- Email: [email protected]
Surface birefringence of self-assembly periodic nanostructures induced on 6H-SiC surface by femtosecond laser
Applied Surface Science 363 (2016) 664–669
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Surface birefringence of self-assembly periodic nanostructures induced on 6H-SiC surface by femtosecond laser Juan Song a,∗ , Ye Dai b , Wenjun Tao a , Min Gong b , Guohong Ma b , Quanzhong Zhao c , Jianrong Qiu d a
School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, China Physics Department, Shanghai University, Shanghai 200444, China c State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China d Key Laboratory of Specially Functional Materials of Ministry of Education and Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510640, China b
a r t i c l e
i n f o
Article history: Received 19 June 2015 Received in revised form 21 November 2015 Accepted 13 December 2015 Available online 15 December 2015 Keywords: Deep-subwavelength periodic ripples Birefringence Retardance Attenuator
a b s t r a c t In this paper, we report the birefringence effect of surface self-assembly periodic nanostructures induced on 6H-SiC by femtosecond laser irradiation. Birefringence characteristic (e.g. cross-polarized image), measured by cross polarized microscopy, was found to be controlled by both single pulse energy and scanning velocity. Comparing birefringence measurement results of nanostructures and morphology characterization by Scanning electron microscopy, it is shown that ∼200 nm-period deep-subwavelength periodic ripples (DSWR) plays a dominating role in the birefringence effect. Raman spectra show that the change of retardance with pulse energy and scanning velocity is most possibly caused by the thickness variation of DSWR. Finally, a light attenuator based on a single layer of DSWR structure on 6H-SiC surface was constructed and tested by light source of 800 nm to have a tunable attenuating ratio of 69–100%. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Study on femtosecond laser induced periodic surface structures (LIPSS) has developed for at least two decades since the pioneering works of Birnbaum [1]. In the early years, more interest was put on analysis of the periodicity origin of LIPSS. The classical near-subwavelength ripples (NSWR) are widely accepted to result from the interaction between the incident light and the surface scattering wave or the surface plasmon wave [2,3]. In recent years, deep-subwavelength ripples (DSWR) were reported on several semiconductor materials as well as metal materials and are considered to be induced by second harmonic wave or selforganization [4–6]. Studies on LIPSS are not only important for deeply understanding of laser-material interaction process but also valuable for developing or improving material functions. For example, LIPSS have been used for luminescence enhancement, optical absorption enhancement, photocurrent enhancement, wettability modification, friction controlling, material colorization and so on
∗ Corresponding author. Tel.: +086 139 1280 8637; fax: +086 0511 88791288. E-mail address: [email protected] (J. Song). http://dx.doi.org/10.1016/j.apsusc.2015.12.096 0169-4332/© 2015 Elsevier B.V. All rights reserved.
[7–11]. Recently, periodic surface nanostructures on hydrogenated amorphous silicon (a-Si: H) induced by either a femtosecond or a picosecond laser, was reported to show giant birefringence and dichroism. It is the first report for the form birefringence of LIPSS [12]. Form birefringence, depending on periodic refractive index modulation, is supposed to be more pronounced on high-refractive-index material. The 6H-SiC polytypes, with refractive index n = 2.6, is a promising photoelectric material for the advantages of wide bandgap, high thermal conductivity, and stable electrical performance at high temperature. Typical LIPSS, near-subwavelength ripples (NSWR) of 700 nm and deepsubwavelength ripples (DSWR) of 200 nm, have already been reported on 6H-SiC [13]. Optical absorption and photocurrent enhancement of femtosecond-laser-induced ripples on 6H-SiC was also reported [8]. In this paper, birefriengence effect of LIPSS induced by femtosecond laser on 6H-SiC was studied. Birefringence retardance was found to be controlled by single pulse energy and scanning velocity used in laser processing. Raman spectra of laser-induced nanostructures on 6H-SiC surface and effective medium theory were employed to analyze the variation of retardance with laser parameters. A light attenuator based on birefringence characteristics of 6H-SiC surface nanostructures was tested.
J. Song et al. / Applied Surface Science 363 (2016) 664–669
665
Fig. 1. Top-view scanning electron microscopy images of the ablation lines inscribed by laser beams with different polarization azimuth angles.
Fig. 2. Birefringence signal of the structures for different polarization azimuth angles measured by orthogonal polarizing microscope: (a) transmission intensity image captured by CCD after Cross-polarizer; (b) variation of transmission intensity with polarization azimuth angle.
2. Experiments A Ti: sapphire regeneratively amplified mode-locked laser system launches ultrashort pulses with 800 nm wavelength and 120 fs pulse duration at a repetition rate of 1 kHz. The laser beam was normally focused by a 5× microscopic objective with NA = 0.15 on the polished (0 0 0 1) crystal plane of 6H-SiC crystal, which was mounted on a 3-axis motorized stage driven by a computer. A neutral filter was used for adjusting the single pulse energy. An electronic shutter was adopted for controlling the pulse number for irradiation. A /2 waveplate were used to adjust the polarization plane azimuth of the laser beam for irradiation. The birefringence characteristics were qualitatively measured by crosspolarized light microscopy. While the laser polarization direction was adjusted to be aligned with the transmission axis of the first polarizer of cross-polarized light microscopy, this special azimuth angle of polarization plane at this moment was defined as 0◦ . All ablation lines in this paper were induced by scanning laser focus perpendicular to the laser propagation direction.
