1. Introduction

Laser-induced structural modification to construct micro-architectures in materials has attracted much attention. Laser micro-fabrication in materials is a simple process compared with photolithographic techniques requiring multiple processing steps and an important technique for high technology devices [1–3]. The present authors' group [4–11] proposed laser-induced crystallization techniques for spatially selected structural modifications in glasses, in which conventional lasers such as continuous wave (cw) Nd:yttrium aluminum garnet (YAG) laser (wavelength: λ = 1064 nm) and Yb:YVO₄ fiber laser (λ = 1080 nm) are used and a non-radiative relaxation process (electron-phonon couplings) of rare-earth ions such as Sm³⁺ and transition metal ions such as Cu²⁺ is an origin of heating. Those techniques have been applied to various glasses, and one-dimensional lines consisting of highly oriented nonlinear optical crystals have been patterned successfully [9–14]. For instance, β-BaB₂O₄ single crystal lines have been patterned on the surface of Sm₂O₃-BaO-B₂O₃ glasses by scanning cw Nd:YAG lasers. Furthermore a combination technique of laser irradiation and chemical etching gives us well-designed and complicated micro-architectures of crystal dots and lines.

Niobate-based crystals such as lithium niobate LiNbO₃ (LN) and strontium barium niobate Sr_xBa_1-xNb₂O₆ (SBN) are important nonlinear optical (NLO) materials, and they have been used in various devices such as surface acoustic wave devices and phase modulator waveguides in integrated optics due to their excellent electro-optical, pyroelectrical, piezoelectrical and photorefractive properties. As a fabrication method of NLO crystals, the crystallization of glasses has received much attention, because transparent and dense condensed materials with desired shapes, nanostructures and highly oriented crystals are fabricated through well-controlled crystallizations of glasses. For instance, crystallized glasses consisting of BaTiO₃, LN, SBN crystals have been fabricated [8,14–16], and some of them exhibit a uni-axis orientation of crystals at the glass surface due to the surface crystallization [16]. However, in such surface crystallized glasses, the existence of grain boundaries causes critical problems for optical device applications. Recently, we succeeded in patterning of LN crystal lines with high orientations on the glass surface by laser-induced crystallization techniques and found that LN crystals have c-axis orientations parallel to the laser scanning direction [6–9]. We also demonstrated that LN crystal lines work as optical waveguides [12]. It is important to pattern two-dimensional planar (area) LN crystals with high orientations, because crystallographic structure axes favorable for device applications can be used more effectively in such planar LN crystals.

In this study, we developed a laser irradiation technique to fabricate two-dimensional planar architectures consisting of LN crystals on the surface of Li₂O-Nb₂O₅-SiO₂ glass and confirmed from polarized micro Raman scattering spectra and azimuthal dependence of second harmonic (SH) intensities that LN crystals with high c-axis orientations (like epitaxial crystal growths) are formed along the laser scanning direction.

2. Experimental

A glass with the target composition of 0.5CuO-40Li₂O-32Nb₂O₅-10B₂O₃-20SiO₂ (mol%) was developed in this study. The glass was prepared using a conventional melt quenching method. Commercial powders of reagent grade CuO, Li₂CO₃, Nb₂O₅, H₃BO₃ and SiO₂ were used as starting materials. A mixed batch of 20 g in weight was melted in a platinum crucible at 1350°C for 40 min in an electric furnace. The melts were poured onto an iron plate and pressed to a thickness of ~1.5 mm with another iron plate. The glass transition, T _g, crystallization onset, T _x, and crystallization peak, T _p, temperatures were determined using a differential thermal analysis (DTA) at a heating rate of 10 K/min. The glasses were mechanically polished to a mirror finish with CeO₂ powders. A cw fiber laser with λ = 1080 nm was focused at the glass surface using a 50X objective lens (numerical aperture: NA = 0.80). The plate-shaped glasses were put on a microscope stage and mechanically moved during laser irradiations to construct crystal lines. The morphology of crystal lines was observed with polarization optical microscopes. Micro-Raman scattering spectra at room temperature for crystal lines were taken in the wave numbers of 300 - 1000 cm⁻¹ with a laser microscope (Tokyo Instruments Co., Nanofinder) operated at Ar⁺ (488 nm, 10 mW) laser.

