1. Introduction

Chirality is a specific inherent feature, typically occurring in natural living organisms at almost all length scales from snails and sea shells to chiral molecules and DNA. Artificially designed nanoscale chiral-shaped structures, mimicking their natural analogues owing to unique ways of interaction with optical radiation, possess remarkable properties as circular dichroism, enhancement of nonlinear signals, highly directional emission and photoactivity [1–4]. Meanwhile, state-of-the-art direct nanofabrication of chiral nanostructures based on ion- or electron-beam milling are rather expensive and small-scale (low-throughput) approaches, which triggers, in its turn, search for alternative technologies. In particular, at nanometer scale, fabrication of chiral nanostructures can be provided by assembly of specifically designed chiral molecules via various self-assembling processes [5–8]. However, the number of the available materials as well as the achievable size of the produced structures are both limited in this case.

Recently, another alternative pioneering approach, employing optical radiation with specially designed intensity-, polarization- or phase states for fast fabrication of chiral structures, was demonstrated in a number of papers [9–19]. Specifically, using inherent chirality of nanoparticles, synthesis of twisted nanoribbons through self-assembly in water solution under irradiation with continuous-wave circularly polarized laser radiation was demonstrated [9]. More importantly, vortex laser pulses, carrying simultaneously orbital (OAM) and spin (SAM) angular momenta, were demonstrated to twist transiently molten metal, producing chiral-shaped micron-size needles [10,11]. Later, similar effects were used to produce chiral structures on the surface polymers [12–14] and semiconductors [15,16]. Based on their experiments with ablation of bulk tantalum (Ta) targets [17], the authors revealed that mass transfer and handedness of the produced surface structures are associated with corresponding phase helicity (or OAM), while the polarization helicity (or SAM) accelerates/decelerates the movement of the molten material.

Despite the emerging potential of such direct and easy-to-implement method in fabrication of different chiral nanostructures, to the best of our knowledge, no experiments showing formation of twisted nanoneedles of variable size on surfaces of such common plasmonic materials, as silver (Ag) or gold (Au), were demonstrated so far. Besides some potential nanophotonic applications of such chiral structures on surfaces of typical plasmonic metals, there is still open fundamental issue of their formation mechanism, which is not well understood yet. Comparing to the well-developed thermocapillary theory of pulsed laser fabrication of central-symmetric micro-structures [20], straight nanoneedles on molten Au, Ag, Cu as well as noble-metal alloys by tightly focused Gaussian beams [21–27], light-pressure effect on laser-induced melt was suggested as driving force of chiral micro- and nanojetting for other materials (silicon (Si), Ta [10,15]).

In this work, we explore formation of twisted nanojets under direct high-NA translative ablation of Ag films of variable thickness, using nanosecond (ns) laser vortex pulses generated from circularly polarized pulses by an S-waveplate [28]. Main geometric parameters of the produced chiral nanoneedles - height, width and aspect ratio - were shown to be tunable in a wide range by varying the thickness of the irradiated metal films, supporting substrates, as well as the optical size of the vortex beam, demonstrating in the high-NA limit a strong analogy with formation of straight nanoneedles by Gaussian beams, apparently, via thermocapillary flows. Moreover, such nanosecond vortex pulses with the donut-shaped intensity distributions, carrying two vortices with opposite handedness, were demonstrated to produce two chiral nanojets twisted in opposite directions. We suggest an alternative explanation of the chiral nanoneedle formation on the Ag film surface based on a combination of straight and twisted thermocapillary flows of the molten material, which are triggered by the characteristic spiral-like intensity distribution, originating from the interference of the incident donut-shape vortex beam and its replica reflected from transiently evolving molten metal surface.

2. Experimental details

A vortex beam, carrying OAM ℓ = ± 1 (per photon, in units of ħ), was generated by transferring second-harmonic radiation from a Nd:YAG laser system (Brio, Quantel: central wavelength – 532 nm, pulse duration – 4 ns, repetition rate – 20 Hz, maximal pulse energy – 50 mJ) through a Glan-Taylor polarizer and a quarter-waveplate (Fig. 1), to produce a circularly-polarized beam, and finally through a commercially available radial polarization converter (S-waveplate, Altechna). Then, generated ns-laser vortex pulses were focused onto the sample surface (Fig. 1(A)) by means of different microscope objectives (Xirox: NA = 0.45, 50x; Nikon Plan Fluor: NA = 0.3, 15x and NA = 0.65, 50x), yielding in donut-shape intensity distributions (Fig. 1(B)) with the outer optical diameter D_vortex of ~2.05 μm, ~2.7 μm and ~3.9 μm for NA = 0.65, 0.45 and 0.3, respectively.

