Using optical vortex laser induced forward transfer to fabricate a twisted ferrite microcrystal array

Multicomponent oxide systems have been intensely investigated because of their scientific and practical importance.^1,2 Iron-based multicomponent oxide systems, referred to as ferrites, are particularly attractive as they possess excellent magnetic properties and exhibit multiple functional crystal forms, including spinel, garnet, orthoferrite, and hexaferrite.^3,4

Nozzle-based ink-jet printing has been well established as a non-contact deposition technology for materials. The technique enables the on-demand development of electronics, photonics, and biomedical devices with cost and energy savings and reduced material waste.^5–7 However, despite its broad applicability, it still struggles to directly print highly viscous or highly dense materials with a high spatial resolution owing to nozzle clogging effects. In addition, it frequently requires additional sintering steps to develop practical devices.

Laser induced forward transfer (LIFT) has been widely studied as an alternative approach to ink-jet printing. The technique enables the selective direct deposition of a myriad of materials (herein referred to as donors), such as solids, liquids, and even high viscosity pastes, onto a receiver substrate simply by irradiation with a laser pulse without nozzles.^8–13 However, LIFT also struggles to directly print donors onto substrates with high spatial resolution at millimeter-scale working distances. Until now, there have been few reports on the direct printing of high viscous multicomponent oxide systems with non-contact deposition technologies (even including LIFT), and the direct printing of micron-scale monocrystalline multicomponent oxide systems has never been reported.

An optical vortex possesses unique features, such as a ring-shaped spatial form and an orbital angular momentum (OAM), characterized by a topological charge ℓ, associated with its helical wavefront with an on-axis phase-singularity.^14–17 It has been intensely studied in a variety of research fields, including nano/micron-scale optical trapping and manipulation,¹⁸ optical/quantum space division multiplexing communications,¹⁹ and super-resolution microscopes with high spatial resolution beyond the diffraction limit.²⁰ In addition, it has been discovered that the OAM of the optical vortex twists the irradiated materials, and it allows for the fabrication of chiral structures with the aid of a spin angular momentum (SAM, s = ±1) owing to the helical electric field of circularly polarized light.^21–23 

In recent years, us and our co-workers have successfully proposed a new LIFT technology with an optical vortex pulse possessing a helical wavefront (instead of a conventional Gaussian beam with a planar wavefront), in which the OAM of the optical vortex pulse twists the irradiated donor material even if it has moderate-to-high viscosity (>1 Pa·s), thus resulting in the ejection of a spinning donor droplet with a perfectly straight flight path, somewhat akin to a rifling bullet.²⁴ 

This technique is herein referred to as optical vortex laser-induced forward transfer (OV-LIFT), and it enables the direct printing of microdots of a variety of donor materials with a high spatial resolution at an extremely long working distance.^25–27 In addition, OV-LIFT is known to produce as-printed dots with excellent physical properties, such as high electric conductivity, without any additional sintering processes. Thus, the OV-LIFT technique has great potential to enable the development of next generation printed devices with time and cost savings.

In this paper, we demonstrate the direct printing of a ferrite nanoparticle ink with OV-LIFT. Surprisingly, we discover that the OV-LIFT produces twisted printed dots possessing spinel Fe₃O₄ monocrystalline properties without any sintering processes. In addition, we find that the helicity of printed dots can be selectively controlled merely by reversing the handedness of the irradiated optical vortex pulse. Furthermore, the diameter of printed dots is observed to be inversely proportional to the numerical aperture (NA) of a focusing lens, and its minimum value was measured to be 1–2 µm, corresponding to that of less than 1/10^th of the irradiated laser spot (diffraction limit). These considerations suggest that the optical vortex twists and confines the sintered nanoparticles within its dark core to form chiral spinel monocrystalline printed dots.

