Microstructure of the Crystals Generated in Borate Glass Irradiated by

Synopsis. In this study, we examined the microstructure of crystals generated in borate glass by femtosecond laser irradiation (FSLI). The distributio...
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Microstructure of the Crystals Generated in Borate Glass Irradiated by Femtosecond Laser Pulses Yu,*,†

Bingkun Bin Xiongwei Jiang‡

Chen,†,‡

Bo

Lu,†,‡

Xiaona

Yan,†

Jianrong

Qiu,‡

Congshan

Zhu,‡

and

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 1 30-34

Department of Physics, College of Sciences, Shanghai UniVersity, Shanghai 200444, China and Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China ReceiVed September 26, 2005; ReVised Manuscript ReceiVed August 30, 2006

ABSTRACT: In this study, we examined the microstructure of crystals generated in borate glass by femtosecond laser irradiation (FSLI). The distribution of the high-temperature and low-temperature phases of barium metaborate crystals produced in the borate glass is analyzed using Raman spectroscopy. We then propose the possible mechanism for the generation of crystals in glass by FSLI. 1. Introduction The use of femtosecond lasers to locally alter the structure of transparent materials has attracted much attention in recent years. Using femtosecond laser irradiation (FSLI) technology, many optical devices have been generated, including threedimensional (3D) binary data storage1-4 and the direct writing of optical waveguides,5-10 waveguide splitters,11-14 waveguide amplifiers,15 gratings,16,17 coupled-mode photonic devices,18 3D colored image,19 3D micro-optical components,20 and a color center laser.21 When a femtosecond laser pulse is focused inside a transparent material, the accumulation of heat intensity in the focal volume can become high enough to induce nonlinear absorption of laser energy by the material.22,23 If sufficient energy is deposited into the focal volume by the nonlinear absorption, permanent structural changes could be produced.24 It has been suggested that femtosecond laser pulses directly triggers the densification of glasses through multiphoton ionization, which may explain the occurrence of structural changes25 and crystallization26 in glasses. In our previous studies, we showed that a femtosecond laser could be used as an irradiation source for the generation of crystals in glasses26 and as a source to trigger phase transition for high-temperature (HT) and low-temperature (LT) barium metaborate.27 Barium metaborate (BaB2O4) crystal, with a melting point of 1095 ( 5 °C, possesses a phase transition temperature at 925 ( 5 °C. While sharing the common unit of the (B3O6)3ring, the HT phase of barium metaborate (R-BaB2O4, R-BBO) and LT phase of barium metaborate (β-BaB2O4, β-BBO) are two structurally distinct types of BaB2O4 crystal. R-BaB2O4 is characteristic for fine birefraction and well transmissibility in ultraviolet band. β-BaB2O4 is an excellent nonlinear optical material for second-harmonic generation (SHG), optoelectronic pulse amplifier (OPA), optical parametric oscillator (OPO). Ever since β-BaB2O4 was first discovered by Chen and associates,28 many interesting studies have focused on analyzing the phase transitions between the HT and LT phases of BaB2O4 crystal.29-33 This work studies the microstructure of the crystals generated in borate glass by femtosecond laser irradiation. Raman * To whom correspondence should be addressed. E-mail: chenfir@ hotmail.com. † Shanghai University. ‡ Chinese Academy of Sciences.

spectroscopy is used to investigate the distribution of the HT and LT phases of barium metaborate crystals produced in the borate glass. We then propose a possible mechanism for such structural transitions in glass. In the future, it is possible to apply FSLI technology in the fabrication of integrated optical devices, such as SHG, OPA, and OPO. 2. Experimental Methods The composition of the borate glass studied was 47.5BaO-47.5B2O35Al2O3 [mol %]. Reagent-grade BaCO3, B2O3, and Al(OH)3 were used as starting materials. A mixed batch was melted in a Pt crucible at 1250 °C for about 2 h and then transferred into cold water to prevent crystallization of the glass. The resulting glass was cut and polished into a cuboid of X × Y × Z ) 5 × 5 × 2 mm. The femtosecond laser system we used is the same as in a previous study.26 It is essential to control the laser intensity to avoid damaging the glass on which the laser is focused. In this work, we set the fluence of the laser irradiation at 4.0 × 1012 W/cm2. The Raman spectral analysis was performed with a Renishaw inVia Spectrometer system using visible excitation at 514 nm. The excitation light source was a Ar ion laser with a power of 20 mW. A 50× objective lens was used to focus the beam onto the sample. The power on the sample was approximately 7 mW. The grating of the spectrometer was 1800 lines/mm. The spectrum’s scattering detection region ranged from 40 to 1600 cm-1 with a standard CCD array detector (576 × 384 pixels). The slit was 20 µm. The scattered radiation was measured at an angle of 180° from the incident laser beam. The sample was exposed for 40 s and scanned twice. All experiments were carried out at a room temperature of 25 °C.

