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Development of metamaterial lenslet arrays for cosmic microwave background experiments

Thompson, Jonathan 2021. Development of metamaterial lenslet arrays for cosmic microwave background experiments. PhD Thesis, Cardiff University.
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The use of sub-wavelength meshes consisting of square metallic patches (and their complement) embedded in a dielectric substrate for the implementation of optical devices in the millimetre wave bands is well established. Many millimetre wave astronomical instruments incorporate low-pass filters and dichroics using this technology while wave plates and graded index lenses are starting to appear. Combining many lenses into a lenslet array as a method for collecting light onto a focal plane sensor matrix for cosmic microwave background instruments is an active area of research. Reported here is work carried out with the aim of improving the properties of graded index lenses constructed using the mesh technology in lenslet arrays. A genetic algorithm was developed to search for patch shapes that would improve the basic low-pass filter. It was discovered that square patches with rounded corners could slightly improve the passband performance of Chebyshev response filters. Deliberately designing a new shape that increases the capacitance of the mesh did lead to a patch with useful characteristics. A patch that starts as a square and then grows fingers into the adjacent patch was proposed. It was shown that this super-capacitive patch could be used to create a low-pass filter with an extended stopband before the diffraction limit and an artificial dielectric with an increased refractive index. Use of the super-capacitive patch in graded index lenslets reduced the number of mesh layers in a polypropylene lens but did not work for a silicon substrate due to the cut-off frequency of the resulting artificial dielectric. A novel curved column design was proposed that better matches the path of light through the lens. This allowed another saving in layers along with the ability to accurately account for the effect of non-normal incidence of waves on the artificial dielectrics and was applicable to silicon.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Physics and Astronomy
Subjects: Q Science > QB Astronomy
Uncontrolled Keywords: Metamaterials, Lenslet Array, GRIN Lens, Millimeter waves, Filters, FDTD, Genetic Algorithm, Artificial Dielectrics
Date of First Compliant Deposit: 19 August 2021
Last Modified: 27 Sep 2022 01:26

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