Polarimetric terahertz imaging systems based on tailored metasurface optics

Nowack, Thomas Stephan (2022) Polarimetric terahertz imaging systems based on tailored metasurface optics. PhD thesis, University of Glasgow.

Abstract

Terahertz radiation has long been highly anticipated for a wide range of advanced imaging applications due to a range of special inherent properties. Spectral fingerprints of many molecules within the terahertz spectrum offer prospects for spectroscopic imaging, while the low energetic impact and biocompatibility of terahertz photons raise prospects for biomedical and security imaging. Furthermore, the high achievable resolution in the submillimetre range together with high transparency for common packaging materials are opening applications in non-destructive testing and (agri-food) production control. However, common obstacles for all these applications are found in the limited signal strength of terahertz sources, the high (thermal) noise levels of (uncooled) detectors, and the lossy optical components in-between. Terahertz setups are thus typically somewhat limited in their complexity and imaging capabilities as a result of this low signal-to-noise ratio (SNR).

This thesis aims to alleviate the SNR constraints of terahertz setups by developing metasurface optics that provides polarimetric imaging capabilities in an ultra-compact, efficient and easy to integrate format. The state of polarisation is expected to be particularly suited as a contrast channel since it is robust to both low-light-level conditions and common sources of noise, such as unpolarised background radiation, white noise, and thermallyinduced noise. Furthermore, the state of polarisation can provide important additional information on the images sample and its microstructure. To demonstrate this concept within two prototype setups, four major research objectives were addressed in this thesis: First, a methodology to design all-dielectric metasurface optics from individual anisotropic meta-pillars to their collective response based on simulations was developed and successfully tested. Such metasurface optics showed unparalleled capabilities in manipulating both the transmitted phase and polarisation front into well-defined shapes that can be tailored to their intended application.

In a second step, a method for the monolithic fabrication of the metasurface designs into low-cost silicon substrates was developed based on a single photolithography step that included an optimised Bosch etch. The presented fabrication technique raises the prospect of mass-production and included the reliable quantification of fabrication tolerances to achieve process control over the dry etch, as well as the well-founded estimation of associated optical performance deviations.

The third research objective aimed to demonstrate beam forming metasurface optics that created a non-diffracting Bessel beam with a polarisation profile that varied in all three spatial dimensions in a well-defined manner. The utility of such 3D-polarised probe beams to measure a sample’s refractive index or polarisation-dependent behaviour was explored in theory and confirmed with both simulations and laser experiments. Polarimetric imaging experiments employing the metasurface to illuminate a resolution target and composite sample showed a remarkable resolution of ≤1.5 λ0 and the ability to identify electrically conductive carbon fibres (measured diameter: 5.1 µm +- 1.4 µm) based on their polarisation signature, respectively.

Finally, a beam analysing metasurface was developed to precisely determine the incident state-of-polarisation within a single image. A sophisticated measurement methodology including an initial calibration ensured a robust, reliable and accurate operation with an averaged measurement accuracy of 92.1 % ± 4.2 % for the incident state of polarisation obtained from 64 measurements.

This thesis thus demonstrates the benefits offered by metasurface optics to create compact optical setups with advanced capabilities for sensing and imaging applications in NDT, as well as the validity of the strategy to utilise the state of polarisation as a contrast channel that remains robust to low-light conditions and the low SNR often encountered at terahertz frequencies. Beyond the presented results, much potential remains to extend the capabilities of both the presented metasurface optics and the optical setups.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: T Technology > T Technology (General)
• Colleges/Schools: College of Science and Engineering > School of Engineering
• Supervisor's Name: Cumming, Professor David R.S. and Wasige, Professor Edward
• Date of Award: 2022
• Depositing User: Theses Team
• Unique ID: glathesis:2022-83373
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 20 Jan 2023 10:31
• Last Modified: 14 Jun 2023 12:25
• Thesis DOI: 10.5525/gla.thesis.83373
• URI: http://theses.gla.ac.uk/id/eprint/83373

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