- Email: [email protected]
Laser-assisted nanostructuring of metal films by means of a fibre dielectric microprobe
+
MODEL
Available online at www.sciencedirect.com
H O S T E D BY
ScienceDirect Pacific Science Review xx (2015) 1e6 www.elsevier.com/locate/pscr
Laser-assisted nanostructuring of metal films by means of a fibre dielectric microprobe Alexander A. Kuchmizhak a,*, Oleg B. Vitrik a,b, Yuri N. Kulchin a,b a
Optoelectronics Lab., Institute of Automation and Control Processes, FEB RAS, Russia b Far Eastern Federal University, FEFU, Russia Available online ▪ ▪ ▪
Abstract A simple apertureless dielectric microprobe in the form of a section of the tapered optical fibre was proposed for surface laser nanomodification. This probe enables surface l/2-localisation of laser beam, as demonstrated both numerically and experimentally. The controllable formation of single through nanoholes with the minimum size down to 35 nm (~l/15) in the 50-nm Au/Pd film was shown using this probe and a 532-nm pump nanosecond laser. We also report for the first time on the formation of micro- and nanobumps, jet-like microstructures and microholes on optically thick gold films using single nanosecond laser pulses focused through the fibre dielectric apertureless probe. Both the shape and the sizes of the obtained microstructures were demonstrated to be determined by the pulse energy and film thickness. Copyright © 2014, Far Eastern Federal University, Kangnam University, Dalian University of Technology, Kokushikan University. Production and Hosting by Elsevier B.V. All rights reserved.
Keywords: Laser-assisted nanostructuring; Nanosecond laser pulses; Fibre apertureless microprobe
Introduction Nanoscale interaction of short and ultrashort laser pulses with solid surfaces is of consistently growing scientific and technological interest. Such interaction typically results in the fabrication of single and periodic surface nanofeatures [1,2], bringing new optical * Corresponding author. E-mail addresses: [email protected] (A.A. Kuchmizhak), [email protected] (O.B. Vitrik), [email protected] (Y.N. Kulchin). Peer review under responsibility of Far Eastern Federal University, Kangnam University, Dalian University of Technology, Kokushikan University.
properties to the nanomodified surfaces [3]. Sub-100nm surface nanofeatures are efficiently fabricated by spatial localisation of a driving laser electric field transmitted through an exit hole of a scanning nearfield optical microscope (SNOM) probe [4,5]. Such SNOM probes are composed of a single-mode optical fibre (OF) with a conically tapered and metal-coated tip, ending up with a nanoscale exit aperture. A SNOM probe can be moved along the modified surface with a high positioning accuracy of tens nanometre. However, the low optical transmittance through the exit nanoaperture of the SNOM probe (~106 at the aperture diameter D ¼ 50 nm) [6] decreases the transmitted intensity to well below the modification
http://dx.doi.org/10.1016/j.pscr.2014.08.012 1229-5450/Copyright © 2014, Far Eastern Federal University, Kangnam University, Dalian University of Technology, Kokushikan University. Production and Hosting by Elsevier B.V. All rights reserved. Please cite this article in press as: A.A. Kuchmizhak et al., Laser-assisted nanostructuring of metal films by means of a fibre dielectric microprobe, Pacific Science Review (2015), http://dx.doi.org/10.1016/j.pscr.2014.08.012
+ 2
MODEL
A.A. Kuchmizhak et al. / Pacific Science Review xx (2015) 1e6
threshold for most of materials. This problem cannot be solved by merely increasing the input energy because this results in strong heating and degradation of the tip due to the high dissipative losses in its metallic coating [7]. Consequently, the output aperture diameter of the SNOM probe, which is used for the surface modification with high energy pulses, typically cannot be made smaller than ~l [4], where l is the laser wavelength. Another laser-based method of formation of surface nanostructures with high spatial resolution consists of using the effect of electromagnetic field amplification near the nanoscale tip [6,8]. An atomic-force microscope probe irradiated with the external laser source can be used, e.g., a nanosized tip [9e11]. A record lateral resolution of ~10 nm was obtained [9] using this method with femtosecond laser pulses. However, the vertical resolution is also limited by this value, which restricts the application of this method. As a result, high-NA focussing optics are still frequently used to localise laser fields on a diffraction scale ~l/2. Nevertheless, sub-100-nm features can be fabricated in such a case via non-linear near-threshold effects (e.g., laser modification or ablation), multibeam interference and excitation of surface plasmon-polaritons [1e3]. However, these methods require additional diffractive optical elements to obtain a homogeneous focal spot with the required energy distribution. In addition, the addressing of light to a specific point on the sample surface with nanometre precision is significantly complicated because of the rather large dimensions of the far-field focussing element. One potential