light passing through the pair of cross polarizers, demonstrated the birefringence effect of the laser-inscribed structures. Birefringence profile extracted from Fig. 2(a) along the horizontal direction is shown in Fig. 2(b). Birefringence signal intensity is periodically changed with laser polarization azimuth angle, and is roughly symmetric with respect to the polarization azimuth angle of 0◦ . The birefringence retardance for ablation lines at different pulse energies are qualitatively represented by grey level of transmission intensity images captured by CCD after cross-polarizers and demonstrated in Fig. 3. Clearly, the ablation line for E = 0.9 J almost has no birefringence signal. With pulse energy increased from
3. Results and discussion With single pulse energy set as 1.6 J and a scanning velocity of 600 m/s used, the obtained ablation lines for the different polarization azimuth angles from −100◦ to 100◦ was demonstrated in Fig. 1. Clearly, periodic ripples appear on all cases. The ripple orientation is synchronously rotated with the laser polarization direction and keeps always perpendicular to the laser polarization. It is worth noting that the structures ablated by laser pulses with polarization azimuth angles from −60◦ to 60◦ are DSWR with period of ∼200 nm, while the structures for the azimuth angles around ±90◦ are ∼200 nm-period DSWR overlapped by ∼580 nm-period NSWR. The cross-polarized images of the structures in Fig. 1 were shown in Fig. 2(a). Clearly, the phenomenon, that there is still transmission intensity left in some ablation lines after illumination
Fig. 3. Qualitative birefringence characteristics of the ablation lines inscribed by pulses with different energies at velocity of 600 m/s. (a) Birefringence intensity image (b) qualitative birefringence signal profile along the white line. (From left to right, E = 0.9 J, 1.1 J, 1.3 J, 1.5 J, 1.7 J, 1.9 J, 2.1 J, 2.3 J, 2.5 J, 3.0 J, 3.5 J, 4.0 J).
666
J. Song et al. / Applied Surface Science 363 (2016) 664–669
Fig. 4. SEM images for the ablation lines induced by laser pulses with different energies.
1.1 J to 1.9 J, the retardance value grows fast to two times the original magnitude. It is worth noting that retardance profile across each ablation line is basically a Gaussian-like distribution with highest retardance in the center section. While the pulse energy continues to increase from 2.5 J to 4.0 J, the retardance profiles are all two-peak curve with a valley in the center. The retardance of the valley is lower and lower as the pulse energy increases. Fig. 4 shows the morphologies of a set of ablation lines inscribed at a scanning velocity of 600 m/s by laser beams with single pulse energy ranging from 0.9 J to 4.5 J. As can be seen, when the energy is as low as 0.9 J, no obvious structures are left in the ablation line. With single pulse energies increased from 1.1 J to 1.7 J, DSWR with period of about 200 nm start to appear in laser-scanned trace and tend to be more consistently aligned perpendicular to the laser polarization. Once the single pulse energy is beyond 1.7 J, ∼580 nm NSWR begin to form overlapping on the as-formed DSWR and have their grooves deepened.
Fig. 5. Qualitative birefringence characteristics of the structures obtained at different scanning velocities (a) birefringence intensity image captured by cross-polarized microscopy (b) birefringence signal profile along the white line. (from left to right, v = 2.0, 1.5, 1.0, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, 0.05, 0.025, 0.01 mm/s).
Besides, dependence of the birefringence intensity on the scanning velocity was also studied and displayed in Fig. 5, where single pulse energy was set as 1.6 J. It is shown that when scanning velocity is decreased from 2 mm/s to 0.3 mm/s, the birefringence signal is roughly linearly increased by several times. However, when the velocity is decreased lower than 0.3 mm/s, the birefringence signal sharply declines. Also, the birefringence signal profile of each ablation line in velocity range of 0.2 mm/s and 2 mm/s is Gaussianlike single-peak curve, while the retardance profile for each line inscribed at velocities between 0.01 m/s and 0.05 m/s appears as a two-peak curve with a valley in between. The morphologies of the ablation lines induced at different scanning velocities are also characterized by SEM. As shown in Fig. 6, when the velocity is between 1.5 mm/s and 2.0 mm/s, DSWR with shallow grooves are induced, the orientation of which is disrupted. As the velocity was decreased from 0.9 mm/s to 0.5 mm/s,
Fig. 6. SEM images of the ablation trace induced at different scanning velocity.
J. Song et al. / Applied Surface Science 363 (2016) 664–669
DSWR tends to be aligned well perpendicular to the laser polarization and have deeper grooves. By further slowing the velocity from 0.4 mm/s to 0.1 mm/s, at the first beginning, NSWR is found to be newly-formed and overlapped on DSWR for v = 0.4 mm/s and v = 0.3 mm/s, and then NSWR once again disappear with only DSWR left for v = 0.2 mm/s and v = 0.1 mm/s. More importantly, the remained DSWR seem to arrange more regularly along the direction perpendicular to laser polarization. Finally, for the slowest velocity between 0.01 mm/s and 0.05 mm/s, superficial DSWR in the center section of the ablation line are gradually damaged and be replaced with a deep trench. By comparing the two-peak birefringence signal profiles of ablation lines in Fig. 3 (E = 2.5–4.0 J) and Fig. 5 (v = 0.01–0.05 mm/s) with the corresponding morphologies in Figs. 4 and 6, whether the pulse energy or the scanning velocity was changed, the form birefringence effect is in fact originate from the formation of DSWR with the period of ∼200 nm rather than NSWR. Sung Hoon Kim once reported the fabrication of parallel deep nanogrooves with periodicity of 200 nm on 6H-SiC by femtosecond laser ablation ( = 785 nm) and employed a focused ion beam to etch the laserablated nanogroove region. More importantly, they found that the nanogrooves have a depth of at least 15 m along the laser propagation direction [14]. Hence, it is reasonable that the DSWR inscribed in this paper is actually a 200 nm-period nanograting consisting of periodic nanoplanes. The retardance of form birefringence is determined by nd, where n = ne − no stands for the refractive index difference between exordinary light and ordinary light, and d represents the thickness of nanograting. Nanograting reported here can be considered to be formed by periodically alternative arrangement of unmodified 6H-SiC nanoplane with width w1 and modified material nanoplane with width w2 . While w1 w2 (satisfied well in the nanograting here), the refractive index difference n of nanograting is approximated as [15,16]: n ≈
2
(n1 − n2 )2 (n1 − n2 )3 + n1 n1 2
w2
(1)
where = w1 + w2 is the period of nanogratings; n1 and n2 is respectively the refractive index of unmodified 6H-SiC nanoplane and modified-material nanoplane. Therefore, the refractive index difference n of nanograting is determined not only by the assembly geometry but also by the material composition of the nanoplanes of nanograting. The micro-Raman spectra of the ablation lines filled with DSWR are obtained at excitation power of 5 mW and 10 mW, and are compared with that of the unirradiated 6H-SiC, as shown in Fig. 7. The transverse optic phonon modes at 766 cm−1 and 788 cm−1 as well as longitudinal optic phonon modes at 968 cm−1 are present as sharp peaks in all Raman profiles, belonging to c-6H-SiC[17]. Although the spectrum of DSWR region seems the same as that of unirradiated sample at the excitation power of 5 mW, a weak broad peak around 1619 cm−1 , which can be ascribed to D’ peak of graphite [18], newly appears by increasing excitation power to 10 mW. Besides, peaks around 504 cm−1 becomes wider with center shifted toward lower wavenumber in comparison to that for 5 mW excitation power. According to Ref. [17], Raman peak around 504 cm−1 can be attributed to A1-FLA mode of c-6H-SiC. We think that the left shift of peak around 504 cm−1 is because it is overlapped by a newly-emerging weak broad peak around 480 cm−1 , which was reported to be from a-Si [19]. Hence, we propose that the modified-material nanoplanes of nanograting most possibly consist of a-Si and graphite with certain ratio. We also measured the duty cycle w1 / of nanogratings as a function of scanning velocity according to Fig. 6, as shown in Fig. 8. It is clear that duty cycle roughly decreases slightly with deceasing scanning
667
Fig. 7. Raman spectra of the ablation traces filled with DSWR and the unirradiatedablation traces.
