We report on the formation and structural evolution of embedded self-organized, polarization-dependent nanogratings in sodium germanate glasses induced by an 800 nm, 1 kHz femtosecond laser. Optical birefringence dependent on the femtosecond laser polarization as well as the sodium oxide content is observed when the sample surface is perpendicular to the laser propagation direction. Scanning electron microscopy images of the written lines reveal the formation of periodic platelet or nanovoid arrays, which are aligned perpendicularly to the laser polarization direction after mechanical polishing. The influences of sodium oxide content on the morphology and period of the nanogratings are discussed.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Modification of transparent materials with ultrafast lasers has been extensively studied in recent decades for its unique advantages in three dimensionally material processing . When a single ultrafast laser beam is tightly focused inside the interior of a transparent solid, various structures have been observed after the laser irradiation near the focal point of the laser beam. These structures are very promising for applications in optical communications, integrated optical circuits and microfluidics .
More than a decade ago, a femtosecond laser-driven nanoscale self-assembly phenomenon was observed in the interior of a bulk silica glass [3,4].The nanostructure, usually referred to as nanograting, exhibits a number of interesting properties, including anisotropic light scattering, wavelength-dependent reﬂectivity, and birefringence with a high resistance to temperature [5–11]. These properties were successfully utilized for multiple practical applications including spatially variant polarization optics, microﬂuidics, polarization selective holography, and ultrastable optical data storage [5, 12–14]. However, this phenomenon was only observed in very limited glass systems, including pure or doped silica glass , binary or multicomponent borosilicate glass  and GeO2 glass, etc . Among these glasses, GeO2 glass shows excellent optical properties and large optical nonlinearity . In addition, it was observed that nanogratings in pure GeO2 glass exhibits even higher retardance compared with SiO2 glass . which may broaden applications of GeO2 glass in nanophotonics and relevant fields .
Up to now, there have been few systematic studies on effect of glass composition on the formation of nanograting in glass [15, 20]. In this paper, in order to discuss the effect of sodium oxide content on the formation of nanograting, femtosecond laser with a repetition rate of 1 kHz is applied to sodium germanate glass. The structural evolution of the nanogratings and its dependence on the glass composition have been analyzed and the results may provide significant information for laser microstructuring in glasses.
Glasses with the composition of xNa2O∙(100-x)GeO2 (x = 0, 5, 10) (mol.%, hereinafter referred to as 100Ge, 95Ge and 90Ge glass) were prepared by a standard melt-quenching technique. Reagent grade GeO2 and Na2CO3 were used as raw materials. In a typical process, an approximately 20 g batch was well homogenized and melted in an alumina crucible at 1450 °C for 30 min in air. The melt was then cast onto a cold steel plate and quickly quenched with another plate to obtain transparent glass samples. After annealing at 500 °C for 8 h, the samples were cut and polished into 10 mm × 10 mm × 2 mm dimensions for further experiment.
A regeneratively amplified 800 nm Ti: sapphire laser with 120 fs, 1 kHz mode-locked pulse was employed as the laser source in the experiment. The laser beam was tightly focused 50 μm beneath the surface of the glass sample translated by an XYZ stage via a 50 × microscope objective (NA = 0.6). The glass samples were scanned perpendicular to the laser propagation direction at a constant speed of 2μm/s. Optical microscope images were captured with a CCD camera connected with a computer using back illumination after the entire laser writing processing. For the scanning electron microscopy (SEM) observation, the samples were mechanically polished to remove the surface layer, so that the laser-modified region was exposed to air. The polished samples were then rinsed with alcohol(AR) for 60 s at room temperature to obtain clear surfaces.
3. Results and discussion
The optical images (transmission mode) of modified area (top view) induced by static femtosecond laser are shown in Fig. 1. A series of lines are written into the 100Ge, 95Ge and 90Ge glass samples by translating the sample perpendicularly to the laser propagation direction. Each line was irradiated with different laser polarizations (indicated by θ in Fig. 1) with the pulse energy of 1.5 μJ. Smooth line structures can be clearly observed in bright field images of Figs. 1(b), 1(d) and 1(f). In Figs. 1(a) and 1(c), the cross-polarized image shows a near-perfect symmetric distribution of birefringence signal on both sides with the center at 90°, which is quite similar to that observed in fused silica , indicating the formation of polarization-dependent microstructure in the induced line structures. However, no birefringence signal is observed in 90Ge glass sample [Fig. 1(e)], which may reveal the inexistence of polarization-dependent microstructures in the irradiated region. In order to compare the birefringence phenomenon between 100Ge and 95Ge glass, measurement of the birefringence signal intensity was carried out by integrating the transmission intensity from the cross-polarized image alone the laser scanning direction [Fig. 1(g)]. Obviously, both 100Ge and 95Ge glass samples exhibit similar polarization dependent tendency, while the birefringence signal intensity in 100Ge is stronger than that in 95Ge.