solution for sub-100-nm surface nanostructuring could be the combination of the high SNOM positioning accuracy and the high transmittance of far-field optics. In particular, a standard SNOM device can be equipped with a piece of a single-mode fibre with a conical output tip (an apertureless dielectric microprobe, ADM) [12e15]. The homogeneous Gaussian-like spatial distribution of the fibre fundamental mode does not require correction and is well suited for further focussing of the transmitted laser energy, due to total internal reflection at the “dielectric/air” interface, into a l/2-spot [12]. Such a probe differs from its apertured SNOM analogue only by the absence of the metallic coating on its tip and exhibits significantly higher optical damage threshold and transmittance than the aperture SNOM probe. As a result, ADMs have been used in laser ablation, photochemical etching, laser-induced breakdown spectroscopy [13e15], etc. However, the
fabrication process of the ADM tip is usually very complex [12]. In this paper, we present a simple fibre dielectric probe in the form of a section of the tapered optical fibre with a tip in the form of a truncated cone, as well as demonstrate the applicability of the developed ADM for precise laser-assisted nanostructuring of metal films surfaces and fabrication of various nanostructures with high spatial resolution. Experimental Linearly polarized second-harmonic (l ¼ 532 nm) pulses of a Nd:YAG laser (Solar LS LQ215) with the FWHM-pulsewidth ~7 ns and maximum energy E < 10 mJ in the TEM00-mode were used for surface nanostructuring (Fig. 1(a)). Each p-polarized laser pulse was focused onto the sample surface (Fig. 1(a)) by means of an apertureless dielectric microprobe (ADM), a tapered 20-mm long section of a singlemode optical fibre (Thorlabs SM400) with a constant taper angle ~12 (Fig. 1(b)) and a flat endface with a diameter ~250 nm [16,17]. In accordance to our finitedifference time-domain (FDTD) simulation such an ADM provides spatial filtering and focussing of a laser beam (Fig. 1(c)), resulting in a diffraction-limited output spot (R1/e z 0.3 mm at normal incidence) with a nearly Gaussian spatial profile and relatively deep focal depth (~l/2) [16]. The laser pulses were effectively coupled to the ADM using a fibre coupler (Thorlabs MBT612D/M). During laser nanostructuring, the ADM tip was located 50 nm above the sample surface and inclined at an angle of 60 (inset in Fig. 1) to the sample surface normal, yielding an elliptical Gaussian spot with Rx,1/e z 0.65 mm and Ry,1/e z 0.4 mm. The probe-to-sample distance was controlled using the tuning fork feedback. Visual control of the ADM motion and observation of the Au/Pd film damage was performed by means of a high-resolution optical microscope Hirox KH7700 (optical magnification ~700e7000, working distance ~3.4 mm). The laser pulse energy entering the fibre was varied by means of a polarizing attenuator. To measure the pulse energy E at the ADM tip, the output laser radiation was collected by means of an objective (Olympus, NA ¼ 0.65) and focused onto a photodetector (J10SI-HE Energy Sensor, Coherent EPM2000). The laser-structured films were characterised using an atomic force microscope (AFM, NanoDST Pacific Nanotechnology) in the close-contact mode and using a scanning electron microscope (Hitachi S3400). All
Please cite this article in press as: A.A. Kuchmizhak et al., Laser-assisted nanostructuring of metal films by means of a fibre dielectric microprobe, Pacific Science Review (2015), http://dx.doi.org/10.1016/j.pscr.2014.08.012
+
MODEL
A.A. Kuchmizhak et al. / Pacific Science Review xx (2015) 1e6
3
Fig. 1. (a) Schematic of the experimental setup for laser-assisted surface nanostructuring. (b) Electron images of the tapered ADM (top) and its 250-nm wide truncated flat endface (bottom). (c) Calculated TE electric field (Еr) distribution at the ADM tip apex for the truncated endface diameter ~250 nm.
of the experiments were performed under ambient conditions. Results and discussion Nanoholes Using the developed ADM for laser pulse focussing, we demonstrated for the first time controllable fabrication of nanoscale through-holes in 50-nm thick Au/Pd film (80/20 wt.%) on the silica substrate under the single-pulse irradiation [17]. Nanoscale through-holes fabricated in the Au/Pd film at variable laser pulse energies above the threshold value Eth are shown in
Fig. 2(a), with a minimal nanohole (marked with (2) at Fig. 2(a)) diameter of 35 nm (~l/15 at l ¼ 532 nm). Fig. 2(b) shows the dependence of the squared nanohole diameter D2 on the pulse energy ln(E) where the linear curve slope represents the squared characteristic Gaussian diameter s21/е of the surface energy distribution. This diameter s1/е ¼ 1.38 mm is several times larger than the ADM output beam 1/е-diameter D1/е ¼ 0.4 mm. Such a large increase in the laser energy deposition scale results from the lateral heat diffusion in the metallic film, which is supported by the thermally isolating glass substrate, during the heating ns-laser pulse. Fig. 2(c) shows the AFM profiles of the nanoholes with the diameters of approximately 95 nm and 35 nm.
