Direct laser writing of LaBGeO5 crystal-in-glass waveguide enabling

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Direct laser writing of LaBGeO crystal-inglass waveguide enabling frequency conversion Alexey S. Lipatiev, Tatiana O. Lipateva, Sergey V. Lotarev, Andrey G. Okhrimchuk, Alexey S. Larkin, Mikhail Yu. Presniakov, and Vladimir N. Sigaev Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00581 • Publication Date (Web): 25 Jul 2017 Downloaded from http://pubs.acs.org on July 28, 2017

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Crystal Growth & Design

Direct laser writing of LaBGeO5 crystal-in-glass waveguide enabling frequency conversion Alexey S. Lipatiev1*, Tatiana O. Lipateva1, Sergey V. Lotarev1, Andrey G. Okhrimchuk1, Alexey S. Larkin2, Mikhail Yu. Presnyakov3, Vladimir N. Sigaev1 1

D. Mendeleyev University of Chemical Technology of Russia, Miusskaya sq. 9, 125047 Moscow, Russia

2

Lomonosov Moscow State University, Leninskie Gory GSP-1, 119992 Moscow, Russia 3

NRC Kurchatov Institute, Akademika Kurchatova pl. 1, 123182 Moscow, Russia

ABSTRACT

We report on a technique of femtosecond laser-induced crystallization of glass allowing to improve the homogeneity of continuous crystalline tracks formed in lanthanum borogermanate glass using a focused beam with the elliptical-shaped waist cross-section instead of a conventional circular one. Second harmonic generation produced by near-infrared femtosecond laser pulses was demonstrated in the waveguiding mode for the laser written crystalline track. The structure of the fabricated single crystalline waveguides is investigated with high resolution transmission electron microscopy and micro-Raman spectroscopy.

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KEYWORDS Femtosecond laser direct writing; space-selective crystallization, waveguide, second harmonic generation, lanthanum borogermanate glass, LaBGeO5.


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INTRODUCTION Femtosecond (fs) laser pulses possess extremely high peak power, which made them a powerful tool for processing transparent solid materials including high-temperature ones such as oxide crystals and glasses1. Huge light intensity around ~ 1014 W/cm2 in the waist of the fs beam tightly focused in the transparent dielectric material gives rise to multiphoton absorption and avalanche photoionization transferring light energy to any glass regardless its absorbance at the fundamental wavelength of the laser. Controllable space-selective precipitation of microcrystals is a promising kind of laser-induced modification of glasses. Methods of direct laser writing of 2D/3D amorphous or crystalline architectures (dots, lines, arrays of crystals, single crystal-like waveguides of a complicated shape) possessing some functional properties such as waveguide ones, second-harmonic generation (SHG), luminescence have been recently developed2-8. Space-selective growth of lines consisting of nonlinear optical crystals in glasses was widely studied by T. Komatsu and coworkers

4-6

who developed techniques of laser-induced crystallization of glasses doped with

rare-earth or transition metal ions by the continuous wave (cw) laser beam. In addition to surface glass crystallization they have recently succeeded in cw laser patterning of high orientated βBaB2O4 crystals inside the glass fiber9. A number of significant works in the field of laser-induced crystallization of glass were carried out on lanthanum borogermanate (LBG) glasses10-12 including 3D space-selective glass crystallization by fs lasers11,12. The interest to LBG glass is specified by the opportunity to precipitate ferroelectric LaBGeO5 crystals with the stillwellite structure (space group P31) on the glass surface as well as in the glass bulk initiating second-order optical nonlinearity13,14. Laserinduced space-selective crystallization of glass opens a prospect to direct laser writing of three-



