well by the simple thermal diffusion model even though the temperature change is more than 1000 K. This suggests that the energy emitted by a thermal radiation within the thermal diffusion time is much smaller than the thermal energy in the photoexcited region so that the thermal diffusion process is determined by phonon diffusion mainly. The determined wth (=1.7 μm) is smaller than the diffraction limited beam diameter; (1.22λex)/NA~2.4 μm. This smaller diameter should be due to nonlinear excitation. The obtained thermal diffusivity (0.75 μm2μs-1) was larger than that of a borosilicate glass at room temperature (0.46 μm2μs-1). This difference should come from the temperature dependence of Dth. By Eqs. (4) and (5), the obtained ΔT0, wth and Dth, the temperature change at the center was calculated and shown in Fig. 4(b). The temperature decreases from ~1800 oC to ~50 oC within 10 μs. Hence, an averaged temperature cooling rate is calculated to be 1.7X108 Ks-1. This cooling rate is much faster than the fastest rate achieved by conventional glass production methods; for example, 105-106 Ks-1 by a roller quenching method [22]. It is expected that the equilibrated structure at high temperature is fixed by the cooling process. The structure of high fictive temperature, which is defined by the temperature at which the structure is in equilibrium [23], is probably created at the laser focal region by this rapid cooling. This mechanism is consistent with the research of Chan et al., in which they observed Raman bands originating from the structure of a fictive temperature of 1500 oC at the fs laser focal region [24]. #87515 - $15.00 USD

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As described in the introduction, the cooling rate is important for determining the heat accumulation. According to Fig. 4(b), the temperature of the material cooled down to