Please cite this article in press as: A.A. Kuchmizhak et al., Laser-assisted nanostructuring of metal films by means of a fibre dielectric microprobe, Pacific Science Review (2015), http://dx.doi.org/10.1016/j.pscr.2014.08.012
+ 4
MODEL
A.A. Kuchmizhak et al. / Pacific Science Review xx (2015) 1e6
Fig. 2. (a) SEM image of the surface nanofeatures e nanoholes (1) and nanohillocks (2), fabricated at decreasing pulse energies (the scale bar corresponds to 100 nm). (b) Dependence of the squared nanohole diameter D2 on ln(E). (c) AFM-profiles of the holes inside the features (1) and (2). (d) AFM image of the periodic array of 90-nm wide nanoholes fabricated in the 50-nm Au/Pd film (the interhole spacing and the scale bar are 500 nm). (e) SEM image of the fibre microaxicon probe (left) coated with the 60-nm thick Au/Pd film and a 100-nm wide nanohole (right) fabricated at its tip by single ns-pulse with E ¼ 4 nJ focused through the ADM.
In accordance with a high contrast in the SEM images (Fig. 2(a)) both features are the through holes. However, as seen from Fig. 2(c) the 35-nm wide hole cannot be profiled with the available 20-nm wide AFM tip. Additionally, one can see that the holes are surrounded by a 200-nm wide and 25-nm height crater. Apparently, one can provide more smooth nanoholes by using of a femtosecond laser pulses because it minimises the heat transport in the Au/Pd film. Additionally, to demonstrate the of the developed nanostructuring method, we fabricated a square regular array of nanoholes with the mean diameter of 90 ± 20 nm, which was produced in the Au/Pd film using the ADM arrangement (Fig. 2(d)). Finally, to demonstrate the possibility of the ADM for local
precise nanostructure fabrication, a single through nanohole with the diameter as small as 100 nm was fabricated at the extremity of the Au-coated (metal film thickness z60 nm) fibre microaxicon, an analogue of the SNOM probe. The nanohole fabrication process in this case is apparently governed by lateral heat diffusion in the metallic film and the centre-symmetrical lateral expulsion of the melt by its vapour recoil pressure. The details of through nanoholes formation in optically “thick” metal films can be found in [17,18]. Thus, the results presented in this section indicated the possibility of using the developed probe for precise laserassisted milling of single and a periodic array of nanoholes in the opaque metal film. We believe that the
Please cite this article in press as: A.A. Kuchmizhak et al., Laser-assisted nanostructuring of metal films by means of a fibre dielectric microprobe, Pacific Science Review (2015), http://dx.doi.org/10.1016/j.pscr.2014.08.012
+
MODEL
A.A. Kuchmizhak et al. / Pacific Science Review xx (2015) 1e6
5
Fig. 3. (a) SEM images of the micro- and nanobumps fabricated on the surface of 100-nm thick Au film by single pulses at increasing pulse energy E. (b) AFM image of the periodic array of nanobumps on the 50-nm thick Au film. (c) SEM image of the nanobumps arranged into the “IACP” letters (scale bar corresponds to 5 mm).
use of advanced fibre probes [18] as well as combined with femtosecond lasers pulses can provide further lateral size and heat-affected zone minimisation for the fabricated nanofeatures. Micro- and nano-bumps Using the developed ADM, we have also demonstrated the fabrication of micro- and nanobumps on optically thick gold films using single nanosecond laser pulses [19]. Fig. 3(a) shows the SEM images of nanobumps fabricated on the surface of a 100-nm thick Au film using single pulses at increasing pulse energy E. The visible modification of Au film starts at pulse energy E 1 nJ. At E z 1.2 nJ (Fig. 3(a)) the nanobumps consist of the set of nanosized protrusions of up to 30 nm in height. For increasing pulse energies, these protrusions merge into the single nanobumps (Fig. 3(a)). As seen, both the height and the diameter of nanobumps increase with increasing pulse energy E.