velocity, leading to the increase of w2 / in Eq. (1) and the resulting increase of refractive index difference n. However, it is noticed that the slight increase of duty cycle is not enough to explain the multiplying growth of birefringence intensity with scanning velocity decrease in Fig. 5. Therefore, the change of birefringence signal (namely, retardance nd) with the scanning velocity is supposed to result from the increased thickness of the nanogratings. We can also estimate the refractive index range of modified nanoplanes. As stated above, the modified nanoplanes of nanogratings most possibly consists of a-Si and graphite. The refractive index of graphite, a-SiC, c-SiC was respectively 2.46, 3.1 and 2.6, which means that n2 in Eq. (1) may be between 2.46 and 3.1 and n1 is 2.6. When n2 was set as the lowest value of 2.46, for producing the retardance ranging from 10 nm to 60 nm (quantitatively measured by 546 nm light), thickness of nanogratings calculated by Eq. (1) must be between 2.25 m and 13.5 m. The upper limit of the calculated thickness range of nanograting is close to the value of 15 m reported by Sung Hoon Kim. Hence, it is reasonable that the relative refractive index difference |n1 −n2 |/n1 of modified nanoplanes to unmodified nanoplanes is larger than 5.3%. On the other hand, Gaussian-like single-peak retardance profiles of ablation traces in Fig. 3 (E = 1.1–2.3 J) and Fig. 5 (v = 0.9–0.1 mm/s) are believed to result from variation of nanograting thickness with local laser fluence which is higher in the center and lower at the lateral edges. Similarly, when pulse energy is increased, nanograting thickness
Fig. 8. The plot of duty cycle w1 / versus scanning velocity.
668
J. Song et al. / Applied Surface Science 363 (2016) 664–669
enhanced electrical conduction, making it promising that highintegrity photoelectric functional device on a single 6H-SiC chip is fabricated by femtosecond laser irradiation.
Acknowledgements The authors greatly appreciate the laser ablation experimental contributions from Ye Dai. Thanks for the experimental conditions provided by the laboratory of ultrafast photonics of Shanghai University. Thanks for the birefringence characterization device provided by Optoelectronics Research Centre of university of Southampton in UK. Thanks for the financial support from the National Natural Science Foundation of China (Grant nos. 61205128, 60908007 and 11374316), the Research Foundation for Advanced Talents of Jiangsu University (no. 09JDG022) and Shanghai Municipal Natural Science Foundation (no. 13ZR1414800).
Fig. 9. Variation of transmission with the rotation angle of DSWR structure in light attenuator.