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dimensional active crystalline waveguides for fabrication of novel functional components for integrated optics, optoelectronics and photonics15. Nevertheless until recently no studies concerning the characterization of waveguiding properties of crystalline tracks fabricated in glass have been reported. In 2012 Feng et al.16 carried out preliminary investigations on the waveguiding behavior of crystalline tracks recorded by UV laser beam on the surface of ribbon (La,Yb)BGeO5 glass fiber. The optical loss of the crystalline waveguide was estimated as 3dB/cm from scattered light measurement, but no detectable SHG was demonstrated in the waveguide output. H. Jain’s group proposed aberration correction method via modification of the laser beam by a spatial light modulator (SLM) for improvement of the laser-written crystalline line cross-section shape in 25La2O3·25B2O3·50GeO2 glass in order to make it more suitable for waveguide applications17. Recently this method was successfully applied to provide waveguiding properties of the laser-written LaBGeO5 crystal-in-glass channels and to fabricate symmetric crystal junctions by laser beam single-pass writing. A single crystal nature of the laser-written waveguides was proved and optical losses for the single-crystal architectures inside glass were shown to be not higher than 2.64 dB/cm at 1530 nm15. In the present study we demonstrate an approach to fs laser-induced crystallization of glass using a laser beam with an elliptical cross-section of the waist that improves the homogeneity of written crystalline tracks. Second harmonic generation in the waveguiding mode has been shown in the laser-written waveguides for the first time to the best of our knowledge.

EXPERIMENTAL


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The LBG glass of the 25La2O3·30B2O3·45GeO2 molar composition slightly shifted from that of LaBGeO5 crystal was fabricated by conventional melt-quenching technique in a platinum crucible. Selected glass composition enables some decrease the glass crystallization ability as compared with the glass of 25La2O3·25B2O3·50GeO2 molar composition used in previous studies10-12, 15 and fabrication of a glass of improved homogeneity in the cylindrical mold with a diameter of 3 cm and height of 5 cm. As-quenched glass samples were annealed for 2 hours at 630ºC. All glass samples were thoroughly laser-grade polished with diamond suspension to meet a requirement of elimination of surface scattering and the laser beam distortion. The Yb-base femtosecond laser system with a regenerative amplifier was used for direct laser writing of crystalline tracks by pulses with 300 fs duration at the wavelength of 1030 nm and a pulse repetition rate up to 200 kHz. The laser beam was focused through Olympus 50X objective lens (N.A. = 0.65) at the depth of 170 µm below the glass surface. Experiments using Olympus objective lens with N.A. of 0.45 were also performed to compare the crystal morphology written by the beam focused with different N.A. by polarized optical microscopy. The pulse energy was varied in the range of 0 - 20 µJ by an attenuator consisting of a half-wave plate and a Glan prism. M2 factor of the laser beam was less than 1.3, and beam diameter was about 4.5 µm. The main driving force of glass crystallization is a heat transfer due to high temperature gradient induced by local nonlinear absorption of the fs laser beam. In order to reduce Kerr self-focusing and get anisotropic temperature gradient in the beam waist region we applied inscription by the laser beam with the elliptical waist cross-section18. We tested three different methods for the laser beam transformation to accomplish this including conversion of the Gaussian laser beam having axial symmetry into a beam with an elliptical or quasi-elliptical cross-section by letting it pass through (1) an anamorphic prism pair (anamorphic magnification 2.5) or (2) a slit (0.42 mm



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width); (3) introducing astigmatism into the beam by a cylindrical lens (f = -400 mm). The beam was focused by high N.A. objective lenses after passing these elements which broke its axial symmetry. The calculated beam waist diameter for the objective lens of N.A. = 0.65 and 0.45 was equaled to 2.3 µm and 3.3 µm respectively. The anamorphic prism pair, the cylindrical lens or the slit were oriented so that to stretch the beam waist cross-section along laser beam scanning direction up to 2.5, 8 and 10 times respectively. The proposed technique was previously used for efficient homogeneous refractive index modification and waveguide writing in crystal media 18,19 and our group tested it for laser-induced crystallization of LBG glass20. The computer-controlled high-precision motorized stage based on stepping motors was used for translating the glass sample relative to the focused laser beam. A live view of the laser writing process important for a real-time inspection of the forming structures was captured by CMOS camera. Crystal growth under the laser beam could be detected in situ due to noticeable SHG in the precipitating LaBGeO5 crystalline phase. Olympus BX-51 optical microscope was used for to analyse the morphology of the crystalline tracks. Identification of the microcrystal type and its orientation was performed using a polarizing confocal micro-Raman spectrometer included in NTEGRA Spectra nanolaboratory (NT-MDT Co.). An argon ion laser with a wavelength of 488 nm was used as the excitation source and the beam diameter at the focus was about 1 µm. A polarized Raman spectra mapping was performed by scanning areas of crystalline tracks from point to point with the step of 0.8 um. A layout of the experimental setup applied for measurement of optical propagation loss in crystalline waveguides and SHG signal registration is shown in Fig. 1.