To further confirm the formation of polarization-dependent structures in the 95Ge glass and reveal their features, SEM images of the lines written with different laser polarization were measured [Fig. 2]. In the low magnification SEM image [Fig. 2(a)], the lines in the modified area can be hardly observed. However, a more explicit character of the formed nanogratings is revealed in the line structures in high magnification images [Figs. 2(b)-2(j)]. Apparently, the fabricated nanogratings are polarization dependent, and their orientations are always perpendicular to the laser polarization.
As shown in Fig. 3, the maximum period of the nanogratings is about 612 nm, which is obtained when incident laser polarization is parallel to the scan direction. The period decreases as the angle (θ) between laser polarization and scan direction increases, until the minimum period of 469 nm is reached at the θ of 90°.
The formation of the nanogratings and the variation of period have been observed in fused silica by Dai et al [22, 23]. It has been attributed to the change of the mutual orientation of the laser polarization and the pulse front tilt (PFT), because varying the laser polarization with respect to the writing direction also changes its orientation against PFT . However, the present result shown in Fig. 3 does not follow this explanation well. The red fitted curve can be expressed as d = λ/(2nsinθ), in which λ/2n is the parameter. The fitting result indicates the value of λ/2n is 392 nm, which has a large deviation with the value of 248 nm under our experiment condition approximately. Furthermore, the period of the nanograting found on 95Ge glass(from 469 nm-612 nm) is far larger than that of all previous gratings formed in pure or Ge-doped SiO2 glass(from 120 nm- 270 nm) [15, 17, 19, 25] On the other hand, taking the glasses with alkaline cations into consideration, nanograting period (from 180 nm- 340 nm) [20, 25, 26] in these glasses are still far more narrow than that in the 95Ge glass. This implies the influence of sodium oxide content on physical mechanism of the nanograting formation in germanate glasses.
To unravel the underlying mechanism accounting for the structural evolution among samples of 100Ge, 95Ge and 90Ge, high magnification optical microscope images and SEM images are recorded, as shown in Fig. 4. The nanograting-like structures can be observed clearly in 95Ge glass via optical microscope, which match the structures observed in the SEM image very well, conforming that the nanograting period is above the diffraction limit (~420 nm) of the objective. In comparison, a blurred vision of nanogratings-like structures is found in 100Ge glass, indicating the period of the nanograting is just below the diffraction limit of the optical microscope. However, the period is hard to define clearly by the SEM image of 100Ge, as the ratio of length to thickness of the nanogratings is too high to support itself after the polishing treatment. No nanograting-like structure was found in both optical and SEM image of 90Ge glass, which means no polarization-dependent microstructure exists in the modified region, in accordance with the cross-image shown in Fig. 1(e).
In order to find out the influences of sodium oxide content on the morphology and formation mechanism of the nanogratings, SEM images of nanograting area in 100Ge and 95Ge glass are shown in Figs. 5(b) and 6(b). The dashed box regions are magnified in Figs. 5(a) and 6(a), where the energy dispersive spectrometer (EDS) line mapping is carried on along the white dotted line. The periodic platelet structures can be clearly observed in Figs. 5(a) and 5(b) . The EDS signal intensity of Ge and O is shown in Fig. 5(c), and the O/Ge ratio corresponding to the position is indicated by white crosses in Fig. 5(a). The blue dotted line demonstrates the average level of O/Ge ratio in glass matrix. To weaken the influence of recessed and projected surface on the EDS signal, the position of white crosses we chosen in Figs. 5 and 6 are mostly on the polishing surface of the sample, which can be confirmed by the SEM image [Figs. 5(a) and 6(a)] and the absolute intensity [Figs. 5(c) and 6(c)]. The EDS signal in these positions may be hardly projected. The micro-irregularities of the surface are unavoidable using of mechanical polishing, but their influence on the EDS signal may be limited, for their scales are much less than the period of nanogratings. Apparently, the O/Ge ratio is higher in the cross section of platelet area of the nanograting than that in glass matrix. Similar processing is also conducted on 95Ge glass [Fig. 6(c)], however, only the edge area between the platelet and the void presents a slightly higher O/Ge ratio.
GeO2 glass has similar network structure with SiO2 glass, in which [GeO4] tetrahedra are corner-connected to form three-dimensional network , and the formation of periodically distributed oxygen-deficient regions (GeO2−x) is expected to be similar to that (SiO2−x) in fused silica [19, 27]. As the oxygen-deficient regions are porous and vulnerable in mechanical polishing process , these regions become voids and the platelet structures remain. In other hand, oxygen is usually induced in the nanograting area of GeO2 glass . Thus, we may conjecture that the diffusion of oxygen may lead to the higher O/Ge ratio in platelet area.
