We present a novel optical element – fiber microaxicon (FMA) for laser radiation focusing into a diffraction-limited spot with Bessel-like profile as well as for precision laser nanostructuring of metal film surfaces. Using the developed FMA for single-pulse irradiation of Au/Pd metal films on quartz substrate we have demonstrated the formation of submicron hollow microbumps with a small spike atop as well as hollow spherical nanoparticles. Experimental conditions for controllable and reproducible formation of ordered arrays of such microstructures were defined. The internal structure of the fabricated nanoparticles and nanobumps was experimentally studied using both argon ions polishing and scanning electron microscopy. These methods reveal a porous inner structure of laser-induced nanoparticles and nanobumps, which presumably indicates that a subsurface boiling of the molten metal film is a key mechanism determining the formation process of such structures.
© 2014 Optical Society of America
Nanostructuring by short and ultra-short laser pulses provides opportunities for fabrication of different micro- and nanoscale structures (nanojets [1,2], nanowhiskers , bumps , spherical nanoparticles [5,6], through nanoholes , nanocrowns , etc.) on sample surfaces. Single or periodically arranged laser-induced nanostructures exhibit local field enhancement and strong plasmonic response [9,10], high electron emission , making them a promising candidates for different sensors as well as nanophotonic and plasmonic devices . Research interest in this area unabated for several decades is largely associated with versatility, non-contact nature, high efficiency and relatively low cost of the laser nanostructuring methods being compared with an electron- and ion-beam milling as well as the possibility to fabricate unique types of nanostructures inherent only for a laser-matter interaction.
Fabrication of the nanostructures on the sample surface is mainly performed by using nano-, pico- and femtosecond pulses as well as classical high-NA focusing optics [1,2,5,6]. The minimal lateral size of the focal spot in this case is limited by a fundamental diffraction limit and at ideal conditions equals to λ/2 (λ - laser wavelength), i.e. ~200 nm for visible light. Fabrication of sub-100 nm structures using focusing optics is still possible and can be performed despite the diffraction limit effect by using a non-linear threshold response of the modified sample. However, in this case requirements for homogeneity and intensity distribution in the focal spot are very high, which often necessitates the use of additional diffraction optical elements. In practice, achievement of such extreme light localization with the high-NA optics is quite a difficult task and requires expensive high-quality focusing lenses. However, several papers [7,13–15] report that the high spatial focusing of the laser radiation into a high-quality focal spot as well as precise positioning of the focal spot on the sample surface can be achieved by using the only optical element – a bare fiber taper (BFT). This element is similar to a standard aperture-type probe of a scanning near-field optical microscopy (SNOM) and differs in absence of a metal coating and a nanosized aperture. Such BFTs, despite the lower focusing capabilities, being compared with conventional SNOM probes, exhibit significantly higher throughput and damage threshold and is widely used in precision laser nanostructuring , laser-induced breakdown spectroscopy , local spectroscopy of quantum objects , reflection-mode SNOM , etc. To achieve the maximal lateral localization of the laser radiation by using the BFT, its tip must be shaped into a truncated cone with an upper cone base diameter ~λ/2 . However, such geometric shape optimization of the probe tip requires not only an expensive and the time-consuming ion-beam milling, which significantly complicates the fabrication process of probes, but also makes the focusing element tied to a specified wavelength of the input laser radiation.
In our previous work  we have demonstrated that near-λ/2 lateral localization of laser radiation can be achieved by using a fiber microaxicon (FMA) fabricated on the flat endface of the single-mode optical fiber (OF) axially symmetric to its core. We have also modified a chemical etching method to fabricate the FMAs  using different commercial OFs. In this paper we will use for the first time the FMA as a compact, universal and highly efficient optical element for a spatial filtering and focusing of the nanosecond laser pulses into the diffraction-limited spot with a Bessel-like spatial distribution, as well as demonstrate the single-pulse laser-assisted fabrication of the various nanostructures on the metal films surface. We also report the fabrication of submicron bumps, through holes and porous spherical nanoparticles as well as discuss possible underlying formation mechanisms of this micron- and nanoscale structures.
2. Experimental details
Linearly-polarized third-harmonic (λ = 355 nm) pulses of a Nd:YAG-laser with the pulsewidth τp~7 ns, maximum energy E~10 mJ and energy stability ~10% were used for surface nanostructuring (Fig. 1(a)). Spatial filtering of the output laser beam and its focusing into the diffraction-limited spot was carried out by means of FMA (Fig. 1(b)) fabricated by the modified chemical etching  on the flat endface of the OF (optical core diameter ~1.7 μm). FMA’s geometric shape (full cone angle θ≈90 ° and cone base diameter D = 2 μm) were optimized to achieve the smallest laser spot  (Fig. 1(b)). The resulting laser beam at the FMA focus plane represents a central spot surrounded by additional maxima with a significantly lower intensity, which do not influence on the laser nanostructuring process. Typical laser intensity distribution at the FMA output measured by using the aperture-type collection-mode SNOM indicates the spatial localization of laser radiation in a highly symmetric focal spot with the FWHM diameter ~180 nm (~λ/2 at λ = 355 nm) (Fig. 1(c)).
