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    Merging van der Waals materials and optical metasurfaces for cavity quantum electrodynamics
    (Wiley, 2026-02-23)
    Sortino, Luca
    ;
    Tittl, Andreas
    ;
    Maier, Stefan A.
    Flat optical metasurfaces are transforming photonics research by enabling new ways to control light in ultrathin, versatile photonic devices. The rise of quasi-bound states in the continuum (qBIC) metasurfaces has enabled tailored high-quality (Q) factor resonances in subwavelength nanostructured thin films, analogous to traditional optical cavities. In this perspective, we explore the emergence of cavity quantum electrodynamics (QED) in optical qBIC metasurfaces, specifically those constructed from van der Waals (vdW) layered materials. Because of their remarkable properties, vdW metasurfaces can support intrinsic optical resonances within the same active material hosting luminescent species, such as excitons or defects, leading to optimal light–matter coupling. This approach of self-hybridizing the cavity-emitter system into a single platform effectively overcomes limitations in on-chip integration of conventional cavities. Combining vdW materials with optically engineered qBIC metasurfaces opens exciting possibilities for exploring nanoscale light–matter interactions. Moreover, the distinctive features of vdW materials, from vertical heterostructures to twist-angle-dependent properties, offer a unique platform bridging the condensed matter physics of 2D materials and engineered nanophotonics. We propose that harnessing strong light–matter coupling in vdW-integrated qBIC metasurfaces will pave the way for next-generation nanoscale polaritonic devices.
    Publicationtype: Journal Article
      12
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    Permittivity-asymmetric qBIC metasurfaces for refractive index sensing
    (2025-11-24)
    Yang, Xingye
    ;
    Antonov, Alexander
    ;
    Hu, Haiyang
    ;
    Tittl, Andreas
    Bound states in the continuum (BICs) provide exceptional light confinement due to their inherent decoupling from radiative channels. Small symmetry breaking transforms BIC into quasi-BIC (qBIC) that couples to free-space radiation enabling ultra-high-quality-factor (Q-factor) resonances desirable for refractive index (RI) sensing. In practical implementations, geometric asymmetry is typically employed. However, since the radiative loss remains fixed once fabricated, such metasurfaces exhibit only a horizontal shift of the resonance spectrum in RI sensing, without modification of its overall shape. Here, we demonstrate a permittivity-asymmetric qBIC (ε-qBIC) metasurface, which encodes environmental refractive index variations directly into the asymmetry factor, resulting in an index response involving both resonance wavelength shift and modulation variation. In addition to exhibiting a competitive transmittance sensitivity of ∼5,300 %/RIU under single-wavelength conditions, the ε-qBIC design provides a substantially improved linear response. Specifically, the linear window area of its sensing data distribution, calculated as the integrated wavelength region where the linearity parameter remains above the preset threshold, is 104 times larger than that of the geometry-asymmetric qBIC (g-qBIC), enabling more robust and reliable single-wavelength signal readout. Additionally, numerical results reveal that environmental permittivity asymmetry can optically restore the g-qBIC to a state with ultra-high-Q (over 10⁷), approaching the BIC condition. Unlike traditional BICs, which are typically inaccessible once perturbed, the permittivity-restored BIC becomes accessible through environmental perturbations. These findings suggest an alternative design strategy for developing high-performance photonic devices for practical sensing applications.
    Publicationtype: Journal Article
      16
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    All-dielectric structural coloration empowered by bound states in the continuum
    (SPIE, 2024-10-07)
    Zheng, Hong
    ;
    Hu, Haiyang
    ;
    Weber, Thomas
    ;
    Wang, Juan
    ;
    Nan, Lin
    The technological requirements of low-power and high-fidelity color displays have been instrumental in driving research into advanced coloration technologies. At the forefront of these developments is the implementation of dye-free approaches, which overcome previous constraints related to color resolution and fading. Resonant dielectric nanostructures have emerged as a promising paradigm, showing great potential for high efficiency, high color saturation, wide gamut palette, and image reproduction. However, they still face limitations related to color accuracy, purity, and simultaneous brightness tunability. Here, we demonstrate an all-dielectric metasurface empowered by photonic bound states in the continuum (BICs), which supports sharp resonances throughout the visible spectral range, ideally suited for producing a wide range of structural colors. The metasurface design consists of TiO<inf>2</inf> ellipses with carefully controlled sizes and geometrical asymmetry, allowing versatile and on-demand variation of the brightness and hue of the output colors, respectively.