The nanobump reaches its maximal sizes (up to hbump ~ 530 nm and dbump ~ 2 mm) at E z 1.7 nJ. When the pulse energy reaches ~2 nJ, the nanobump collapses at its bottom forming an asymmetric jet-like microstructure (not shown here). The shapes of such microstructures are poorly reproducible at energies in the range from 2 to 3 nJ, which apparently can be explained by the oblique laser irradiation of the film surface. Note that similar nanofeatures were fabricated using tightly focused femtosecond laser pulse [20,21]; however, the fabrication of nanobumps under the single nanosecond-pulse irradiation was performed for the first time, apparently due to the extremely high focussing performance of the developed ADM. Fig. 3(bec) shows the 3 3 array of the nanobumps on the 50-nm thick supported Au film and “IACP” demonstrating the reproducibility of the nanofeatures fabrication using the developed ADM. The small difference in nanobump height can be explained by the low pulse energy stability (~5%) of the Nd:YAG-laser
Please cite this article in press as: A.A. Kuchmizhak et al., Laser-assisted nanostructuring of metal films by means of a fibre dielectric microprobe, Pacific Science Review (2015), http://dx.doi.org/10.1016/j.pscr.2014.08.012
+ 6
MODEL
A.A. Kuchmizhak et al. / Pacific Science Review xx (2015) 1e6
used. We carefully studied the mechanisms of nanobumps formation and found that the nanofeatures size significantly depends on the supported film thickness [19]. Acknowledgements This project was financially supported by the Russia Federation Ministry of Science and Education, Contract N 02.G25.31.0116 of 14.08.2014 between the Open Joint Stock Company “Ship Repair Center “Dalzavod” and the RF Ministry of Science and Education. References [1] P.P. Pronko, et al., Machining of sub-micron holes using a femtosecond laser at 800 nm, Opt. Commun. 114 (1995) 106. [2] A.G. Vorobiev, C. Guo, Direct femtosecond laser surface nano/ microstructuring and its applications, Laser Photon. Rev. 7 (2013) 385. [3] T.C. Chong, et al., Laser precision engineering: from microfabrication to nanoprocessing, Laser Photon. Rev. 4 (2010) 123. [4] S. Nolte, et al., Nanostructuring with spatially localized femtosecond laser pulses, Opt. Lett. 24 (1999) 914. [5] Y. Lin, et al., Sub-30 nm lithography with near-field scanning optical microscope combined with femtosecond laser, Appl. Phys. A 80 (2005) 461. [6] L. Novotny, B. Hecht, Principles of Nano-optics, Cambridge Press, 2006. [7] R.M. Stockle, et al., Brighter near-field optical probes by means of improving the optical destruction threshold, J. Microsc. 194 (1999) 378. [8] F. Keilmann, R. Hillenbrand, Near-field microscopy by elastic light scattering from a tip, Philos. Trans. R. Soc. Lond. A 362 (2004) 787.
[9] M.H. Hong, et al., Laser assisted surface nanopatterning, Sens. Actuators A 108 (2003) 69. [10] A. Chimmalgi, et al., Femtosecond laser aperturless near-field nanomachining of metals assisted by scanning probe microscopy, Appl. Phys. Lett. 82 (2003) 1146. [11] A. Chimmalgi, et al., Surface nanostructuring by nano-/femtosecond laser-assisted scanning force microscopy, J. Appl. Phys. 97 (1993) 104319. [12] S. Yakunin, J. Heitz, Microgrinding of lensed fibers by means of a scanning-probe microscope setup, Appl. Opt. 48 (2009) 6172. [13] G. Wysocki, et al., Near-field optical nanopatterning of crystalline silicon, Appl. Phys. Lett. 84 (2004) 2025. [14] G. Wysocki, et al., Etching of crystalline Si in atmosphere by means of an optical fiber tip, Appl. Phys. Lett. 79 (2001) 159. [15] J. Heitz, et al., Laser-induced nanopatterning, ablation, and plasma spectroscopy in the near-field of an optical fiber tip, in: Proceedings of SPIE 71311W, 2009. [16] A.A. Kuchmizhak, et al., Optical apertureless fiber microprobe for surface laser modification of metal films with sub-100 nm resolution, Opt. Commun. 308 (2013) 125e129. [17] Yu N. Kulchin, et al., Through nanohole formation in thin metallic film by single nanosecond laser pulses using optical dielectric apertureless probe, Opt. Lett. 38 (2013) 1452e1454. [18] A.A. Kuchmizhak, et al., High-quality fiber microaxicons fabricated by a modified chemical etching method for laser focusing and generation of Bessel-like beams, Appl. Opt. 53 (2014) 937e943. [19] Yu. N. Kulchin, et al., Formation of nanobumps and nanoholes in thin metal films by strongly focused nanosecond laser pulses, J. Expr. Theor. Phys. 119 (2014) 15e23. [20] Y. Nakata, et al., Solideliquidesolid process for forming freestanding gold nanowhisker superlattice by interfering femtosecond laser irradiation, Appl. Surf. Sci. 274 (2013) 27e32. [21] J. Koch, et al., Nanotexturing of gold films by femtosecond laser-induced melt dynamics, Appl. Phys. A 81 (2005) 325e328.
