Growth of fresnoite single crystal tracks inside glass using

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Growth of fresnoite single crystal tracks inside glass using femtosecond laser beam followed by heat treatment Alexey S. Lipatiev, Ivan A. Moiseev, Sergey V. Lotarev, Tatiana O. Lipateva, Mikhail Yu. Presnyakov, Sergey S. Fedotov, and Vladimir N. Sigaev Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01358 • Publication Date (Web): 19 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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Crystal Growth & Design

Growth of fresnoite single crystal tracks inside glass using femtosecond laser beam followed by heat treatment Alexey S. Lipatiev1*, Ivan A. Moiseev1, Sergey V. Lotarev1, Tatiana O. Lipateva1, Mikhail Yu. Presnyakov2,Sergey S. Fedotov1, Vladimir N. Sigaev1 1Mendeleev

University of Chemical Technology of Russia, Miusskaya sq. 9, 125047

Moscow, Russia 2NRC

Kurchatov Institute, Akademika Kurchatova pl. 1, 123182 Moscow, Russia *Corresponding

author e-mail: [email protected]

ABSTRACT

We have studied the space-selective micro-crystallization of barium titanium silicate glass under the femtosecond laser beam. The glass composition (mol.%) was 40BaO·20TiO2·40SiO2 corresponding to the composition of polar fresnoite phase known for its large second-order optical susceptibility. Optical and micro-Raman microscopy confirmed the laser-induced formation of the Ba2TiSi2O8 crystalline tracks and their morphology was investigated by

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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transmission electron microscopy. The possibility of a 20-fold increase in laser scanning speed providing homogeneous crystal track growth using the laser beam with an elliptical cross-section as compared to the conventional Gaussian beam has been shown. It has been demonstrated for the first time that annealing of laser-written crystalline tracks leads to substantial improvement of the quality of tracks structure tending to that of the fresnoite single crystal, which can be an important step in the development of single crystal optical waveguide components for photonics.

KEYWORDS Femtosecond laser direct writing; beam shaping; laser-induced crystal growth; space-selective crystallization; barium titanium silicate glass; fresnoite; second harmonic generation.


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INTRODUCTION Vast prospects in engineering sciences greatly depend on the development of new functional materials possessing some functional properties such as second-order optical nonlinearity and photoluminescence. In particular, glass-ceramic materials with high optical nonlinearity have significant potential for the development of optical devices such as broadband optical amplifiers, converters, shutters and sensors1. An effective method of fabrication of glass-ceramic materials is a crystallization of glass by thermal treatment in the furnace. Many studies on the precipitation of functional micro- and nanocrystals inside glasses for various glass-forming systems such as Li2O-Nb2O3-SiO22-4, Ln2O3-B2O3-GeO25-7, BaO-TiO2-SiO28-10 were carried out. Much attention has been paid to polar fresnoite (Ba2TiSi2O8 further referred to as BTS) crystal possessing piezoelectric, pyroelectric

11,12

and stimulated Raman scattering13 properties, as well

as a very large second-order optical nonlinearity and high transmittance over a wide wavelength range from 340 to 2500 nm.14 Its strong polarity is determined by TiO5 square pyramidal polyhedra being a part of the fresnoite structure related to P4bm group15 while crystalline BTS is not ferroelectric and cannot be poled16. Fresnoite is considered for applications in surfaceacoustic-wave devices17 and solid-state Raman laser applications.13 The more complete data on the structure, properties and applications of BTS crystal as well as physicochemical properties of BTS-glasses and glass-ceramics are summarized and presented in the review by Wisniewski et al.16 who also introduced a standardized nomenclature for glass compositions. Hereinafter, the nomenclature proposed by Wisniewski et al. is used to denote glass compositions, BTS, BT1S and BT1G meaning, respectively, 40BaO·20TiO2·40SiO2, 33.3 BaO·16.7TiO2·50SiO2 and 33.3BaO·16.7TiO2·50GeO2. Importantly, stochiometric BTS composition belongs to the glassforming region, which enables fabrication of glass exactly corresponding to fresnoite in its


