November 15, 2008 / Vol. 33, No. 22 / OPTICS LETTERS
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Nanograting formation on the surface of silica glass by scanning focused femtosecond laser pulses Quan Sun,* Feng Liang, Réal Vallée, and See Leang Chin Centre d’Optique, Photonique et Laser (COPL) and Department of Physics, Université Laval, Québec, G1V 0A6, Canada *Corresponding author:
[email protected] Received July 2, 2008; revised October 7, 2008; accepted October 14, 2008; posted October 21, 2008 (Doc. ID 98297); published November 14, 2008 We have investigated the formation of nanogratings induced by femtosecond laser pulses on the surface of silica glass blocks. The nanograting period ranges between 170 and 340 nm, depending on the pulse-topulse spacing, whereas for a given spacing the period remains constant upon changing the laser pulse energy. Intensity clamping is proposed as the mechanism that is responsible for such independence of the grating period from pulse energy. © 2008 Optical Society of America OCIS codes: 320.7110, 140.3390.
Laser-induced periodic surface structures (LIPSSs) were discovered soon after the laser was invented. Early in 1965, Birnbaum first observed regular LIPSSs on the GaAs surface [1]. After that, LIPSSs, also called ripples, have been reported on various materials such as metals [2,3], semiconductors [4,5], and dielectrics [6–8], not only by cw or long pulsed lasers but also by ultrashort pulsed lasers. Earlier, most LIPSSs had spacing similar to an applied laser wavelength [2,4,6] and were referred to as lowspatial-frequency LIPSSs (LSFLs) [5]. Recently, several groups reported high-spatial-frequency LIPSSs (HSFLs) with spacing substantially smaller than the writing wavelength, especially with ultrashort pulse irradiation [3,5,7]. Because the spacing of subwavelength ripples could reach a few hundreds of nanometers, they were also called nanogratings [9]. The mechanism responsible for the formation of nanogratings induced by femtosecond laser pulses is still a matter of debate. An interference model, relying on the interference between the incident laser beam and the surface scattered waves leading to inhomogeneous absorption of the laser energy, has been developed [4,6,10,11]. This model was quite successful in the formation of LSFLs, especially for long pulses. But it could not explain most of the HSFL (nanograting) formation in the case of femtosecond laser irradiation. More recently, nanograting formation inside bulk dielectric materials was also reported [9,12]. Two models were proposed for femtosecond-pulse-induced nanogratings. One was put forward by Shimotsuma et al. as the interference of the incident laser field with the electron plasma wave [9], where the period of the nanogratings was thought to be dependent on the density and temperature of the electron plasma, which were both related to the pulse intensity. Another proposal by Bhardwaj et al. was based on local field enhancement occurring during inhomogeneous breakdown [12]. They proposed that the growth of underdense nanoplasmas into sheets would cause light to adopt modes similar to those in planar waveguides, leading to a / 2n period (where is the laser wavelength and n is the refractive index of the medium). According to this 0146-9592/08/222713-3/$15.00
model, the nanograting period would be independent of the pulse intensity and the scanning speed (or equivalently, the pulse-to-pulse spacing). This would be in contradiction with some experimental reports where, for instance, the nanograting period was shown to decrease with larger pulse numbers in the case of static focusing [9]. However, Wagner et al. reported that the period was independent of the scanning speed in their experiment [7]. There was also some contradiction between the two reports [9,12] regarding the pulse energy dependence of the nanograting period. In addition, Costache et al. reported that the period could depend on local laser intensity [8]. In this Letter, nanogratings were fabricated by the irradiation of femtosecond laser pulses on the surface of a silica glass plate. The sample was scanned perpendicular to the laser propagation direction so that long nanogratings could be fabricated. Specifically, the dependence of the nanograting period on the pulse-to-pulse spacing and the pulse energy were investigated. A schematic of our experimental setup is shown in Fig. 1. A Ti:sapphire chirped-pulse amplification laser system (Spitfire, Spectra-Physics) was used in our experiments. It provided 1 kHz 40 fs laser pulses at 800 nm with 2 mJ maximum pulse energy. The laser beam was focused on the surface of a silica glass plate (Corning 7980-UV) by a 25⫻ microscope objective (f = 6.6 mm, NA= 0.50). The silica glass plate with well-polished surfaces was mounted on a motorized Nanomover translation stage (Melles Griot, Nanomotion II). The speed of the translation stage could be varied from 5 m / s (minimum) to 2.5 mm/ s. A halfwave plate was used to control the polarization of the incident laser. A variable metallic neutral density fil-