Please cite this article in press as: A.A. Kuchmizhak et al., Laser-assisted nanostructuring of metal films by means of a fibre dielectric microprobe, Pacific Science Review (2015), http://dx.doi.org/10.1016/j.pscr.2014.08.012
MODEL
Available online at www.sciencedirect.com
H O S T E D BY
ScienceDirect Pacific Science Review xx (2015) 1e6 www.elsevier.com/locate/pscr
Laser-assisted nanostructuring of metal films by means of a fibre dielectric microprobe Alexander A. Kuchmizhak a,*, Oleg B. Vitrik a,b, Yuri N. Kulchin a,b a
Optoelectronics Lab., Institute of Automation and Control Processes, FEB RAS, Russia b Far Eastern Federal University, FEFU, Russia Available online ▪ ▪ ▪
Abstract A simple apertureless dielectric microprobe in the form of a section of the tapered optical fibre was proposed for surface laser nanomodification. This probe enables surface l/2-localisation of laser beam, as demonstrated both numerically and experimentally. The controllable formation of single through nanoholes with the minimum size down to 35 nm (~l/15) in the 50-nm Au/Pd film was shown using this probe and a 532-nm pump nanosecond laser. We also report for the first time on the formation of micro- and nanobumps, jet-like microstructures and microholes on optically thick gold films using single nanosecond laser pulses focused through the fibre dielectric apertureless probe. Both the shape and the sizes of the obtained microstructures were demonstrated to be determined by the pulse energy and film thickness. Copyright © 2014, Far Eastern Federal University, Kangnam University, Dalian University of Technology, Kokushikan University. Production and Hosting by Elsevier B.V. All rights reserved.
Keywords: Laser-assisted nanostructuring; Nanosecond laser pulses; Fibre apertureless microprobe
Introduction Nanoscale interaction of short and ultrashort laser pulses with solid surfaces is of consistently growing scientific and technological interest. Such interaction typically results in the fabrication of single and periodic surface nanofeatures [1,2], bringing new optical * Corresponding author. E-mail addresses: [email protected] (A.A. Kuchmizhak), [email protected] (O.B. Vitrik), [email protected] (Y.N. Kulchin). Peer review under responsibility of Far Eastern Federal University, Kangnam University, Dalian University of Technology, Kokushikan University.
properties to the nanomodified surfaces [3]. Sub-100nm surface nanofeatures are efficiently fabricated by spatial localisation of a driving laser electric field transmitted through an exit hole of a scanning nearfield optical microscope (SNOM) probe [4,5]. Such SNOM probes are composed of a single-mode optical fibre (OF) with a conically tapered and metal-coated tip, ending up with a nanoscale exit aperture. A SNOM probe can be moved along the modified surface with a high positioning accuracy of tens nanometre. However, the low optical transmittance through the exit nanoaperture of the SNOM probe (~106 at the aperture diameter D ¼ 50 nm) [6] decreases the transmitted intensity to well below the modification
http://dx.doi.org/10.1016/j.pscr.2014.08.012 1229-5450/Copyright © 2014, Far Eastern Federal University, Kangnam University, Dalian University of Technology, Kokushikan University. Production and Hosting by Elsevier B.V. All rights reserved. Please cite this article in press as: A.A. Kuchmizhak et al., Laser-assisted nanostructuring of metal films by means of a fibre dielectric microprobe, Pacific Science Review (2015), http://dx.doi.org/10.1016/j.pscr.2014.08.012
+ 2
MODEL
A.A. Kuchmizhak et al. / Pacific Science Review xx (2015) 1e6
threshold for most of materials. This problem cannot be solved by merely increasing the input energy because this results in strong heating and degradation of the tip due to the high dissipative losses in its metallic coating [7]. Consequently, the output aperture diameter of the SNOM probe, which is used for the surface modification with high energy pulses, typically cannot be made smaller than ~l [4], where l is the laser wavelength. Another laser-based method of formation of surface nanostructures with high spatial resolution consists of using the effect of electromagnetic field amplification near the nanoscale tip [6,8]. An atomic-force microscope probe irradiated with the external laser source can be used, e.g., a nanosized tip [9e11]. A record lateral resolution of ~10 nm was obtained [9] using this method with femtosecond laser pulses. However, the vertical resolution is also limited by this value, which restricts the application of this method. As a result, high-NA focussing optics are still frequently used to localise laser fields on a diffraction scale ~l/2. Nevertheless, sub-100-nm features can be fabricated in such a case via non-linear near-threshold effects (e.g., laser modification or ablation), multibeam interference and excitation of surface plasmon-polaritons [1e3]. However, these methods require additional diffractive optical elements to obtain a homogeneous focal spot with the required energy distribution. In addition, the addressing of light to a specific point on the sample surface with nanometre precision is significantly complicated because of the rather large