The donor used in the experiment was formed of magnetic nanoparticles [∼1.0 wt. % (FeO, MnO) · Fe₂O₃, 100–300 nm] in suspension with a 57:43 mixture of purified water and glycerin (∼6 mPa s). Figure 1(a) shows a scanning electron microscope (SEM) image of magnetic nanoparticles. The donor solution was dropped onto a glass substrate to form a thin film with a thickness of ∼26 µm (an optical density of ∼0.3 at 532 nm) and a roughness of ∼6 µm.

Figure 1(b) shows a schematic diagram of the experimental setup for OV-LIFT used in this study. A Q-switched nanosecond green laser with a wavelength of 532 nm and a pulse duration of 10 ns was used as a light source, and its output was converted into a circularly polarized optical vortex pulse [J = ±2 (ℓ = ±1, s = ±1)] by employing a spatial light moderator and a quarter-wave plate. The generated optical vortex was loosely focused to be an annular spot with a diameter of 75 µm onto the donor film by using a lens with a focal length of 300 mm. This was two-dimensionally scanned by using a galvanometer scanner. The fluence of the irradiated laser beam was then fixed to be ∼0.28 J/cm². The distance between the donor film and the receiver substrate was controlled to be 0.6–1.5 mm.

With this system, a single droplet was launched and printed onto the receiver substrate. The temporal dynamics of the droplet ejection were observed from the side by employing an ultrahigh-speed camera with a frame rate of 2 × 10⁶ frames/s. The printed dots were then annealed at ∼300 °C to remove the solvent.

A single optical vortex pulse created a blister near its central dark core (within ∼3 µs). Subsequently, a rotating jet was launched to eject a single droplet (a diameter of ∼11 µm) owing to the Plateau–Rayleigh instability at ∼30 µs after the pulse deposition [Fig. 2(a)]. Interestingly, a twisted gray core (herein referred to as a “magnetic core”) with a diameter of 7 µm (one-tenth of that of the irradiated laser spot) was deposited at the center of a printed dot of 20 µm in diameter (Fig. 3). The core diameter was then the same at any working distance (0.6–1.5 mm). Note that the twisted direction of the magnetic core was reversed by inverting the handedness of the optical vortex (OAM) and circular polarization (SAM). Conversely, an equivalent Gaussian pulse produced simultaneously multiple droplets, resulting in a non-uniform dot printed with more debris and without any core on the receiver [Figs. 2(b) and 3(b)]. The OV-LIFT includes many steps: laser-induced cavitation, OAM transfer effect, and cavitation pressure induced mass transport and ejection of spinning droplets through Plateau–Rayleigh instability (microsecond-scale hydro-fluid dynamics). Such microsecond-scale hydro-fluid dynamics should play an important role in the phenomenon, as shown in Fig. 2.

The diameters of magnetic cores and printed dots were approximately inversely proportional to the numerical aperture (NA) (Fig. 4). The minimum diameters of the magnetic core and printed dot were measured to be 2.6 and 10.7 µm at NA = 0.04. It is worth noting that the height of the magnetic core was typically in the range of 0.6–2 µm, and its minimum value was 0.6 µm at NA of 0.04. These results show that OV-LIFT can sinter and twist ferrite nanoparticle ink to form chiral magnetic cores owing to helicity transfer from the irradiated optical vortex pulse.

The fabricated magnetic core was sliced into a 100 nm thick film by employing a focused ion beam milling method, and its physical properties were observed using a transmission electron microscope (TEM) (Fig. 5). Note that a carbon-coated layer was deposited on the magnetic core to prevent surface damage. Surprisingly, an ultrafine, hexagonal electron diffraction pattern, originating from the (122) face, was observed in more than 90% (surrounded by the red dashed line) of the entire core film, thus demonstrating that the magnetic core exhibits a well-formed octahedral single-crystal with a cubic spinel Fe₃O₄ structure (Fig. 5, Table I). The experimental spot intervals and lattice angles in the electron diffraction pattern of the film were fully identical to those of spinel-type crystals simulated by the open software ReciPro.²⁸ The magnetic core (surrounded by the red dashed line) was formed of a single domain without any grain boundaries.