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Figure 1. Experimental setup of the propagation losses measurements and SHG detecting. To observe the waveguiding effect and to measure propagation losses, we polished the facets perpendicular to crystalline waveguides after laser-induced crystallization and removed the ends of the laser-written crystalline track. So by the moment of optical loss analysis, the start point and the end point of the waveguide had the same structure as other parts of the waveguide. We used pulses attenuated and stretched to 600 fs duration at pulse repetition rate of 10 kHz as input light for measurement of light transmittance of crystalline waveguides. They were focused into waveguide input end by an objective lens with N.A. of 0.1 aligned by 6-axis translation table for light coupling. The sample with crystalline waveguides was set on 3-axis adjustable glass sample holder. Ophir PD-30R power meter head was used for measurement of output power of light passed the waveguide. SHG signal measurement was made with the same setup. We inserted a visible filter in front of the power meter and used a spectrometer ASP-150 with Czerny-Turner optical circuit for analyzing SHG spectrum of the output. The laser beam profiler (Ophir Spiricon SP 620U) replacing the power meter head was used to study crystalline waveguide propagation modes. A detailed analysis of a longitudinal cross section of crystalline waveguides formed in the LBG glass was performed by means of transmission electron microscopy (TEM) using Titan 80300 S/TEM system (FEI). TEM images were got in 300 kV mode also by using of STEM bright-



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field (BF) and high angle annular dark-field (HAADF) detectors. A longitudinal cross-section slice was prepared as a specimen for TEM analysis by ion beam etching.

RESULTS AND DISCUSSION As in previous studies15,17, the formation of crystalline tracks under fs laser writing started from the growth of the crystal seed from which a crystal of predetermined length was later grown by moving the sample relative to the focused laser beam. The appearance of the crystal seed was detected by the SHG effect of due to the second-order nonlinear optical response of the LaBGeO5 phase. While the average power of the laser beam is constant, the dependence of the crystal seed appearance time on the laser pulse energy has an exponential character21. However, the crystal seed nucleation process has a statistical nature due to fluctuations in the glass composition21,22. Even a slight reduction of pulse energy can cause a significant increase of the crystal seed formation time. We have proposed an optimized technique of laser-induced nucleation of a crystalline seed, ensuring its steady growth time. The technique constitutes a gradual increase of the laser pulse energy at a constant rate during glass exposure by focused laser beam until the appearance of the microcrystal seed. It is based on the assumption that crystal seed growth occurs at the interface between glass melt in the region of the beam waist and glass heated above the glass transition temperature due to the heat transfer. The increase of the laser pulse energy results in movement of the glass melt – heated glass interface and growth of the initial nanometer sized seed to the microcrystal seed possessing strong SHG activity that coincides with previously obtained data on crystallization of BaO-TiO2-GeO2 glass using a fs laser23. Proposed technique enables reduction of the crystal seed formation time as compared to


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irradiation by the laser beam with constant pulse energy and determines the pulse energy close to optimal one for writing crystalline waveguide structures. The Raman spectra and top views of the crystalline tracks inscribed in LBG glass by the circular and elliptical laser beam waists are shown in Fig. 2. Crystalline nature of obtained tracks was checked for several regions throughout the entire length of the tracks, and optical axis c orientation coinciding with the laser beam scanning direction was confirmed by polarized Raman spectra (Fig.2a). In addition, Raman spectra mapping of crystalline tracks was performed for the area of 384-404 covered a peak at 394 cm-1 that is characteristic for LaBGeO5 single crystal when c-axis is in the polarization plane of excitation and detection radiation24. It was determined that the Raman spectra map for crystalline track grown by elliptical laser beam waist is almost identical to the one for crystalline track grown by the Gaussian laser beam (Fig. 2b-c, insets) and all Raman spectra maps are in good agreement with early obtained results11.



