Strategies for Design and Fabrication of Nanoplasmonic Metasurfaces with Extreme Geometric Features
Creators: Semple, Mitchell J T
Publisher: name: University of Alberta Library
Year: 2022

<p>The ever-increasing integration density of optical components has pushed engineers and physicists to rethink the fundamentals of bulky dielectric lensing systems. Metasurfaces, which are two-dimensional, periodic arrays of metallic (plasmonic) or dielectric scatterers that exhibit exotic interactions with light, have been suggested as a robust and easily-integrated alternative to perform a variety of optical functions. In particular, plasmonic metasurfaces have a number of advantages when applied to imaging systems and sensors due to their small size and their ability to strongly enhance electric fields. Unfortunately, intuitive design methods have failed to meet physical performance requirements. Computational inverse design methods are now being explored to approach these limits, but are hindered by practical nanofabrication considerations. One of the main enabling technologies for nanofabrication of plasmonic metasurfaces has been the Ga+ focused ion beam, which uses a stream of heavy ions to knock away sample material. The recently commercialized He+ focused ion beam is able to pattern metals more accurately, but thus far, has suffered from low reproducibility, which has made fabricating large devices such as metasurfaces nearly impossible.</p>

<p>This thesis pushes the boundaries of top-down plasmonic gold metasurface nanofabrication to a feature size of 10 nm using helium ion beam milling. Metasurfaces demand stringent fabrication performance, often requiring hundreds of identical elements with fine, complex features and large aspect ratios (depth:feature size). As examples, three metasurfaces were designed using conventional design methods, two to make full use of the potential of 10-nm features -- a mere 60 atoms across -- with the helium ion microscope, and one contrasting design making use of 100-nm features patterned by electron beam lithography.</p>

<p>The first design acts as a bandpass filter for near-infrared wavelengths, making use of fine plasmonic kinetic inductive nanowires lining an aperture. An equivalent circuit model is developed using generalizable techniques that can predict the performance of the metasurface near resonance. The designed metasurface uses 10-nm wide and 50-nm thick nanowires that meet at the centre, separated by 10-nm wide nanogaps. The structure is used to study the ion beam control parameters in detail, and the most impactful parameters to increase reliability are found to be the ion dosage and ion beam current. A prototype is fabricated over a large 225 um^2 (&gt;90 lambda^2) area, and fabrication defects are analyzed in detail using representative simulations to show that even with tuned beam parameters, grain defects from the gold crystal structure, redeposition of the milled material, and poor substrate contrast of the nanogaps suppress the metasurface performance. A subwavelength imaging metasurface making use of this design is proposed, which can achieve a subdiffraction resolution of at least lambda/3.6 in simulation.</p>

<p>The second design acts as a polarization filter for visible wavelengths and is accurately modelled using the same equivalent-circuit approach developed for the previous design. It is shown that the aforementioned fabrication challenges can be overcome by patterning single-crystal gold films specially grown on lattice-matched LiF crystals and by removing the LiF substrate through dissolution in HF. The fabricated prototypes show reliable 10-nm features with a variation on the order of 1 nm, fabricated over an area of &gt;12um^2 (&gt;16 lambda^2). The metasurface is experimentally characterized and shows a clear separation between orthogonal polarizations. This design is then used to simulate a metasurface that refracts a normal-incidence plane wave to 48.5 degrees with an efficiency above 2%. </p>

<p>Finally, the third design acts as a CO2 sensor in the mid-infrared. Plasmonic nanowires are combined with meandered capacitors with a 100-nm feature size to strongly enhance the local electric field around the metasurface, showing a nearly $5\times$ enhancement over the conventional approach. A simple electron-beam lithography lift-off process is developed and validated. The fabricated metasurfaces are integrated into a custom gas cell and experimentally show a strong enhancement of the bending resonance of CO2 at a wavelength of 15 um. The addition of a thin functionalization layer is studied numerically, showing a significant increase in the absorption enhancement.</p>