To measure the pulse energy at the FMA output the Bessel-like beam is filtered to cut additional maxima and then focused using lens onto the sensitive photodetector (J-10SI-HE Energy Sensor, Coherent EPM2000). We used Au/Pd (80/20 wt.%) films with different thickness deposited by a magnetron sputtering (Quorum Technologies) on the smooth cleaved OF endface. This material is not inferior to the pure gold in terms of chemical stability as well as possibility to create unique nanostructures (nanojets, nanoparticles, etc.) under the tightly focused single-pulse irradiation , however, the average nanocrystallites grain size of this film deposited by the magnetron sputtering is smaller than that of pure Au film. Meanwhile, features of laser nanostructuring of the Au/Pd film in comparison with the pure material are poorly studied. During laser nanostructuring the FMA was placed normally to the sample surface (inset in Fig. 1(a)). The probe-to-sample distance was controlled at a constant level (~0.3λ ) using tuning fork feedback. Visual control of the FMA movement was carried out using a high-resolution optical microscope (Hirox KH7700). All laser-induced structures were fabricated by single-pulse irradiation under ambient conditions and then were characterized using electron (SEM, Hitachi S3400N) and atomic force microscopes (AFM, NanoDST Pacific Nanotechnology).
3. Results and discussions
Figure 2 shows the main types of laser-induced structures fabricated on the 40-nm thick Au/Pd film by the single-pulse irradiation with the pulse energy E ranging from 1 to 4 nJ. Visible modification of the metal film is observed at E>1 nJ and represents a 400-nm wide microbump with a height hbump up to 30 nm (see inset in Fig. 2(a)). Note that the minimum lateral size of the microbump is approximately 2 times larger than the initial optical spot (~180 nm) on the Au/Pd film surface presumably indicating a lateral heat transfer. At further pulse energy increase the lateral size Dbump and the height hbump of the microbump grow, reaching its maximum values (Dbump = 600 nm and hbump = 60 nm) at E~2 nJ (Fig. 2(b)). Both darker areas in the SEM images (marked by the red arrows in Fig. 2(b)) and the absence of any local convexities or dents in the corresponding AFM images presumably indicate a local thinning of the Au/Pd film and formation of nanoscale cavities inside the microbump, pointed to the subsurface boiling process occurred at the “film – substrate” interface.
When the pulse energy reaches approximately E≈2.3 nJ, one of the inner cavities inside the microbumps collapses forming a through (in accordance with a high SEM images contrast) nanohole (Fig. 2(c)). The diameter of the fabricated nanohole may be very small reaching ~35 nm . However, the formation mechanism of the through nanohole seems to be a result of an increasing vapor recoil pressure, which raises with E. Thus, the nanohole can appear randomly at the microbump center or at the interface of the melted and unmodified film (see Fig. 2(c)). We assume that the fabrication of the sub-100 nm through holes in  was also associated with a similar mechanism. Note also that this process can also result in the simultaneous formation of several nanoholes in the microbump area (not shown in the figures).
Further increase of the pulse energy up to E≈3,7 nJ leads to the film break by vapor recoil pressure and formation of sufficiently large 600-nm wide through hole, with the molten material being tended to form a nanoparticle or spread in different directions forming frozen edge microstructures (Fig. 2(d)). It is noteworthy that an attempt to calculate and compare volumes of the spherical nanoparticles and the corresponding through nanoholes (Fig. 2(d)) leads to some discrepancy. Thus, the average nanoparticle diameter ranges from 350 to 420 nm with its volume being about Vs = 0.03 μm3, respectively. Similarly, the volume of the through hole with an average diameter Dhole = 800 nm fabricated in a 40-nm thick Au/Pd film (Fig. 2(d)) is about Vhole~0.02 μm3, Thus, even without assuming the small frozen edge microstructures the Vs value is about 1.5 times larger than the corresponding through-hole volume, which presumably indicates the hollow internal structure of the nanoparticle. When the pulse energy exceeds E≈4.1 nJ, almost all molten material ejected from the hole (not shown in Fig. 2). It is noteworthy that some other formation mechanism is apparently realized at femtosecond single-pulse irradiation of the Au/Pd film, as it evidenced by the recent publication , where absence of pores in the particle concludes from matching of volume of the laser-induced nanoparticle and the through hole in the metal film.