The presence of alkaline cations causes the large modifications in the glassy structure by interconverting some of the [GeO4] tetrahedra into [GeO6] octahedral structural units , which improves the integration of the glass network and consequently increases the density . The rigid network structure and the migration of Na ions may decrease the defects induced by single laser pulse and weaken the coupling mechanism between the laser pulses [20, 30]. leading to the shrink of nanograting formation as the content of Na ions increase. Also as the doping of alkaline cations, lower melting point and consequently lower viscosity will occur when heating in the focal region, and germanate glass would react plasticly on the thermoelastic forces. The more content of Na ions, the lower melt viscosity during laser heating, the formed nanograting in germanate glass tends to be erased easier . The morphology and birefringence signal intensity evolution of 100Ge, 95Ge and 90Ge may attribute to all above related reasons. We can further predict that other alkali cations or alkali earth cations may weaken the birefringence signal in the germanate glasses.
In conclusion, self-organized, polarization-dependent nanogratings were formed inside sodium germanate glass after irradiation with a 1 kHz, 800 nm femtosecond laser. Under the same laser irradiation condition, the microstructure formed shows a strong dependence on the composition of the glass, and nanograting has only been observed in 100Ge and 95Ge glasses, while no apparent birefringence is observed in 90Ge glass. The period of the fabricated nanogratings changes with the laser polarization direction, and these results can be related partly with the effect of pulse front tilt. The sodium oxide content plays an important role on the morphology and birefringence signal intensity of the nanogratings. Our results demonstrated here may have strong implications in the application of laser-fabricated GeO2 glass based nanostructures in photonics areas.
National Natural Science Foundation of China. (51472091, 51772270, 11774220); and National Key R&D Program of China (YS2018YFB110012).
References and links
1. K. N. Tan, K. N. Sharafudeen, Y. Yue, and J. Qiu, “Femtosecond laser induced phenomena in transparent solid materials: Fundamentals and applications,” Prog. Mater. Sci. 76, 154–228 (2016). [CrossRef]
3. P. G. Kazansky, H. Inouye, T. Mitsuyu, K. Miura, J. Qiu, K. Hirao, and F. Starrost, “Anomalous anisotropic light scattering in Ge-doped silica glass,” Phys. Rev. Lett. 82(10), 2199–2202 (1999). [CrossRef]
6. Y. Shimotsuma, K. Hirao, P. G. Kazansky, and J. Qiu, “Three-Dimensional Micro- and Nano-Fabrication in Transparent Materials by Femtosecond Laser,” Jpn. J. Appl. Phys. 44(7A), 4735–4748 (2005). [CrossRef]
7. C. Hnatovsky, R. S. Taylor, P. P. Rajeev, E. Simova, V. R. Bhardwaj, D. M. Rayner, and P. B. Corkum, “Pulse duration dependence of femtosecond-laser-fabricated nanogratings in fused silica,” Appl. Phys. Lett. 87(1), 014104 (2005). [CrossRef]
9. S. Juodkazis, K. Nishimura, S. Tanaka, H. Misawa, E. G. Gamaly, B. Luther-Davies, L. Hallo, P. Nicolai, and V. T. Tikhonchuk, “Laser-induced microexplosion confined in the bulk of a sapphire crystal: Evidence of multimegabar pressures,” Phys. Rev. Lett. 96(16), 166101 (2006). [CrossRef] [PubMed]
10. E. G. Gamaly, S. Juodkazis, K. Nishimura, H. Misawa, B. Luther-Davies, L. Hallo, P. Nicolai, and V. T. Tikhonchuk, “Laser-matter interaction in the bulk of a transparent solid: Confined microexplosion and void formation,” Phys. Rev. B 73(21), 214101 (2006). [CrossRef]
11. S. K. Sundaram, C. B. Schaffer, and E. Mazur, “Microexplosions in tellurite glasses,” Appl. Phys., A Mater. Sci. Process. 76(3), 379–384 (2003). [CrossRef]
12. Y. Shimotsuma, M. Sakakura, P. G. Kazansky, M. Beresna, J. R. Qiu, K. Miura, and K. Hirao, “Ultrafast Manipulation of Self-Assembled Form Birefringence in Glass,” Adv. Mater. 22, 5442 (2010). [CrossRef]