    Publicationtype: Journal Article
      9
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    Si metasurface supporting multiple quasi-BICs for degenerate four-wave mixing
    (SPIE, 2024-06-05)
    Moretti, Gianni Q.
    ;
    Weber, Thomas
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    Possmayer, Thomas
    ;
    Cortés, Emiliano
    ;
    Menezes, Leonardo de S.
    Dielectric metasurfaces supporting quasi-bound states in the continuum (qBICs) enable high field enhancement with narrow-linewidth resonances in the visible and near-infrared ranges. The resonance emerges when distorting the meta-atom's geometry away from a symmetry-protected BIC condition and, usually, a given design can sustain one or two of these states. In this work, we introduce a silicon-on-silica metasurface that simultaneously supports up to four qBIC resonances in the near-infrared region. This is achieved by combining multiple symmetry-breaking distortions on an elliptical cylinder array. By pumping two of these resonances, the nonlinear process of degenerate four-wave mixing is experimentally realized. By comparing the nonlinear response with that of an unpatterned silicon film, the near-field enhancement inside the nanostructured dielectric is revealed. The presented results demonstrate independent geometric control of multiple qBICs and their interaction through wave mixing processes, opening new research pathways in nanophotonics, with potential applications in information multiplexing, multi-wavelength sensing and nonlinear imaging.
    Publicationtype: Journal Article
      12
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    Plasmonic gas and chemical sensing
    (2014-06-01)
    Tittl, Andreas
    ;
    Giessen, Harald
    ;
    Liu, Na
    Sensitive and robust detection of gases and chemical reactions constitutes a cornerstone of scientific research and key industrial applications. In an effort to reach progressively smaller reagent concentrations and sensing volumes, optical sensor technology has experienced a paradigm shift from extended thin-film systems towards engineered nanoscale devices. In this size regime, plasmonic particles and nanostructures provide an ideal toolkit for the realization of novel sensing concepts. This is due to their unique ability to simultaneously focus light into subwavelength hotspots of the electromagnetic field and to transmit minute changes of the local environment back into the farfield as a modulation of their optical response. Since the basic building blocks of a plasmonic system are commonly noble metal nanoparticles or nanostructures, plasmonics can easily be integrated with a plethora of chemically or catalytically active materials and compounds to investigate processes ranging from hydrogen absorption in palladium to the detection of trinitrotoluene (TNT). In this review, we will discuss a multitude of plasmonic sensing strategies, spanning the technological scale from simple plasmonic particles embedded in extended thin films to highly engineered complex plasmonic nanostructures. Due to their flexibility and excellent sensing performance, plasmonic structures may open an exciting pathway towards the detection of chemical and catalytic events down to the single molecule level.
    Publicationtype: Review Article
      11
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    Label-free photonic biosensors fabricated with low-loss hydrogenated amorphous silicon resonators
    (SPIE, 2013-11-05)
    Lipka, Timo
    ;
    Wahn, Lennart
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    Trieu, Hoc Khiem
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    Hilterhaus, Lutz
    ;
    Müller, Jörg
    The precise detection of chemicals and biomolecules is of great interest in the areas of biotechnology and medical diagnostics. Thus, there is a need for highly sensitive, small area, andlow-cost sensors. We fabricated and optically characterized hydrogenated amorphous silicon photonic resonators for label-free lab-on-chip biosensors. The sensing was performed with small-footprint microdisk and microring resonators that detect a refractive-index change via the evanescent electric field. Homogeneous sensing with NaCl and surface-sensing experiments with immobilized bovine serum albumin (BSA) were carried out. A sensitivity as high as 460 nm/RIU was measured for NaCl dissolved in deionized water for the disk, whereas about 50 nm/RIU was determined for the ring resonator. The intrinsic limits of detection were calculated to be 3.3 × 10-4 and 3.2 × 10-3 at 1550-nm wavelength. We measured the binding of BSA to functionalized ring resonators and found that molecular masses can be detected down to the clinically relevant femtogram regime. The detection and quantification of related analytes with hydrogenated amorphous silicon photonic sensors can be used in medical healthcare diagnostics like point-of-care-testing and biotechnological screening.
    Publicationtype: Journal Article
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