is also supposed to be enhanced for higher laser fluence. Once the pulse energy is increased till NSWR appears in the central section of ablation lines, surficial DSWR in the center will be destroyed or erased making its total thickness shortened. This is why the valley of the two-peak profile still keeps a certain amount of retardance, although lower than the retardance at the lateral parts. Finally, a laser-modified zone with 200nm-period DSWR filling in an area of 4 mm × 4 mm were prepared by scanning 6H-SiC surface line by line with line spacing about 8 m. Every line was inscribed by laser beam with single pulse energy of E = 1.6 J and the scanning velocity of 0.6 mm/s. The sample with this kind of large-area DSWR on surface was then mounted on a rotatable holder and sandwiched between two crossed polarizers to form an attenuator. Linearly-polarized femtosecond laser beam with 800 nm wavelength passed through the attenuator and finally reached the laser powermeter. The transmission T of laser beam, which is defined as dividing the laser power Pout after the attenuator by the incident laser power Pin , is shown as a function of the rotation angle of sample with respect to the light-extinction position in Fig. 9. It is shown that the profile in Fig. 9 accords well with the theoretical prediction of T = 1/2 sin2 2 1 − cos ı [16], indicating the perfect birefringence characteristics of DSWR structure on 6H-SiC. Besides, the retardance can be deferred from the obtained maximum transmission of 0.31 to be about 147 nm, which is close to the measured retardance of 135 nm using 546 nm light as testing source. Fig. 9 demonstrates a practicable attenuation with adjustable attenuating rate from 69% to 100%. Much-wider attenuating range is possibly obtained by further optimizing the laser microprocessing parameters or inscribing DSWR structures on both upper surface and lower surface to increase the retardance. 4. Conclusion In summary, birefringence effect was found to be present on laser-induced surface nanostructures on 6H-SiC crystal. Comparison of retardance of nanostructures with their morphologies indicates that DSWR rather than NSWR is responsible for birefringence effect. The retardance of DSWR can be adjusted by single pulse energy and scanning velocity. Our research extend the material type demonstrating birefringence effect after laser ablation from fused silica, borosilicate glass, germanium dioxide glass to 6H-SiC crystal [20–22]. Besides, the birefringence effects of lasermodified 6H-SiC is a valuable supplement to previously-reported
References [1] M. Birnbaum, Semiconductor surface damage produced by ruby lasers, J. Appl. Phys. 36 (1965) 3688–3689. [2] J.E. Sipe, Jeff F. Young, J.S. Preston, H.M. van Driel, Laser-induced periodic surface structure. I. Theory, Phys. Rev. B: Condens. Matter 27 (1983) 1141–1154. [3] M. Huang, F. Zhao, Y. Cheng, N. Xu, Z. Xu, Origin of laser-induced near-subwavelength ripples: interference between surface plasmons and incident laser, ACS Nano 3 (2009) 4062–4070. [4] T.Q. Jia, H.X. Chen, M. Huang, F.L. Zhao, J.R. Qiu, R.X. Li, Z.Z. Xu, X.K. He, J. Zhang, H. Kuroda, Formation of nanogratings on the surface of a ZnSe crystal irradiated by femtosecond laser pulses, Phys. Rev. B: Condens. Matter 72 (2005) 125429. [5] O. Varlamova, F. Costache, J. Reif, M. Bestehorn, Self-organized pattern formation upon femtosecond laser ablation by circularly polarized light, Appl. Surf. Sci. 252 (2006) 4702–4706. [6] J. Bonse, J. Krüger, S. Höhm, A. Rosenfeld, Femtosecond laser-induced periodic surface structures, J. Laser Appl. 24 (2012) 241–247. [7] P. Xiong, T. Jia, X. Jia, D. Feng, S. Zhang, L. Ding, Z. Sun, J. Qiu, Z. Xu, Ultraviolet luminescence enhancement of ZnO two-dimensional periodic nanostructures fabricated by the interference of three femtosecond laser beams, New J. Phys. 13 (2011) 023044. [8] Q.Z. Zhao, F. Ciobanu, S. Malzer, L.J. Wang, Enhancement of optical absorption and photocurrent of 6H-SiC by laser surface nanostructuring, Appl. Phys. Lett. 91 (2007) 1107. [9] V. Zorba, L. Persano, D. Pisignano, et al., Making silicon hydrophobic: wettability control by two-lengthscale simultaneous patterning with femtosecond laser irradiation, Nanotechnology 17 (13) (2006) 3234. [10] Y.N. Miyazaki, K.J. Kiuchi, Control of tribological properties of diamond-like carbon films with femtosecond-laser-induced nanostructuring, Appl. Surf. Sci. 254 (2008) 2364–2368. [11] A.Y. Vorobyev, C. Guo, Colorizing metals with femtosecond laser pulses, Appl. Phys. Lett. 92 (2008) 041914. [12] R. Drevinskas, M. Beresna, M. Geceviˇcius, M. Khenkin, A.G. Kazanskii, I. ˙ G. Niaura, O.I. Konkov, E.I. Terukov, Y.P. Svirko, P.G. Kazansky, Matulaitiene, Giant birefringence and dichroism induced by ultrafast laser pulses in hydrogenated amorphous silicon, Appl. Phys. Lett. 106 (2015) 171106. [13] S.H. Kim, K.H. Byun, I.B. Sohn, S.H. Jeong, Progressive formation of fine and coarse ripples on SiC surface by repeated irradiation of femtosecond laser pulses, Appl. Phys., B 113 (2013) 395–402. [14] S.H. Kim, I.B. Sohn, S. Jeong, Fabrication of uniform nanogrooves on 6H-SiC by femtosecond laser ablation, Appl. Phys., A 102 (2011) 55–59. [15] R. Taylor, C. Hnatovsky, E. Simova, Applications of femtosecond laser induced self-organized planar nanocracks inside fused silica glass, Laser Photonics Rev. 2 (2008) 26. [16] L.P.R. Ramirez, M. Heinrich, S. Richter, F. Dreisow, R. Keil, A.V. Korovin, U. Peschel, S. Nolte, A. Tünnermann, Tuning the structural properties of femtosecond-laser-induced nanogratings, Appl. Phys., A 100 (2010) 1–6. [17] S. Nakashima, H. Harima, T. Tomita, T. Suemoto, Raman intensity profiles of folded longitudinal phonon modes in SiC polytypes, Phys. Rev. B: Condens. Matter 62 (2000) 16605. [18] K. Haubner, J. Murawski, P. Olk, L.M. Eng, C. Ziegler, B. Adolphi, E. Janhne, The route to functional graphene oxide, ChemPhysChem, A 11 (2010) 2131–2139. [19] M. Yamaguchi, S. Ueno, R. Kumai, K. Kinoshita, T. Murai, T. Tomita, S. Matsuo, S. Hashimoto, Raman spectroscopic study of femtosecond laser-induced phase transformation associated with ripple formation on single-crystal SiC, Appl. Phys., A 99 (2010) 23.
