Gallium phase change as a mechanism for resonance tuning in photonic nanostructures

Abstract

This thesis covers contributions to the field of electromagnetic metamaterials, a broad, dynamic area that has produced a considerable array of new phenomena and is now looking to the prospect of tuning and switching the characteristics of novel materials and surfaces, using many means including thermal and electromagnetic excitation, in order to extend their usefulness and to enable their application in practical devices. In particular, this work develops materials that allow electromagnetic and thermal control of material optical properties and methods that facilitate the use of metasurface physics on macroscopic scales.
The reversible photoconductivity effect produced by phase coexistence in an elemental metal has been enhanced in a metamaterial for the first time. A gap plasmon absorber type metasurface was developed to enhance the photoconductivity of a gallium metal interface under near infrared illumination, achieving an order of magnitude increase in the strength of the phenomenon in a pump-probe experiment. The enhanced nonlinearity displayed at the pump wavelength of 1310nm corresponds to an effective nonlinear dielectric susceptibility x⁽³⁾ of greater than 1 × 10⁻⁸ m³ V⁻².
Competing descriptions of the mechanism behind the reversible photoconductivity effect in elemental gallium, including an analytic model based on heat transfer, a non-thermal model of light-induced excitation, full finite element simulation and a novel cellular automaton method have been implemented and compared. These models have been adapted and applied to the metasurface-enhanced interface for the enhancement of the effect. It was found that an exclusively thermodynamic theoretical treatment of laser heating is insufficient to explain the magnitude of the observed excitation and hence an additional possibly non-thermal photoexcited mechanism must contribute.
Processing methods were developed for the testing of nano-imprint metamaterial manufacturing with liquid metals under controlled environmental conditions, using a custom-built pressure controlled processing chamber. Using this methodology a feasibility study of the production of a nanoimprinted liquid metal photonic structure was conducted, successfully demonstrating the infiltration of nanoscale gratings of period 600nm with liquid gallium and optical switching of the resulting composite interface.
The templated deposition of ordered arrays of gallium nanoparticles on structured substrates has been demonstrated for the first time. Bulk gallium was thermally evaporated onto substrates structured with gratings and checkerboard-type patterns of period 200 to 1600nm. Fourier and image analysis of the produced ordering shows a pronounced tendency for the nanoparticle growth to align with the substrate patterns in both positional frequency and diameter. Such ordered arrays of metallic particles could form an active medium as a component of a plasmonic metamaterial.

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Identifiers

ORCID for Nikolai Zheludev: orcid.org/0000-0002-1013-6636

ORCID for Kevin MacDonald: orcid.org/0000-0002-3877-2976

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