# Harnessing Multi-Photon Absorption to Produce Three-Dimensional Magnetic Structures at the Nanoscale

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

## 1. Introduction

_{3}particles (particle size: 1.5–7.5 nm) which could self-assemble into complex 3D colloidal crystals [19]. In the same year, 3D cobalt nanocrystals were produced by application of a ferrofluid to oriented pyrolytic graphite and applying a magnetic field [20]. Such methods were innovative and provided a first taste of the interesting properties that may arise when nanostructuring magnets in three-dimensions. Perhaps what was lacking was a controlled means to produce nanomagnets of arbitrary 3D geometry.

## 2. Experimental Setup and Physics of Two-Photon Lithography

_{th}, the exposed area is polymerised. The exposure dose is approximately proportional to the generated free radicals and therefore to the excited photoinitiator molecules and thus to the number of absorbed photons. In order to write arbitrary structures, it is necessary to only expose desired areas. This is trivial to do for 2D fabrication by using a simple optical mask. For more complex 3D geometries, it becomes necessary to focus the light using a high numerical aperture (NA) lens, so that only a limited volume (‘voxel’) of the photoresist around the focal point is exposed with enough intensity to reach the threshold dose.

^{2}∙mol

^{−1}), c is the concentration of photoinitiator (mol∙m

^{−3}) and z is the depth into the photoresist (m). The absorbance A(z) is related to the transmittance T(z) (ratio of transmitted intensity I(z) to incident intensity I

_{0}) by:

_{i}so that Σhν

_{i}≥ ΔE (multi-photon absorption) (Figure 2). In the case of multi-photon absorption, the photons must arrive at the photoinitiator within a small enough time window. It is therefore less likely than 1PA. The probability of absorption (absorption rate) is proportional to I

^{n}where n is the number of absorbed photons (assuming photons of the same frequency ν) and I the intensity of the light at the given frequency ν. Two-photon absorption (2PA) therefore requires much stronger intensities than one-photon absorption to make the process efficient enough for lithography. Table 1 summarises the properties of 1PA and 2PA.

^{2}instead of I, the exposed area is much smaller. In addition, when integrating over the x-y plane, one finds a peak in the focal plane. This allows setting the power so that only the volume enclosed within the focal point (the voxel) receives sufficient exposure to be polymerised, thus enabling the creation of the desired 3D plane or any arbitrary 3D structure.

_{0}is the beam waist (minimum radius of beam at focus), σ

_{2}is the effective two-photon radical absorption cross-section, I

_{0}is the photon flux at the beam centre, τ is the laser pulse width and C is defined as the following equation:

_{0}is the density of free radicals at the focal plane and ρ

_{th}is the threshold density of free radicals.

_{R}is the Rayleigh length, which is defined as the following equation:

_{laser}= incident laser power, t = processing time, f = repetition frequency of the laser, τ = pulse width, ν = frequency of light, M

_{0}= initial concentration of photoinitiator in the ground state, M

_{th}= threshold amount of dissolvable photoinitiator. The factor C’ relates the quantum efficiency (Φ), two-photon absorption cross-section (δ), the Einstein coefficient of absorption (A

_{E}) and a rate constant (C

_{0}) as shown below:

**Table 1.**Comparison of optical properties for single photon absorption (1PA) versus two-photon absorption (2PA).

Optical Property | 1PA | 2PA |
---|---|---|

transmitted intensity, I [48] | $I\left(z\right)=\text{}{I}_{0}{e}^{-\mathrm{ln}\left(10\right)\epsilon cz\left(z\right)}$ | $I\left(z\right)=\text{}\frac{{I}_{0}}{1+\beta z{I}_{0}}$ |

$\frac{\mathit{d}\mathit{I}\left(\mathit{z}\right)}{\mathit{d}\mathit{z}}$ [49] | $\frac{dI\left(z\right)}{dz}=\text{}-\mathrm{ln}\left(10\right)\epsilon c\times I\left(z\right)$ | $\frac{dI\left(z\right)}{dz}=\left(-\beta \right)\times I{\left(z\right)}^{2}$ |

transmittance [50] $\mathit{T}\left(\mathit{z}\right)=\text{}\frac{\mathit{I}\left(\mathit{z}\right)}{{\mathit{I}}_{0}}=\text{}{10}^{-\mathit{A}\left(\mathit{z}\right)}$ | $T\left(z\right)=\text{}{e}^{-\mathrm{ln}\left(10\right)\epsilon cz\left(z\right)}$ | $T\left(z\right)=\text{}\frac{1}{1+\beta z{I}_{0}}$ |

decadic absorbance [51,52] $\mathit{A}\left(\mathit{z}\right)=\text{}\frac{-\mathrm{ln}\left(\mathit{T}\right)}{\mathrm{ln}\left(10\right)}$ | $A\left(z\right)=\text{}\epsilon cz$ | $A\left(z\right)=\text{}\frac{\mathrm{ln}\left(1+\beta z{I}_{0}\right)}{\mathrm{ln}\left(10\right)}$ $\cong \text{}\frac{\beta z}{\mathrm{ln}\left(10\right)}\times {I}_{0}$ |

absorption probability/rate [44,50] $\mathit{P}\text{}\propto \text{}{\mathit{I}}^{\mathit{n}}$ | $P\text{}\propto \text{}I$ | $P\text{}\propto \text{}{I}^{2}$ |