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composition and the opportunity of efficient crystallization with the precipitation of the fresnoite-type Ba2TiSi2O8 nano- or microcrystals exhibiting relatively strong second harmonic generation (SHG)18. Recently Fang et al.19 successfully prepared glass-ceramic fibers containing Ba2TiSi2O8 nanocrystals using the melt-in-tube method and heat treatment and demonstrated the possibility of second harmonic generation induced by a 1030 nm femtosecond laser beam in the fiber. The evolution of integrated optics or photonics gives rise to a demand for the development of novel principles of fabrication of miniaturized devices. In this regard, it looks promising to develop methods of fabrication the glass-ceramic architectures which can be used as active optical waveguides, couplers, gratings etc. in the volume or on the surface of a single piece of glass.20 Substantial efforts in this field were made in the last three decades using a laser beam as a tool for fast and precise space-selective crystallization of glasses.21 BTS-glasses are among the most promising materials in which nonlinear optical crystals can be precipitated during continuous wave (cw) laser irradiation and form active local patterns with complex architecture on their surface.22,23 Komatsu et al. applied cw Nd:YAG laser for writing two-dimensional Ba2TiGe2O8 crystal straight, bending or curved lines with highly oriented crystals in BT1G+0.06Cu and BT1G+0.06Ni glass doped with transition metals for absorption of laser energy.23,24 The appearance and fast progress of femtosecond (fs) lasers made a real revolution in the micromodification of various materials providing the ability to rapidly deliver energy inside materials with high spatial resolution through the multiphoton avalanche ionization and absorption. In particular, femtosecond pulses allow for local heating and crystallization and inscription 3D crystalline patterns inside glasses.25-33 Some studies on the formation of fresnoite nanocrystalline dots in rare-earth ion-doped (BTS+1.5Dy2O3) and (BTS+3Nd2O3)-glasses under


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cw and fs laser irradiation were carried out by Martin et al.34,35 who showed clear evidence of spectroscopic changes induced by laser exposure. The space-selective precipitation of Ba2TiSi2O8 microcrystal dots and lines inside BT1S-glass using tightly focused femtosecond laser pulses was also demonstrated.36, 37 Zhu et al. revealed the Sm3+ and Er3+ ions luminescence resulting from the upconversion of the fs Ti:sapphire laser second harmonic due to precipitation of nonlinear Ba2TiSi2O8 crystals in (BTS+0.5Sm2O3) and (BT1S+0.5Eu2O3)-glasses.38,39 Nevertheless, the morphology, structure and quality of laser-written crystalline patterns have not been studied in details as only optical microscopy and Raman spectroscopy were used to characterize them although understanding and control of these features is crucial both for waveguide performance and for spectroscopic characteristics. The task of direct laser writing of a high-quality crystalline structure in BTS-glass is still a challenge. In our previous study concerning BTS-glass, we investigated the influence of femtosecond laser pulse energy and laser beam scanning speed on the formation and morphology of crystalline tracks containing fresnoite microcrystals written by the conventional Gaussian laser beam.40 Due to extremely high nucleation rate of BTS-glass of the chosen composition in comparison with other silicate glasses17 there is no need in the preliminary growing of a seed crystal by the stationary beam which generally takes a certain time and is necessary in case of laser writing continuous crystalline tracks in lanthanum borogermanate41 or lithium niobium silicate glasses26. At 200 kHz pulse repetition rate, the pulse energy of 150 nJ and low laser beam scanning speed of 5 μm/s were found to provide the growth of oriented crystalline tracks with the smoother structure while higher values of these parameters gave rise to the evident disorientation of crystalline track parts or an increase of polycrystallinity indicated by disordering of the slow axis of birefringence.40 The favorable scanning speed coincides with the value earlier reported


