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@article{nokey,

title = {A Comparative Analysis of Plasmonic and Dielectric Metasurface Sensing Platforms Powered by Bound States in the Continuum},

author = {T Jiang and A Bhattacharya and M Barkey and A Aigner and L Rohrer and T Weber and J Wang and S A Maier and A Tittl},

url = {\<Go to ISI\>://WOS:001558957300001},

doi = {10.1002/adfm.202516021},

issn = {1616-301X},

year  = {2025},

date = {2025-08-28},

journal = {Advanced Functional Materials},

abstract = {Nanophotonic platforms based on surface-enhanced infrared absorbance spectroscopy (SEIRAS) have emerged as an effective tool for molecular detection. Sensitive nanophotonic sensors with robust resonant modes and amplified electromagnetic near fields are essential for spectroscopy, especially in lossy environments. Metasurfaces driven by bound state in the continuum (BICs) have unlocked a powerful platform for molecular detection due to their exceptional spectral selectivity. While plasmonic BIC metasurfaces are preferred for molecular spectroscopy due to their high surface fields, enhancing the interaction with analytes, dielectric BICs have become popular due to their high-quality factors and, thus, high sensitivity. However, their sensing performance has largely been demonstrated in air, neglecting the intrinsic infrared (IR) losses found in common solvents. This study evaluates the suitability of plasmonic versus dielectric platforms for in situ molecular spectroscopy. Here, the sensing performance of plasmonic (gold) and dielectric (silicon) metasurfaces is assessed across liquid environments with varying losses resembling typical solvents. The results show that dielectric metasurfaces excel in dry conditions, while plasmonic BIC metasurfaces outperform them in lossy solvents, with a distinct crossover point where both show similar performance. The results provide a framework for selecting the optimal metasurface material platform for SEIRAS studies based on environmental conditions.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Optical control of resonances in temporally symmetry-broken metasurfaces},

author = {A Aigner and T Possmayer and T Weber and A A Antonov and L D Menezes and S A Maier and A Tittl},

url = {\<Go to ISI\>://WOS:001545391600001},

doi = {10.1038/s41586-025-09363-7},

issn = {0028-0836},

year  = {2025},

date = {2025-08-06},

journal = {Nature},

abstract = {Tunability in active metasurfaces has mainly relied on shifting the resonance wavelength1,2 or increasing material losses3,4 to spectrally detune or quench resonant modes, respectively. However, both methods face fundamental limitations, such as a limited Q factor and near-field enhancement control and the inability to achieve resonance on-off switching by completely coupling and decoupling the mode from the far field. Here we demonstrate temporal symmetry breaking in metasurfaces through ultrafast optical pumping, providing an experimental realization of radiative-loss-driven resonance tuning, allowing resonance creation, annihilation, broadening and sharpening. To enable this temporal control, we introduce restored symmetry-protected bound states in the continuum. Even though their unit cells are geometrically asymmetric, coupling to the radiation continuum remains fully suppressed, which, in this work, is achieved by two equally strong antisymmetric dipoles. By using selective Mie-resonant pumping in parts of these unit cells, we can modify their dipole balance to create or annihilate resonances as well as tune the linewidth, amplitude and near-field enhancement, leading to potential applications in optical and quantum communications, time crystals and photonic circuits.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Fabrication Optimization of van der Waals Metasurfaces: Inverse Patterning Boosts Resonance Quality Factor},

author = {J Biechteler and C Heimig and T Weber and D Gryb and L Sortino and S A Maier and L S De Menezes and A Tittl},

url = {\<Go to ISI\>://WOS:001508031900001},

doi = {10.1002/adom.202500920},

issn = {2195-1071},

year  = {2025},

date = {2025-06-09},

journal = {Advanced Optical Materials},

abstract = {Van der Waals (vdW) materials have garnered growing interest for use as nanophotonic building blocks that offer precise control over light-matter interaction at the nanoscale, such as optical metasurfaces hosting sharp quasi-bound states in the continuum resonances. However, traditional fabrication strategies often rely on lift-off processes, which inherently introduce imperfections in resonator shape and size distribution, ultimately limiting the resonance performance. Here, an optimized fabrication approach for vdW-metasurfaces is presented that implements inverse patterning of the etching mask, resulting in increased resonator quality solely limited by the resolution of the electron beam lithography resist and etching. Applying this inverse fabrication technique on hexagonal boron nitride (hBN), quality (Q) factors exceeding 103 in the visible spectral range are demonstrated, significantly surpassing previous results shown by lift-off fabricated structures. Additionally, the platform's potential as a biosensor is displayed, achieving remarkable sensitivity and figure of merit of 220 in a refractive index sensing experiment. The inverse technique is applied to create chiral metasurfaces from hBN, using a two-height resonator geometry to achieve up to 50% transmittance selectivity. This inverse lithography technique paves the way toward high-performances vdW-devices with high-Q resonances, establishing hBN as a cornerstone for next-generation nanophotonic and optoelectronic devices.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Atomic-layer assembly of ultrathin optical cavities in van der Waals heterostructure metasurfaces},