13. F. Zhang, A. Cerkauskaite, R. Drevinskas, P. G. Kazansky, and J. Qiu, “Microengineering of Optical Properties of GeO2 Glass by Ultrafast Laser Nanostructuring,” Adv. Opt. Mater. 5(23), 1700342 (2017). [CrossRef]
14. Y. Liao and Y. Cheng, “Femtosecond Laser 3D Fabrication in Porous Glass for Micro- and Nanofluidic Applications,” Micromachines (Basel) 5(4), 1106–1134 (2014). [CrossRef]
15. F. Zimmermann, M. Lancry, A. Plech, S. Richter, B. H. Babu, B. Poumellec, A. Tünnermann, and S. Nolte, “Femtosecond laser written nanostructures in Ge-doped glasses,” Opt. Lett. 41(6), 1161–1164 (2016). [CrossRef] [PubMed]
16. S. S. Fedotov, R. Drevinskas, S. V. Lotarev, A. S. Lipatiev, M. Beresna, A. Čerkauskaitė, V. N. Sigaev, and P. G. Kazansky, “Direct writing of birefringent elements by ultrafast laser nanostructuring in multicomponent glass,” Appl. Phys. Lett. 108(7), 071905 (2016). [CrossRef]
17. F. Zhang, H. Zhang, G. Dong, and J. Qiu, “Embedded nanogratings in germanium dioxide glass induced by femtosecond laser direct writing,” J. Opt. Soc. Am. B 31(4), 860–864 (2014). [CrossRef]
19. T. Asai, Y. Shimotsuma, T. Kurita, A. Murata, S. Kubota, M. Sakakura, K. Miura, F. Brisset, B. Poumellec, and M. Lancry, “Systematic Control of Structural Changes in GeO2 Glass Induced by Femtosecond Laser Direct Writing,” J. Am. Ceram. Soc. 98(5), 1471–1477 (2015). [CrossRef]
20. S. Lotarev, S. Fedotov, A. Lipatiev, M. Presnyakov, P. Kazansky, and V. Sigaev, “Light-driven nanoperiodical modulation of alkaline cation distribution inside sodium silicate glass,” J. Non-Cryst. Solids 479, 49–54 (2018). [CrossRef]
21. F. Zhang, Y. Yu, C. Cheng, Y. Dai, and J. Qiu, “Fabrication of polarization-dependent light attenuator in fused silica using a low-repetition-rate femtosecond laser,” Opt. Lett. 38(13), 2212–2214 (2013). [CrossRef] [PubMed]
23. Y. Dai, J. Ye, M. Gong, X. Ye, X. Yan, G. Ma, and J. Qiu, “Forced rotation of nanograting in glass by pulse-front tilted femtosecond laser direct writing,” Opt. Express 22(23), 28500–28505 (2014). [CrossRef] [PubMed]
24. P. G. Kazansky, Y. Shimotsuma, M. Sakakura, M. Beresna, M. Gecevičius, Y. Svirko, S. Akturk, J. Qiu, K. Miura, and K. Hirao, “Photosensitivity control of an isotropic medium through polarization of light pulses with tilted intensity front,” Opt. Express 19(21), 20657–20664 (2011). [CrossRef] [PubMed]
25. M. Lancry, F. Zimmerman, R. Desmarchelier, J. Tian, F. Brisset, S. Nolte, and B. Poumellec, “Nanogratings formation in multicomponent silicate glasses,” Appl. Phys. B-Lasers. Opt. 122, 66 (2016).
26. J. Cao, B. Poumellec, L. Mazerolles, F. Brisset, A. Helbert, S. Surble, X. He, and M. Lancry, “Nanoscale Phase Separation in Lithium Niobium Silicate Glass by Femtosecond Laser Irradiation,” J. Am. Ceram. Soc. 100(1), 115–124 (2017). [CrossRef]
27. M. Lancry, J. Canning, K. Cook, M. Heili, D. R. Neuville, and B. Poumellec, “Nanoscale femtosecond laser milling and control of nanoporosity in the normal and anomalous regimes of GeO2-SiO2 glasses,” Opt. Mater. Express 6(2), 321 (2016). [CrossRef]
28. Y. Shimotsuma, T. Asai, M. Sakakura, and K. Miura, “Femtosecond-laser Nanostructuring in Glass,” J. Laser Micro Nanoeng. 9(1), 31 (2014). [CrossRef]
29. M. Rada, N. Aldea, Z. H. Wu, Z. Jing, S. Rada, E. Culea, S. Macavei, R. Balan, R. C. Suciu, R. V. Erhan, and V. Bodnarchuk, “Evolution of the germanium–oxygen coordination number in lithium–lead–germanate glasses,” J. Non-Cryst. Solids 437, 10–16 (2016). [CrossRef]
30. S. Richter, F. Jia, M. Heinrich, S. Döring, U. Peschel, A. Tünnermann, and S. Nolte, “The role of self-trapped excitons and defects in the formation of nanogratings in fused silica,” Opt. Lett. 37(4), 482–484 (2012). [CrossRef] [PubMed]
31. F. Zimmermann, A. Plech, S. Richter, A. Tünnermann, and S. Nolte, “Ultrashort laser pulse induced nanogratings in borosilicate glass,” Appl. Phys. Lett. 104(21), 211107 (2014). [CrossRef]