## 3. Fabrication of Magnetic Nanostructures with Two-Photon Lithography

#### 3.1. TPL and Electrodeposition

#### 3.2. TPL and Line of Sight Deposition

_{81}Fe

_{19}(permalloy) nanowires with a novel curved cross-section, at a resolution of ~200 nm (Figure 7a–c). With LOS deposition, the permalloy was deposited upon the uppermost unit cell without depositing magnetic material on the lower scaffold layers, thereby minimising dipolar interactions between the magnetic nanowires and the sheet film upon the substrate. Additionally, the buried polymer layers act as optical scattering centres, allowing MOKE measurements to be performed with minimal contribution from the sheet film (Figure 7d). This study reports a first example showing that TPL and LOS deposition can rapidly yield extended arrays of single-domain magnetic nanowires in complex 3D geometries. Standard measurement techniques such as MOKE and magnetic force microscopy (MFM) were used to probe the magnetic properties, providing an exciting new path of investigation for 3D magnetic nanowires and artificial frustrated systems. Utilisation of other imaging techniques such as scanning transmission x-ray microscopy (STXM) [77] and high resolution Hall sensing [78] is expected to yield even more fruitful results.

## 4. Methods to Reduce Feature Size

_{xy}for NA values > 0.7 has been shown to be [81]:

## 5. Outlook

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

- Andrus, J.; Bond, W.L. Photoengraving in transistor fabrication. Transistor Technol.
**1958**, 3, 151–162. [Google Scholar] - Lojek, B. History of Semiconductor Engineering; Springer Science & Business Media: Berlin, Germany, 2007; p. 193. [Google Scholar]
- Bobeck, A.; Danylchuk, I. Characterization and test-results for a 272k bubble memory package. IEEE Trans. Magn.
**1977**, 13, 1370–1372. [Google Scholar] [CrossRef] - Thompson, D.; Romankiw, L.; Mayadas, A. Thin-film magnetoresistors in memory, storage, and related applications. IEEE Trans. Magn.
**1975**, 11, 1039–1050. [Google Scholar] [CrossRef] - Baibich, M.N.; Broto, J.M.; Fert, A.; Vandau, F.N.; Petroff, F.; Eitenne, P.; Creuzet, G.; Friederich, A.; Chazelas, J. Giant Magnetoresistance of (001)Fe/(001) Cr Magnetic Superlattices. Phys. Rev. Lett.
**1988**, 61, 2472–2475. [Google Scholar] [CrossRef] [PubMed] [Green Version] - Julliere, M. Tunneling between Ferromagnetic-Films. Phys. Lett. A
**1975**, 54, 225–226. [Google Scholar] [CrossRef] - Tsang, C.; Decker, S.K. The Origin of Barkhausen Noise in Small Permalloy Magnetoresistive Sensors. J. Appl. Phys.
**1981**, 52, 2465–2467. [Google Scholar] [CrossRef] - Shinjo, T.; Okuno, T.; Hassdorf, R.; Shigeto, K.; Ono, T. Magnetic vortex core observation in circular dots of permalloy. Science
**2000**, 289, 930–932. [Google Scholar] [CrossRef] [PubMed] [Green Version] - Cowburn, R.P.; Allwood, D.A.; Xiong, G.; Cooke, M.D. Domain wall injection and propagation in planar Permalloy nanowires. J. Appl. Phys.
**2002**, 91, 6949–6951. [Google Scholar] [CrossRef] - Cowburn, R.P.; Koltsov, D.K.; Adeyeye, A.O.; Welland, M.E.; Tricker, D.M. Single-domain circular nanomagnets. Phys. Rev. Lett.
**1999**, 83, 1042–1045. [Google Scholar] [CrossRef] - Slonczewski, J.C. Current-driven excitation of magnetic multilayers. J. Magn. Magn. Mater.
**1996**, 159, L1–L7. [Google Scholar] [CrossRef] - Berger, L. Emission of spin waves by a magnetic multilayer traversed by a current. Phys. Rev. B
**1996**, 54, 9353–9358. [Google Scholar] [CrossRef] [PubMed] [Green Version] - Lewis, E.R.; Petit, D.; O’Brien, L.; Fernandez-Pacheco, A.; Sampaio, J.; Jausovec, A.V.; Zeng, H.T.; Read, D.E.; Cowburn, R.P. Fast domain wall motion in magnetic comb structures. Nat. Mater.
**2010**, 9, 980–983. [Google Scholar] [CrossRef] [PubMed] - Parkin, S.S.P.; Hayashi, M.; Thomas, L. Magnetic domain-wall racetrack memory. Science
**2008**, 320, 190–194. [Google Scholar] [CrossRef] [PubMed] - Meier, J.; Doudin, B.; Ansermet, J. Magnetic properties of nanosized wires. J. Appl. Phys.
**1996**, 79, 6010–6012. [Google Scholar] [CrossRef] - Ferre, R.; Ounadjela, K.; George, J.; Piraux, L.; Dubois, S. Magnetization processes in nickel and cobalt electrodeposited nanowires. Phys. Rev. B
**1997**, 56, 14066–14075. [Google Scholar] [CrossRef] - Metzger, R.; Konovalov, V.; Sun, M.; Xu, T.; Zangari, G.; Xu, B.; Benakli, M.; Doyle, W. Magnetic nanowires in hexagonally ordered pores of alumina. IEEE Trans. Magn.
**2000**, 36, 30–35. [Google Scholar] [CrossRef] - García, J.; Vega, V.; Iglesias, L.; Prida, V.M.; Hernando, B.; Barriga-Castro, E.D.; Mendoza-Reséndez, R.; Luna, C.; Görlitz, D.; Nielsch, K. Template-assisted Co–Ni alloys and multisegmented nanowires with tuned magnetic anisotropy. Phys. Status Solidi A
**2014**, 211, 1041–1047. [Google Scholar] [CrossRef] - Shevchenko, E.V.; Talapin, D.V.; Rogach, A.L.; Kornowski, A.; Haase, M.; Weller, H. Colloidal synthesis and self-assembly of CoPt
_{3}nanocrystals. J. Am. Chem. Soc.**2002**, 124, 11480–11485. [Google Scholar] [CrossRef] - Petit, C.; Legrand, J.; Russier, V.; Pileni, M.P. Three dimensional arrays of cobalt nanocrystals: Fabrication and magnetic properties. J. Appl. Phys.