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for direct laser writing of fresnoite lines in BT1S-glass by Y. Dai et al.37 in spite of the different laser treatment parameters and glass composition. Recently, we have proposed a technique of improvement of the laser-written continuous crystalline tracks homogeneity by applying a focused fs laser beam with an elliptical cross-section of the beam waist in lanthanum borogermanate glass.41 In the present study, we checked the effect of this technique on the space-selective crystallization of BTS-glass, compared the morphology of the crystalline tracks written by the femtosecond laser beams possessing the beam waist with elliptical cross section (BWECS) or the conventional Gaussian beam waist (GBW) and succeeded in a multiple increase of the writing speed in case of using BWECS, which enables highly homogeneous crystalline tracks with the amorphous core. We also revealed that additional annealing of the laser-written crystalline tracks greatly enhanced the quality of their structure tending to the single-crystal one. Applying highresolution transmission electron microscopy (TEM) provided information on the crystalline structure of the tracks written in BTS-glass at different conditions.

EXPERIMENTAL BTS-glass

with

40BaO·20TiO2·40SiO2

molar

composition

corresponding

to

the

stoichiometrical composition of fresnoite was fabricated by conventional melt-quenching technique. Chemically pure BaCO3, TiO2 and amorphous SiO2 were used as components for glass batch preparation. Glass melting was carried out in a platinum crucible at 1500 ºC for 1 hour and the practical yield of glass was more than 98.5% from the calculated one. Glass melt was poured on a steel plate and then quickly pressed by another steel plate. The resulting glass cast was annealed for 2 hours at the temperature of 640 ºC and then cut into plane-parallel plates


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with an area of ~2 cm2 and thickness of ~2 mm and thoroughly mirror-grade polished with CeO2 suspension to meet a requirement of elimination of surface scattering and the laser beam distortion. The glass transition temperature Tg and the temperature of the crystallization peak maximum Tc were determined using differential scanning calorimeter NETZSCH Jupiter STA 449 F1 (Fig. S1). Glass density was determined by the method of Archimedes using distilled water as an immersion liquid. Commercial Yb femtosecond laser Pharos SP (Light Conversion Ltd.) operating at the wavelength of 1030 nm was used for glass crystallization. In our experiments, pulse duration and the pulse repetition rate were set to 300 fs and 200 kHz, respectively. M2 parameter of the laser beam did not exceed 1.25. Pulse energy measured after the focusing lens could be gradually varied up to 2 μJ. The laser beam was focused at a depth of 100 μm below the surface of a glass sample by a near-IR-optimized Olympus LCPLN-IR 50X microscope objective (N.A. = 0.65). The beam waist radius was calculated in paraxial approximation as low as 0.7 μm. An APP J (TOPTICA Photonics AG) anamorphic prism pair with adjustable magnification was placed before the focusing objective and provided the ellipticity of the laser beam cross-section (Fig. 1) and, consequently, of the cross-section of the beam waist. The magnification of the anamorphic prism pair was adjusted to ~3.5 providing the same ratio of the axes of the elliptical cross-section of the waist. A computer-controlled high-precision motorized stage ABL1000 (Aerotech Ltd.) was used for translating of the glass sample in the plane perpendicular to the focused laser beam. A live view of the laser exposure process was delivered by Retiga 3000 camera allowing immediate detection of the Ba2TiSi2O8 crystal precipitation by the green light due to noticeable second


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harmonic generation (SHG) signal at 515 nm wavelength (Fig. S2). We started writing the tracks in BTS-glass at the pulse energy of 150 nJ and the beam scanning speed of 5 μm/s and then varied both parameters. The nonlinear absorption of laser energy was measured in single pulse regime using two energy meter heads Ophir PD-10-C one of which was placed after glass sample and the second one served as a reference.