J. Song et al. / Applied Surface Science 363 (2016) 664–669 [20] E. Bricchi, B.G. Klappauf, P.G. Kazansky, Form birefringence and negative index change created by femtosecond direct writing in transparent materials, Opt. Lett. 29 (2004) 119–121. [21] S. Richter, C. Miese, S. Döring, F. Zimmermann, M.J. Withford, A. Tünnermann, S. Nolte, Laser induced nanogratings beyond fused silica-periodic
669
nanostructures in borosilicate glasses and ULETM , Opt. Mater. Express 3 (2013) 1161–1166. [22] F. Zhang, H. Zhang, G. Dong, J. Qiu, Embedded nanogratings in germanium dioxide glass induced by femtosecond laser direct writing, JOSA B 31 (2014) 860–864.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Surface birefringence of self-assembly periodic nanostructures induced on 6H-SiC surface by femtosecond laser Juan Song a,∗ , Ye Dai b , Wenjun Tao a , Min Gong b , Guohong Ma b , Quanzhong Zhao c , Jianrong Qiu d a
School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, China Physics Department, Shanghai University, Shanghai 200444, China c State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China d Key Laboratory of Specially Functional Materials of Ministry of Education and Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510640, China b
a r t i c l e
i n f o
Article history: Received 19 June 2015 Received in revised form 21 November 2015 Accepted 13 December 2015 Available online 15 December 2015 Keywords: Deep-subwavelength periodic ripples Birefringence Retardance Attenuator
a b s t r a c t In this paper, we report the birefringence effect of surface self-assembly periodic nanostructures induced on 6H-SiC by femtosecond laser irradiation. Birefringence characteristic (e.g. cross-polarized image), measured by cross polarized microscopy, was found to be controlled by both single pulse energy and scanning velocity. Comparing birefringence measurement results of nanostructures and morphology characterization by Scanning electron microscopy, it is shown that ∼200 nm-period deep-subwavelength periodic ripples (DSWR) plays a dominating role in the birefringence effect. Raman spectra show that the change of retardance with pulse energy and scanning velocity is most possibly caused by the thickness variation of DSWR. Finally, a light attenuator based on a single layer of DSWR structure on 6H-SiC surface was constructed and tested by light source of 800 nm to have a tunable attenuating ratio of 69–100%. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Study on femtosecond laser induced periodic surface structures (LIPSS) has developed for at least two decades since the pioneering works of Birnbaum [1]. In the early years, more interest was put on analysis of the periodicity origin of LIPSS. The classical near-subwavelength ripples (NSWR) are widely accepted to result from the interaction between the incident light and the surface scattering wave or the surface plasmon wave [2,3]. In recent years, deep-subwavelength ripples (DSWR) were reported on several semiconductor materials as well as metal materials and are considered to be induced by second harmonic wave or selforganization [4–6]. Studies on LIPSS are not only important for deeply understanding of laser-material interaction process but also valuable for developing or improving material functions. For example, LIPSS have been used for luminescence enhancement, optical absorption enhancement, photocurrent enhancement, wettability modification, friction controlling, material colorization and so on
∗ Corresponding author. Tel.: +086 139 1280 8637; fax: +086 0511 88791288. E-mail address: [email protected] (J. Song). http://dx.doi.org/10.1016/j.apsusc.2015.12.096 0169-4332/© 2015 Elsevier B.V. All rights reserved.
[7–11]. Recently, periodic surface nanostructures on hydrogenated amorphous silicon (a-Si: H) induced by either a femtosecond or a picosecond laser, was reported to show giant birefringence and dichroism. It is the first report for the form birefringence of LIPSS [12]. Form birefringence, depending on periodic refractive index modulation, is supposed to be more pronounced on high-refractive-index material. The 6H-SiC polytypes, with refractive index n = 2.6, is a promising photoelectric material for the advantages of wide bandgap, high thermal conductivity, and stable electrical performance at high temperature. Typical LIPSS, near-subwavelength ripples (NSWR) of 700 nm and deepsubwavelength ripples (DSWR) of 200 nm, have already been reported on 6H-SiC [13]. Optical absorption and photocurrent enhancement of femtosecond-laser-induced ripples on 6H-SiC was also reported [8]. In this paper, birefriengence effect of LIPSS induced by femtosecond laser on 6H-SiC was studied. Birefringence retardance was found to be controlled by single pulse energy and scanning velocity used in laser processing. Raman spectra of laser-induced nanostructures on 6H-SiC surface and effective medium theory were employed to analyze the variation of retardance with laser parameters. A light attenuator based on birefringence characteristics of 6H-SiC surface nanostructures was tested.
J. Song et al. / Applied Surface Science 363 (2016) 664–669
665
Fig. 1. Top-view scanning electron microscopy images of the ablation lines inscribed by laser beams with different polarization azimuth angles.
Fig. 2. Birefringence signal of the structures for different polarization azimuth angles measured by orthogonal polarizing microscope: (a) transmission intensity image captured by CCD after Cross-polarizer; (b) variation of transmission intensity with polarization azimuth angle.
2. Experiments A Ti: sapphire regeneratively amplified mode-locked laser system launches ultrashort pulses with 800 nm wavelength and 120 fs pulse duration at a repetition rate of 1 kHz. The laser beam was normally focused by a 5× microscopic objective with NA = 0.15 on the polished (0 0 0 1) crystal plane of 6H-SiC crystal, which was mounted on a 3-axis motorized stage driven by a computer. A neutral filter was used for adjusting the single pulse energy. An electronic shutter was adopted for controlling the pulse number for irradiation. A /2 waveplate were used to adjust the polarization plane azimuth of the laser beam for irradiation. The birefringence characteristics were qualitatively measured by crosspolarized light microscopy. While the laser polarization direction was adjusted to be aligned with the transmission axis of the first polarizer of cross-polarized light microscopy, this special azimuth angle of polarization plane at this moment was defined as 0◦ . All ablation lines in this paper were induced by scanning laser focus perpendicular to the laser propagation direction.