**2002**, 91, 1502–1508. [Google Scholar] [CrossRef] - Maruo, S.; Nakamura, O.; Kawata, S. Three-dimensional microfabrication with two-photon-absorbed photopolymerization. Opt. Lett.
**1997**, 22, 132–134. [Google Scholar] [CrossRef] [Green Version] - Sun, H.B.; Matsuo, S.; Misawa, H. Three-dimensional photonic crystal structures achieved with two-photon-absorption photopolymerization of resin. Appl. Phys. Lett.
**1999**, 74, 786–788. [Google Scholar] [CrossRef] - Borisov, R.A.; Dorojkina, G.N.; Koroteev, N.I.; Kozenkov, V.M.; Magnitskii, S.A.; Malakhov, D.V.; Tarasishin, A.V.; Zheltikov, A.M. Fabrication of three-dimensional periodic microstructures by means of two-photon polymerization. Appl. Phys. B Lasers Opt.
**1998**, 67, 765–767. [Google Scholar] [CrossRef] - Chen, L.; Lopez-Garcia, M.; Taverne, M.; Zheng, X.; Ho, Y.; Rarity, J. Direct wide-angle measurement of a photonic band structure in a three-dimensional photonic crystal using infrared Fourier imaging spectroscopy. Opt. Lett.
**2017**, 42, 1584–1587. [Google Scholar] [CrossRef] [Green Version] - Hu, Y.; Miles, B.; Ho, Y.; Taverne, M.; Chen, L.; Gersen, H.; Rarity, J.; Faul, C. Toward Direct Laser Writing of Actively Tuneable 3D Photonic Crystals. Adv. Opt. Mater.
**2017**, 5, 1600458. [Google Scholar] [CrossRef] [Green Version] - Chen, L.; Morgan, K.; Alzaidy, G.; Huang, C.; Ho, Y.; Taverne, M.; Zheng, X.; Ren, Z.; Feng, Z.; Zeimpekis, I.; et al. Observation of Complete Photonic Bandgap in Low Refractive Index Contrast Inverse Rod-Connected Diamond Structured Chalcogenides. ACS Photonics
**2019**, 6, 1248–1254. [Google Scholar] [CrossRef] [Green Version] - Kumi, G.; Yanez, C.O.; Belfield, K.D.; Fourkas, J.T. High-speed multiphoton absorption polymerization: Fabrication of microfluidic channels with arbitrary cross-sections and high aspect ratios. Lab Chip
**2010**, 10, 1057–1060. [Google Scholar] [CrossRef] - Song, J.X.; Michas, C.; Chen, C.S.; White, A.E.; Grinstaff, M.W. From Simple to Architecturally Complex Hydrogel Scaffolds for Cell and Tissue Engineering Applications: Opportunities Presented by Two-Photon Polymerization. Adv. Healthc. Mater.
**2019**, 9, 1901217. [Google Scholar] [CrossRef] - Thompson, J.R.; Worthington, K.S.; Green, B.J.; Mullin, N.K.; Jiao, C.H.; Kaalberg, E.E.; Wiley, L.A.; Han, I.C.; Russell, S.R.; Sohn, E.H.; et al. Two-photon polymerized poly(caprolactone) retinal cell delivery scaffolds and their systemic and retinal biocompatibility. Acta Biomater.
**2019**, 94, 204–218. [Google Scholar] [CrossRef] - Worthington, K.; Thompson, J.; Shrestha, A.; Jiao, C.H.; Kaalberg, E.; Green, B.; Wiley, L.; Guymon, C.A.; Russell, S.; Sohn, E.; et al. Two-Photon Polymerized Poly(Caprolactone) as a High-Resolution, 3D-Printed Cell Delivery Scaffold. Mol. Ther.
**2018**, 26, 427. [Google Scholar] - Worthington, K.S.; Thompson, J.R.; Green, B.J.; Bunn, S.J.; Kaalberg, E.E.; Johnston, R.M.; Wiley, L.A.; Mullins, R.F.; Stone, E.M.; Guymon, C.A.; et al. Two-Photon Polymerization of High-Resolution 3D, Biodegradable Photoreceptor Cell Scaffolds. Investig. Ophthalmol. Vis. Sci.
**2017**, 58, 12300. [Google Scholar] - Fernandez-Pacheco, A.; Streubel, R.; Fruchart, O.; Hertel, R.; Fischer, P.; Cowburn, R. Three-dimensional nanomagnetism. Nat. Commun.
**2017**, 8, 1–14. [Google Scholar] [CrossRef] [PubMed] [Green Version] - Da Col, S.; Jamet, S.; Rougemaille, N.; Locatelli, A.; Mentes, T.; Burgos, B.; Afid, R.; Darques, M.; Cagnon, L.; Toussaint, J.; et al. Observation of Bloch-point domain walls in cylindrical magnetic nanowires. Phys. Rev. B
**2014**, 89, 180405. [Google Scholar] [CrossRef] [Green Version] - Schobitz, M.; De Riz, A.; Martin, S.; Bochmann, S.; Thirion, C.; Vogel, J.; Foerster, M.; Aballe, L.; Mentes, T.O.; Locatelli, A.; et al. Fast Domain Wall Motion Governed by Topology and OErsted Fields in Cylindrical Magnetic Nanowires. Phys. Rev. Lett.
**2019**, 123, 217201. [Google Scholar] [CrossRef] [PubMed] [Green Version] - Yan, M.; Andreas, C.; Kákay, A.; García-Sánchez, F.; Hertel, R. Fast domain wall dynamics in magnetic nanotubes: Suppression of Walker breakdown and Cherenkov-like spin wave emission. Appl. Phys. Lett.
**2011**, 99, 122505. [Google Scholar] [CrossRef] [Green Version] - Xia, J.; Zhang, X.; Yan, M.; Zhao, W.; Zhou, Y. Spin-Cherenkov effect in a magnetic nanostrip with interfacial Dzyaloshinskii-Moriya interaction. Sci. Rep.