3. Results and discussion

From the DTA pattern for the bulk glass, the values of T _g = 554°C, T _x = 670°C and T _p = 694 °C were obtained. It seems that the doping of CuO does not affect the thermal behavior of the glass, because these values are almost the same as those for CuO-undoped glass. The color of glass plate was light green and absorption peak was found at ~800 nm. In the detal of glass preparation and optical properties were described in another reference [11]. Figure 1 shows the polarized optical micrographs for the laser scanned regions with an area of 20 × 20 μm², in which lasers were scanned with different steps, i.e., the pitches of 2, 1, 0.5, and 0.3 μm between lines. The laser power (P) and scanning speeds (S) were fixed to P = 0.9 W and S = 7 μm/s, respectively. Symbols of * indicates the initial laser focal point, and the scanning was started from this point with steps toward the direction perpendicular to the scanning direction. As shown in Fig. 1, the straight line patterned with the step of 2 μm gives the width of ~1 μm. In the writing of β-BaB₂O₄ in glass [5], putting of crystal nuclei was needed to make crystal line patterns but in the present study crystal growth was easily progress during laser irradiation without any external crystal nuclei. In this step with 2 μm, each line is surrounded by the glass phase (non-laser irradiated parts), and any overlaps (interactions) between the lines were not observed. In the step with 1 μm, the lines are still separated from each other, but slight interactions might be induced between the lines. in contrast, in the steps with 0.5 and 0.3 μm, the morphology of the laser-scanned regions is largely different from that of straight lines, and a smooth surface (homogeneous colors) was obtained, suggesting the presence of interactions between laser-irradiated parts and the formation of two-dimensional planar architectures. However some cracks were found in the planar LN architecture. These cracks were generated after laser irradiation stopping due to the presence of thermal shock and stress between crystal and glass boundary. To prevent crack in LN architecture preheating of glass substrate would be helpful [5]. It was confirmed from micro-Raman scattering spectra that LN crystals are formed in the laser-irradiated parts patterned with all steps. It should be also pointed out that a small amount (0.5 mol%) of CuO is effective in the laser-induced crystallization of 40Li₂O-32Nb₂O₅-10B₂O₃-20SiO₂ glass.

 

Fig. 1 (Color online) Polarized optical micrographs for the regions (area: 20 × 20 μm²) patterned by laser irradiations with different steps (pitches) of 2, 1, 0.5, and 0.3 μm between lines on the surface of the glass. The laser power and scanning speeds were fixed to 0.9 W and 7 μm/s, respectively, and laser irradiations were started from the point marked by the symbols of *.

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The polarization dependence in the optical microscope observations of the two-dimensional planar (50 × 100 μm²) architecture patterned with the step of 0.5 μm is shown in Fig. 2 . By using a sensitive color-plate in the polarized microscope, the uniform yellow and blue colors depending on the angle between the polarizer and sample are observed, indicating the formation of highly oriented LN crystals with optical nonlinearities (birefringence). In order to clarify the crystal growth direction of LN crystals in the planar architectures (Fig. 2), linearly polarized micro-Raman scattering spectra were measured and the results are shown in Fig. 3 . One corner of the rectangle in Fig. 2 shows no birefringence and it is found that this point is still remained as glassy phase due to some crystal growth obstruction during laser scanning. The data for the LN line patterned by laser irradiations with the step of 2 μm and for a commercially available y-cut LN single crystal are also shown in Fig. 3 for comparison. It should be pointed out that the crystallographic direction of y-cut LN single crystal corresponds to the c-axis. In the previous studies [11,12], it was found that LN crystals in lines grow along laser scanning direction and the crystallographic growth direction was determined to be the c-axis by means of polarized micro-Raman scattering spectra, SH microscope observations, and electron backscattering diffraction (EBSD). In the measurements (Fig. 3), the direction of Z-axis in the configurations corresponds to the line growth direction, i.e., the laser scanning direction. For instance, the configuration of Y(ZZ)Y means that the incident laser introduced from the Y-axis direction has a polarization (electric vector) of Z-axis and Raman light with polarization of Z-axis is detected from –Y direction (backscattering arrangement). As can be seen in Fig. 3, several sharp peaks are observed at 333, 431, 631, and 878 cm⁻¹ in all samples.

