[Paper Review] Influence of symmetry breaking on Fano-like resonances in high Figure of Merit planar terahertz metafilms

Joshua A. Burrow, Riad Yahiaoui+8 more|arXiv (Cornell University)|Dec 5, 2018

Microwave Engineering and Waveguides4 citations

TL;DR

This study investigates symmetry-breaking techniques in planar terahertz metafilms based on 4-gap split ring resonators to enhance Fano-like resonances for sensing applications. By combining adjacent L-bracket translation and increased gap width, the Figure of Merit (FoM) reaches 6× higher than the fundamental dipole mode, enabling high-sensitivity, multi-mode THz sensing with superior quality factors and intensity.

ABSTRACT

It is well established that nearly all high-quality (Q) Fano-like resonances in terahertz (THz) metasurfaces broaden as asymmetry increases, resulting in a decline of Q-factor and an increase in the resonance intensity. Therefore, in order to determine the optimal design for applications in THz sensing, a Figure of Merit (FoM) is required. Previous studies have identified the asymmetry regimes at which the peak FoM occurs for various, specific unit cell geometries. However to date, there is no systematic comparison of the resulting FoMs for common and novel geometries. Here, a THz planar metafilm featuring split ring resonators with four distributed capacitive gaps is investigated to compare three unique methods of implementing asymmetry: (1) adjacent L-bracket translation, (2) capacitive gap translation and (3) increasing gap width. The results obtained find that by translating two gaps and increasing the bottom gap width of the unit cell, the high-Q Fano-like resonances are $6 imes$ higher than the FoM for the fundamental dipole mode. This work further informs the design process for THz metasurfaces and as such will help to define their applications in photonics and sensing.

Motivation & Objective

• To systematically compare different symmetry-breaking methods in high-Figure of Merit terahertz metafilms for enhanced Fano-like resonances.
• To identify optimal geometric asymmetry configurations that maximize the Figure of Merit (FoM) while maintaining high Q-factors.
• To develop a semi-analytical RLC circuit model to explain the coupling dynamics and resonance behavior in asymmetric metastructures.
• To enable multi-mode, high-sensitivity THz sensing and filtering by engineering sharp, high-intensity resonances through controlled asymmetry.
• To provide a design framework for future THz metasurfaces in sensing, modulation, and narrowband filtering applications.

Proposed method

• Employed a 4-gap square split ring resonator (SRR) metafilm with fourfold symmetry as the base geometry for high-Q Fano-like resonances.
• Implemented three distinct symmetry-breaking techniques: (1) adjacent L-bracket translation, (2) capacitive gap translation, and (3) widening of the bottom gap.
• Used full-wave electromagnetic simulations (CST Microwave Studio) to model transmission spectra and extract resonance characteristics.
• Developed a semi-analytical transmission-line RLC circuit model with mutual inductance coupling to explain the excitation of Fano-like modes.
• Validated results using a linearly polarized continuous-wave THz time-domain spectroscopy system for experimental characterization.
• Fitted circuit parameters (R, L, C, M) to simulation and experimental data to achieve good agreement in transmission response.

Experimental results

Research questions

• RQ1 How does different symmetry-breaking geometry affect the Figure of Merit (FoM) of Fano-like resonances in terahertz metafilms?
• RQ2 What is the optimal combination of asymmetry mechanisms that maximizes FoM while maintaining high Q-factors and strong resonance intensity?
• RQ3 Can a semi-analytical RLC circuit model accurately predict the excitation and coupling of Fano-like modes in asymmetric metastructures?
• RQ4 How does the number of accessible high-FoM resonant modes scale with increasing asymmetry in the metafilm design?
• RQ5 What is the relationship between asymmetry parameters (e.g., gap shift, width) and the resulting FoM for higher-order Fano resonances?

Key findings

• The combination of adjacent L-bracket translation and increased bottom gap width produced a FoM 6× higher than the fundamental dipole mode.
• The double-asymmetric configuration (technique 1 + 3) yielded the highest FoM and supported four distinct resonant modes within a 300 GHz bandwidth.
• Frequencies f₂ and f₃ exhibited a proportional increase in FoM with increasing asymmetry, indicating tunable sensitivity.
• The semi-analytical RLC circuit model successfully predicted resonance frequencies and transmission spectra, showing good agreement with simulations and experiments.
• The highest FoM was achieved at δx = 29 μm and w₂ = 122 μm, demonstrating a clear design window for optimal performance.
• The proposed metafilm supports multi-spectral sensing applications due to multiple, sharp, high-FoM resonances in a compact frequency range.