Investigating the use of two-photon polymerisation for the creation of gradient index optics

Woods, Emma (2022) Investigating the use of two-photon polymerisation for the creation of gradient index optics. PhD thesis, University of Nottingham.

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Metamaterials consist of repeating unit cells resulting in a homogeneous averaged electromagnetic (EM) response. Spatially varying EM properties can be created by introducing variations within each unit cell. In this way gradient index (GRIN) metamaterials offer a route to developing GRIN optics with larger refractive index (RI) gradients compared to traditional GRIN optics. By altering the length scale of the features within the metamaterial, the lens can be designed to work in different parts of the EM spectrum opening up new avenues for telecommunications and integrated optics.

The aim of this work was to investigate the use of two-photon polymerisation (TPP), an additive manufacturing technique, to fabricate a GRIN metamaterial lens. The lens was to work in the 1µm−2µm wavelength region (corresponding to 150THz−300THz) necessitating the nanoscale feature sizes TPP provides. Compared to conventional nanofabrication techniques, TPP is a truly 3D fabrication process that can open up the design space of metamaterial geometries.

Firstly, the line widths of a commercial resin, IP-L (Nanoscribe GmbH), at different laser powers and scan speeds were measured by fabricating ascending scan arrays using the commercial Nanoscribe GmbH Photonic Professional GT. By changing the laser dose, the focal spot of the laser is changed which results in different cured line dimensions. The minimum achievable line width determines the lowest wavelength the metamaterial would work at. Whereas the range of line widths determines the amount by which the metamaterial unit cells can be altered to produce the spatially varying properties. This work found the maximum line width to be 561 nm ± 21 nm and the minimum to be 273 nm ± 32 nm, resulting in a range of 228 nm ± 53 nm.

The capabilities of the TPP system were investigated by fabricating woodpile structures with different laser power parameters across the devices. Two different substrate preparation techniques were compared, as well as fabricating structures with and without frames for added structural stability. The initial investigation led to the design of the metamaterial structure used in this work; the cylindrical fishnet. This structure is made up of layers of concentric rings filled in by an increasing number of spokes in successive rings. This results in a structure somewhat like a circularly deformed fishnet structure.

Analytical calculations were undertaken to estimate the RI based on the filling fraction of the cylindrical lens structure for different line widths and number of spokes between each ring. A hyperbolic secant (sech) profile is normalised to the maximum and minimum analytical RI results and different line width / number of spokes combinations were selected which fall along this curve. Four lenses were designed in this way, and the 7 selected unit cells for each were simulated using finite element method modelling in COMSOL to investigate the geometry dependent RI. A rectangular cut out geometry was used to approximate the unit cells for simulation to reduce computational requirements. The results were compared to the ideal sech curve as well as the analytical RI calculations for both the full cylindrical fishnet and the rectangular cut out. The analytical calculations for the rectangular cut out did not appear to predict the simulated results better than the cylindrical fishnet analytical calculations.

Using the analytical and simulated RI results, the focal lengths of the four lenses were predicted analytically. COMSOL simulations were used to investigate the focal length and relative focal spot size for each of the four lenses for the ideal and simulated profiles. Finally, proof of concepts lenses were fabricated highlighting the current capabilities of TPP and further areas of research for fabrication.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Tuck, Christopher
Fromhold, Mark
Wildman, Ricky
Keywords: Two-photon polymerisation, metamaterials, GRIN metamaterial lens, 3D printing
Subjects: Q Science > QC Physics > QC350 Optics. Light, including spectroscopy
T Technology > TS Manufactures
Faculties/Schools: UK Campuses > Faculty of Engineering
Item ID: 71748
Depositing User: Woods, Emma
Date Deposited: 13 Dec 2022 04:40
Last Modified: 13 Dec 2022 04:40

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