4PI Two-photon Lithography for Isotropic 3D Nanostructure Fabrication

Lead Research Organisation: Cardiff University
Department Name: School of Physics and Astronomy

Abstract

Optical lithography is a process that utilises light to define a specific pattern within a material. Standard optical lithography is capable of patterning materials in two dimensions and the possible feature size scales with the wavelength of the light. It is research into this process and associated techniques that has been one of the main drivers of the technological revolution, is partly responsible for the reduction of areal density within computer hard drives and the doubling of processor power every 18 months (Moore's Law).

As we progress through the 21st century it is likely that 3D architectures on the nanoscale will become important in developing advanced materials for future data processing and storage technologies. Two-photon lithography is a 3D fabrication methodology that has recently been commercialised and is having a huge impact upon science, allowing the fabrication of bespoke 3D geometries on a length-scale of 200nm horizontally and 500nm vertically. Commercial two-photon lithography has made the fabrication of 3D systems on the several-100nm scale accessible to scientists in a variety of fields allowing the realisation of swimming micro-robots for targeted drug delivery, bioscaffolds and a range of photonic and mechanical metamaterials. A significant setback with two-photon lithography is the asymmetry in the lateral and vertical resolution, which limits both the absolute size and the type of geometry that can be realised.

In this proposal, we are going to utilise our world-leading expertise in non-linear microscopy to modify a commercial two-photon lithography system and obtain enhanced resolution. We will utilise techniques that have already significantly improved the resolution in fluorescence microscopy in order to achieve a 100nm isotropic resolution. The newly built system will be used by our team to fabricate two types of 3D nanoscale magnetic materials, in geometries and on length-scales that are difficult to achieve using other fabrication methodologies. Our work in this area will pave the way for next generation 3D memory technolgies such as magnetic racetrack memory and help us to understand magnetic charge transport in novel magnetic materials.

In addition, we will be working with project partners in the regenerative medicine and photonics communities in order to realise a number of novel 3D nanostructured materials. Firstly, we will work with stem cell researchers in order to fabricate artificial tissues that will be used in stem cell differentiation experiments. Our work here will provide a fascinating insight into the role of nanoscale topography upon stem cell differentiation and may eventually have applications in tissue/organ growth. Secondly, we will work with academics studying photonic crystals - artificial materials that are capable of blocking electromagnetic radiation within a certain range of the spectrum. The majority of 3D photonic crystals that have been made to date are capable of attenuating electromagnetic waves that are outside the visible range of the spectrum, limiting applications in optoelectronics. Our work here will allow the fabrication and measurement of photonic crystals that can be used with visible and infra-red light. This work may pave the way to next generation three-dimensional optical circuits that can be utilised by telecommunication industries.

Overall, this project will build an internationally unique instrument and utilise it to fabricate a range of advanced materials. This will put the U.K. at the forefront of 3D lithography technologies and the associated biomedical, magnetic and photonic materials that will be realised using our newly built instrument.

Planned Impact

Impact for this project will be realised by many different routes.

U.K. Availability: The commissioned 4Pi two-photon lithography system will be capable of a 100nm isotropic resolution and will be internationally unique. We have already identified a number of project partners that will utilise this tool to fabricate a wide-range of advanced materials for biomedical and photonic applications. In addition we will advertise instrument capability at relevant conferences and allocate 1 day of instrument time per week to work with project partners.

Society and Economy: By focussing upon the fabrication of world-leading advanced materials we will maximise societal impact. One of the main project objectives is to study 3D nanomagnetic wires and structures that have the potential to be used within a next generation data storage device known as magnetic racetrack memory. The amount of data stored by an individual is increasing rapidly and over the next 20 years it is likely that a paradigm shift in data storage technology will occur. Once realised, magnetic racetrack memory has the potential to replace the computer hard drive, offering ultrahigh storage densities and orders of magnitude increase in data read-out.

Societal impact is also likely to be achieved by the work done in collaboration with project partners.The use of stem cells within medicine is likely to produce a revolution in medical treatment. One of the key underlying questions in stem cell research today is the factors that govern differentiation into the various cell types. Recent studies have suggested both the nanoscale topography and mechanical properties of tissue infrastructure play a role in differentiation. Here, by fabricating a range of 3D cell scaffolds with different topography and micro-mechanical properties we will be provided with an unparalleled insight into the dominant factors that influence stem cell differentiation. This work may lead to new therapies to regenerate damaged tissues or eventually to grow entire organs.
The photonics industry is a rapidly growing industry and that is expected to have market volume of 650 Euro by 2020. Our project partners at Bristol University will be fabricating 3D photonic crystals with band-gaps in the visible. These devices have the potential to be be used as next generation switches in high-end 3D optical networks.