**2016**, 6, 25189. [Google Scholar] [CrossRef] [Green Version] - Sheka, D.D.; Kravchuk, V.P.; Gaididei, Y. Curvature effects in statics and dynamics of low dimensional magnets. J. Phys. A Math. Theor.
**2015**, 48, 125202. [Google Scholar] [CrossRef] [Green Version] - Streubel, R.; Fischer, P.; Kronast, F.; Kravchuk, V.; Sheka, D.; Gaididei, Y.; Schmidt, O.; Makarov, D. Magnetism in curved geometries. J. Phys. D Appl. Phys.
**2016**, 49, 363001. [Google Scholar] [CrossRef] - Yershov, K.; Kravchuk, V.; Sheka, D.; Gaididei, Y. Curvature-induced domain wall pinning. Phys. Rev. B
**2015**, 92, 104412. [Google Scholar] [CrossRef] [Green Version] - Gaididei, Y.; Kravchuk, V.P.; Sheka, D.D. Curvature Effects in Thin Magnetic Shells. Phys. Rev. Lett.
**2014**, 112, 257203. [Google Scholar] [CrossRef] - Pimpin, A.; Srituravanich, W. Review on Micro- and Nanolithography Techniques and their Applications. Eng. J.
**2012**, 16, 37. [Google Scholar] [CrossRef] [Green Version] - Silva, F.A. Smart Sensors and MEMS: Intelligent Devices and Microsystems for Industrial Applications; IEEE: Piscataway, NJ, USA, 2014. [Google Scholar]
- Shaw, J.; Gelorme, J.; LaBianca, N.; Conley, W.; Holmes, S. Negative photoresists for optical lithography. IBM J. Res. Dev.
**1997**, 41, 81–94. [Google Scholar] [CrossRef] - Fischer, J. Three-dimensional Optical Lithography beyond the Diffraction Limit. Ph.D. Thesis, Karlsruhe Institute of Technology, Karlsruhe, Germany, 2012. [Google Scholar] [CrossRef]
- Swinehart, D.F. The Beer-Lambert Law. J. Chem. Educ.
**1962**, 39, 333. [Google Scholar] [CrossRef] - Zhou, X.; Hou, Y.; Lin, J. A review on the processing accuracy of two-photon polymerization. AIP Adv.
**2015**, 5, 030701. [Google Scholar] [CrossRef] - Cao, H.Z.; Zheng, M.L.; Dong, X.Z.; Jin, F.; Zhao, Z.S.; Duan, X.M. Two-photon nanolithography of positive photoresist thin film with ultrafast laser direct writing. Appl. Phys. Lett.
**2013**, 102, 201108. [Google Scholar] [CrossRef] - Nikogosyan, D.; Angelov, D. Formation of free-radicals in water under high-power laser uv irradiation. Chem. Phys. Lett.
**1981**, 77, 208–210. [Google Scholar] [CrossRef] - Reintjes, J.; Eckardt, R. Two-photon absorption an ADP and KD*P at 266.1 nm. IEEE J. Quantum Electron.
**1977**, 13, 791–795. [Google Scholar] [CrossRef] - Nathan, V.; Guenther, A.; Mitra, S. Review of multiphoton absorption in crystalline solids. J. Opt. Soc. Am. B Opt. Phys.
**1985**, 2, 294–316. [Google Scholar] [CrossRef] - Fox, M. Optical Properties of Solids; AAPT: College Park, MD, USA, 2002. [Google Scholar]
- Mills, I. Quantities, Units and Symbols in Physical Chemistry/Prepared for Publication by Ian Mills...[et al.]; Blackwell Science: Oxford, MI, USA; Boston, MA, USA; CRC Press: Boca Raton, FL, USA, 1993. [Google Scholar]
- Da Col, S.; Jamet, S.; Stano, M.; Trapp, B.; Le Denmat, S.; Cagnon, L.; Toussaint, J.; Fruchart, O. Nucleation, imaging, and motion of magnetic domain walls in cylindrical nanowires. Appl. Phys. Lett.
**2016**, 109, 4961058. [Google Scholar] [CrossRef] [Green Version] - Staňo, M.; Fruchart, O. Chapter 3—Magnetic Nanowires and Nanotubes. In Handbook of Magnetic Materials; Brück, E., Ed.; Elsevier: Amsterdam, The Netherlands, 2018; pp. 155–267. [Google Scholar]
- Bran, C.; Berganza, E.; Fernandez-Rolden, J.A.; Palmero, E.M.; Meier, J.; Calle, E.; Jaafar, M.; Foerster, M.; Aballe, L.; Rodriguez, A.F.; et al. Magnetization ratchet in cylindrical nanowires. ACS Nano