 

Fig. 1 (A) Schematic of the experimental setup for nanostructuring with the nanosecond vortex beams. (B) Vortex-beam intensity distributions measured in the focal plane of the microscope objectives with NA = 0.3 (top), NA = 0.45 (middle) and NA = 0.65 (bottom). (C) Sketch schematically showing the formation process of twisted Ag nanojets under vortex pulse ablation. Right-most side-view SEM image shows twisted nanojet with the red arrows indicating the rotation direction. (D,E) Normal-view SEM images of twisted Ag nanojets produced under single-pulse ablation of 500-nm thick Ag film with ns vortex pulses having opposite signs of vortex helicity.

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As samples for nanostructuring, Ag films of variable thickness, ranging from 100 to 1000 nm, were used, being deposited by e-beam evaporation (Ferrotec EV M-6) at the pressure of 5·10⁻⁶ bar and the average deposition rate of 0.5 nm/s onto optically smooth silica glass substrates. The resulting film thicknesses were pre-controlled by a calibrated quartz-crystal microbalance system (Sycon STC-2002), and then were verified by atomic force microscopy. Moreover, in order to study a substrate effect on formation of twisted nanojets, two types of substrates for 100-nm thick Ag films – silica glass and poly(methyl methacrylate) – were used. The samples were arranged on a PC-driven micropositioning platform (Newport XM and GTS series), providing a minimal translation step of 50 nm along each axis. Pulse energy E was varied by means of a transmission filter and controlled by a pyroelectric photodetector. All ablation experiments were performed under ambient conditions in the single-pulse mode. The morphology of the produced nanostructures was characterized by high-resolution scanning electron microscopy (SEM, Carl Zeiss Ultra 55 + ).

3. Results and discussions

Series of energy-resolved SEM images (Figs. 2(A)-2(D)) illustrates the formation and evolution of the twisted nanojets produced under single-pulse irradiation of the 500-nm thick Ag film, covering the silica glass substrate, with ns-laser vortex pulse focused at NA = 0.3. Besides the evident twisted shape of the produced nanojets with their clockwise rotational symmetry (the sign of vortex helicity), expectedly coinciding with the vortex pulse handedness (Figs. 1(C)-1(E)), their formation process appears to be very similar to that for common nanojets fabricated under single-pulse ablation of noble (semi-noble) metal films with Gaussian-shape (zero-OAM) ns-laser pulses [21–27].

 

Fig. 2 (A-D) Side-view (view angle of 40°) and normal-view (in false color) SEM image pairs of twisted Ag nanojets produced under single-pulse ablation of 500-nm thick Ag film with the vortex pulses focused at NA = 0.3. Similar side-view (view angle of 40°) SEM images showing twisted nanojets fabricated on the surface of the same Ag film with single vortices focused with microscope objective at NA = 0.45 (E-H) and NA = 0.65 (I-L). The pulse energy in each image row increases from left to right. Insets in figures (C) and (L) show the magnified view of the nanojet tip indicating the typical curvature radii smaller than 12 nm.

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The main energy-resolved steps include (i) accumulation of the molten material at the spatial center of the beam through thermocapillary forces and melt flows, which rapidly thin the peripheral part, (ii) formation of the liquid nanojet, which undergoes a Rayleigh-Plateau hydrodynamic instability, resulting in appearance and ejection of the molten droplets and, finally, (iii) formation of the through hole by breaking the significantly thinned area, surrounding the nanojet, at increased pulse energy (this stage is not shown in the SEM images). The tipped twisted nanojets are expectedly fabricated for typical pulse energies sufficient to trigger the ejection of all molten droplets from the resolidifying tip. Under such condition, the produced twisted nanojet can be characterized by very fine tips with the typical averaged curvature radius R_tip~12 ± 6 nm, as it was shown by the high-resolution SEM imaging (see inset in Fig. 2(C)).

