Prior to the laser patterning of Bi₂ZnB₂O₇ crystal lines in Bi₂O₃–ZnO–B₂O₃ glasses, it is necessary to confirm the formation of Bi₂ZnB₂O₇ crystals in the crystallization of a given glass. Basically, the crystalline phase formed by laser irradiations in glasses is the same as that formed by heat treatments in an electric furnace. The glass with the composition of 33.3Bi₂O₃-33.3ZnO-33.3B₂O₃ corresponding to the stoichiometric composition of the Bi₂ZnB₂O₇ crystalline phase is designated here as 111BiZnB glass.

Figure 1 shows the DTA patterns for the bulk and powdered samples of as-quenched 111BiZnB glasses. In both samples, endothermic dips due to the glass transition and exothermic peaks due to the crystallization are detected clearly, providing the values of T_g = 395°C and T_p = 562°C for the bulk sample and T_g = 395°C and T_p = 551°C for the powdered sample. It is noted that the crystallization peak temperature (T_p = 562°C) for the bulk sample is close to that (T_p = 551°C) for the powdered sample, implying the possibility of the so-called bulk crystallization in the inside of glass. In Figure 1, the optical microscope photograph for the base glass is included. The glass is colorless in the visible region. The density (d) and refractive index (n) of 111BiZnB glass at room temperature are d = 6.53 g/cm³ and n = 2.06, respectively.

Figure 2 shows the XRD patterns at room temperature for the 111BiZnB samples obtained by different heat treatment temperatures for 3 h in air. In these XRD measurements, the bulk samples were used without any pulverizing and thus information on the crystalline phase being present at and near the glass surface is obtained. As seen in Figure 2, the formation of Bi₂ZnB₂O₇ crystals is confirmed in the samples heat-treated at 470, 500, and 530°C. Another crystalline phase is formed initially, which has not been identified (i.e., unknown crystalline phase) in our study. It has been reported that the β-BiB₃O₆ crystalline phase is formed initially in the crystallization of 111BiZnB glass (Shanmugavelu and Kumar, 2012). In Figure 2, the optical microscope photograph for the sample heat-treated at 470°C is included. It is seen that the surface of the bulk sample is rounded. This behavior indicates that the softening/deformation of samples and crystallization occur almost simultaneously during heat treatments in 111BiZnB glass (Kanenishi et al., 2012). The high thermal stability against the crystallization in 111BiZnB glass, i.e., ΔT = T_p−T_g = 562°C−395°C = 167°C, would be one of the reasons for the sample deformation during heat treatments. As seen in Figure 2, the formation of unknown crystalline phase is detected in the sample heat-treated at 450°C from the XRD pattern; but in the sample heat-treated at 470°C, the formation of such an unknown crystalline phase is not confirmed. The crystallization at the glass surface is influenced by various factors such as surface quality, tips, cracks and scratches, foreign particles, and surrounding atmosphere in a given glass, i.e., the surface state (Müller et al., 2000). The shape of the sample heat-treated at 470°C was changed, i.e., the rounded shape (see Figure 2). This means that the surface state is largely changed in the heat treatment at 470°C, and thus the surface crystallization behavior would be influenced, providing the formation behavior of the unknown crystalline phase.

Figure 3 shows the XRD patterns at room temperature for the powdered samples, in which bulk plate-shape glasses were heat-treated and the samples were pulverized after heat treatments. Figure 3 provides, therefore, information mainly on the crystalline phase present in the interior of samples. All XRD peaks in Figure 3 are assigned to the Bi₂ZnB₂O₇ crystalline phase (ICDD: 74–7878), indicating that only the Bi₂ZnB₂O₇ crystalline phase is formed in all samples heat-treated at 450–530°C for 3 h in air. SHGs were confirmed in the crystallized samples (data are not shown here), indicating that the Bi₂ZnB₂O₇ crystalline phase formed in 111BiZnB glass exhibits the second-order optical non-linearity. Figure 4 shows the Raman scattering spectra at room temperature for the base glass and crystallized (530°C, 3 h) samples. The glass shows very broad peaks being typical for glassy materials in the Raman scattering spectrum, i.e., at ~350, ~600, ~920, 1150 ~ 1400, and ~1550 cm⁻¹. From the Raman scattering spectra for various Bi₂O₃–ZnO–B₂O₃ glasses such as 30Bi₂O₃-30ZnO-40B₂O₃, the formation of B–O–Bi and B–O–Zn bridging bonds in the glass structure has been proposed (Inoue et al., 2010). The broad peaks observed in Figure 4 would be assigned as follows: (1) the band at 350 cm⁻¹ can be connected with a symmetric stretching motion in Bi–O–Bi bonds between BiO₆ groups; (2) the band at 600 cm⁻¹ could be assigned to symmetric stretching motions in B–O–Bi and B–O–Zn bonds; (3) the band at 920 cm⁻¹ can be related mainly to asymmetric stretching motions in B–O–Bi and B–O–Zn bonds; (4) the bands at 1150 ~ 1400 and ~1550 cm⁻¹ can be assigned to an asymmetric stretching vibration of B–O bonds in BO₃ groups. On the other hand, the crystallized sample shows several sharp peaks at 347, 382, 497, 537, 577, 632, 699, and 741 cm⁻¹ together with broad and weak peaks at 877, 1243, 1340, and 1548 cm⁻¹. Because Bi₂ZnB₂O₇ crystals are formed in the crystallized sample, it is obvious that the Raman scattering spectrum shown in Figure 4 is for the Bi₂ZnB₂O₇ crystalline phase. In Bi₂ZnB₂O₇ with an orthorhombic structure (a = 1.0827 nm, b = 1.10329 nm, and c = 0.48848 nm) (Barbier et al., 2005), octahedral BiO₆ units, tetrahedral ZnO units, BO₃ and BO₄ units are present. ZnO₄ units and BO₃/BO₄ units are connected through a corner-shared bonding, i.e., the presence of B–O–Zn bonds, and are creating layers. The Bi₂ZnB₂O₇ crystalline phase is, therefore, constructed from the stacking of BiO₆, ZnO₄, BO₃, and BO₄ units. At this moment, the peaks shown in Figure 4 have not been assigned in detail. However, the Raman scattering spectrum for Bi₂ZnB₂O₇ crystals would be used for the confirmation of the presence of Bi₂ZnB₂O₇ crystals in laser-patterned lines.

