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Optical Materials xxx (2005) xxx–xxx www.elsevier.com/locate/optmat
Creation of tailored features by laser heating of Nd0.2La0.8BGeO5 glass P. Gupta a, H. Jain a
a,*
, D.B. Williams a, J. Toulouse b, I. Veltchev
b
Center for Optical Technologies and, Department of Materials Science and Engineering, PA 18015, USA b Physics Department, Lehigh University, PA 18015, USA Received 11 January 2005; accepted 6 August 2005
Abstract The creation of tailored micron-size structures by a simple method has been demonstrated in Nd0.2La0.8BGeO5 glass. Polycrystalline lines and dots are produced in predetermined regions of the glass by selective heating with 800 nm radiation from a Ti-Sapphire laser. The X-ray diffraction and energy dispersive spectroscopy results indicate that Nd3+ enters the crystal structure substitutionally. The glass crystallizes congruently with the crystalline spot diameter increasing / time1/3, while the crystal structure remains same as of LaBGeO5. Ó 2005 Elsevier B.V. All rights reserved. PACS: 42.70; 42.60
1. Introduction Recently, laser assisted modification of materials has attracted much attention because of the spatial selectivity of the process and the ability to generate micro-features inside and on the surface of the sample. Much work has been done on the refractive index change induced by UV excimer, femtosecond and picosecond lasers on Ge-doped SiO2 fibers to form fiber Bragg gratings [1–5] as well as the laser crystallization of the amorphous silicon films [6–9] for applications in micro-electronics. Similar laser processing techniques have been attempted on several different materials such as excimer laser crystallization of amorphous indium–tin oxide thin films [10], formation of photonic crystals on Ge-doped silica [11], formation of three-dimensional array of crystallites inside the Ag-doped NaF glass using femtosecond lasers [12], etc. An optical waveguide on the glass has been produced using an ultrashort pulse laser [13]. In general, most of the laser crystal-
*
Corresponding author. Tel.: +1 610 758 4217; fax: +1 610 758 4244. E-mail address:
[email protected] (H. Jain).
0925-3467/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.08.036
lizations or other modifications of the material have been achieved using either UV excimer lasers or femtosecond lasers, but very recently single-crystal formation has been reported by continuous Nd:YAG laser irradiation on b-BaB2O4, a nonlinear optical material [14–16]. This development is important because it opens up a way of forming nonlinear single crystal lines on different kinds of glasses using easily available laser radiation. A very unusual glass forming material, LaBGeO5 (LBGO) is ferroelectric in its single crystal form. LBGO is considered a model glass-ceramic material to study the structural origins of the nonlinear optical effects because it forms a glass easily and crystallizes without a change in chemical composition [17,18]. Also, this material is ideal for laser crystallization experiments because Nd3+ incorporation, useful for the purpose of crystallization, does not significantly change either the devitrification behavior or the ferroelectric properties of the crystallites produced [19]. In addition, Nd3+-doped LBGO single crystal is reported to be a self-frequency-doubling laser [20–22] material and so single crystal lines fabricated on the glass would provide an inexpensive and efficient method for making mini-lasers.
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In the present paper, we report a simple writing method and discuss the mechanism of laser-induced formation of polycrystalline lines, dots and micron size features on Nd0.2La0.8BGeO5 (NLBGO) glass samples. 2. Experiment NLBGO glass was prepared using conventional meltquenching technique. High-purity powders of Nd2O3 (99.99%), La2O3 (99.99%), H3BO3 (99.99%) and GeO2 (99.95%) were mixed in the stoichiometric ratio to provide a 40 g batch. The batch was mixed for 24 h and then melted in a platinum crucible for 1 h at 1200 °C. The melt was poured into a stainless steel mould kept at 400 °C to form 3 mm thick glass plates, which were then mechanically polished using cerium oxide powder to produce an optical quality finish. The setup for laser irradiation of the glass consisted of a titanium-sapphire continuous wave (CW) light source of wavelength k = 800 nm, a focusing lens, a tube furnace and a translation stage. The glass sample was kept inside a furnace at 455 °C and laser light was focused on the surface of the sample by objective lens. The high temperature of the furnace relieves thermal stresses generated in the sample due to heating or crystallization from laser irradiation, and saves it from cracking. During irradiation the sample and with the furnace were moved laterally to produce crystalline lines or other microfeatures. X-ray diffraction (XRD) studies on the crystalline dots and lines were carried out by Bruker Analytical X-ray Systems using Co Ka radiation. A JEOL 6300 scanning electron microscope was employed to obtain energy dispersive spectrum (EDS) from the glass and crystallites. Polarized-light optical microscopy was performed using an Olympus BH2-UMA microscope.