For more tightly focused vortex pulses, similar chiral nanojets, having ultrafine tips, can be produced on the surface of the 500-nm thick Ag film under the single-pulse ablation (see Figs. 2(E)-2(L)). The main difference, evident from the analysis of the presented SEM images, consists in the amount of the molten material involved in the formation process, yielding in variation of the main geometrical parameters of the produced twisted conically shaped nanojets – height (H), width (w), base diameter D and height-to-width (aspect) ratio A. By varying the focusing conditions and systematically studying the formation process of the nanojets under single-pulse ablation of such films, we identify the several general trends characterizing the effect of the objective’s NA. First, the formation of the thinner chiral jets (simultaneous decrease of w and D), providing evident increase of their aspect ratio A (Figs. 3(A)-3(D)), is observed for the more tightly focused vortex pulses. With the typical width w of the Ag nanojets being varied between 0.95 and 0.68 μm for the tested NA range (almost 4-fold smaller values, comparing to those reported for nanojets produced on the bulk Ta surface [10]), the aspect ratio A increases two-fold, in its turn (orange circles in Fig. 3(D)). Second, the average height H accessed for the tipped nanojets without the droplets atop was found to be slightly affected by the NA value (Fig. 3(C)). Similar observations were reported in [10]. Finally, the chirality of the nanojets decreases versus NA. In more general words, the chiral nanojets produced under high-NA focusing (NA = 0.65) become less twisted (averaged number of the turnovers per single nanojet decreases) and more similar to those produced under zero-OAM Gaussian pulse ablation [21–27]. In particular, this can indicate that the formation process is directly connected with the resolidification time, which is expectedly smaller for the structures produced under the tight focusing conditions.

 

Fig. 3 (A-D) Averaged thickness- and NA-dependent width w (A), base diameter D (B), height H (C) and aspect ratio A, of the chiral nanojets produced under single-pulse ablation of the 500- and 100-nm thick Ag films, covering the silica glass (orange and grey circles, respectively) and PMMA (green circles) substrates. (E) Diameter of the damaged area on the surface of the 500-nm Ag film versus NA. (F-H) Side-view (view angle of 40°) and top-view (in false color) SEM image pairs of the twisted nanojets produced under vortex-pulse ablation of the 100-nm thick Ag film, covering the silica glass (F-H) and PMMA (I-L) substrates. The pulse energy in each image row increases from left to right. The vortex pulses were focused by the 0.65-NA objective. Inset in the figure (H) shows the magnified view of the nanojet tip.

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Similar NA-dependent trends were observed for the chiral nanojets produced on the surface of the 100-nm thick Ag film, covering the glass substrate (grey circles in the Figs. 3(A)-3(D)). Meanwhile, excluding the aspect ratio value A, all other geometric parameters of such nanojets demonstrate evident decrease, comparing to those measured for nanojets produced on the 500-nm thick Ag film, apparently owing to smaller amount of the involved molten material. The SEM study of the nanojets produced on the 100-nm thick Ag film at tight focusing (NA = 0.65) indicates their chiral shape as well as pronounced shift of the nanojet position, comparing to the center of both the optical spot and the damaged area (marked by the orange circles in Figs. 3(F)-3(H)), indicating the rotational motion in the direction coincided with the vortex handedness. Also, regardless of the tested experimental parameters (either the objective NA or the film thickness), the average curvature radius R_tip of the nanojet tip was found to be smaller than 17 nm in all cases.

In terms of vortex pulse printing of twisted nanojet arrays, it is also important to evaluate their ultimate package density, measuring the size of the film damaged area. Figure 3(E) shows the typical lateral size of the twisted nanojets as a function of the objective NA, demonstrating the linear scalability and potential few-micron package density at high-NA focusing, which will be studied in details in our forthcoming papers. Finally, since the thickness of the metal film in the microbump area is very small, its exposure to the second laser pulse usually results in destruction of the microbump and appearance of a through hole. In this respect, it seems impossible to use multi-pulse irradiation to reshape or enhance the chirality of the produced jets [10]. Meanwhile, as it was shown in this paper, the large set of geometrical parameters can be realized for nanojets via tuning corresponding focusing conditions and film thicknesses.