Figure 1. Laser beam profiles before the focusing objective lens without (a) and with (b) anamorphic prism pair in the optical path

Optical polarizing microscope Olympus BX-51 was used for the study of laser-written dots and tracks written in glass by focused fs laser beam scanning. Additionally, quantitative microanalysis of tracks birefringence by means of optical microscope Olympus BX61 equipped with CRi Abrio imaging system was performed. Raman spectra were collected by a confocal micro-Raman spectrometer of NTEGRA Spectra system (NT-MDT Co.). The argon ion laser with a wavelength of 488 nm was used as the excitation source and the microscope objective Mitutoyo MPlan 100X provided the beam diameter of about ~0.8 µm at the focal point. The Raman spectra maps of the laser-written track cross-section were recorded by scanning the sample with a step of 0.6 μm.


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The internal structure of the laser-written crystalline tracks was investigated by TEM using Titan 80-300 S/TEM system (Thermo Fisher Scientific, USA) in TEM 300 kV mode. Longitudinal cross-section slices were cut from the polished end faces of the crystalline tracks and thinned by the gallium focused ion beam (FIB) technique for TEM observation.

RESULTS AND DISCUSSION The results of the qualitative birefringence analysis of the laser-induced tracks written by BWECS and by GBW at a pulse energy of 150 nJ and a low scanning speed range are shown in Fig. 2(a). Since fresnoite is a negative uniaxial crystal, the polarization of the slow wave is perpendicular to its c-axis. The slow axis of birefringence of the laser-written fresnoite tracks in BTS-glass was earlier confirmed to be perpendicular to the c-axis of fresnoite.40 The stricter slow axis orientation in the crystalline track written by GBW was observed for a scanning speed of 5 μm/s while increasing the scanning speed up to 20 μm/s resulted in increasing of crystal disorientation in the tracks indicated by fluctuations of the slow axis of birefringence. In both cases of BWECS and GBW, scanning speed of 40 μm/s and higher does not allow fresnoite crystal precipitation. Raman spectra mapping showed the presence at ~600 cm-1 of a characteristic peak of fresnoite42 for cross-sections of the crystalline tracks written at 5 μm/s by GBW and BWECS (Fig. 2(c,d)) and confirmed Ba2TiSi2O8 phase formation that inherently occurs closer to the top of the track. A weaker Raman signal of the BWECS-written track crosssection with respect to GBW-written one likely indicates a smaller fraction of the precipitated crystals due to a lower power density of BWECS. Transmission optical images (Fig. 2(b)) of cross-sections of the crystalline tracks give evidence of light absorption increase in crystallized area.


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Figure 2. (a) Pseudocolor map of the orientation of the slow axis of birefringence in the laserwritten crystalline tracks at 150 nJ pulse energy. (b) Transmission-mode optical images of crosssections of crystalline tracks written with BWECS and GBW at 5 µm/s scanning speed and 150 nJ pulse energy. (c) Corresponding maps of the integral intensity of Raman spectra in the range


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590–610 cm−1. (d) Raman spectra of the crystalline track cross-sections written by GBW (upper spectrum) and BWECS (middle spectrum) and BTS-glass outside the track (lower spectrum). Peaks indicated by rhombs are assigned to fresnoite single crystal42. Blue, green and red crosses indicate points of registration of the μ-Raman spectra shown in the map (c). To evaluate the temperature distribution in the laser-modified region, we used a simplified finite-difference thermal diffusion model proposed by Eaton et al.43 For solving the heat equation in spherical coordinates which has a general form as follows44:

1 T 1  2 T (r ),   t r 2 r r

(1)

where    / (  C p ) - thermal diffusivity (m2/s), T – temperature (K), r – radial direction (m), t – time (s), λ – thermal conductivity (W/(m·K)), ρ – density (kg/m3),