Fig. 1. (Color online) Schematic of the experimental setup. © 2008 Optical Society of America
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ter was used to adjust the pulse energy. After writing, the sample was cleaned with methanol in an ultrasonic bath. Then the irradiated surface was analyzed by a scanning electronic microscope (SEM, JEOL 840-A), showing nanogratings. Figure 2 shows the typical morphology of the nanogratings that were fabricated with the translation stage moving transversely to the incident laser beam (in the direction of x in Fig. 1) at the scanning speed of 60 m / s, which nominally corresponds to 60 nm pulse-to-pulse spacing at a 1 kHz repetition rate. The surface of the sample was irradiated at the energy of 300 nJ/pulse. In Fig. 2(a), the polarization was parallel to the scanning direction, while in Fig. 2(b) it was perpendicular to it. Thus, as reported before, the direction of the fabricated nanogratings was always perpendicular to the laser polarization [2–12]. Also, we could observe some debris around the nanogratings that could not be removed, even after ultrasonic cleaning for 30 min. The evolution of the nanograting period as a function of the pulse-to-pulse spacing is shown in Fig. 3, where three distinct regions can be identified. In Reg. I, where pulse-to-pulse spacing ranges between 5 nm and 20 nm, the nanograting period remains constant to a value of 170 nm, which appears to be close to / 3n. Accordingly, we could detect third-harmonic signal generation from the surface with a spectrometer during the writing process. In Reg. II, where the pulse-to-pulse spacing varied between 40 and 200 nm, the nanograting period increased with the pulse-to-pulse spacing with a slope of approximately 0.35 (on the log–log plot) over this intermediate pulse-to-pulse overlap region. In fact, defining the overlap factor as “1-d / D,” where d is the pulse-topulse spacing and D is the diameter of the focal spot (estimated to be 1 m) for pulse-to-pulse spacing below 20 nm (Reg. I), the pulse overlap is larger than 98%. For pulse-to-pulse spacing varying between 40 and 200 nm (Reg. II), the pulse overlap ranged between 96% and 80%, corresponding to the observed increase of the nanograting period. Finally, for pulseto-pulse spacing exceeding 400 nm (Reg. III) where practically no sufficient pulse overlap occurs, the spacing of the modified structures correspond, as expected, to the pulse-to-pulse spacing, which is in agreement with Wagner et al. [7]. In this range,
Fig. 2. Typical SEM pictures of nanograting on silica glass. The incident laser polarizations were (a) parallel and (b) perpendicular to the writing direction, respectively. The pulse-to-pulse spacing and pulse energy were set to 60 nm and 300 nJ, respectively.
Fig. 3. (Color online) Nanograting period vs. pulse-topulse spacing. In Reg.III, the period of the modifications is directly related to pulse-to-pulse spacing (diagonal line), instead of nanograting formation. The inset is a typical SEM image of the modification zone at pulse-to-pulse spacing of 1200 nm. The pulse energy was set to 300 nJ.
single-shot modifications were observed, as shown in the inset in Fig. 3. We note that for application purposes, Reg. I affords a flexible condition in which to get constant period nanogratings, whereas in Reg. II, the pulse spacing can be used as an alternative parameter to induce variable period nanogratings. Finally, Reg. III corresponds to point-by-point writing conditions. The sharp transitions that were observed among these three regions, especially between Reg. I and Reg. II, are very important for understanding the underlying the physical mechanisms responsible for nanograting formation. The dependence of the nanograting period on laser pulse energy was also investigated. The results are shown in Fig. 4. The period appeared to remain constant for pulse energies ranging from 170 to 870 nJ. Above 900 nJ, severe damage occurred so that nanograting formation could not be observed. The pulseto-pulse spacing was fixed at 40 nm for all of the investigated laser pulse energy. Note that the previous results are consistent with the results reported by Bhardwaj et al. [12] but not with those of [9], where a nanograting period increase with laser pulse energy
Fig. 4. Nanograting period versus pulse energy. The pulse-to-pulse spacing was set to 40 nm.