We also note that similar microstructures are also fabricated by the single pulse irradiation of the 80-nm thick Au/Pd film, however, the threshold energies for formation of the corresponding microstructures are about 2 times higher in comparison with the E values measured for 40-nm thick film. This result is in good agreement with our earlier experiments carried out on pure gold films, demonstrating the influence of the film thickness d on the size of the obtained laser-induced nanostructures and the threshold pulse energy Eth required for fabrication of such structures (“size effect”) . The microbumps fabrication resulted from the subsurface boiling and the vapor recoil pressure was also observed for 160-nm thick Au/Pd film irradiated by the single nanosecond pulses at E>10 nJ. However, at increasing pulse energy no nanosized through-holes were observed. Instead, when the pulse energy reaches ≈15 nJ the microbump walls thinned down as it evidenced by a noticeable darkening in the SEM image, and the formation of the spherical protrusions at the microbump apexes were observed. Apparently, it is associated with both increasing vapor recoil pressure and molten material accumulation caused by the surface tension gradient (Marangoni effect [1,3,5]). Note that these microstructures resemble the initial stage of nanojets formation with a spherical nanoparticles ejection under the action of single tightly focused femtosecond laser pulse [1–3]. However, at further pulse energy increase no nanojets are observed, while the microbumps collapse forming the submicron holes with frozen structures at its edges (Fig. 3(d)). Note that despite the high uniformity and symmetry of the output FMA beam the frozen edge structures are poorly reproducible, with the microhole diameter Dhole being significantly varied from 400 to 1200 nm.
To confirm the subsurface boiling mechanism leading to the microstructures formation, we have studied their internal structure using a slow polishing by unfocused Ag+ ions (Hitachi IM4000). This technique allows one to remove layers of a metal film with an average polishing rate ~0.33 nm/sec. In order to minimize the possible melting of the metal film under the action of the heating Ag+ beam, the polishing process was performed for several consecutive irradiation cycles each of which does not exceed 15 seconds followed by metal film cooling during 1 min after each cycle. The result of 160-nm thick Au/Pd film polishing for a total exposure time ~300 sec is shown in Fig. 4 for some selected microstructures (bottom row). The same figure (top row) also shows corresponding microstructures before their Ag+ beam processing. As seen the laser-induced microbumps have a highly porous internal structure with the molten film material being mainly concentrated at the center of the irradiated laser spot, which confirms the findings discussed in this work. We also note that the appearance of the local thickness inhomogeneities after the single-pulse irradiation may be associated with an explosive boiling as well as a formation of cavitation bubbles with a fast expansion of the molten material the under the laser pulse action.
Similar experiments made for Au/Pd films with the thicknesses d = 40 and 80 nm also show similar porous internal structure of the fabricated spherical nanoparticles and microbumps. To the best of our knowledge, the controllable fabrication of laser-induced hollow nanoparticles is reported for the first time. Plasmonic properties of such hollow nanoparticles seem to significantly differ from the properties of bulk nanoparticles demonstrating the local plasmon resonance shifts or even additional resonances . However, these studies are clearly outside the frameworks of this paper and will be presented in our forthcoming paper. Dependencies of geometric dimensions of the laser-induced microstructure on the pulse energy E for different film thicknesses d summarizing the experimental results of this paper is shown in Fig. 5(a). As seen the film thickness d is a determining parameter for controllable fabrication of the desired microstructure.
Thus, at the film thickness ranging from 40 to 80 nm the porous spherical nanoparticles with the diameters Dsphere = 350-420 nm located within the through-holes (Fig. 2(d)) as well as microbumps with the lateral size down to Dbump = 400-600 nm can be fabricated with a sufficiently good reproducibility even for relatively low energy stability of the used laser source ~10%. At the same time, for d = 160 nm reproducible arrays of submicron microbumps as well as the hollow microbumps with a small spike atop (see Figs. 3(a)–3(c)) can be created by using the FMA scanning the surface of the metal film (here, we have used the pulse frequency up to 10 Hz and the FMA’s movement speed ~10-20 μm/s). Note that the fabrication of the ordered microstructures arrays with a substantially higher efficiency can be performed by using the interfering laser beams [2–4]. However, this method is not applicable in the case when a complex-shaped array of microstructures as well as a single surface nanomodification at a given point on the sample surface must be performed, while FMA provides opportunities for solving such problems. Figures 5(b) and 5(c) show SEM image of the microbump array ordered into “IACP” letters, as well as the single 100-nm wide through-hole fabricated at the apex of another FMA coated with 80-nm thick Au/Pd film.
In conclusion, this paper presents a novel optical element – fiber microaxicon for laser radiation focusing into the diffraction-limited spot with Bessel-like spatial distribution as well as for precision laser nanostructuring of the metal film surfaces. Using the developed FMA for single-pulse irradiation of Au/Pd metal films on quartz substrate we have demonstrated the formation of submicron hollow microbumps with a small spike atop as well as hollow spherical nanoparticles. Experimental conditions for controllable and reproducible formation of ordered arrays of corresponding microstructures were defined. The internal structure of the fabricated nanoparticles and nanobumps was experimentally studied by using the accelerated argon ions polishing as well as scanning electron microscopy. These methods reveal a porous inner structure of laser-induced nanoparticles and nanobumps, which presumably indicates that a subsurface boiling of the molten metal film is a key mechanism determining the formation of such structures.
The authors acknowledge partial support from Russian Foundation for Basic Research (Projects nos. 14-02-31323-mol_a, 14-02-00205-a).
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