**2018**, 12, 5932–5939. [Google Scholar] [CrossRef] - Zhang, H.; Zhang, X.; Zhang, J.; Li, Z.; Sun, H. Template-Based Electrodeposition Growth Mechanism of Metal Nanotubes. J. Electrochem. Soc.
**2013**, 160, D41–D45. [Google Scholar] [CrossRef] - Gliga, S.; Seniutinas, G.; Weber, A.; David, C. Architectural structures open new dimensions in magnetism Magnetic buckyballs. Mater. Today
**2019**, 26, 100–101. [Google Scholar] [CrossRef] - Bran, C.; Ivanov, Y.P.; Kosel, J.; Chubykalo-Fesenko, O.; Vazquez, M. Co/Au multisegmented nanowires: A 3D array of magnetostatically coupled nanopillars. Nanotechnology
**2017**, 28, 095709. [Google Scholar] [CrossRef] [PubMed] - Ferain, E.; Legras, R. Track-etch templates designed for micro- and nanofabrication. Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms
**2003**, 208, 115–122. [Google Scholar] [CrossRef] - Apel, P.Y.; Dmitriev, S.N. Micro-and nanoporous materials produced using accelerated heavy ion beams. Adv. Nat. Sci. Nanosci. Nanotechnol.
**2011**, 2, 013002. [Google Scholar] [CrossRef] - Tsutsumi, N.; Fukuda, A.; Nakamura, R.; Kinashi, K.; Sakai, W. Fabrication of three-dimensional microstructures in positive photoresist through two-photon direct laser writing. Appl. Phys. A
**2017**, 123, 553. [Google Scholar] [CrossRef] - MicroChemicals. Development of Photoresists. Available online: http://www.microchemicals.com (accessed on 26 October 2019).
- Zeeshan, M.A.; Grisch, R.; Pellicer, E.; Sivaraman, K.M.; Peyer, K.E.; Sort, J.; Özkale, B.; Sakar, M.S.; Nelson, B.J.; Pané, S. Hybrid Helical Magnetic Microrobots Obtained by 3D Template-Assisted Electrodeposition. Small
**2014**, 10, 1284–1288. [Google Scholar] [CrossRef] - Alcantara, C.; Kim, S.; Lee, S.; Jang, B.; Thakolkaran, P.; Kim, J.; Choi, H.; Nelson, B.; Pane, S. 3D Fabrication of Fully Iron Magnetic Microrobots. Small
**2019**, 15, 1805006. [Google Scholar] [CrossRef] [Green Version] - Williams, G.I.; Hunt, M.; Boehm, B.; Ho, D.; Taverne, M.; Allenspach, R.; Rarity, J.; Ladak, S. Two photon lithography for 3D Magnetic Nanostructure Fabrication. Nano Res.
**2018**, 11, 845–854. [Google Scholar] [CrossRef] - Sahoo, S.; Mondal, S.; Williams, G.; May, A.; Ladak, S.; Barman, A. Ultrafast magnetization dynamics in a nanoscale three-dimensional cobalt tetrapod structure. Nanoscale
**2018**, 10, 9981–9986. [Google Scholar] [CrossRef] [Green Version] - Schurch, P.; Petho, L.; Schwiedrzik, J.; Michler, J.; Philippe, L. Additive Manufacturing through Galvanoforming of 3D Nickel Microarchitectures: Simulation-Assisted Synthesis. Adv. Mater. Technol.
**2018**, 3, 1800274. [Google Scholar] [CrossRef] - Richardson, D.; Kingston, S.; Rhen, F.M.F. Synthesis and Characterization of Ni-Fe-B Nanotubes. IEEE Trans. Magn.
**2015**, 51, 1–4. [Google Scholar] [CrossRef] - Tierno, P.; Goedel, W.A. Using Electroless Deposition for the Preparation of Micro Sized Polymer/Metal Core/Shell Particles and Hallow Metal Spheres. J. Phys. Chem. B
**2006**, 110, 3043–3050. [Google Scholar] [CrossRef] [PubMed] - Yan, Y.; Rashad, M.; Teo, E.; Tanoto, H.; Teng, J.; Bettiol, A. Selective electroless silver plating of three dimensional SU-8 microstructures on silicon for metamaterials applications. Opt. Mater. Express
**2011**, 1, 1548–1554. [Google Scholar] [CrossRef] - Wang, W.-K.; Sun, Z.-B.; Zheng, M.-L.; Dong, X.-Z.; Zhao, Z.-S.; Duan, X.-M. Magnetic Nickel–Phosphorus/Polymer Composite and Remotely Driven Three-Dimensional Micromachine Fabricated by Nanoplating and Two-Photon Polymerization. J. Phys. Chem. C
**2011**, 115, 11275–11281. [Google Scholar] [CrossRef] - Kavaldzhiev, M.; Perez, J.; Ivanov, Y.; Bertoncini, A.; Liberale, C.; Kosel, J. Biocompatible 3D printed magnetic micro needles. Biomed. Phys. Eng. Express
**2017**, 3, 025005. [Google Scholar] [CrossRef] - Kim, S.; Qiu, F.; Kim, S.; Ghanbari, A.; Moon, C.; Zhang, L.; Nelson, B.J.; Choi, H. Fabrication and Characterization of Magnetic Microrobots for Three-Dimensional Cell Culture and Targeted Transportation. Adv. Mater.
**2013**, 25, 5863–5868. [Google Scholar] [CrossRef] [PubMed] [Green Version] - Tottori, S.; Zhang, L.; Qiu, F.; Krawczyk, K.; Franco-Obregon, A.; Nelson, B. Magnetic Helical Micromachines: Fabrication, Controlled Swimming, and Cargo Transport. Adv. Mater.
**2012**, 24, 811–816. [Google Scholar] [CrossRef] - May, A.; Hunt, M.; van den Berg, A.; Hejazi, A.; Ladak, S. Realisation of a 3D frustrated magnetic nanowire lattice. Commun. Phys.
**2019**, 2, 13. [Google Scholar] [CrossRef] [Green Version] - Donnelly, C.; Scagnoli, M.G.