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was reported. In order to understand such behavior, we first note that, since the peak powers corresponding to the pulse energy range found in our experiment were all above the critical power for selffocusing (⬃2 MW [13]), we verified that the filamentation process was indeed accompanying the nanograting formation. This was not surprising given the relatively loose focusing conditions found in our experiment. It is therefore very likely that the intensity clamping process was also occurring [14,15], thus providing a mechanism for limiting the local intensity in the glass, whereas the energy was increased. This would also explain why in [9], where very tight focusing conditions were used 共100⫻, NA = 0.95), the self-focusing was suppressed by geometrical focusing [16,17] so that the intensity in the focal region could increase with the pulse energy, resulting in a dependence of the nanograting period. Also note that in our experiment the nanograting period was significantly smaller than the laser wavelength throughout the range, i.e., from 170 to 340 nm, where it was shown to vary. So, neither the interference model [4,6,10,11] (close to the laser wavelength at normal incidence) nor that proposed by Bhardwaj et al. [12] (period as / 2n) could really account for the nanograting formation in our experiment. As for the model proposed by Shimotsuma et al. [9], it requires the temperature of the electron plasma to reach 107 K – 108 K 共103 – 104 eV兲. Now, as pointed out in [12], this high temperature cannot be reached within the range of pulse energies typically used for nanograting formation. Furthermore, the two models [9,12] were both proposed for nanograting inside silica glass, which presents significant differences with the nanograting formation at the surface of the glass studied here. For instance, the nanograting period formed inside the glass was reported to be roughly constant at ⬃270 nm, which was close to ⬃ / 2n [12]. However, in our experiment the grating period could be close to 170 nm, which actually corresponds to / 3n. Thus, as mentioned above, third-harmonic generation might play a major role in nanograting formation at the surface, as previously suggested [18]. This is currently under investigation. In conclusion, we experimentally demonstrated that the period of nanograting on the surface of silica glass induced by a 1 kHz femtosecond laser was ⬃170 nm 共 / 3n兲 when the pulse-to-pulse spacing was 20 nm or below, while the period increased with pulse spacing ranging between 20 and 200 nm. The nan-
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ograting period was also found to be independent of the laser pulse energy owing to the intensity clamping accompanying the filamentation process. This work was supported by the Natural Sciences and Engineering Research Council of Canada, Defence R&D Canada—Valcartier, Le Fonds Québécois de la Recherche sur la Nature et les Technologies, Canada Research Chairs, Canada Foundation for Innovation, Canadian Institute for Photonic Innovations, and Ministère du Développement économique, de l’Innovation et de l’Exportation. The authors appreciate the technical support of M. Martin and M. D’Auteuil. References 1. M. Birnbaum, J. Appl. Phys. 36, 3688 (1965). 2. S. R. J. Brueck and D. J. Ehrlich, Phys. Rev. Lett. 48, 1678 (1982). 3. J. Wang and C. Guo, Appl. Phys. Lett. 87, 251914 (2005). 4. D. C. Emmony, R. P. Howson, and L. J. Willis, Appl. Phys. Lett. 23, 598 (1973). 5. A. Borowiec and H. K. Haugen, Appl. Phys. Lett. 82, 4462 (2003). 6. P. A. Temple and M. J. Soileau, IEEE J. Quantum Electron. QE-17, 2067 (1981). 7. R. Wagner, J. Gottmann, A. Horn, and E. W. Kreutz, Appl. Surf. Sci. 252, 8576 (2006). 8. F. Costache, M. Henyk, and J. Reif, Appl. Surf. Sci. 186, 352 (2002). 9. Y. Shimotsuma, P. G. Kazansky, J. Qiu, and K. Hirao, Phys. Rev. Lett. 91, 247405 (2003). 10. G. Zhou, P. M. Fauchet, and A. E. Siegman, Phys. Rev. B 26, 5366 (1982). 11. J. E. Sipe, J. F. Young, J. S. Preston, and H. M. van Driel, Phys. Rev. B 27, 1141 (1983). 12. V. R. Bhardwaj, E. Simova, P. P. Rajeev, C. Hnatovsky, R. S. Taylor, D. M. Rayner, and P. B. Corkum, Phys. Rev. Lett. 96, 057404 (2006). 13. A. Couairon, L. Sudrie, M. Franco, B. Prade, and A. Mysyrowicz, Phys. Rev. B 71, 125435 (2005). 14. W. Liu, O. Kosareva, I. S. Golubtsov, A. Iwasaki, A. Becker, V. P. Kandidov, and S. L. Chin, Appl. Phys. B 76, 215 (2003). 15. S. L. Chin, F. Théberge, and W. Liu, Appl. Phys. B 86, 477 (2007). 16. C. B. Schaffer, A. Brodeur, and E. Mazur, Meas. Sci. Technol. 12, 1784 (2001). 17. W. Liu, Q. Luo, and S. L. Chin, Chin. Opt. Lett. 1, 56 (2003). 18. J. Bonse, M. Munz, and H. Sturm, J. Appl. Phys. 97, 013538 (2005).