dimensions of the far-field focussing element. One potential solution for sub-100-nm surface nanostructuring could be the combination of the high SNOM positioning accuracy and the high transmittance of far-field optics. In particular, a standard SNOM device can be equipped with a piece of a single-mode fibre with a conical output tip (an apertureless dielectric microprobe, ADM) [12e15]. The homogeneous Gaussian-like spatial distribution of the fibre fundamental mode does not require correction and is well suited for further focussing of the transmitted laser energy, due to total internal reflection at the “dielectric/air” interface, into a l/2-spot [12]. Such a probe differs from its apertured SNOM analogue only by the absence of the metallic coating on its tip and exhibits significantly higher optical damage threshold and transmittance than the aperture SNOM probe. As a result, ADMs have been used in laser ablation, photochemical etching, laser-induced breakdown spectroscopy [13e15], etc. However, the
fabrication process of the ADM tip is usually very complex [12]. In this paper, we present a simple fibre dielectric probe in the form of a section of the tapered optical fibre with a tip in the form of a truncated cone, as well as demonstrate the applicability of the developed ADM for precise laser-assisted nanostructuring of metal films surfaces and fabrication of various nanostructures with high spatial resolution. Experimental Linearly polarized second-harmonic (l ¼ 532 nm) pulses of a Nd:YAG laser (Solar LS LQ215) with the FWHM-pulsewidth ~7 ns and maximum energy E < 10 mJ in the TEM00-mode were used for surface nanostructuring (Fig. 1(a)). Each p-polarized laser pulse was focused onto the sample surface (Fig. 1(a)) by means of an apertureless dielectric microprobe (ADM), a tapered 20-mm long section of a singlemode optical fibre (Thorlabs SM400) with a constant taper angle ~12 (Fig. 1(b)) and a flat endface with a diameter ~250 nm [16,17]. In accordance to our finitedifference time-domain (FDTD) simulation such an ADM provides spatial filtering and focussing of a laser beam (Fig. 1(c)), resulting in a diffraction-limited output spot (R1/e z 0.3 mm at normal incidence) with a nearly Gaussian spatial profile and relatively deep focal depth (~l/2) [16]. The laser pulses were effectively coupled to the ADM using a fibre coupler (Thorlabs MBT612D/M). During laser nanostructuring, the ADM tip was located 50 nm above the sample surface and inclined at an angle of 60 (inset in Fig. 1) to the sample surface normal, yielding an elliptical Gaussian spot with Rx,1/e z 0.65 mm and Ry,1/e z 0.4 mm. The probe-to-sample distance was controlled using the tuning fork feedback. Visual control of the ADM motion and observation of the Au/Pd film damage was performed by means of a high-resolution optical microscope Hirox KH7700 (optical magnification ~700e7000, working distance ~3.4 mm). The laser pulse energy entering the fibre was varied by means of a polarizing attenuator. To measure the pulse energy E at the ADM tip, the output laser radiation was collected by means of an objective (Olympus, NA ¼ 0.65) and focused onto a photodetector (J10SI-HE Energy Sensor, Coherent EPM2000). The laser-structured films were characterised using an atomic force microscope (AFM, NanoDST Pacific Nanotechnology) in the close-contact mode and using a scanning electron microscope (Hitachi S3400). All
Please cite this article in press as: A.A. Kuchmizhak et al., Laser-assisted nanostructuring of metal films by means of a fibre dielectric microprobe, Pacific Science Review (2015), http://dx.doi.org/10.1016/j.pscr.2014.08.012
+
MODEL
A.A. Kuchmizhak et al. / Pacific Science Review xx (2015) 1e6
3
Fig. 1. (a) Schematic of the experimental setup for laser-assisted surface nanostructuring. (b) Electron images of the tapered ADM (top) and its 250-nm wide truncated flat endface (bottom). (c) Calculated TE electric field (Еr) distribution at the ADM tip apex for the truncated endface diameter ~250 nm.
of the experiments were performed under ambient conditions. Results and discussion Nanoholes Using the developed ADM for laser pulse focussing, we demonstrated for the first time controllable fabrication of nanoscale through-holes in 50-nm thick Au/Pd film (80/20 wt.%) on the silica substrate under the single-pulse irradiation [17]. Nanoscale through-holes fabricated in the Au/Pd film at variable laser pulse energies above the threshold value Eth are shown in
Fig. 2(a), with a minimal nanohole (marked with (2) at Fig. 2(a)) diameter of 35 nm (~l/15 at l ¼ 532 nm). Fig. 2(b) shows the dependence of the squared nanohole diameter D2 on the pulse energy ln(E) where the linear curve slope represents the squared characteristic Gaussian diameter s21/е of the surface energy distribution. This diameter s1/е ¼ 1.38 mm is several times larger than the ADM output beam 1/е-diameter D1/е ¼ 0.4 mm. Such a large increase in the laser energy deposition scale results from the lateral heat diffusion in the metallic film, which is supported by the thermally isolating glass substrate, during the heating ns-laser pulse. Fig. 2(c) shows the AFM profiles of the nanoholes with the diameters of approximately 95 nm and 35 nm.