-S.V.; Holler, M.; Huthwelker, T.; Menzel, A.; Vartiainen, I.; Müller, E.; Kirk, E.; Gliga, S.; Raabe, J.; et al. Element-Specific X-Ray Phase Tomography of 3D Structures at the Nanoscale. Phys. Rev. Lett.
**2015**, 114, 115501. [Google Scholar] [CrossRef] - Fischer, P.; Ohldag, H. X-rays and magnetism. Rep. Prog. Phys.
**2015**, 78, 094501. [Google Scholar] [CrossRef] - Gilbertson, A.M.; Benstock, D.; Fearn, M.; Kormanyos, A.; Ladak, S.; Emeny, M.T.; Lambert, C.J.; Ashley, T.; Solin, S.A.; Cohen, L.F. Sub-100-nm negative bend resistance ballistic sensors for high spatial resolution magnetic field detection. Appl. Phys. Lett.
**2011**, 98, 062106. [Google Scholar] [CrossRef] [PubMed] [Green Version] - Liao, P.; Li, J.; Zhang, S.; Sun, D. A Fish-like Magnetically Propelled Microswimmer Fabricated by 3D Laser Lithography. In Proceedings of the 2018 IEEE International Conference on Robotics and Automation (Icra), Brisbane, Australia, 21–25 May 2018; pp. 3581–3586. [Google Scholar]
- Spanos, I.; Selimis, A.; Farsari, M. 3D magnetic microstructures. Procedia CIRP
**2018**, 74, 349–352. [Google Scholar] [CrossRef] - Zipfel, W.R.; Williams, R.M.; Webb, W.W. Nonlinear Magic: Multiphoton Microscopy in the Biosciences. Nat. Biotechnol.
**2003**, 21, 1369–1377. [Google Scholar] [CrossRef] [PubMed] - Fischer, J.; Wegener, M. Three-dimensional optical laser lithography beyond the diffraction limit. Laser Photonics Rev.
**2013**, 7, 22–44. [Google Scholar] [CrossRef] - Mueller, P.; Thiel, M.; Wegener, M. 3D direct laser writing using a 405 nm diode laser. Opt. Lett.
**2014**, 39, 6847–6850. [Google Scholar] [CrossRef] - Park, S.; Lim, T.; Yang, D.; Kim, R.; Lee, K. Improvement of spatial resolution in nano-stereolithography using radical quencher. Macromol. Res.
**2006**, 14, 559–564. [Google Scholar] [CrossRef] - Wollhofen, R.; Katzmann, J.; Hrelescu, C.; Jacak, J.; Klar, T.A. 120 nm resolution and 55 nm structure size in STED-lithography. Opt. Express
**2013**, 21, 10831–10840. [Google Scholar] [CrossRef] - Fischer, J.; Mueller, J.B.; Quick, A.S.; Kaschke, J.; Barner-Kowollik, C.; Wegener, M. Exploring the Mechanisms in STED-Enhanced Direct Laser Writing. Adv. Opt. Mater.
**2015**, 3, 221–232. [Google Scholar] [CrossRef] - Seniutinas, G.; Weber, A.; Padeste, C.; Sakellari, I.; Farsari, M.; David, C. Beyond 100 nm Resolution in 3D Laser Lithography—Post Processing Solutions. Microelectron. Eng.
**2018**, 191, 25–31. [Google Scholar] [CrossRef] [Green Version] - Vyatskikh, A.; Delalande, S.; Kudo, A.; Zhang, X.; Portela, C.; Greer, J. Additive manufacturing of 3D nano-architected metals. Nat. Commun.
**2018**, 9, 1–8. [Google Scholar] [CrossRef] - Ruiz-Clavijo, A.; Ruiz-Gomez, S.; Caballero-Calero, O.; Perez, L.; Martin-Gonzalez, M. Tailoring Magnetic Anisotropy at Will in 3D Interconnected Nanowire Networks. Phys. Status Solidi Rapid Res. Lett.
**2019**, 13, 1900263. [Google Scholar] [CrossRef] - Huth, M.; Porrati, F.; Schwalb, C.; Winhold, M.; Sachser, R.; Dukic, M.; Adams, J.; Fantner, G. Focused electron beam induced deposition: A perspective. Beilstein J. Nanotechnol.
**2012**, 3, 597–619. [Google Scholar] [CrossRef] [PubMed] [Green Version] - Van Dorp, W.F.; van Someren, B.; Hagen, C.W.; Kruit, P.; Crozier, P.A. Approaching the resolution limit of nanometer-scale electron beam-induced deposition. Nano Lett.
**2005**, 5, 1303–1307. [Google Scholar] [CrossRef] [PubMed] - Skoric, L.; Sanz-Hernández, D.; Meng, F.; Donnelly, C.; Merino-Aceituno, S.; Fernández-Pacheco, A. Fernández-Pacheco. Layer-by-layer growth of complex-shaped three-dimensional nanostructures with focused electron beams. Nano Lett.
**2020**, 20, 184–191. [Google Scholar] [CrossRef] - Cordoba, R.; Sharma, N.; Kolling, S.; Koenraad, P.M.; Koopmans, B. High-purity 3D nano-objects grown by focused-electron-beam induced deposition. Nanotechnology
**2016**, 27, 355301. [Google Scholar] [CrossRef] - Sanz-Hernandez, D.; Hamans, R.F.; Liao, J.W.; Welbourne, A.; Lavrijsen, R.; Fernandez-Pacheco, A. Fabrication, Detection, and Operation of a Three-Dimensional Nanomagnetic Conduit. ACS Nano
**2017**, 11, 11066–11073. [Google Scholar] [CrossRef]