 

Fig. 2 (Color online) Polarization dependence in the optical microscope observations of the two-dimensional planar (50 × 100 μm²) architecture patterned by laser irradiations with the step of 0.5 μm on the glass surface. The configuration of micro Raman measurement is also shown.

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Fig. 3 (Color online) Linearly polarized micro-Raman scattering spectra at room temperature for laser-patterned single straight line, two-dimensional planar pattern (Fig. 2), and y-cut LiNbO₃ single crystal. X-axis is parallel to the laser step direction and Z-axis is parallel to the laser scanning direction.

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The Raman scattering spectra for LN crystals have been studied so far [17,18], and all Raman peaks observed for laser-patterned LN lines have been assigned to the vibrations of Nb-O bonds [6–9]. The correlations between the configuration (X, Y, Z) and crystallographic (a, b, c) axes are as follows; X//a, Z//c, and Y is perpendicular to the ac plane. The results shown in Fig. 3 indicate that the relative peak intensities change largely depending on the configuration. It is clear that the Raman scattering spectra (Fig. 3) for the line and two-dimensional planar architecture patterned by laser irradiations are almost similar to y-cut LN single crystal. We, therefore, propose that LN crystals in two-dimensional planar architectures patterned in this study also have the c-axis orientation along the laser scanning direction.

In order to confirm the quality of the orientation of LN crystals in planar architectures (Fig. 2), the azimuthal dependence of SH intensities was measured, and the results are shown in Fig. 4 . In this experiment, linearly polarized Q-switched Nd:YAG fundamental laser waves with λ = 1064 nm were introduced, and SH waves with λ = 532 nm were detected in the same polarization, i.e., H-H configuration. And, the angle of 0° means that the laser scanning direction is parallel to the electric vector of the incident laser light. Clear SH generations and a unique azimuthal dependence of SH intensities were observed. That is, the maximum SH intensity exists at the angle of 0°. A similar azimuthal dependence of SH intensities was observed for LN lines [11]. LN crystal belongs to the space group of R3c and has d tensors as d ₃₃ > 34 and d ₃₁ ~6.1 pm/V. The solid line is theoretical curve calculated using the d ₃₁ and d ₃₃ values for y-cut LN single crystal, i.e., for the c-axis orientation. The azimuthal dependence of experimentally obtained SH intensities is well consistent with that of theoretical predictions. Again, it is demonstrated that LN crystals in two-dimensional planar patterns have high c-axis orientations. Furthermore, because extremely homogeneous colors are observed in polarized optical microscopes (Fig. 1), c-axis orientations of LN crystals in two-dimensional planar patterns might be uniform. That is, epitaxial LN crystal architectures might be fabricated on the glass surface by just scanning laser irradiations with narrow steps.

 

Fig. 4 Second harmonic intensity as a function of sample rotation

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At this moment, the growth mechanism in two-dimensional planar LN crystals with high c-axis orientations has not been clarified. We need to study the phenomenon that is taking place at the laser-irradiated parts with small steps such as 0.5 and 0.3 μm, i.e., at the overlapped laser-irradiated parts. However, it should be emphasized that the success in planar LN crystals on the glass surface by laser irradiations would be a great progress in the field of laser patterning of functional crystals.

4. Conclusion

In conclusion, we succeeded in fabricating two-dimensional planar LN crystal architectures on the glass surface by using a laser-induced crystallization technique. It was confirmed from polarized micro-Raman scattering spectra and azimuthal dependence of SH intensities that LN crystals in planar architectures have the c-axis orientation along the laser scanning direction. The present study proposes that the laser irradiation technique gives us uniform planar LN crystal films on the glass surface.