light passing through the pair of cross polarizers, demonstrated the birefringence effect of the laser-inscribed structures. Birefringence profile extracted from Fig. 2(a) along the horizontal direction is shown in Fig. 2(b). Birefringence signal intensity is periodically changed with laser polarization azimuth angle, and is roughly symmetric with respect to the polarization azimuth angle of 0◦ . The birefringence retardance for ablation lines at different pulse energies are qualitatively represented by grey level of transmission intensity images captured by CCD after cross-polarizers and demonstrated in Fig. 3. Clearly, the ablation line for E = 0.9 J almost has no birefringence signal. With pulse energy increased from
3. Results and discussion With single pulse energy set as 1.6 J and a scanning velocity of 600 m/s used, the obtained ablation lines for the different polarization azimuth angles from −100◦ to 100◦ was demonstrated in Fig. 1. Clearly, periodic ripples appear on all cases. The ripple orientation is synchronously rotated with the laser polarization direction and keeps always perpendicular to the laser polarization. It is worth noting that the structures ablated by laser pulses with polarization azimuth angles from −60◦ to 60◦ are DSWR with period of ∼200 nm, while the structures for the azimuth angles around ±90◦ are ∼200 nm-period DSWR overlapped by ∼580 nm-period NSWR. The cross-polarized images of the structures in Fig. 1 were shown in Fig. 2(a). Clearly, the phenomenon, that there is still transmission intensity left in some ablation lines after illumination
Fig. 3. Qualitative birefringence characteristics of the ablation lines inscribed by pulses with different energies at velocity of 600 m/s. (a) Birefringence intensity image (b) qualitative birefringence signal profile along the white line. (From left to right, E = 0.9 J, 1.1 J, 1.3 J, 1.5 J, 1.7 J, 1.9 J, 2.1 J, 2.3 J, 2.5 J, 3.0 J, 3.5 J, 4.0 J).
666
J. Song et al. / Applied Surface Science 363 (2016) 664–669
Fig. 4. SEM images for the ablation lines induced by laser pulses with different energies.
1.1 J to 1.9 J, the retardance value grows fast to two times the original magnitude. It is worth noting that retardance profile across each ablation line is basically a Gaussian-like distribution with highest retardance in the center section. While the pulse energy continues to increase from 2.5 J to 4.0 J, the retardance profiles are all two-peak curve with a valley in the center. The retardance of the valley is lower and lower as the pulse energy increases. Fig. 4 shows the morphologies of a set of ablation lines inscribed at a scanning velocity of 600 m/s by laser beams with single pulse energy ranging from 0.9 J to 4.5 J. As can be seen, when the energy is as low as 0.9 J, no obvious structures are left in the ablation line. With single pulse energies increased from 1.1 J to 1.7 J, DSWR with period of about 200 nm start to appear in laser-scanned trace and tend to be more consistently aligned perpendicular to the laser polarization. Once the single pulse energy is beyond 1.7 J, ∼580 nm NSWR begin to form overlapping on the as-formed DSWR and have their grooves deepened.
Fig. 5. Qualitative birefringence characteristics of the structures obtained at different scanning velocities (a) birefringence intensity image captured by cross-polarized microscopy (b) birefringence signal profile along the white line. (from left to right, v = 2.0, 1.5, 1.0, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, 0.05, 0.025, 0.01 mm/s).
Besides, dependence of the birefringence intensity on the scanning velocity was also studied and displayed in Fig. 5, where single pulse energy was set as 1.6 J. It is shown that when scanning velocity is decreased from 2 mm/s to 0.3 mm/s, the birefringence signal is roughly linearly increased by several times. However, when the velocity is decreased lower than 0.3 mm/s, the birefringence signal sharply declines. Also, the birefringence signal profile of each ablation line in velocity range of 0.2 mm/s and 2 mm/s is Gaussianlike single-peak curve, while the retardance profile for each line inscribed at velocities between 0.01 m/s and 0.05 m/s appears as a two-peak curve with a valley in between. The morphologies of the ablation lines induced at different scanning velocities are also characterized by SEM. As shown in Fig. 6, when the velocity is between 1.5 mm/s and 2.0 mm/s, DSWR with shallow grooves are induced, the orientation of which is disrupted. As the velocity was decreased from 0.9 mm/s to 0.5 mm/s,
Fig. 6. SEM images of the ablation trace induced at different scanning velocity.
J. Song et al. / Applied Surface Science 363 (2016) 664–669
DSWR tends to be aligned well perpendicular to the laser polarization and have deeper grooves. By further slowing the velocity from 0.4 mm/s to 0.1 mm/s, at the first beginning, NSWR is found to be newly-formed and overlapped on DSWR for v = 0.4 mm/s and v = 0.3 mm/s, and then NSWR once again disappear with only DSWR left for v = 0.2 mm/s and v = 0.1 mm/s. More importantly, the remained DSWR seem to arrange more regularly along the direction perpendicular to laser polarization. Finally, for the slowest velocity between 0.01 mm/s and 0.05 mm/s, superficial DSWR in the center section of the ablation line are gradually damaged and be replaced with a deep trench. By comparing the two-peak birefringence signal profiles of ablation lines in Fig. 3 (E = 2.5–4.0 J) and Fig. 5 (v = 0.01–0.05 mm/s) with the corresponding morphologies in Figs. 4 and 6, whether the pulse energy or the scanning velocity was changed, the form birefringence effect is in fact originate from the formation of DSWR with the period of ∼200 nm rather than NSWR. Sung Hoon Kim once reported the fabrication of parallel deep nanogrooves with periodicity of 200 nm on 6H-SiC by femtosecond laser ablation ( = 785 nm) and employed a focused ion beam to etch the laserablated nanogroove region. More importantly, they found that the nanogrooves have a depth of at least 15 m along the laser propagation direction [14]. Hence, it is reasonable that the DSWR inscribed in this paper is actually a 200 nm-period nanograting consisting of periodic nanoplanes. The retardance of form birefringence is determined by nd, where n = ne − no stands for the refractive index difference between exordinary light and ordinary light, and d represents the thickness of nanograting. Nanograting reported here can be considered to be formed by periodically alternative arrangement of unmodified 6H-SiC nanoplane with width w1 and modified material nanoplane with width w2 . While w1 w2 (satisfied well in the nanograting here), the refractive index difference n of nanograting is approximated as [15,16]: n ≈
2
(n1 − n2 )2 (n1 − n2 )3 + n1 n1 2
w2
(1)
where = w1 + w2 is the period of nanogratings; n1 and n2 is respectively the refractive index of unmodified 6H-SiC nanoplane and modified-material nanoplane. Therefore, the refractive index difference n of nanograting is determined not only by the assembly geometry but also by the material composition of the nanoplanes of nanograting. The micro-Raman spectra of the ablation lines filled with DSWR are obtained at excitation power of 5 mW and 10 mW, and are compared with that of the unirradiated 6H-SiC, as shown in Fig. 7. The transverse optic phonon modes at 766 cm−1 and 788 cm−1 as well as longitudinal optic phonon modes at 968 cm−1 are present as sharp peaks in all Raman profiles, belonging to c-6H-SiC[17]. Although the spectrum of DSWR region seems the same as that of unirradiated sample at the excitation power of 5 mW, a weak broad peak around 1619 cm−1 , which can be ascribed to D’ peak of graphite [18], newly appears by increasing excitation power to 10 mW. Besides, peaks around 504 cm−1 becomes wider with center shifted toward lower wavenumber in comparison to that for 5 mW excitation power. According to Ref. [17], Raman peak around 504 cm−1 can be attributed to A1-FLA mode of c-6H-SiC. We think that the left shift of peak around 504 cm−1 is because it is overlapped by a newly-emerging weak broad peak around 480 cm−1 , which was reported to be from a-Si [19]. Hence, we propose that the modified-material nanoplanes of nanograting most possibly consist of a-Si and graphite with certain ratio. We also measured the duty cycle w1 / of nanogratings as a function of scanning velocity according to Fig. 6, as shown in Fig. 8. It is clear that duty cycle roughly decreases slightly with deceasing scanning
667
Fig. 7. Raman spectra of the ablation traces filled with DSWR and the unirradiatedablation traces.