**Figure 1.**Illustration of laser intensity profile for (

**a**) resist with high-concentration photoinitiator; (

**b**) resist with low-concentration photoinitiator; (

**c**) two-photon absorption profile [44].

**Figure 2.**Energy level schematic for single photon absorption and two-photon absorption processes. Dotted line indicates imaginary intermediary state.

**Figure 3.**Use of TPL and electrochemical deposition to fabricate 3D magnetic nanostructures. (

**a**) Spin-coating of a positive resist onto a conductive substrate; (

**b**) Two-photon lithography of a 3D structure into the positive resist; (

**c**) Electrodeposition of magnetic material into the channels; (

**d**) Lift off of the resist.

**Figure 4.**(

**a**) Tilted SEM image of a single Co tetrapod structure; (

**b**) Top-down SEM of a single Co tetrapod structure; (

**c**) SEM micrograph of an individual wire within a tetrapod structure (left) and spin-polarised SEM images showing x and y-components of magnetisation in an as-deposited sample (middle and right); (

**d**) Longitudinal MOKE loop obtained from tetrapod array with field applied along the projection of the lower wires onto the substrate [65].

**Figure 5.**Illustration depicting the fabrication of a 3D arrangement of magnetic nanowires, via TPL and LOS deposition. (

**a**) Exposure of photoresist during TPL; (

**b**) Polymer scaffold after development of unexposed photoresist; (

**c**) Resulting magnetic nanowires and sheet film, following deposition of a thin magnetic film.

**Figure 6.**Physical characterisation of a cobalt-coated buckyball. (

**a**) SEM images displaying the buckyball mounted upon a tomography pin (left) and a magnified image of the fabricated structure (right); (

**b**) 3D rendering of the buckyball composition, obtained by x-ray tomography, cobalt is indicated by orange contrast whilst photoresist is blue; (

**c**) Fluorescence spectra of cobalt deposited upon the polymer scaffold, upon the pillar and cobalt oxide detected in transmission [76].

**Figure 7.**Structural and magnetic characterisation of a 3D N

_{i81}Fe

_{19}nanowire lattice. (

**a,b**) SEM images of the nanowire lattice observed from top view and a 45° tilt respectively; (

**c**) Schematic of the Ni

_{81}Fe

_{19}nanowires (grey) upon a polymer scaffold (yellow), where the effects of shadowing by upper nanowire layers during LOS deposition is evident. Inset: Nanowire cross-sectional geometry; (

**d**) MOKE data captured from the sheet film (blue) and nanowire lattice—this is separated into up-sweep (black) and down-sweep (red) [75].

**Figure 8.**SEM image of a woodpile structure fabricated by TPL with a laser of wavelength 405 nm. Smallest line width is reported to be 68 nm. Reprinted with permission from ref [83] © The Optical Society.

**Figure 9.**Schematic showing the point spread functions in stimulated emission depletion two-photon lithography [82]. Here the excitation beam is red, depletion beam taking the form of a bottle-beam mode is green, and this yields an effective exposure as shown in purple.

**Figure 10.**Illustration of reduction in feature size by separate use of oxygen plasma etching, pyrolysis and then a combination of the two techniques [87].

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## Share and Cite

**MDPI and ACS Style**

Hunt, M.; Taverne, M.; Askey, J.; May, A.; Van Den Berg, A.; Ho, Y.-L.D.; Rarity, J.; Ladak, S.
Harnessing Multi-Photon Absorption to Produce Three-Dimensional Magnetic Structures at the Nanoscale. *Materials* **2020**, *13*, 761.
https://doi.org/10.3390/ma13030761

**AMA Style**

Hunt M, Taverne M, Askey J, May A, Van Den Berg A, Ho Y-LD, Rarity J, Ladak S.
Harnessing Multi-Photon Absorption to Produce Three-Dimensional Magnetic Structures at the Nanoscale. *Materials*. 2020; 13(3):761.
https://doi.org/10.3390/ma13030761

**Chicago/Turabian Style**

Hunt, Matthew, Mike Taverne, Joseph Askey, Andrew May, Arjen Van Den Berg, Ying-Lung Daniel Ho, John Rarity, and Sam Ladak.
2020. "Harnessing Multi-Photon Absorption to Produce Three-Dimensional Magnetic Structures at the Nanoscale" *Materials* 13, no. 3: 761.
https://doi.org/10.3390/ma13030761