Cp

– specific heat

(J/(kg·K)) we used the measured Tg value as 710±2°С and BTS-glass density as 4.25±0.01 g/cm3. The value of Tg agrees well with previously obtained data16,18, while the density is slightly divergent from the results reported by Shinozaki et al.45 likely due to somewhat different melting and forming conditions and widens the range of reported values of BTS-glass density.16 Since we could not find data on specific heat

Cp

and thermal conductivity λ of BTS-glass in available

sources, we used Cp and λ values measured for Ba2TiSi2O8 crystal46 in our estimative calculations taking into account exact match of the chemical composition of BTS-glass to fresnoite. The dependence of nonlinear absorption on laser pulse energy for BTS-glass (Fig. 3a) is a monotonically increasing function typical for glass.47 The absorbed pulse energy calculated taking into account the Fresnel losses at the upper surface of glass sample allows estimation of an instantaneous temperature increase occurring when a laser pulse arrives and calculation of the radius of the modified region. The glass


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transition temperature Tg of BTS-glass was taken as a characteristic temperature of the boundary condition of glass modification that does not contradict the results obtained by Sakakura et al.48 who calculated the characteristic temperature for soda-lime glass as 560±20 °С while glass Tg equals to 533 °C. The close correspondence between the modified dot diameter observed for exposure of glass to a various number of the laser pulses with 150 nJ energy and the calculated data (Fig. 3b) confirms the adequacy of the finite-difference thermal diffusion model for our laser irradiation conditions in spite of uncertainties of thermal characteristics of glass, especially in high-temperature range. Indeed, the modified dot diameter practically does not change after the impact of 100 pulses while at 50·103 pulses crystallization of glass is initiated resulting in a detectable SHG signal by CCD camera. It should also be taken into account that the simplified temperature model cannot explain the full representation of laser-induced crystallization of glass including different thermodynamic and kinetic factors.


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Figure 3. (a) Dependence of absorption of the laser beam focused in BTS-glass on the laser pulse energy. (b) Numerical simulation (solid line) and experimental data (solid rhomb) on modified dot diameter versus number of pulses with 150 nJ pulse energy. The inset shows optical micrographs of laser-written dots at different number of pulses, the yellow scale bar being equalled to 5 µm. (c) Finite-difference evaluation of glass temperature versus number of pulses with 150 nJ pulse energy at a radial position of 1 µm from the center of the conventional Gaussian beam waist (GBW) and the beam waist with elliptical cross section (BWECS).

We evaluated the temperature of glass for GBW and BWECS assuming the radius of the sphere calculated from the volume of the ellipsoid of the focused laser beam for the latter case. Fig. 3c shows the temperature dynamics calculated at a position of 1 µm from the center of the


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laser beam waist on the basis of the width of the crystalline track as small as 2 µm. The mean temperature exceeds the peak crystallization temperature of BTS-glass which is 783±2 °C (Fig. S1) when using both GBW and BWECS. It can be seen that using BWECS significantly reduces the temperature gradient from pulse to pulse (Fig. 3c) and provides a “smoother” temperature regime. TEM investigation (Fig. 4) of the longitudinal cross-sections of the tracks written at 5 µm/s scanning speed and 150 nJ pulse energy confirms that BWECS-written tracks have more homogeneous structure than GBW-written ones due to “smoother” temperature regime provided by BWECS as compared to GBW (Fig. 3c). Thus, using BWECS is favorable for uniform crystalline tracks growth. Nevertheless, all crystalline tracks manifest noticeable grain boundaries inside them. The drop in the temperature below Tg in crystal growth region from laser pulse to pulse (Fig. 3c) can be an possible reason for the following alternative scenarios of the grain boundaries formation. The first possibility is that high stress field in the laser-induced crystal growth region can lead to the formation of dislocation structure which is responsible for grain boundaries appearance. Another possible scenario based on the close resemblance the TEM images of laser-written tracks and TEM images obtained for a textured crystalline fresnoite thin film fabricated on the substrate by the pulsed laser deposition49 suggests that the crystalline tracks possess textured microstructure formed when many crystalline nuclei emerge and grow coalescing into a consistent track with high degree of polar axis orientation along laser beam scanning direction which was confirmed by strict orientation of the slow axis of birefringence throughout the track (Fig. 2a).


