Dynamic Metasurfaces: From Tunability to Parametric Modulation

Abstract

Metamaterials, artificially designed composite media made of subwavelength structural elements, enable new ways of controlling wave-matter interactions. Their elements, or meta-atoms, through tailored geometry, arrangement, and collective response, provide precise control over electromagnetic waves across different frequency ranges. Their planar analogues, metasurfaces, are particularly attractive because they combine subwavelength thickness with powerful wave-manipulation capabilities. Current developments in metamaterials and metasurfaces focus on tunability and approaches that enable dynamic changes of their parameters over time. This thesis investigates methods for dynamically tuning metasurface properties and controlling their electromagnetic responses through spatial and temporal changes. While the advanced functionality of the dynamic metasurfaces studied here is realised through control of their resonant responses, the proposed parametric effects are based on temporal modulation of physical parameters induced by external energy sources. Chapter 1 reviews the development of metamaterials and methods for their dynamic tuning, followed by an analysis of progress in tunable and time-varying metasurfaces. It concludes with the thesis overview. Chapter 2 examines mechanical tuning of metasurface resonant responses, focusing on tunable filtering based on extraordinary optical transmission (EOT) and bound states in the continuum (BIC) enabled by microelectromechanical systems (MEMS). We first review mechanically tunable metasurfaces and then demonstrate spectrally tunable metasurface filters for hyperspectral infrared imaging, realised through adjustment of EOT resonances by varying the spacing between a dielectric membrane and a nanohole-patterned gold layer. We then discuss advances enabled by integrating metasurfaces into optical fibres and introduce EOT-based metasurface filters for fluorescence fibre-optic imaging. Finally, we introduce a new concept of dynamic control of quasi-BIC resonances through MEMS-enabled hybridisation of one-dimensional (1D) and two-dimensional (2D) metasurfaces, achieving ultranarrow tunable filtering for spectroscopy, remote sensing, and imaging. Chapter 3 explores control of resonances in toroidal metasurfaces using phase change materials for applications such as terahertz detectors and modulators. After reviewing recent progress in active toroidal metasurfaces, we demonstrate quasi-BIC resonances in gold split-ring resonator (SRR) arrays and analyse their dependence on geometrical parameters. We then examine the role of toroidal dipoles in scattering using multipole expansion and introduce tunability by integrating thin film patches of the phase-change material vanadium dioxide into meta-atoms. Chapter 4 investigates parametric metasurfaces for amplification and frequency conversion of free-space electromagnetic waves in the microwave range. It begins with an introduction of time-varying metasurfaces based on SRRs with variable capacitances and an analytical model of wave amplification through electrical modulation of metasurface parameters. We then analyse the results of full-wave numerical simulations and compare them with analytical predictions. In the final section, we propose a dual-resonance parametric metasurface for amplified up-conversion, demonstrating significant efficiency beyond Manley-Rowe limits. The chapter concludes with experimental verification of amplified up-conversion efficiency and metasurface performance. Chapter 5 summarises the research findings and concludes the thesis. Show more