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Typical polarized Raman spectra (a) and crоssed-polarizers optical microscope images of crystalline tracks written by the Gaussian beam with axial symmetry (b), by the laser beam with elliptical cross section of the waist formed with the negative cylindrical lens (c), anamorphic prism pairs (d) and the slit (e). Optical transmission micrographs of crystalline waveguides cross section and maps of integral intensity in the range of 384-404 cm-1 of polarized Raman spectra registered in the Z(XX)Ż geometry are shown on the insets. The laser beam scanning and incidence direction are indicated by arrows v and k respectively. The laser beam scanning speed and pulse energy for crystal growth (b-e) were 45 µm/s, 0.9 µJ; 40 µm/s, 2.1 µJ; 40 µm/s, 0.9 µJ and 42 µm/s, 2.3 µJ respectively.


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All crystalline tracks are colored when viewed in crossed polarizers due to their strong birefringence (Fig. 2b-d). It is evident that crystalline waveguides formed by elliptical laser beam waist in contrast to ones written by the Gaussian laser beam have a more uniform distribution of retardance produced by birefringence indicating that orientation and thickness of the crystal in the track cross-section is unchanged along a waveguide. This result is similar to one obtained for the single crystal-like waveguides formed by SLM-corrected laser beam15. The self-focusing critical power is 0.94 MW, as calculated by the formula25: Pcr_G =λ2/(2πn0n2) (1), where λ is laser wavelength, n0 and n2 are the linear and non-linear refractive index of LaBGeO5 crystal according to the works16,26. Pulses with energies of hundreds of nanoJoules and duration of 300 fs, which is necessary for local heating and crystallization of glass, have a peak power of more than 3 MW that is high enough to cause Kerr self-focusing of the Gaussian beam. The selffocusing distorts the beam, causes filamentation and disrupts the uniformity of the cross section shape of the crystalline track. An optical transmission micrograph of a cross section of the crystalline waveguide written by Gaussian beam (Fig. 2, inset) shows a significant elongation of crystal track in its bottom part confirming self-focusing phenomenon. The beam with the elliptical beam waist has increased critical power of self-focusing that facilitates more uniform and deterministic absorption of the laser beam18. The proposed technique is technically simpler than those based on high-cost SLM and also provides more uniform local heating which results in sustainable growth of a uniform crystal track with cross section shape closer to circular one. Optical microscopy is not sufficient for general characterization of crystal quality of a crystalline track. Taking into account that the track length exceeds several mm, such



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characterization can take much time. Considering this point we estimated the optical propagation losses of obtained waveguides of about 9.1 mm length which appears to be very sensitive to crystal structure quality. We used the direct method of measurement of the propagation loss (PL) (Fig. 1). The power of the beam which passed the optical system with (Ps) and without the sample (P0) was measured, and then the propagation loss were calculated by the formula, taking into account the Fresnel reflection losses at both ends of the waveguide: PL = -10/l*lg(Ps/P0/Ts2)

(2),

where l is a length of the waveguide (cm), Ts is a Fresnel transmission coefficient determined as: Ts = 4n0/(n0+1)2

(3).

Due to the poor quality of crystal tracks formed by Gaussian beam waist, we did not find waveguiding with them. In contrast, we observed multimode waveguiding behavior for the crystalline track (Fig. 3) written by elliptical beam waist with propagation loss estimated as less than 3.1 dB/cm at the wavelength of 1030 nm. These propagation losses are of an order of magnitude comparable to the ones obtained for single crystal waveguides15.


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Figure 3. Optical transmission micrograph of crystalline waveguide cross section (a) and waveguide fundamental (b) and SHG (c) modes intensity distribution at the output end of the waveguide.

SHG in crystalline waveguide written using the anamorphic prism pair was detected (Fig. 3c). The dependence of SHG intensity on laser beam input power follows quadratic power law I2ω.~ Iω2, which is corresponded to the regime of low conversion coefficient (Fig.4). Calculated value of SHG conversion efficiency was found to be as high as ~0.91 %/W, taking into account the transmission of visible filter and Fresnel reflections of both ends of the crystal waveguide. The measurable value of the conversion coefficient indicates that there is phase matching condition for SHG. The angle of collinear phase matching in LaBGeO5:Nd3+ single crystal is nearly 54 degree27. Obviously, that the collinear phase matching is unlikely to take place in the laser-written crystalline waveguide because polar axis of LaBGeO5 phase coincides with the waveguide axis. The birefringence–induced non-collinear phase matching is moreover impossible because it requires even higher propagation angles with respect to the optical axis. One of mechanism which can be assumed for significant SHG activity of the crystal waveguide is Cherenkov-type phase matching, which was found for LaBGeO5 single crystal28. SHG with such type of phase matching could have significant efficiency in a multimode waveguide, because, for example, the waveguide could insure overlap between IR fundamental mode and a higher order mode of the second harmonic, that is required for Cherenkov-type phase matching conditions29. SHG under the condition of phase matching between different transverse modes assumes that light of fundamental and second harmonic frequencies belongs to different transverse modes. This circumstance is reflected in Fig.3b,c, where maxima of intensity