One additional experimental aspect, uncovering formation and enabling extra tunability of nanojet shapes, which will be only briefly and phenomenologically addressed in this paper, consists in variation of supporting substrate, which determines adhesion and acoustic impedance matching between the film and substrate. The “substrate effect” was studied by comparing typical geometrical parameters of the twisted nanojets produced on the surface of 100-nm thick Ag films on the silica glass and PMMA substrates at different pulse energies. With almost twice lower ablation threshold measured for the case of PMMA substrate, using the common linear approximation of the squared diameters of through holes D² versus natural logarithm of applied pulse energy ln(E) [29], the typical width of the nanojets decreases considerably (Fig. 3(A)), yielding in averaged 2.2-fold increase in the aspect ratio A values (see Fig. 3(D)). Similar “substrate effect” was found for the ordinary non-chiral nanojets produced under ablation of the same Ag film with zero-OAM pulses, pointing out the adhesion as one of the key factors, affecting the nanojet formation process and achievable parameter range, in its turn. In our ongoing research, the acoustic impedance and adhesion effects will be studied more systematically, using in some control experiments an additional chromium layer to provide high film adhesion and to test particularly the acoustic impedance influence. Also, it should be stressed that ablation experiments undertaken for the 1000-nm thick Ag film, covering the glass substrate, did not demonstrate the formation of regular microscale nanojets for the whole tested NA range, while some rotational movement “fingerprints” were identified from the SEM analysis of the produced surface structures (not shown here).

As it was mentioned above, the S-waveplate was used to generate the ns-laser vortex pulses, carrying OAM ℓ = ± 1, from circularly polarized ones. It is known that under illumination of the S-waveplate with elliptically polarized light, two optical vortices with opposite SAM handedness and opposite OAM handedness can appear [30,31]. This inherent feature of the S-waveplate was used in this paper to generate a donut-shaped beam, carrying two optical vortices with opposite handedness. To do this, for the fixed position of the S-waveplate, we rotated the quarter-wave plate to generate elliptically polarized light, while simultaneously detecting the interference pattern produced by the generated donut-beam and the plane wave from the reference arm of the common Mach-Zehnder interferometer scheme. For a certain position of the quarter-wave plate, the single “fork” in the interference pattern converts into two opposite “forks”, indicating the formation of the vortices with opposite helicity signs (Figs. 4(A) and 4(B)) without strong deformations of the donut-shaped intensity distribution.

 

Fig. 4 Fringe patterns of the interfering plane wave and its donut-shaped replica generated by S-waveplate for the elliptical (A) and circular (B) polarizations. The circles indicate the phase discontinuities. (C) Side-view (view angle of 40°) SEM images of doubled twisted Ag nanojets produced under single-pulse ablation of 500-nm thick Ag film with donut-shaped pulses carrying two vortices with the opposite signs of vortex helicity. (D) Top-view SEM image of similar double-jet structure with the magnified images of both nanojets given in the insets. The vortex pulses were focused by the 0.3-NA objective. (E) Top-view high-resolution SEM image, showing the package of nanocrystallites within the single chiral nanojet. Several nanocrystallites were marked by false colors to highlight the nanocrystallite “bending” effect (red-colored crystallite) in the area affected by distinct helical thermocapillary flows and central-symmetric arrangement of crystallites (blue colored) outside this area.

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Surprisingly, the ablation of the 500-nm thick Ag film, covering the glass substrate with the donut-beam, carrying two vortices, produces two chiral nanojets with opposite handedness, as it is indicated by the series of the energy-resolved SEM images (Figs. 4(C) and 4(D)). At the increasing pulse energy, each separate nanojet undergoes the evolution, similar to those observed for single twisted nanojet, finally evolving into tipped nanoneedle with the pronounced chirality. This remarkable demonstration also indicates that, by tailoring complex intensity and phase distributions via DOEs or other optical elements, the complex patterns with multiple chiral plasmonic nanoneedles can be produced under single-pulse ablation.

Similarity of our present observations and the previous extensive experience in fabrication of thin-film nanojets, using short- and ultrashort laser pulses [25–27,32], provokes us to compare formation of the non- and chiral nanoneedles. In comparison to previously reported results [10,15,17], where no nanojets were observed under irradiation of the bulk Ta and Si targets with the zero-OAM Gaussian pulses, for Ag films studied in this paper, both Gaussian- and donut-shaped beams produce nanojets, having non- and chiral shapes, respectively. In particular, this indicates that for almost similar general mechanism, underlying the formation of the nanojets on the noble-metal film surface and attributed to the temperature-gradient driven thermocapillary flow of the molten material, strong rotational movement appears under vortex-pulse irradiation, twisting molten material in the direction, coinciding with the vortex helicity sign. Fingerprints of such rotational movement can be found on both microscale - in the chiral shape of the resolidified nanojets and in the central symmetry breaking of the nanojet spatial position (Figs. 3(F)-3(H)), and nanoscale – in the bending of some nanocrystallites (highlighted with red colors in the Fig. 4(E)), typically having radial-symmetry arrangement under zero-OAM Gaussian pulse irradiation [25]. High-resolution SEM imaging of produced twisted nanojets shows that such effect is observed only near the nanojet areas, where twisting thermocapillary flows, strong enough to perturb the recrystallization wave, appear (Fig. 4(E)). The helical thermocapillary melt flows within the evolving nanojet and surrounding microbump area possibly can originate from the corresponding characteristic spiral-like (or more complex [12],) intensity distribution. For the noble-metal films considered in this study, the origin of such spiral-shaped intensity distribution and corresponding temperature profile on the metal film surface can be explained in terms of “pure” optical processes as interference of the incident donut-shaped vortex beam with the zero-OAM beam, having a spherical (or more complex) wavefront. Similar principle was recently realized in coaxial-interference scheme for fabrication of chiral structures with a two-photon photopolymerization technique [33]. It is clear that appearance of such perturbed reflected wave, providing the desired “spiral” component of the corresponding surface intensity distribution, can be associated with the transient nanosecond evolution of the metal film surface, which can be triggered via its heating, melting and thermal expansion by the incident donut-shaped laser pulse.

