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Abstract
A nanoscale twisting of aluminum (Al) is demonstrated by irradiation with a single picosecond optical vortex pulse with relatively low energy near the ablation threshold, due to the orbital angular momentum (OAM) transfer effects. The twisting needle is easily transformed into a microscale non-twisting needle by only the deposition of several overlaid optical vortex pulses. Irradiation with a picosecond/nanosecond optical vortex pulse with a millijoule level pulse energy also enabled the fabrication of a microscale non-twisting needle. Such nano/microstructuring of Al provides a new physical insight for the interaction between OAM and materials, and it also offers an entirely new nano/microfabrication technique towards ultraviolet plasmonic devices.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
An optical vortex [1–5] carries a donut-shaped spatial profile and an orbital angular momentum (OAM) due to its helical wavefront with an on-axis phase singularity characterized by a 2ℓπ azimuthal phase (ℓ termed as topological charge), and it is currently being explored for various applications, including optical and quantum communications [6–8], quantum information technology [9–13], and optical tweezers and manipulation [14–20].
In recent years, we have proposed optical vortex laser ablation [21], in which the irradiated optical vortex twists a target material, such as tantalum [22,23], silicon [24,25] or an organic polymer [26], to establish sub-microscale (i.e., nanoscale) structures; for example, twisting and non-twisting microneedles [27], chiral surface reliefs [28], and chiral fibers [29]. The formation of nanoscale twisting plasmonic needles on Ag and Au films [30], and on a copper substrate have been demonstrated by irradiation with visible vortex pulses [31]. These nanoscale twisting structures, which we have termed nanotwists, are still difficult to fabricate using conventional laser processing.
Going beyond the conventional laser processing for material fabrication, optical vortex laser ablation should also provide a new physical insight in the interaction between OAM of light and matter [32–34].
Aluminum (Al), commonly used as a standard industrial metal, acts as a plasmonic material in both the ultraviolet (UV) and visible regions, and it has been widely investigated in biotechnology, device engineering, nanomaterial technology, and solar energy harvesting [35–37]. A nanotwist of Al would be expected to significantly enhance the plasmonic properties of Al.
Here, we report on the fabrication of a nanotwist of plasmonic Al by irradiation with a single picosecond low energy optical vortex pulse.
2. Experiments
The laser source used in this experiment was a picosecond 1064 nm Nd:YAG laser (PL2210C-1K, EKSPLA Co.) with a pulse width of ca. 25 ps. A spiral phase plate (SPP) with a 2π azimuthal phase and a quarter-wave plate were used to convert the laser output beam into a circularly polarized first-order optical vortex beam possessing OAM with ℓ=1 and a spin angular momentum (SAM) with s=1. The generated optical vortex beam was loosely focused by an objective lens (Mitutoyo M Plan APO NIR 20×, NA=0.2) to be an annular spot with a diameter of ca. 20 µm on a Al plate. All experiments were performed at room temperature and atmospheric pressure. The aluminum plate used in this experiment was polished, so that its surface roughness was less than 400 nm.
The fabricated Al structures were observed using a scanning electron microscope (SEM, JEOL JSM 2010LA) with a spatial resolution of 8 nm and a laser confocal microscope (Keyence VK-9700/VK9710GS) with a depth resolution of 14 nm.
3. Results and discussion
Al exhibits a large extinction coefficient (ca. 9.99) for a 1 µm laser and a low melting temperature (ca. 933 K), which enables an extremely shallow absorption depth and a relatively low ablation threshold. The experimental absorption threshold was measured to be approximately 3 µJ (Fig. 1).
Fig. 1. (a) Height and FWHM of the fabricated Al nanotwist as a function of the irradiated optical vortex pulse energy. (b) Fabricated crater depth as a function of the irradiated optical vortex pulse energy. The ablation threshold was estimated to be 3 μJ by the crater method.
A single optical vortex pulse deposition allowed the fabrication of a nanotwist with a height of less than 1.6 µm and a thickness of 600 nm, defined as the full width at 50% height of the structure (Fig. 2). The laser pulse energy used was measured to be approximately 8.3 times (ca. 25 µJ) for the ablation threshold. Also, note that the depth of the ablated region was then measured to be <1 µm. The irradiated optical vortex pulse forces the molten Al to rotate azimuthally owing to the OAM transfer effects. Also, its non-uniform forward scattering force then collects the molten Al in the dark core of the optical vortex, so as to form the twisting needle.