In the laser-induced crystallization technique proposed and applied by our group (Komatsu et al., 2007; Komatsu and Honma, 2013), some amounts of rare-earth ions such as Sm³⁺ or transition metal ions such as Ni²⁺ must be included into the base glass for the absorption of laser. In this study, the glasses with the compositions of xSm₂O₃-(33.3-x)Bi₂O₃-33.3ZnO-33.3B₂O₃ (x = 1, 3, and 5) were prepared. The melt-quenched samples with these compositions were confirmed to be glasses from XRD and DTA measurements.

For the glass with 1Sm₂O₃ (x = 1), it was found that laser irradiations with different conditions such as the laser powers of P = 1.3–1.5 W and the laser scanning speed of S = 1.5 μm/s did not induce any crystallization. Furthermore, for the glass with 5Sm₂O₃ (x = 5), it was difficult to pattern crystal lines with a homogeneous morphology. Here, therefore, the results for the glass with 3Sm₂O₃ (x = 3), i.e., 3Sm₂O₃-30.3Bi₂O₃-33.3ZnO-33.3B₂O₃ (designated here as 3Sm-111BiZnB glass) are presented. The 3Sm-111BiZnB glass has the values of T_g = 410°C and T_p = 553°C. Because of the substitution of Sm₂O₃ for Bi₂O₃, the glass transition temperature is becoming high and the crystallization temperature is low compared with the 111BiZnB glass (T_g = 395^°C and T_p = 562^°C) with no Sm₂O₃. The value of the optical absorption coefficient (α) at λ = 1080 nm (Yb:YVO₄ laser) was α = 10.5 cm⁻¹ for 3Sm-111BiZnB glass. The reflection of light at the glass surface calculated using the Fresnel’s equation was considered in the estimation of α (i.e., Lambert–Beer law).

Figure 5 shows the polarized optical microphotographs (POMs) for the samples obtained by laser irradiations. The laser scanning speed was fixed to S = 1.5 μm/s, and the laser power was changed in the range of P = 1.30–1.50 W. The laser was focused at the surface (not in the inside). The lines patterned with P = 1.30–1.45 W show a homogeneous distribution in color along the laser scanning direction, indicating that a stable structural change is taking place under these laser irradiation conditions. The morphology of the line patterned with P = 1.50 W is not homogeneous. Figure 6 shows the micro-Raman scattering spectra at room temperature for the lines patterned by laser irradiations with P = 1.35 and 1.50 W. In both lines, several sharp peaks are observed and the positions of these peaks are almost similar to the Bi₂ZnB₂O₇ crystalline phase (Figure 4), indicating that the lines patterned with P = 1.35 and 1.50 W consist of Bi₂ZnB₂O₇ crystals. In Figure 6, the SHG microscopy images of these lines are presented, indicating that SHG emissions are founded to be caused by only the crystal lines. Similar results were obtained for other lines patterned with P = 1.30, 1.40, and 1.45 W.