velocity, leading to the increase of w2 / in Eq. (1) and the resulting increase of refractive index difference n. However, it is noticed that the slight increase of duty cycle is not enough to explain the multiplying growth of birefringence intensity with scanning velocity decrease in Fig. 5. Therefore, the change of birefringence signal (namely, retardance nd) with the scanning velocity is supposed to result from the increased thickness of the nanogratings. We can also estimate the refractive index range of modified nanoplanes. As stated above, the modified nanoplanes of nanogratings most possibly consists of a-Si and graphite. The refractive index of graphite, a-SiC, c-SiC was respectively 2.46, 3.1 and 2.6, which means that n2 in Eq. (1) may be between 2.46 and 3.1 and n1 is 2.6. When n2 was set as the lowest value of 2.46, for producing the retardance ranging from 10 nm to 60 nm (quantitatively measured by 546 nm light), thickness of nanogratings calculated by Eq. (1) must be between 2.25 m and 13.5 m. The upper limit of the calculated thickness range of nanograting is close to the value of 15 m reported by Sung Hoon Kim. Hence, it is reasonable that the relative refractive index difference |n1 −n2 |/n1 of modified nanoplanes to unmodified nanoplanes is larger than 5.3%. On the other hand, Gaussian-like single-peak retardance profiles of ablation traces in Fig. 3 (E = 1.1–2.3 J) and Fig. 5 (v = 0.9–0.1 mm/s) are believed to result from variation of nanograting thickness with local laser fluence which is higher in the center and lower at the lateral edges. Similarly, when pulse energy is increased, nanograting thickness
Fig. 8. The plot of duty cycle w1 / versus scanning velocity.
668
J. Song et al. / Applied Surface Science 363 (2016) 664–669
enhanced electrical conduction, making it promising that highintegrity photoelectric functional device on a single 6H-SiC chip is fabricated by femtosecond laser irradiation.
Acknowledgements The authors greatly appreciate the laser ablation experimental contributions from Ye Dai. Thanks for the experimental conditions provided by the laboratory of ultrafast photonics of Shanghai University. Thanks for the birefringence characterization device provided by Optoelectronics Research Centre of university of Southampton in UK. Thanks for the financial support from the National Natural Science Foundation of China (Grant nos. 61205128, 60908007 and 11374316), the Research Foundation for Advanced Talents of Jiangsu University (no. 09JDG022) and Shanghai Municipal Natural Science Foundation (no. 13ZR1414800).
Fig. 9. Variation of transmission with the rotation angle of DSWR structure in light attenuator.
is also supposed to be enhanced for higher laser fluence. Once the pulse energy is increased till NSWR appears in the central section of ablation lines, surficial DSWR in the center will be destroyed or erased making its total thickness shortened. This is why the valley of the two-peak profile still keeps a certain amount of retardance, although lower than the retardance at the lateral parts. Finally, a laser-modified zone with 200nm-period DSWR filling in an area of 4 mm × 4 mm were prepared by scanning 6H-SiC surface line by line with line spacing about 8 m. Every line was inscribed by laser beam with single pulse energy of E = 1.6 J and the scanning velocity of 0.6 mm/s. The sample with this kind of large-area DSWR on surface was then mounted on a rotatable holder and sandwiched between two crossed polarizers to form an attenuator. Linearly-polarized femtosecond laser beam with 800 nm wavelength passed through the attenuator and finally reached the laser powermeter. The transmission T of laser beam, which is defined as dividing the laser power Pout after the attenuator by the incident laser power Pin , is shown as a function of the rotation angle of sample with respect to the light-extinction position in Fig. 9. It is shown that the profile in Fig. 9 accords well with the theoretical prediction of T = 1/2 sin2 2 1 − cos ı [16], indicating the perfect birefringence characteristics of DSWR structure on 6H-SiC. Besides, the retardance can be deferred from the obtained maximum transmission of 0.31 to be about 147 nm, which is close to the measured retardance of 135 nm using 546 nm light as testing source. Fig. 9 demonstrates a practicable attenuation with adjustable attenuating rate from 69% to 100%. Much-wider attenuating range is possibly obtained by further optimizing the laser microprocessing parameters or inscribing DSWR structures on both upper surface and lower surface to increase the retardance. 4. Conclusion In summary, birefringence effect was found to be present on laser-induced surface nanostructures on 6H-SiC crystal. Comparison of retardance of nanostructures with their morphologies indicates that DSWR rather than NSWR is responsible for birefringence effect. The retardance of DSWR can be adjusted by single pulse energy and scanning velocity. Our research extend the material type demonstrating birefringence effect after laser ablation from fused silica, borosilicate glass, germanium dioxide glass to 6H-SiC crystal [20–22]. Besides, the birefringence effects of lasermodified 6H-SiC is a valuable supplement to previously-reported