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Figure 4. TEM HAADF images of a longitudinal slice of the crystalline track laser-written by GBW (a) and BWECS (b) at 5 µm/s scanning speed and 150 nJ pulse energy. Arrows v and k show directions of laser scanning and laser beam propagation, respectively. Writing continuous crystalline tracks at higher scanning speed requires increasing pulse energy but the tracks written by GBW at high scanning speed (tens or hundreds µm/s) and large pulse energy are strongly inhomogeneous as we showed earlier41. However, we succeeded in forming a smooth, optically homogeneous crystalline track using BWECS and the pulse energy of 350 nJ which is confirmed by birefringence microanalysis (Fig. 5a). Thus, applying BWECS allows for 20-fold acceleration of writing homogeneous fresnoite tracks. Optical and Raman microscopy (Fig. 5b,c), and TEM data (Fig. 6a,c) showed that the central zone of the obtained track included an amorphous phase. Considering Gaussian energy distribution in the laser beam profile we can admit that overheating of glass directly in the laser beam waist causes melting of crystalline nuclei precipitated at the leading edge of BWECS and thus hinders crystallization in the central part of the track. At the same time, high scanning speed may prevent the growth of the lateral crystalline parts into the track center unlike in the case of slow scanning at 5 µm/s (Fig. 3, 4).


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A TEM image of the longitudinal cross-section of the crystalline track verified its symmetrical view and pointed out three zones with different morphology for both lateral crystalline parts. The first crystalline zone, nearest to the amorphous core of the track is presented by grain-oriented fresnoite and is similar to the structure of tracks written at low scanning speed. Many vacancytype and dislocation defects are found in the second (interstitial) crystalline zone (Fig. 6b). The third (periphery) crystalline zone is the most homogeneous part of the track. The maximal temperature and cooling rate go down with increasing the distance from the track center, which resulted in crystal precipitation and growth in strongly nonequilibrium conditions. The most favorable conditions for the sustained single crystal growth evidently occur in the periphery zone of the laser-written track, similar to what was earlier shown for lanthanum borogermanate glass.50 Besides that, we assume that the tensile stresses were arisen in the interstitial zones of the lateral parts of the crystalline track during rapid quenching. That can be responsible for their highly defective structure.51,52 To remove strain in the crystalline track, we performed fine annealing of the glass sample with the laser-written tracks at the glass annealing temperature of 640 °C during 4 hours with subsequent controlled cooling at the rate of 0.4 °C/min.


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Figure 5. (a) Pseudocolor map of the orientation of the slow axis of birefringence in the crystalline tracks written by BWECS at the scanning speed of 100 μm/s and the pulse energy of 300-400 nJ. (b) Transmission optical image of a cross-section of crystalline track BWECSwritten at a speed of 100 μm/s and pulse energy of 350 nJ. (c) Corresponding map of the integral intensity of Raman scattering in the range 590–610 cm−1 including a characteristic peak of fresnoite. Arrows v and k show directions of laser scanning and laser beam propagation respectively. TEM and high-resolution TEM images (Fig.6e-g) of the annealed crystalline track laserwritten at the scanning speed of 100 μm/s and the pulse energy of 350 nJ reveal almost complete elimination of imperfections in the structure of the right lateral part of the crystalline track while defects and grain boundaries still remained in the left one. A pattern shown in the Fourier transform image (Fig 6g, inset) clearly confirms a single-crystal structure of the right lateral part


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which can be regarded as a continuous single-crystal fresnoite track. The presence of residual defects in the structure of the left lateral part of the crystalline track can be associated with insufficiently high annealing temperature which was 70 °C lower than Tg and excludes any glass nucleation or crystal growth.

