patterning of two-dimensional planar lithium niobate architectures on glass surface by laser...

Patterning of two-dimensional planar lithium

niobate architectures on glass surface by laser

scanning

Tsuyoshi Honma,1,*

and Takayuki Komatsu1

1Department of Materials Science and Technology, Nagaoka University of Technology, Kamitomioka 1603-1

Nagaoka, Niigata, 940-2188, Japan

*[email protected]

Abstract: Two-dimensional planar LiNbO3 (LN) crystal architectures are

patterned on the surface of Li2O-Nb2O5-B2O3-SiO2 glass by continuous

wave ytterbium YVO4 fiber laser (wavelength: 1080 nm) irradiations, in

which lasers are scanned continuously with narrow steps (pitches: 0.3 and

0.5 µm) and thus with overlaps of laser irradiated parts. For the planar LN

crystals (area: 50 µm × 100 µm) patterned by laser scanning with a power of

0.9 W and a speed of 7 µm/s, it is demonstrated from polarized micro-

Raman scattering spectra and azimuthal dependence of second harmonic

intensities that the c-axis orientation of LN crystals is established along the

laser scanning direction. The present study proposes that the laser

irradiation technique gives us uniform LN crystal films on the glass surface.

©2010 Optical Society of America

OCIS codes: (160.2750) Glass and other amorphous materials; (140.3390) Laser materials

processing; (160.4330) Nonlinear optics materials; (220.4000) Microstructure fabrication

References and links

1. M. C. Gower, “Industrial applications of laser micromachining,” Opt. Express 7(2), 56–67 (2000).

2. Y. Yonesaki, K. Miura, R. Araki, K. Fujita, and K. Hirao, “Space-selective precipitation of non-linear optical

crystals inside silicate glasses using near-infrared femtosecond laser,” J. Non-Cryst. Solids 351(10-11), 885–892

(2005).

3. Y. Dai, H. Ma, B. Lu, B. Yu, B. Zhu, and J. Qiu, “Femtosecond laser-induced oriented precipitation of

Ba2TiGe2O8 crystals in glass,” Opt. Express 16(6), 3912–3917 (2008).

4. R. Sato, Y. Benino, T. Fujiwara, and T. Komatsu, “YAG laser-induced crystalline dot patterning in samarium

tellurite glasses,” J. Non-Cryst. Solids 289(1-3), 228–232 (2001).

5. T. Honma, Y. Benino, T. Fujiwara, R. Sato, and T. Komatsu, “Technique for writing of nonlinear optical single-

crystal lines in glass,” Appl. Phys. Lett. 83(14), 2796–2798 (2003).

6. T. Honma, Y. Benino, T. Fujiwara, and T. Komatsu, “Transition metal atom heat processing for writing of crystal

lines in glass,” Appl. Phys. Lett. 88(23), 231105 (2006).

7. T. Honma, N. Hirokawa, and T. Komatsu, “Micro-architecuture of nonlinear optical Ba2TiGe2O8 crystal dots and

lines on the surface of laser-induced crystallized glasses by chemical etching,” Appl. Surf. Sci. 255(5), 3126–

3131 (2008).

8. N. Chayapiwut, T. Honma, Y. Benino, T. Fujiwara, and T. Komatsu, “Synthesis of Sm3+-doped strontium barium

niobate crystals in glass by samarium atom heat processing,” J. Solid State Chem. 178(11), 3507–3513 (2005).

9. M. Sato, T. Honma, Y. Benino, and T. Komatsu, “Line patterning of (Sr,Ba)Nb2O6 crystals in borate glasses by

transition metal atom heat processing,” J. Solid State Chem. 180(9), 2541–2549 (2007).

10. H. Sugita, T. Honma, Y. Benino, and T. Komatsu, “Formation of LiNbO3 crystals at the surface of TeO2-based

glass by YAG laser-induced crystallization,” Solid State Commun. 143(6-7), 280–284 (2007).

11. T. Honma, K. Koshiba, Y. Benino, and T. Komatsu, “Writing of crystal lines and its optical properties of rare-

earth ion (Er3+ and Sm3+) doped lithium niobate crystal on glass surface formed by laser irradiation,” Opt. Mater.

31(2), 315–319 (2008).

12. T. Honma, T. Komatsu, D. Zhao, and H. Jain, “Writing of rare-earth ion doped lithium nibate line patterns in

glass by laser scanning,” IOP Conf. Series: Mater. Sci. Eng. 1, 012006 (2009).

13. Y. Tsukada, T. Honma, and T. Komatsu, “Self-organized periodic domain structure for second harmonic

generations in ferroelastic β’-(Sm,Gd)2(MoO4)3 crystal lines on glass surface,” Appl. Phys. Lett. 94(4), 041915

(2009).

14. T. Komatsu, H. Tawarayama, H. Mohri, and K. Matusita, “Properties and crystallization behaviors of TeO2-

LiNbO3 glasses,” J. Non-Cryst. Solids 135(2-3), 105–113 (1991).

#124027 - $15.00 USD Received 9 Feb 2010; revised 8 Mar 2010; accepted 24 Mar 2010; published 31 Mar 2010(C) 2010 OSA 12 April 2010 / Vol. 18, No. 8 / OPTICS EXPRESS 8019

15. T. Komatsu, H. Tawarayama, and K. Matusita, “Preparation and optical properties of transparent TeO2-based

glasses containing BaTiO3 crystals,” J. Ceram. Soc. Jpn. 101, 46–50 (1993).

16. Y. Ding, A. Osaka, Y. Miura, H. Toratani, and Y. Matsuoka, “Second order optical nonlinearity of surface

crystallized glass with lithium niobate,” J. Appl. Phys. 77(5), 2208–2210 (1995).

17. H. R. Xia, S. Q. Sun, X. F. Cheng, S. M. Dong, H. Y. Xu, L. Gao, and D. L. Cui, “Lattice vibrations and phase-

transition soft mode in near stoichiometric lithium niobate crystals,” J. Appl. Phys. 98(3), 033513 (2005).

18. P. Galinetto, M. Marinone, D. Grando, G. Samoggia, F. Caccavale, A. Morbiato, and M. Musolino, “Micro-

Raman analysis on LiNbO3 substrates and surfaces: Compositional homogeneity and effects of etching and

polishing processes on structural properties,” Opt. Lasers Eng. 45(3), 380–384 (2007).

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

SrxBa1-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, Tg,

crystallization onset, Tx, and crystallization peak, Tp, 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 Tg = 554°C, Tx = 670°C and Tp = 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 *.

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.

400 600 800 1000

y-cut LiNbO3

2D pattern

Line pattern

333 878431

631 Y(ZZ)Y

Y(XX)Y

Inte

nsity

(arb

. u

nits)

Raman shift (cm-1)

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.

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

d33 > 34 and d31 ~6.1 pm/V. The solid line is theoretical curve calculated using the d31 and d33

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.

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.

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

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.

Acknowledgments

This work was supported by the Grant-in-Aid for Scientific Research from the Ministry of

Education, Science, Sports, Culture and Technology, Japan.


patterning of two-dimensional planar lithium niobate architectures on glass surface by laser...

Documents