Training of young scientists: This grant will employ two postdoctoral workers that will be trained in a variety of experimental techniques and will also gain experience with the building of bespoke optical equipment. In addition, the postdoctoral workers will be given the opportunity to co-supervise a PhD student that will be working upon the project. This PhD student, that is being funded by Cardiff University, will be trained in a variety of fabrication and characterisation methodologies within magnetism and will also be given the opportunity to visit IBM, Zurich and carry out imaging studies upon samples.

Exploitation: Since we will be building an internationally unique instrument as well as a number of industrially relevant magnetic nanostructures, we will carefully monitor and manage any exploitable knowledge with help and advise from Cardiff University partner Fusion IP. When possible, we will exploit our results via the filing of patents, licensing of our technologies, joint ventures or via industrial-led deals.

Dissemination: By building an internationally unique facility and by utilising it to manufacture a range of advanced materials we will maximise the possibility of publishing in high impact journals. We will also target leading international conferences in magnetism, photonics and regenerative medicine.

Communication and Engagement: We will work with our departmental outreach officer in order to produce material that can be presented both at national science festivals and also in local schools. We expect to present to 3 schools per year and 2 national festivals.
 
Title Realisation of a frustrated 3D magnetic nanowire lattice 
Description Magneto-optical Kerr Effect Data A 150 mW, 650 nm laser was attenuated to a power of approximately 50 mW, expanded to a diameter of 1 cm, and passed through a Glan-Taylor polarizer to obtain an s-polarized beam. The beam was then focused onto the sample using an achromatic doublet (f= 30 cm), to obtain a spot size of approximately 50 ┬Ám2. The reflected beam was collected using an achromatic doublet (f = 10 cm) and passedthrough a second Glan-Taylor polarizer, from which the transmitted and reflected beams were directed onto two amplified Si photodetectors, yielding the Kerr and reference signals, respectively. A variable neutral density filter was used to ensure that the reference and Kerr signals were of similar values. Subtraction of the reference from the Kerr signal compensates for any change in the laser intensity drift and also eliminates any small transverse Kerr effect from the signal. Here, we provide MOKE data for when the laser spot was placed onto the lattice (Lattice_MOKE.dat) and the film (Film_MOKE.dat). In both cases the first column is the magnetic field in units of mT and the second column is a scaled detector voltage in Volts. Finite element simulations A series of micro-magnetic simulations using finite element method discretisation were performed using the NMAG code [41]. These simulations are performed by numerical integration of the Landau-Liftshitz equation upon a finite element mesh. Typical Ni81Fe19 parameters were used with zero magnetocrystalline anisotropy. The simulations were performed at a temperature of 0 K that has previously been shown to capture the correct spin-texture seen in room temperature measurements but a systematic difference in coercivity (factor of ~5) is observed. The wire cross section is a crescent shape where the arcs subtend a 160 degree angle. The inner arc is defined from a circle with 80 nm radius corresponding to the 160 nm lateral feature size of the TPL system. Line of sight deposition results in a film thickness proportional to the scalar product of the deposition direction and the surface normal therefore, the outer arc is based on an ellipse with an 80 nm minor radius and 130 nm major radius. The length of the wires is set to 780nm, due to computational restraints and the wires are arranged as single wires, bipod and tetrapod structures. Here we present remnant states for the bipod and tetrapod structures. These are in the vtk file format and can be viewed with free software such as Paraview. Bipod files: 180404_l_l_bipod_00-000000. vtk : Two-out state with vortex wall 180404_l_l_bipod_01-000000. vtk : One-in / One-out state Tetrapod files: Here the naming convention follows as described in the supplementary Fig 7 of the paper: Here the ID is a 4-bit binary number where each bit corresponds to one of the wires in the tetrapod and a 1 indicates that the wire is magnetised towards the vertex and a 0 indicates the wire magnetisation away from the vertex. Starting from the left most bit, each bit refers to: Lower wire along [1,1,-1] Lower wire along [-1,-1,-1] Upper wire along [-1,1,1] Upper wire along [1,-1,1] Filenames: tetra_0001_final-000004.vtk tetra_0100_final-000003.vtk tetra_1001_final-000002.vtk tetra_1100_final-000001.vtk 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes