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

Abstract

Three-dimensional nanostructured magnetic materials have recently been the topic of intense interest since they provide access to a host of new physical phenomena. Examples include new spin textures that exhibit topological protection, magnetochiral effects and novel ultrafast magnetic phenomena such as the spin-Cherenkov effect. Two-photon lithography is a powerful methodology that is capable of realising 3D polymer nanostructures on the scale of 100 nm. Combining this with postprocessing and deposition methodologies allows 3D magnetic nanostructures of arbitrary geometry to be produced. In this article, the physics of two-photon lithography is first detailed, before reviewing the studies to date that have exploited this fabrication route. The article then moves on to consider how non-linear optical techniques and post-processing solutions can be used to realise structures with a feature size below 100 nm, before comparing two-photon lithography with other direct write methodologies and providing a discussion on future developments.

Keywords: magnetism; nanoscale; nanostructures; three-dimensional; two-photon lithography.

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_i81Fe₁₉ nanowire lattice. (a,b) SEM images of the nanowire lattice observed from top view and a 45° tilt respectively; (c) Schematic of the Ni₈₁Fe₁₉ 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].