3. Microstructral Changes in the Radial Direction The femtosecond laser beam was focused into the glass on a plane 100 µm below the surface of the sample. After irradiation for various lengths of time, a spherical area around the focal plane was analyzed under a microscope with a 10× objective lens (see Figure 1). The microstructral changes in the radial direction of the spherical areas irradiated for 10 and 15 min in Figure 1 were studied by Raman spectroscopy. 3.1. The Spherical Area Irradiated for 10 min. Figure 2 shows the microscopic image of the spherical area irradiated for 10 min and was observed under crossed polarized light. From point A to point B in the figure, Raman spectra are obtained every 1 µm, and they are shown in Figure 3. It can be seen that the Raman peaks are intensified between -20 µm and 45 µm (in the ruler, relative position) in comparison with the

10.1021/cg0505012 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/13/2006

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Figure 1. Microscopic images of the borate glass on the focal plane after being irradiated for different lengths of time by femtosecond laser. Images are taken under (A) white light or (B) crossed polarized light.

Figure 4. Relative radial changes of some characteristic peaks and their assignments in the spherical area irradiated for 10 min in borate glass.

Figure 2. Microscopic image of the spherical area irradiated for 10 min that is observed under crossed polarized light. The arrow indicates the scanning direction with the start (A) and stop (B) points in the Raman scattering experiment.

Figure 3. Raman spectra obtained every 1 µm from point A to B (in Figure 2).

area outside. The peaks are weaker in the middle (∼10 µm in the ruler) than those on both sides. In Figure 3, graphic curve at -40 µm shows the Raman spectra of borate glass in the unirradiated area. The broad bands represent the vibrations of borate groups in the network of borate glass. The peak at 214 cm-1 originates from the translation of Ba2+ and Al3+ ions in the glass. The peak at 498 cm-1 is assigned as the asymmetrical transforming vibration of the BO4 units. The weak band at 630 cm-1 is from the breathing vibration of (B3O6)3- rings. The peak at 755 cm-1 is attributed to the vibration of borate units within BO4 tetrahedron.34 We therefore conclude that the unirradiated glass is made up of various

groups, such as the triborate groups, the di-triborate groups, the orthoborate groups, and the boroxol rings, in a disordered network. In curves between “40 µm” and “-20 µm” (Figure 3), narrow Raman peaks emerge, which are then intensified between “-10 µm” and “30 µm”. In the low-frequency region, the intensity increases at 179, 202, and 244 cm-1. The peak at 179 cm-1 is assigned to the “A” symmetry vibrational mode of metaborate ring (B3O6)3-, while the peaks at 202 and 244 cm-1 are the translational modes of (B3O6)3- ring. The peak at 482 cm-1 is due to (B3O6)3- ring vibration of “E” symmetry. The peaks at 599 and 621 cm-1 of “A” symmetry are ascribed to the vibration of (B3O6)3- in the LT phase of BaB2O4 crystals. The Raman spectra also indicate the presence of a strong peak at approximately 637 cm-1, which arises from the breathing vibration of the (B3O6)3- ring and is the fingerprint peak of BaB2O4 crystals.29-31 Therefore, in summary, the appearance of the narrow peaks indicates the formation of β-BaB2O4 crystals in the focal area. In Figure 4, from ∼ -25 µm to ∼45 µm, the intensities of the peaks, which could be assigned to the crystals, show the progress of fade in, intensify, weaken, and then fade out. Two characteristic Raman peaks of BaB2O4 crystals are selected to illustrate this process. The change of intensities of these peaks with the relative position (in ruler) are shown in Figure 4. The most obvious peak in Figure 4 is at 637 cm-1, which is the fingerprint band of R-BaB2O4 and β-BaB2O4 and indicates the proportion of the crystals in the irradiated glass (spherical areas in Figure 1). It could be seen in Figure 4 that the peak at 637 cm-1 emerges and disappears at “-25 µm” and “45 µm”, respectively, which therefore indicates that positions “-25 µm” and “45 µm” are the boundaries of glass and the generated crystals. As a result, the diameter of the spherical area in Figure 2 is estimated to be at 70 µm in length. The transformations of the intensity of Raman peaks of R-BaB2O4 and β-BaB2O4 are different in Figure 4. An obvious change is the intensity of Raman peak at 664 cm-1, which is assigned to be β-BaB2O4 crystals. Enhanced peak values are observed within the boundary of the spherical area (from -25 µm to 45 µm) as a result of femtosecond laser irradiation. In contrast, much weaker bands are detected outside of that area.