These results suggest that optical vortex beams enable the close-packing and sintering effects of magnetic nanoparticles to form the monocrystalline magnetic core via the OV-LIFT process. Assuming that the latent heat for the phase transition of ferrites is negligible, the single optical vortex pulse allows for a temperature rise of the donor by ∼2400 K (the melting point of ferrites is ∼850 °C). The optical density and the specific heat of the donor film were then assumed to be 0.4 and 670 J/kg·K,²⁹ respectively. In addition, the irradiated laser fluence was 0.28 J/cm². Such a high temperature rise should be sufficient to sinter ferrite nanoparticles.

Furthermore, a 2-dimensional array of magnetic cores was fabricated by employing OV-LIFT in combination with a galvanometer scanner. The optical vortex pulse was then focused to be a 34 µm spot on the film, and the working distance between the donor and receiver was also fixed to be 0.4 mm. Note that the as-printed dots were annealed at ∼300 °C below the melting point (∼850 °C) of ferrite to remove the solvent (Fig. 7). The magnetic cores were fabricated at a positional error of ∼5.6 µm (4.9 and 1.8 µm along the x and y axes, respectively). This value was limited by the nonuniform dispersion (nonuniform absorption of the film) of ferrite nanoparticles in the solvent. Further improvement of positional accuracy will be possible with the refinement of the fabrication technique for the donor film.

We have discovered, for the first time, that OV-LIFT enables the direct printing of monocrystalline magnetic cores with a spinel structure. The effect occurs via cavitation dynamics, in which ferrite nanoparticles are aggregated and crystallized by optical vortex induced negative cavitation pressure. In addition, we have discovered that the magnetic cores are twisted by OAM transfer effects. The magnetic core diameter was measured to be typically one-tenth of that of the irradiated laser spot, even at larger and more practical working distances (>1 mm). The minimum diameter was then measured to be 2.6 µm at NA = 0.04.

Furthermore, a 2-dimensional array of magnetic cores can be fabricated simply by employing a galvanometer scanner. The positional accuracy of the printed cores was limited by the nonuniform dispersion (nonuniform absorption of the film) of ferrite nanoparticles in the solvent. An improvement in positional accuracy will be possible with the refinement of the fabrication technique for the donor film. The nanosecond OV-LIFT has been demonstrated in our current study. Femto/pico-second OV-LIFT via two photon absorption allows the spatially localized cavitation of donors, and it will further enable the super-resolution direct print of monocrystalline magnetic cores.

Such a crystallization phenomenon via OV-LIFT could be potentially extended to directly print and pattern a variety of crystals, including dielectrics, semiconductors, and even biomaterials, beyond the limits of conventional pulsed laser ablation^32,33 and pulsed laser melting techniques in liquid.^34–36 The resulting structures offer opportunities for new fundamental physics for the study of light–matter interactions based on the interaction of OAM with light.

The authors acknowledge the support in the form of Kakenhi Grants-in-Aid (Grant Nos. JP16H06507, JP17K19070, JP18H03884, JP18H05242, JP22H05131, JP22H05138, JP22K18981, and JP23H00270) from the Japan Society for the Promotion of Science (JSPS) and from the Core Research for Evolutional Science and Technology program (Grant No. JPMJCR1903) of the Japan Science and Technology Agency (JST).

The authors have no conflicts to disclose.

Akihiko Kaneko: Conceptualization (equal); Data curation (lead); Formal analysis (lead); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Software (supporting); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Muneaki Iwata: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Software (lead); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Rong Wei: Data curation (equal); Software (equal); Visualization (equal). Ken-ichi Yuyama: Conceptualization (equal); Data curation (lead); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Software (supporting); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Takashige Omatsu: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Software (lead); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding authors upon reasonable request.