Fig. 2. (a) Nanotwist fabricated by irradiation with an optical vortex pulse at an energy of 25 µJ. The height and thickness of the nanotwist were measured to be 1.6 µm and 600 nm, respectively. Several surplus droplets were also observed outside the ablated zone, and their diameter was typically measured to be ca. 1.0 μm. Magnified (b) side and (c) top views of the nanotwist.
A single vortex pulse with a relatively high energy of >100 µJ (>33 times ablation threshold) collected the molten Al in the dark core of the irradiated vortex, so as to form a microscale non-twisting needle with a rounded tip (Fig. 3). The height and thickness of the fabricated Al needle ranged within 2.5–6.5 µm and 2–4 µm, respectively.
Fig. 3. Non-twisting rounded Al needles fabricated at the pulse energy of (a) >0.1 mJ (b) 0.2 mJ (c) 0.4 mJ.
Overlaid vortex pulses with energies of ca. 25 µJ also caused the nanotwist to transform into a microscale non-twisting needle. Eight overlaid vortex pulses produced a rounded needle with a height of 5 μm and a thickness of 3.7 μm. The twist of the fabricated Al needles had disappeared (Fig. 4), while a tantalum (Ta) needle was still pointed and twisted even after deposition of four vortex pulses.
Fig. 4. (a) Height and FWHM of the fabricated structures as a function of the number of overlaid pulses. (b) Non-twisting needle fabricated by 8 overlaid pulses.
The fabrication of such non-twisting rounded needle via increase of the pulse energy or the number of depositions of overlaid vortex pulses can be mainly explained by the Ohnesorge number, $Oh = \frac{\mu }{{\sqrt {\rho \sigma L} }}$, in which$\; \mu $ is the viscosity, $\rho $ is the density, $\sigma $ is the surface tension, and L is the diameter of the surplus droplet of the irradiated materials. The molten Al has a lower Ohnesorge number (0.03) than that of molten Ta (0.04). The diameter of the surplus Al (Ta) droplets was experimentally determined, and it was then assumed to be 1.0 µm (1.7 µm).
Irradiation of a single nanosecond optical vortex pulse (wavelength: 1064 nm, pulse width: ca. 20 ns) onto the Al substrate was also conducted. The experimental ablation threshold was measured to be ca. 4.5 µJ, which was approximately twice that in the picosecond pulse deposition. The fabricated needles had rounded tips (>1 µm thick) and a height of <2-3 µm (Fig. 5). Significant thermal diffusion during nanosecond pulse deposition may be the main contribution to the microscale structuring of Al. An irradiated nanosecond optical vortex pulse should produce a few microscale heat-affected zones (HAZ) with radii of ca. 1.5 μm (the HAZ radius is estimated as $\sqrt {\kappa t} $, where $\kappa $ is the thermal diffusivity and t is the pulse width of the irradiated optical vortex pulse), so as to prevent efficient accumulation of the molten Al. It should be noted that the fabricated needles were never twisted with nanosecond pulse deposition at any energy level.
Fig. 5. (a) Fabricated crater depth as a function of the nanosecond optical vortex pulse energy. The ablation threshold was determined to be ca. 4.5 μJ. (b) Side and (c) top views of a needle fabricated by irradiation with a nanosecond laser.
4. Conclusion
The formation of a nanotwist of Al was demonstrated by irradiation with a single picosecond optical vortex pulse. Only a single low energy vortex pulse enabled the formation of nanotwists. An increase of the pulse energy or the deposition of overlaid vortex pulses produced non-twisting microneedles due to the fluid dynamics originated from the surface tension and viscosity of the irradiated Al. Irradiation with a single nanosecond optical vortex pulse was also performed; however, it was difficult to produce nanoscale twisting needles due to significant thermal diffusion effects.
The nanotwist formation of low viscosity Al in this present work should provide a new physical insight beyond the micron-scale twisting structures in tantalum and silicon previously reported by optical vortex illumination.
Such Al nanotwists formed by irradiation with a single picosecond optical vortex pulse at relatively low energy will also pave the way towards the development of advanced UV plasmonic devices. The transformation of nanotwists to microscale non-twisting needles by the deposition of a high energy vortex pulse or several overlaid optical vortex pulses also provides a new physical insight for the interaction between OAM and materials.
Funding
Core Research for Evolutional Science and Technology (JPMJCR1903); Japan Society for the Promotion of Science (JP16H06507, JP17K19070, JP18H03884).
Disclosures
The authors declare no conflicts of interest.
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