3. Results and discussion A polycrystalline line, generated by the Ti-Sapphire CW laser at the power of 1.1 W and scanning speed of 70 lm/s, is shown in Fig. 1. The line is surface crystallized with grain size gradually decreasing from its center to the edges. Fig. 2 shows laser crystallized dots of various sizes and relative crystallinity produced by varying the laser power and irradiation time. It is clear that the size of dots increases as we increase either one or both of the laser irradiation parameters viz. the laser power or the irradiation time. Fig. 3 shows the absorption spectrum obtained from the NLBGO glass using a Cary 3E Spectrophotometer. The 800 nm radiation is absorbed by the Nd3+ ions exciting electrons from the ground energy level to the 4F5/2 level. Nonradiative transitions result when electrons relax to the ground level via photon–phonon coupling which heats up the material. Honma et al. [15] have called it ‘‘selective atom heat process’’, which facilitates crystallization. The sample is kept inside a furnace at high temperature where the cooling rate depends upon the difference between the temperature of the irradiated area of the sample and temperature of the furnace (overall sample is at the furnace temperature); higher the furnace temperature lower is the cooling rate of the laser heated region. This allows the cooling rate and local temperature profile to be controlled in the predetermined region of the sample by changing the furnace temperature, resulting into varying crystallite size and degree of crystallization within the tailored micro-features. To understand the effect of the replacement of La3+ by Nd3+ on the crystal structure and the phase(s) developed, X-ray diffraction studies were carried out on the crystallized line. Fig. 4 shows XRD pattern obtained from one such line, which is compared with the standard LBGO power diffraction pattern on the same plot. The diffraction patterns have the same d-values within 0.004 nm, but rela-
Fig. 1. Polarized optical micrograph showing a crystalline line made using 800 nm continuous wave radiations at 1.1 W laser power and 20 lm/s scanning speed.
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Fig. 2. Optical micrograph of dots made on the surface of the NLBGO glass sample by varying either the laser power or the irradiation time.
590
805.67
578
6
748
4
877.33
526
3
358.67
2
1
0 330
430.33
Optical Density (a.u.)
5
430
530
630
730
830
Wavelength (nm) Fig. 3. The optical absorption spectrum of NLBGO glass.
tive intensities differ for some peaks. This close matching of the powder diffraction patterns of LBGO and NLBGO indicates that the crystal structure essentially remains the same when La3+ is replaced by Nd3+. A supporting observation of the formation of solid solution in the NdxLa1xBGeO5 (0.1 > x > 0.6) system has been reported by Taibi et al. [23]. Energy dispersive spectrometry (EDS) results obtained from the glass and crystallites are shown in Fig. 5 with the peaks marked with the elements according to their energy. It is evident from NLBGO spectra of the glass and crystalline regions that there is no observable difference between them suggesting that the overall chemical
composition of the glass and crystalline regions is the same. This similarity in composition and crystal structure suggests that Nd3+ enters the lattice substitutionally and does not change the crystallization behavior of the LBGO glass (LBGO glass crystallizes congruently [17]). The crystallization of the NLBGO glass proceeds in a way similar to that reported by Kawasaki et al. [16] for a BaO Æ Sm2O3 Æ TeO2 glass. In both cases there is a t1/3 dependence of the spot diameter on the irradiation time (Fig. 6) and the devitrification product is the same irrespective of the crystallization method (laser heating or furnace heating). Thus, if we follow Kawasaki et al., the laser heating of the sample should be melting the irradiated area,
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λ
Spot diameter in µ m
100
Power 0.80 W 0.65 W 800 nm
90 80 70 60 50 40 30 20 1.0
1.2
1.4
1.6 1/3
Fig. 4. The comparison of the XRD pattern from crystalline line on the surface of NLBGO glass and the powder diffraction pattern of LBGO.
t
1.8
2.0
2.2
(time)
Fig. 6. The variation of the laser spot diameter with the irradiation time at two different laser powers.
then nucleation occurs during the cooling of the melted part and the nuclei grow to become micron-size crystallites. Further studies on the crystal growth kinetics of the laser heating will be required to clarify the underlying devitrification mechanism. 4. Summary We have successfully created crystalline dots and lines in predetermined regions of an NLBGO glass by using easily available Ti-Sapphire laser source. The laser irradiation produced polycrystalline dots and lines. It is evident from the XRD and EDS results that the crystallization occurs congruently similar to the case of devitrification by furnace heating. Therefore, this glass is an attractive candidate for fabricating single-crystal, ferroelectric micro-structures for photonics applications. Acknowledgements This work was supported by the Pennsylvania Department of Community and Economic Development [DCED] through the Ben Franklin Technology Development Authority [BFTDA]. We wish to thank Prof. B.R. Reddy for his help with spectroscopy, and Dr. Jimmy Wang for helpful discussions. References
Fig. 5. EDS spectra from NLBGO (a) glass, (b) crystalline regions formed by laser irradiation.
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