In order to examine the morphology and orientation of Bi₂ZnB₂O₇ crystals in the line patterned by laser irradiations with S = 1.5 μm/s and P = 1.35 W by using TEM observations, the line part was processed to a thin foil (~100 nm) shape using a FIB method. Figure 7 shows the bright field image and selected area electron diffraction (SAED) pattern in TEM observations for the cross-section of the line. Almost a homogeneous bright field image is observed over the whole crystallized part. In the SAED, the diffraction spots are observed clearly. These results suggest that Bi₂ZnB₂O₇ crystals in the cross-section (i.e., in the line) are highly oriented, and the line is not constructed from the assembly of Bi₂ZnB₂O₇ polycrystals. In order to determine definitely the orientation direction/quality of Bi₂ZnB₂O₇ crystals in the lines patterned, further TEM observations are required (Fan et al., 2012). In the present study, the laser focal position was the surface. However, as seen in Figure 7, Bi₂ZnB₂O₇ crystals were formed in the inside of the glass, i.e., at the beneath of 4 μm from the surface. Usually, if the laser focal position is the surface of a given glass, crystals are basically formed at the glass surface [e.g. Komatsu et al. (2007) and Komatsu and Honma (2013)]. The crystal growth in the inside of the glass (Figure 7) observed in this study might be closely related to the high thermal stability (i.e., ΔT = T_p−T_g = 167^°C) against the crystallization in the glass of 3Sm₂O₃-30.3Bi₂O₃-33.3ZnO-33.3B₂O₃. As shown in Figures 2 and 3, although unknown crystals are formed initially at the glass surface, the main crystalline phase formed in the inside is the Bi₂ZnB₂O₇ crystalline phase. This would be one of the reasons for the formation of Bi₂ZnB₂O₇ crystals in the lines patterned by laser irradiations. Recently, non-linear optical β-BaB₂O₄ (β-BBO) crystal lines have been successfully patterned in the inside (up to the beneath of ~200 μm) of 8Sm₂O₃-42BaO-50B₂O₃ glass by laser irradiations, in which the laser focal position was moved gradually from the surface to the inside (Nishii et al., 2015). The present study suggests that the patterning of Bi₂ZnB₂O₇ crystal lines would be possible even in the inside of more deep beneath around 100 μm for Bi₂O₃–ZnO–B₂O₃ glasses by changing the laser focal position.

Nowadays, it has been well recognized that Bi₂O₃ oxide is one of the important constituents for the design and control of optical, electrical, and thermal properties of oxide glasses, and numerous studies on Bi₂O₃-based glasses have been studied so far (Dumbaugh, 1986; Maeder, 2013). The most attractive features of Bi₂O₃ oxide are a large electronic polarizability and a weak single bond strength of Bi–O bonds (Dimitrov and Komatsu, 1999, 2002, 2005, 2010; Komatsu et al., 2010). Indeed, glasses with Bi₂O₃ exhibit large third order non-linear optical susceptibilities χ⁽³⁾ of the order 10⁻¹² esu (Terashima et al., 1997). Recently, Bi₂O₃–ZnO–B₂O₃ glasses have received much attention (Kim et al., 2007; Doweidar and Saddeek, 2009; Inoue et al., 2010; Hashimoto et al., 2011), in particular, because they are low melting glasses with high thermal stability and have been used as new sealing glasses instead of PbO-containing low melting glasses. The crystallization of Bi₂O₃-based glasses has also received much attention, because some functional crystals containing Bi₂O₃ are formed through the crystallization. For instance, high-T_c Bi₂O₃-based copper superconducting glass-ceramics have been synthesized through the crystallization of glasses with Bi₂O₃ (Komatsu et al., 1988, 1990). Non-linear optical RE_xBi_1–xBO₃ crystals (RE: Gd and Sm) showing strong SHGs have been also synthesized from RE₂O₃–Bi₂O₃–B₂O₃ glasses (Honma et al., 2002; Ihara et al., 2004; Koshiba et al., 2007). The design and control of new Bi₂O₃-based glasses and their crystallization are at the frontiers of the glass science and technology.

In this study, we demonstrated that non-linear optical Bi₂ZnB₂O₇ crystal lines with a homogeneous morphology and a high orientation are patterned in 3Sm₂O₃-30.3Bi₂O₃-33.3ZnO-33.3B₂O₃ glass for the first time. Crystallized glasses (glass-ceramics) are composites of the glassy phase and crystalline phase, i.e., glass/crystal hybrid materials. In particular, the combination of glass and single crystal line would have a high potential for device applications. It is emphasized that a new potential for optical device applications is added in Bi₂O₃-based glasses from the present study. A large single crystal and crystal fibers of Bi₂ZnB₂O₇ have been fabricated in order to develop new solid state lasers with high transparency in the ultra-violet region (Kozhayam et al., 2013; Su et al., 2013). The laser patterning of Bi₂ZnB₂O₇ crystal lines in glasses is a new and innovative approach for the single crystal growth technique. Very recently, non-linear optical β-BBO crystal lines with bending and curved shapes have been patterned at the glass surface, and it has been demonstrated from birefringence imaging measurements that highly c-axis oriented β-BBO crystals grow along laser scanning direction even if the laser scanning direction changes (Ogawa et al., 2013). As further studies, the patterning of bending and curved Bi₂ZnB₂O₇ crystal lines is strongly required in Bi₂O₃–ZnO–B₂O₃ glasses.