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distributions of fundamental and SHG light are positioned in different sites of the crystalline waveguide output.

Figure 4. Dependence of the SHG intensity on the input radiation power. Inset shows the SHG spectrum at the crystalline waveguide output.

A more detailed analysis of the waveguide crystal structure was performed by means of TEM. For this purpose a longitudinal slice of crystal waveguide (Fig. 2d) was cut from its cross-section and polished by argon ion beam. TEM (Fig. 5) reveals an asymmetric crystal growth exhibiting the one-sided periphery part with single crystal nature and the central part of crystal waveguide at first view resembling LaBGeO5 structure resulting from grain-oriented crystallization of LBG glass30. However, the crystal phase in central part of the waveguide virtually doesn’t change its polar axis orientation. It remains very close to the laser scanning direction which is confirmed by diffraction pattern (ICDD #39262, the angle of crystal disorientation is less than 4º). Despite, all parts of crystal waveguide have a well-formed crystal lattice (Fig. 5b) but the revealed grain boundaries can greatly influence on propagation loss.


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Figure 5. (a) STEM HAADF image of lateral slice of the laser-induced crystalline waveguide and diffraction patterns of its different parts corresponding to LaBGeO5 crystal oriented on the laser scanning direction. Arrows v and k show directions of laser scanning and laser beam propagation respectively. (b) HR TEM image of crystal lattice typical for all parts of the crystalline waveguide.

Thus TEM data discover a complex crystal growth dynamics due to the considerable difference between temperature distribution in beam waist and periphery region during laser irradiation of LBG glass. The problem of single crystal formation and optical losses reducing is still challenging and requires more precise control of spatio-temporal characteristics of the laser beam

SUMMARY In conclusion, we propose a technique improving the morphology of the fs laser-induced crystalline track in the LBG glass by means of applying the laser beam with an elliptical crosssection of the waist, whose larger diameter is aligned along the writing direction. This technique enables writing a crystalline waveguide with propagation loss less than 3.1 dB/cm at the wavelength of 1030 nm. The second harmonic generation in the crystalline laser-written waveguide is demonstrated for the first time. TEM analysis shows that the structure of LaBGeO5



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crystalline phase forming waveguides contains the homogeneous periphery region with the orientation of the polar axis aligned exactly along the laser beam scanning direction and the central region formed by needle-like crystals growing simultaneously with the homogeneous region under polar axis orientation within 4º to the waveguide direction. Further investigation of the influence of spatio-temporal characteristics of the laser beam on the laser-induced crystal structure is required for the improving its crystallinity and homogeneity and reducing the propagation loss of the crystal-in-glass waveguide.

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

ACKNOWLEDGEMENTS This work has been supported by the Russian Foundation for Basic Research (grants 16-3360081, 16-33-01050, 16-03-00541) and the Ministry of Science and Education of Russia (grant 14.Z50.31.0009).


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Direct laser writing of LaBGeO5 crystal-in-glass waveguide enabling frequency conversion Alexey S. Lipatiev, Tatiana O. Lipateva, Sergey V. Lotarev, Andrey G. Okhrimchuk, Alexey S. Larkin, Mikhail Yu. Presnyakov, Vladimir N. Sigaev

A technique of direct femtosecond laser-induced crystallization of glass enhancing homogeneity of continuous crystalline tracks formed in lanthanum borogermanate glass using a beam with the waist cross-section of an elliptical form instead of a conventional circular one was proposed. Obtained crystalline tracks give evidence of the waveguiding effect and the frequency conversion ability.


Direct laser writing of LaBGeO5 crystal-in-glass waveguide enabling

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