Figure 6. TEM HAADF image of the lateral slice of the laser-induced crystalline track laserwritten at the scanning speed of 100 μm/s and the pulse energy of 350 nJ (a) high-resolution TEM image of its defective part (b) and diffraction patterns of its glassy (c) and crystalline (d) parts; TEM HAADF image of the lateral slice of the laser-induced crystalline track annealed at 640 °C for 4 hours (e), high-resolution TEM images of the crystalline part with residual defects (f) and the single-crystal part (an inset shows its Fourier transform image) (g). Arrows v and k indicate directions of laser scanning and laser beam propagation, respectively.


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Thus, we believe that further optimization of fine annealing regime would enable the fabrication of the laser-written crystalline tracks with the single crystal structure. To the best of our knowledge, annealing as a method to improve the quality of femtosecond laser-written crystal-in-glass tracks has not been reported for so far, but since it is widely applied for the elimination of some structural defects in single crystals, it may have general nature and be efficient for crystals other than fresnoite too. Since the thermal treatment-induced improvement of the laser-written crystal track quality is expected to facilitate the realization of fresnoite channel waveguides in BTS-glass, analysis of light guiding ability and evaluation of propagation losses in femtosecond laser-written fresnoite tracks in BTS-glass are planned as the next step of the present study.

SUMMARY We have studied the features of fresnoite track writing by the femtosecond laser beam in BTSglass. It has been confirmed that using BWECS instead of GBW allows for smoothing the structure of the laser-written crystalline track presented by the grain-oriented microstructure with a high degree of orientation and moreover enables a 20-fold increase of homogeneous fresnoite track writing speed, which drastically improves the efficiency of this technique and is an important step towards its feasibility. The morphology of laser-written crystalline tracks has been shown to strongly depend on the pulse energy and laser beam scanning speed. For example, laser writing at 150 nJ and 5 µm/s resulted in the formation of entire crystalline tracks while the crystalline track written at 350 nJ and 100 µm/s by BWECS contained an amorphous core. Combination of µ-Raman, quantitative birefringence and TEM analysis showed that the laserwritten crystalline tracks possessed highly oriented fresnoite structure which though contained


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numerous defects and grain boundaries. It has been demonstrated for the first time that annealing of laser-written crystalline tracks can eliminate the defects in the laser-written crystalline tracks and greatly enhance the quality of their structure turning it into a single-crystal one. The obtained results are very promising for the fabrication of active single crystal optical waveguide components in photonics devices.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Notes The authors declare no competing financial interests.

SUPPORTING INFORMATION Differential thermal analysis pattern for the bulk sample of BTS-glass. Scheme of the setup for spectral measurement of SHG signal appearing during laser writing of Ba2TiSi2O8 crystalline track and its SHG spectrum. The Supporting Information is available free of charge on the ACS Publications website http://pubs.acs.org

ACKNOWLEDGEMENTS This work has been supported by the Russian Foundation for Basic Research (grants 16-3360081, 16-03-00541). The microbirefringence and spectroscopic analyses were carried out with the support of the Russian Science Foundation (grant 17-73-20324). REFERENCES


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For Table of Contents Use Only

Growth of fresnoite single crystal tracks inside glass using femtosecond laser beam followed by heat treatment Alexey S. Lipatiev, Ivan A. Moiseev, Sergey V. Lotarev, Tatiana O. Lipateva, Mikhail Yu. Presnyakov, Sergey S. Fedotov, Vladimir N. Sigaev

SYNOPSIS: We showed the formation of fresnoite single crystal tracks inside barium titanium silicate glass by direct femtosecond laser writing followed by the heat treatment. The variation of laser beam exposure parameters provides the possibility to control the morphology of the crystalline patterns written in glass.


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Growth of fresnoite single crystal tracks inside glass using

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