Please cite this article in press as: A.A. Kuchmizhak et al., Laser-assisted nanostructuring of metal films by means of a fibre dielectric microprobe, Pacific Science Review (2015), http://dx.doi.org/10.1016/j.pscr.2014.08.012
+ 4
MODEL
A.A. Kuchmizhak et al. / Pacific Science Review xx (2015) 1e6
Fig. 2. (a) SEM image of the surface nanofeatures e nanoholes (1) and nanohillocks (2), fabricated at decreasing pulse energies (the scale bar corresponds to 100 nm). (b) Dependence of the squared nanohole diameter D2 on ln(E). (c) AFM-profiles of the holes inside the features (1) and (2). (d) AFM image of the periodic array of 90-nm wide nanoholes fabricated in the 50-nm Au/Pd film (the interhole spacing and the scale bar are 500 nm). (e) SEM image of the fibre microaxicon probe (left) coated with the 60-nm thick Au/Pd film and a 100-nm wide nanohole (right) fabricated at its tip by single ns-pulse with E ¼ 4 nJ focused through the ADM.
In accordance with a high contrast in the SEM images (Fig. 2(a)) both features are the through holes. However, as seen from Fig. 2(c) the 35-nm wide hole cannot be profiled with the available 20-nm wide AFM tip. Additionally, one can see that the holes are surrounded by a 200-nm wide and 25-nm height crater. Apparently, one can provide more smooth nanoholes by using of a femtosecond laser pulses because it minimises the heat transport in the Au/Pd film. Additionally, to demonstrate the of the developed nanostructuring method, we fabricated a square regular array of nanoholes with the mean diameter of 90 ± 20 nm, which was produced in the Au/Pd film using the ADM arrangement (Fig. 2(d)). Finally, to demonstrate the possibility of the ADM for local
precise nanostructure fabrication, a single through nanohole with the diameter as small as 100 nm was fabricated at the extremity of the Au-coated (metal film thickness z60 nm) fibre microaxicon, an analogue of the SNOM probe. The nanohole fabrication process in this case is apparently governed by lateral heat diffusion in the metallic film and the centre-symmetrical lateral expulsion of the melt by its vapour recoil pressure. The details of through nanoholes formation in optically “thick” metal films can be found in [17,18]. Thus, the results presented in this section indicated the possibility of using the developed probe for precise laserassisted milling of single and a periodic array of nanoholes in the opaque metal film. We believe that the
Please cite this article in press as: A.A. Kuchmizhak et al., Laser-assisted nanostructuring of metal films by means of a fibre dielectric microprobe, Pacific Science Review (2015), http://dx.doi.org/10.1016/j.pscr.2014.08.012
+
MODEL
A.A. Kuchmizhak et al. / Pacific Science Review xx (2015) 1e6
5
Fig. 3. (a) SEM images of the micro- and nanobumps fabricated on the surface of 100-nm thick Au film by single pulses at increasing pulse energy E. (b) AFM image of the periodic array of nanobumps on the 50-nm thick Au film. (c) SEM image of the nanobumps arranged into the “IACP” letters (scale bar corresponds to 5 mm).
use of advanced fibre probes [18] as well as combined with femtosecond lasers pulses can provide further lateral size and heat-affected zone minimisation for the fabricated nanofeatures. Micro- and nano-bumps Using the developed ADM, we have also demonstrated the fabrication of micro- and nanobumps on optically thick gold films using single nanosecond laser pulses [19]. Fig. 3(a) shows the SEM images of nanobumps fabricated on the surface of a 100-nm thick Au film using single pulses at increasing pulse energy E. The visible modification of Au film starts at pulse energy E 1 nJ. At E z 1.2 nJ (Fig. 3(a)) the nanobumps consist of the set of nanosized protrusions of up to 30 nm in height. For increasing pulse energies, these protrusions merge into the single nanobumps (Fig. 3(a)). As seen, both the height and the diameter of nanobumps increase with increasing pulse energy E.