Scalar diffraction theory calculations [34] show that structure of a vortex beam reflected from a metal film can be perturbed during its reflection from a film, containing multiple sub-microscale inhomogeneities with average size more than 20 nm. In this case, interference of the incident vortex beam and the reflected speckle-modulated zero-OAM one produces the spiral-like intensity distribution on the metal film surface, which handedness coincides with that one of the incident vortex beam (Fig. 5). To the contrary, similar calculation performed for a smooth metal mirror shows a common annular-shape interference pattern, originating from two counter-propagating vortex beams, retaining with opposite helicity and the similar OAM [35] (this intensity distribution is equal to that one presented in left-most image of the Fig. 5). As any substantial surface evolution of the initially smooth metal film starts after electron-phonon relaxation (typical timescale - few picoseconds for both noble metal film and Si), the “heating spiral” as well as the helical shape of the nanojets are expected to be missing for ultra-fast energy deposition with the pulse durations shorter, than this characteristic time. Similar pulse-width dependent character of the helicity was reported for vortex-beam ablation of bulk Si targets [15], however, considering strong differences in the physical characteristics of the Si and Ag materials, only qualitative comparison will hold true in this case.

 

Fig. 5 Intensity distributions of the incident focused first-order vortex beam (left), distorted beam reflected from the rough metal surface (middle) and their interference pattern (right). The white arrows in the right-most image indicate the direction of the intensity gradient, which leads to the appearance of a “thermal spiral”.

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Ordinary jetting behavior of thin metal films irradiated by zero-OAM Gaussian pulses was predicted theoretically [36] only for the group of noble metals (Ag, Au, Cu and their alloys), possessing high natural plasticity and relatively low (~1300 K) melting temperature, and then justified experimentally in a number of papers [21–27]. Based on these previous observations and considering the similarity of the formation process for the non- and chiral nanoneedles demonstrated in this study for the thin Ag films, formation of the twisted nanojets can be predicted also for other plasmonic materials, opening a way for high-performance single-step fabrication of chiral nanostructures for real applications in plasmonics and nanophotonics. From the other hand, the twisted nanoneedles were first observed for bulk Ta target [11], a refractory metal with the high melting temperature (~3300 K), showing jetting only under donut-shape vortex beam irradiation [17]. In particular, this can indicate different general mechanisms and processes, underlying formation of the nanojets on plasmonic metals relative to other materials. Meanwhile, we believe that our possible alternative explanation of the nanojet helicity for plasmonic materials, together with the present one [15,17] based on OAM transfer, contributes to the basic understanding and supports new elucidating studies of matter structuring by structured light.

4. Conclusions

To conclude, donut-shaped nanosecond vortex pulses generated from circularly polarized ones by an S-waveplate, were found to produce twisted nanojets under single-pulse translative ablation of Ag films. Main geometric parameters of the produced chiral nanojets, such as height, width and an aspect ratio, were shown to be tunable in a wide range by varying metal film thickness, supporting substrates, and the optical size of the vortex beam. Such vortex laser pulses, carrying two vortices with opposite handedness, were demonstrated to produce two chiral nanojets twisted in opposite directions. We suggest an alternative explanation of the chiral nanoneedle formation on the silver film based on a combination of straight and twisted thermocapillary flows of the molten material, which are triggered by the characteristic spiral-like intensity distribution originating from interference of the incident donut-shape vortex beam and its replica reflected from the transiently evolving molten metal surface.