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Yu et al.

Figure 7. The distribution of the intensity of Raman peak at 637 cm-1.

Figure 5. Microscopic image of the spherical area irradiated for 15 min that is observed under crossed polarized light. The arrow shows the direction (C: start; D: stop) of the Raman scattering experiment. Two curves show the change of the intensities of two peaks: 637 and 609 cm-1 (and their assignments) with the diameter of the spherical area. “S” is the starting point to do the Raman scattering experiment in depth.

Figure 6. Raman spectra obtained every 2 µm from point C to point D (in Figure 6).

Similar change is also discovered in Raman peaks at 64, 122, 179, 202, 244, 482, 599, 621, 637, 664, 779, 789, 1541 cm-1, although the intensities of these peaks are different. Hence, it could be concluded that β-BaB2O4 crystals are generated regularly in the spherical area. In comparison with the characteristic peak of β-BaB2O4 crystals, the peaks of R-BaB2O4 are unnoticeable, and a small wave could be seen in the intensity of the peak at 103 cm-1 in Figure 4. In curve “-10 µm” of Figure 3, the emergence of the peaks at 103, 122, and 1541 cm-1 shows the generation of R-BaB2O4 crystals, even though they are all very weak. 3.2. The Spherical Area Irradiated for 15 min. The spherical area irradiated for 15 min in Figure 1 is also examined in detail (Figure 5). A confocal Raman experiment was conducted every 2 µm from point C (start) to point D (stop), and the Raman spectra are shown in Figure 5. On the basis of previous studies,29-31 several characteristic Raman peaks are selected to show the transition of the intensities of the peaks along the detection line in Figure 6. The intensity of the band 637 cm-1 in Figure 6, which is the fingerprint peak of R-BaB2O4 and β-BaB2O4, changes dramatically from -45 µm to 30 µm. It is intensified from -45 µm and reaches the maximum at -40 µm, weakens quickly near

the minimum at about -10 µm, and then strengthens and weakens again. A small dip is seen at about -35 µm, which will be discussed later. Similar changes are also taking place in other characteristic peaks of β-BaB2O4 crystals, such as at 56, 176, 482, 597, 620, 664, 777, 788, 1526, and 1541 cm-1. However, these peaks are not strong in the Raman spectra of original β-BaB2O4 crystals, and the variations are not as obvious as the peak at 637 cm-1. Figure 5 also shows a dark center of the spherical area, when the intensities of the two peaks reach their minimum value. This dark center is considered to be a void produced after irradiation. However, the void is presumed to be nonexisting in the spherical area irradiated for 10 min (Figure 4), because a much smaller dent is observed. The characteristic Raman peaks of R-BaB2O4 crystals, such as at 148, 409, 609, 1502, and 1511 cm-1, intensified when the Raman bands of β-BaB2O4 crystals are all relative strong. It should be noted that when the intensities of these peaks reach their maximum value, dents are discovered at about -35 µm in the curve of the characteristic Raman peak of β-BaB2O4 crystals, such as at 599, 621, 789, and 1541 cm-1, as mentioned above. The Raman spectra are also obtained in the spherical area at 3 µm intervals as shown in Figure 7A. The peak at 637 cm-1 in Figure 7B is the characteristic peak of both phases of barium metaborate crystals and indicates the amount of crystals generated in the glass. 4. Microstructral Changes in Depth The spherical area irradiated for 15 min by a femtosecond laser was analyzed by Raman spectroscopy to study any microstructral changes in depth. “S” is the surface of the sample and is the starting point (Figure 5) to do the Raman scattering experiment. Raman spectra are obtained at 1 µm intervals from the surface and are shown in Figure 8. An intense Raman peak is observed at around the “20 µm” position in Figure 8. In addition, several characteristic Raman peaks of BaB2O4 crystals are illustrated in Figure 8 to show the changes of the intensities of these peaks along with the relative position (in ruler). Since the area affected by femtosecond laser pulses is spheroidal,24 the curves in Figure 8 are all eudipleural. We therefore conclude that crystals are generated from the relative position “-32 µm” (point F, calculated) to position “48 µm” (point H). 5. Discussion A. Void Formed. It can be concluded from Figure 1 that, beyond 1 min of irradiation, the radius of the spheroidal areas affected tends to be independent of irradiation time. The growth

Crystals Generated in Borate Glass

Figure 8. Raman spectra obtained at 1 µm intervals.