The nanobump reaches its maximal sizes (up to hbump ~ 530 nm and dbump ~ 2 mm) at E z 1.7 nJ. When the pulse energy reaches ~2 nJ, the nanobump collapses at its bottom forming an asymmetric jet-like microstructure (not shown here). The shapes of such microstructures are poorly reproducible at energies in the range from 2 to 3 nJ, which apparently can be explained by the oblique laser irradiation of the film surface. Note that similar nanofeatures were fabricated using tightly focused femtosecond laser pulse [20,21]; however, the fabrication of nanobumps under the single nanosecond-pulse irradiation was performed for the first time, apparently due to the extremely high focussing performance of the developed ADM. Fig. 3(bec) shows the 3 3 array of the nanobumps on the 50-nm thick supported Au film and “IACP” demonstrating the reproducibility of the nanofeatures fabrication using the developed ADM. The small difference in nanobump height can be explained by the low pulse energy stability (~5%) of the Nd:YAG-laser
Please cite this article in press as: A.A. Kuchmizhak et al., Laser-assisted nanostructuring of metal films by means of a fibre dielectric microprobe, Pacific Science Review (2015), http://dx.doi.org/10.1016/j.pscr.2014.08.012
+ 6
MODEL
A.A. Kuchmizhak et al. / Pacific Science Review xx (2015) 1e6
used. We carefully studied the mechanisms of nanobumps formation and found that the nanofeatures size significantly depends on the supported film thickness [19]. Acknowledgements This project was financially supported by the Russia Federation Ministry of Science and Education, Contract N 02.G25.31.0116 of 14.08.2014 between the Open Joint Stock Company “Ship Repair Center “Dalzavod” and the RF Ministry of Science and Education. References [1] P.P. Pronko, et al., Machining of sub-micron holes using a femtosecond laser at 800 nm, Opt. Commun. 114 (1995) 106. [2] A.G. Vorobiev, C. Guo, Direct femtosecond laser surface nano/ microstructuring and its applications, Laser Photon. Rev. 7 (2013) 385. [3] T.C. Chong, et al., Laser precision engineering: from microfabrication to nanoprocessing, Laser Photon. Rev. 4 (2010) 123. [4] S. Nolte, et al., Nanostructuring with spatially localized femtosecond laser pulses, Opt. Lett. 24 (1999) 914. [5] Y. Lin, et al., Sub-30 nm lithography with near-field scanning optical microscope combined with femtosecond laser, Appl. Phys. A 80 (2005) 461. [6] L. Novotny, B. Hecht, Principles of Nano-optics, Cambridge Press, 2006. [7] R.M. Stockle, et al., Brighter near-field optical probes by means of improving the optical destruction threshold, J. Microsc. 194 (1999) 378. [8] F. Keilmann, R. Hillenbrand, Near-field microscopy by elastic light scattering from a tip, Philos. Trans. R. Soc. Lond. A 362 (2004) 787.
[9] M.H. Hong, et al., Laser assisted surface nanopatterning, Sens. Actuators A 108 (2003) 69. [10] A. Chimmalgi, et al., Femtosecond laser aperturless near-field nanomachining of metals assisted by scanning probe microscopy, Appl. Phys. Lett. 82 (2003) 1146. [11] A. Chimmalgi, et al., Surface nanostructuring by nano-/femtosecond laser-assisted scanning force microscopy, J. Appl. Phys. 97 (1993) 104319. [12] S. Yakunin, J. Heitz, Microgrinding of lensed fibers by means of a scanning-probe microscope setup, Appl. Opt. 48 (2009) 6172. [13] G. Wysocki, et al., Near-field optical nanopatterning of crystalline silicon, Appl. Phys. Lett. 84 (2004) 2025. [14] G. Wysocki, et al., Etching of crystalline Si in atmosphere by means of an optical fiber tip, Appl. Phys. Lett. 79 (2001) 159. [15] J. Heitz, et al., Laser-induced nanopatterning, ablation, and plasma spectroscopy in the near-field of an optical fiber tip, in: Proceedings of SPIE 71311W, 2009. [16] A.A. Kuchmizhak, et al., Optical apertureless fiber microprobe for surface laser modification of metal films with sub-100 nm resolution, Opt. Commun. 308 (2013) 125e129. [17] Yu N. Kulchin, et al., Through nanohole formation in thin metallic film by single nanosecond laser pulses using optical dielectric apertureless probe, Opt. Lett. 38 (2013) 1452e1454. [18] A.A. Kuchmizhak, et al., High-quality fiber microaxicons fabricated by a modified chemical etching method for laser focusing and generation of Bessel-like beams, Appl. Opt. 53 (2014) 937e943. [19] Yu. N. Kulchin, et al., Formation of nanobumps and nanoholes in thin metal films by strongly focused nanosecond laser pulses, J. Expr. Theor. Phys. 119 (2014) 15e23. [20] Y. Nakata, et al., Solideliquidesolid process for forming freestanding gold nanowhisker superlattice by interfering femtosecond laser irradiation, Appl. Surf. Sci. 274 (2013) 27e32. [21] J. Koch, et al., Nanotexturing of gold films by femtosecond laser-induced melt dynamics, Appl. Phys. A 81 (2005) 325e328.
Please cite this article in press as: A.A. Kuchmizhak et al., Laser-assisted nanostructuring of metal films by means of a fibre dielectric microprobe, Pacific Science Review (2015), http://dx.doi.org/10.1016/j.pscr.2014.08.012