[Paper Review] Plasmonic Bound States in the Continuum to Tailor Light-Matter Coupling

Andreas Aigner, Andreas Tittl+5 more|arXiv (Cornell University)|Jul 21, 2022

Plasmonic and Surface Plasmon Research4 citations

TL;DR

This paper presents 3D laser-nanoprinted plasmonic nanofin metasurfaces that support high-quality-factor (up to Q = 180) bound states in the continuum (BICs) via out-of-plane symmetry breaking. By tuning the triangle angle of the nanofin meta-atoms, the researchers precisely control the radiative-to-intrinsic loss ratio, enabling access to under-, critical-, and over-coupled regimes for pixelated molecular sensing with analyte-induced negative, zero, or positive modulation.

ABSTRACT

Plasmon resonances play a pivotal role in enhancing light-matter interactions in nanophotonics, but their low-quality factors have hindered applications demanding high spectral selectivity. Even though symmetry-protected bound states in the continuum with high-quality factors have been realized in dielectric metasurfaces, impinging light is not efficiently coupled to the resonant metasurfaces and is lost in the form of reflection due to low intrinsic losses. Here, we demonstrate a novel design and 3D laser nanoprinting of plasmonic nanofin metasurfaces, which support symmetry-protected bound states in the continuum up to 4th order. By breaking the nanofins out-of-plane symmetry in parameter space, we achieve high-quality factor (up to 180) modes under normal incidence. We reveal that the out-of-plane symmetry breaking can be fine-tuned by the triangle angle of the 3D nanofin meta-atoms, opening a pathway to precisely control the ratio of radiative to intrinsic losses. This enables access to the under-, critical-, and over-coupled regimes, which we exploit for pixelated molecular sensing. Depending on the coupling regime we observe negative, no, or positive modulation induced by the analyte, unveiling the undeniable importance of tailoring light-matter interaction. Our demonstration provides a novel metasurface platform for enhanced light-matter interaction with a wide range of applications in optical sensing, energy conversion, nonlinear photonics, surface-enhanced spectroscopy, and quantum optics.

Motivation & Objective

• To overcome the low quality factors of conventional plasmonic resonances that limit spectral selectivity in nanophotonic devices.
• To enable efficient excitation of high-Q plasmonic bound states in the continuum (BICs) under normal incidence, which are typically inaccessible due to symmetry constraints.
• To develop a design strategy that allows precise control over the radiative and intrinsic loss channels in plasmonic metasurfaces.
• To demonstrate tunable light-matter coupling regimes—under-, critical-, and over-coupled—using a single platform for advanced sensing applications.
• To realize a 3D nanoprinted plasmonic metasurface platform with high spectral and spatial control for applications in sensing, energy conversion, and quantum optics.

Proposed method

• The authors employ 3D laser nanoprinting to fabricate plasmonic nanofin metasurfaces with precisely controlled geometry, including triangle angles of the meta-atoms.
• They engineer out-of-plane symmetry breaking in the nanofin structure to lift degeneracy and enable high-Q BIC modes under normal incidence.
• The triangle angle of the nanofins is used as a tunable parameter to control the ratio of radiative to intrinsic losses, thereby adjusting the coupling regime.
• Theoretical modeling and simulations are used to predict the Q factors and coupling behavior across different geometric parameters.
• Experimental characterization confirms the presence of high-Q BIC modes (up to Q = 180) and validates the transition between under-, critical-, and over-coupled regimes.
• Pixelated molecular sensing is demonstrated by measuring analyte-induced modulation in transmission, which varies from negative to positive depending on the coupling regime.

Experimental results

Research questions

• RQ1 Can high-quality-factor plasmonic bound states in the continuum be achieved under normal incidence using symmetry-protected designs?
• RQ2 How can the radiative and intrinsic loss channels in plasmonic metasurfaces be independently tuned to control light-matter coupling?
• RQ3 What is the role of out-of-plane geometric asymmetry in enabling high-Q BIC modes in plasmonic nanostructures?
• RQ4 Can the coupling regime (under-, critical-, over-coupled) be experimentally accessed and controlled via nanofin geometry?
• RQ5 How does the coupling regime affect the response of plasmonic metasurfaces to molecular adsorption in sensing applications?

Key findings

• The fabricated plasmonic nanofin metasurfaces support bound states in the continuum with quality factors up to 180, significantly enhancing spectral selectivity.
• Out-of-plane symmetry breaking via nanofin triangle angle tuning enables control over the radiative-to-intrinsic loss ratio, allowing access to all three coupling regimes.
• The system demonstrates analyte-induced transmission modulation that is negative, zero, or positive depending on the coupling regime, confirming tunable light-matter interaction.
• 3D laser nanoprinting enables precise fabrication of complex plasmonic meta-atoms with high structural fidelity and reproducibility.
• The platform enables pixelated molecular sensing with spatially addressable coupling regimes, offering new capabilities for label-free biosensing.
• The results establish a new class of plasmonic metasurfaces for applications in nonlinear photonics, surface-enhanced spectroscopy, and quantum optics.