References [1] M. Birnbaum, Semiconductor surface damage produced by ruby lasers, J. Appl. Phys. 36 (1965) 3688–3689. [2] J.E. Sipe, Jeff F. Young, J.S. Preston, H.M. van Driel, Laser-induced periodic surface structure. I. Theory, Phys. Rev. B: Condens. Matter 27 (1983) 1141–1154. [3] M. Huang, F. Zhao, Y. Cheng, N. Xu, Z. Xu, Origin of laser-induced near-subwavelength ripples: interference between surface plasmons and incident laser, ACS Nano 3 (2009) 4062–4070. [4] T.Q. Jia, H.X. Chen, M. Huang, F.L. Zhao, J.R. Qiu, R.X. Li, Z.Z. Xu, X.K. He, J. Zhang, H. Kuroda, Formation of nanogratings on the surface of a ZnSe crystal irradiated by femtosecond laser pulses, Phys. Rev. B: Condens. Matter 72 (2005) 125429. [5] O. Varlamova, F. Costache, J. Reif, M. Bestehorn, Self-organized pattern formation upon femtosecond laser ablation by circularly polarized light, Appl. Surf. Sci. 252 (2006) 4702–4706. [6] J. Bonse, J. Krüger, S. Höhm, A. Rosenfeld, Femtosecond laser-induced periodic surface structures, J. Laser Appl. 24 (2012) 241–247. [7] P. Xiong, T. Jia, X. Jia, D. Feng, S. Zhang, L. Ding, Z. Sun, J. Qiu, Z. Xu, Ultraviolet luminescence enhancement of ZnO two-dimensional periodic nanostructures fabricated by the interference of three femtosecond laser beams, New J. Phys. 13 (2011) 023044. [8] Q.Z. Zhao, F. Ciobanu, S. Malzer, L.J. Wang, Enhancement of optical absorption and photocurrent of 6H-SiC by laser surface nanostructuring, Appl. Phys. Lett. 91 (2007) 1107. [9] V. Zorba, L. Persano, D. Pisignano, et al., Making silicon hydrophobic: wettability control by two-lengthscale simultaneous patterning with femtosecond laser irradiation, Nanotechnology 17 (13) (2006) 3234. [10] Y.N. Miyazaki, K.J. Kiuchi, Control of tribological properties of diamond-like carbon films with femtosecond-laser-induced nanostructuring, Appl. Surf. Sci. 254 (2008) 2364–2368. [11] A.Y. Vorobyev, C. Guo, Colorizing metals with femtosecond laser pulses, Appl. Phys. Lett. 92 (2008) 041914. [12] R. Drevinskas, M. Beresna, M. Geceviˇcius, M. Khenkin, A.G. Kazanskii, I. ˙ G. Niaura, O.I. Konkov, E.I. Terukov, Y.P. Svirko, P.G. Kazansky, Matulaitiene, Giant birefringence and dichroism induced by ultrafast laser pulses in hydrogenated amorphous silicon, Appl. Phys. Lett. 106 (2015) 171106. [13] S.H. Kim, K.H. Byun, I.B. Sohn, S.H. Jeong, Progressive formation of fine and coarse ripples on SiC surface by repeated irradiation of femtosecond laser pulses, Appl. Phys., B 113 (2013) 395–402. [14] S.H. Kim, I.B. Sohn, S. Jeong, Fabrication of uniform nanogrooves on 6H-SiC by femtosecond laser ablation, Appl. Phys., A 102 (2011) 55–59. [15] R. Taylor, C. Hnatovsky, E. Simova, Applications of femtosecond laser induced self-organized planar nanocracks inside fused silica glass, Laser Photonics Rev. 2 (2008) 26. [16] L.P.R. Ramirez, M. Heinrich, S. Richter, F. Dreisow, R. Keil, A.V. Korovin, U. Peschel, S. Nolte, A. Tünnermann, Tuning the structural properties of femtosecond-laser-induced nanogratings, Appl. Phys., A 100 (2010) 1–6. [17] S. Nakashima, H. Harima, T. Tomita, T. Suemoto, Raman intensity profiles of folded longitudinal phonon modes in SiC polytypes, Phys. Rev. B: Condens. Matter 62 (2000) 16605. [18] K. Haubner, J. Murawski, P. Olk, L.M. Eng, C. Ziegler, B. Adolphi, E. Janhne, The route to functional graphene oxide, ChemPhysChem, A 11 (2010) 2131–2139. [19] M. Yamaguchi, S. Ueno, R. Kumai, K. Kinoshita, T. Murai, T. Tomita, S. Matsuo, S. Hashimoto, Raman spectroscopic study of femtosecond laser-induced phase transformation associated with ripple formation on single-crystal SiC, Appl. Phys., A 99 (2010) 23.
J. Song et al. / Applied Surface Science 363 (2016) 664–669 [20] E. Bricchi, B.G. Klappauf, P.G. Kazansky, Form birefringence and negative index change created by femtosecond direct writing in transparent materials, Opt. Lett. 29 (2004) 119–121. [21] S. Richter, C. Miese, S. Döring, F. Zimmermann, M.J. Withford, A. Tünnermann, S. Nolte, Laser induced nanogratings beyond fused silica-periodic
669
nanostructures in borosilicate glasses and ULETM , Opt. Mater. Express 3 (2013) 1161–1166. [22] F. Zhang, H. Zhang, G. Dong, J. Qiu, Embedded nanogratings in germanium dioxide glass induced by femtosecond laser direct writing, JOSA B 31 (2014) 860–864.