Figure 9. The area affected by long-time femtosecond laser pulses is spheroidal. Point O is the focal point of femtosecond laser.

of the radius stops at about 35 µm. So the radius grows with time according to the relation in ref 35. As mentioned above, the area affected by femtosecond laser pulses is spheroidal in shape24,36 and is shown as Φ in Figure 9. Point O is the focal point of the femtosecond laser. From our data, the major axis (54 µm) and minor axis (35 µm) could be calculated. The dotted line from S to G is the track of the analyzed points by Raman spectroscopy. B. Explosive Mechanism Concerned. Results shown in Figures 5-7 indicate the generation of a microvoid in the center of the spherical area surrounded by densified glass, which may be explained by an “explosive mechanism”. We reason that, after applying irradiation for a long time (e.g., 15 min), a void may form as the result of an “explosive expansion” of the highly energetic electron-ion plasma formed by a tightly focused femtosecond laser against the surrounding materials.37-39 The initial equilibrium state made up of hot plasma and densified glass could not be maintained during long-time FSLI. It is a dynamic equilibrium process composed of multiple equilibrium states. A relative small void could be observed in the initial equilibrium state when a limited amount of hot plasma is formed. Subsequently, with more laser energy absorbed by the glass, additional densified glass material is merged into the plasma, which then results in the generation of higher pressure vapor and more densified material. Consequently, high-tem-

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perature and high-pressure vapor generated by the laser heating expands rapidly into the ambient gas and creates a strong shock wave so that the material near the center could be densified again. Small undulations generated by this process could be seen in Figure 4. In the end, a final equilibrium state is reached when the radius is at approximately 35 µm as depicted in Figure 9. In addition to a 15-min irradiation time, we have also studied the spherical area irradiated for 20 min. We find that the only difference between the two is that more HT phase crystals are generated when a longer irradiation time is used. C. Bond Breaking. The cumulative heating due to FSLI could be modeled as a thermal diffusion equation.24,40 After 10 or 15 min of irradiation, the glass near the focal point of the laser could reach a high enough temperature to generate a strong temperature gradient. Since the melting point of glass is rather low (at around 1200 K), glass melt could be formed in the spherical area. As a result, structural rearrangements and bond breakings start to take place.25 Consequently, a clear border could be seen between the glass and the newly formed crystals. D. Groups Recombination. It is known that the borate glass is made up of triborate, di-triborate, orthoborate and diborate, and tetraborate groups. The Raman spectral changes shown above indicate severe structural damage to the structure of glass, and at the same time, the β-BaB2O4 crystal in the form of (B3O6)3- is formed. That is to say that it is the HT condition that leads to the bond breaking, the readjustment of geometry of anionic borate group (B3O6)3-, and the formation of symmetrical anion-cation crystal units. It is important to point out that, due to the differences in bond energies, different lengths of irradiation time are needed for different glasses for bond breaking and readjustment of geometry of anionic groups. E. Nonuniform Crystallization. The process of crystal formation in glass follows a dynamic course. However, after 15-min of FSLI treatment, the dynamic equilibrium process is replaced by a final equilibrium state, in which a prominent temperature gradient is established. Therefore, R-BaB2O4 crystals are generated under the HT conditions. After the laser pulse is turned off, and also because of strong gradients in temperature, the resolidification process is nonuniform. Thus, two phases of crystallization are produced at different areas (Figure 5). 6. Conclusions In this paper, we show that after 10 min of FSLI treatment, crystals are produced in the borate glass, which is confirmed with a confocal Raman spectrometer. We also show that a void is formed inside the borate glass after being irradiated for 15 min. In both cases, two phases of crystallization (R-BaB2O4 and β-BaB2O4 crystals) are discovered in different locations within the spherical irradiated areas. We propose that the bond breaking and group recombination events are the result of intense heat accumulation through a “microexplosive” mechanism. Acknowledgment. This study is supported by the Shanghai Leading Academic Discipline Program T0104, by the National Sciences Foundation of China under Grants 59832080 and 60377017, and by the ‘‘Hundred Persons” Program of the Chinese Academy of Sciences. References (1) Glezer, E. N.; Milosavljevic, M.; Huang, L.; Finlay, R. J.; Her, T. H.; Callan, J. P.; Mazur, E. Opt. Lett. 1996, 21, 2023.

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