author = {L Sortino and J Biechteler and L Lafeta and L K\"{u}hner and A Hartschuh and L D Menezes and S A Maier and A Tittl},

url = {\<Go to ISI\>://WOS:001495232400001},

doi = {10.1038/s41566-025-01675-4},

issn = {1749-4885},

year  = {2025},

date = {2025-05-26},

journal = {Nature Photonics},

abstract = {Photonics has been revolutionized by advances in optical metasurfaces, unlocking design and engineering opportunities for flat optical components. Similarly, layered two-dimensional materials have enabled breakthroughs in physics via the deterministic assembly of vertical heterostructures, allowing precise control over the atomic composition of each layer. However, integrating these fields into a single system has remained challenging, limiting progress in atomic-scale optical cavities and metamaterials. Here we demonstrate the concept of van der Waals heterostructure metasurfaces, where ultrathin multilayer van der Waals material stacks are shaped into precisely engineered resonant nanostructures for enhancing light-matter interactions. By leveraging quasi-bound states in the continuum physics, we create intrinsic high-quality-factor resonances originating from WS2 monolayers encapsulated in hexagonal boron nitride at thicknesses below 130 nm, achieving room-temperature strong coupling and polaritonic photoluminescence emission. Furthermore, the metasurface-coupled exciton-polaritons exhibit strong nonlinearities, leading to a saturation of the strong-coupling regime at ultralow fluences of \<1 nJ cm(-2), three orders of magnitude lower than in previous two-dimensional-material-based cavity systems. Our approach monolithically integrates metasurfaces and van der Waals materials and can be extended to the vast library of existing two-dimensional materials, unlocking new avenues for ambient operation of ultrathin polaritonic devices with atomic-scale precision and control.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Molecular spectroscopies with semiconductor metasurfaces: towards dual optical/chemical SERS},

author = {A Berestennikov and H Y Hu and A Tittl},

url = {\<Go to ISI\>://WOS:001492589700001},

doi = {10.1039/d4tc05420b},

issn = {2050-7526},

year  = {2025},

date = {2025-05-22},

journal = {Journal of Materials Chemistry C},

abstract = {Surface-enhanced Raman spectroscopy (SERS) has emerged as a powerful technique for the ultra-sensitive detection of molecules and has been widely applied in many fields, ranging from biomedical diagnostics and environmental monitoring to trace-level detection of chemical and biological analytes. While traditional metallic SERS substrates rely predominantly on electromagnetic field enhancement, emerging semiconductor SERS materials have attracted growing interest because they offer the additional advantage of simultaneous chemical and electromagnetic enhancements. Here, we review some of the recent advancements in the design and optimization of semiconductor SERS substrates, with a focus on their dual enhancement mechanisms. We also discuss the transition from nanoparticle-based platforms to more advanced nanoresonator-based SERS metasurfaces, highlighting their superior sensing performance.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Angular dispersion suppression in deeply subwavelength phonon polariton bound states in the continuum metasurfaces},

author = {L Nan and A Mancini and T Weber and G L Seah and E Cort\'{e}s and A Tittl and S A Maier},

doi = {10.1038/s41566-025-01670-9},

issn = {1749-4885 

1749-4893},

year  = {2025},

date = {2025-05-16},

journal = {NATURE PHOTONICS},

abstract = {Quasi-bound states in the continuum (qBICs) achieved through symmetry breaking in photonic metasurfaces are a powerful approach for engineering resonances with high quality factors and tunability. However, miniaturization of these devices is limited as the in-plane unit-cell size typically scales linearly with the resonant wavelength. By contrast, polariton resonators can be deeply subwavelength, offering a promising solution for achieving compact devices. Here we demonstrate that low-loss mid-infrared surface phonon polaritons enable metasurfaces supporting qBICs with unit-cell volumes up to 105 times smaller than the free-space volume lambda 03documentclass[12pt]minimal usepackageamsmath usepackagewasysym usepackageamsfonts usepackageamssymb usepackageamsbsy usepackagemathrsfs usepackageupgreek setlength\oddsidemargin-69pt begindocument$$\lambda_0\<^\>3$$enddocument. Using 100-nm-thick free-standing silicon carbide membranes, we achieve highly confined qBIC states with exceptional robustness against incident-angle variations, a feature unique among qBIC systems. This absence of angular dispersion enables mid-infrared vibrational sensing of thin, weakly absorbing molecular layers using a reflective objective, a method that typically degrades resonance quality in standard qBIC metasurfaces. We introduce surface-phonon-polariton-based qBICs as a platform for ultraconfined nanophotonic systems, advancing the miniaturization of mid-infrared sensors and devices for thermal radiation engineering.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Control and enhancement of optical nonlinearities in plasmonic semiconductor nanostructures},

author = {A Rossetti and H T Hu and T Venanzi and A Bousseksou and F De Luca and T Deckert and V Giliberti and M Pea and I Sagnes and G Beaudoin and P Biagioni and E Ba\`{u} and S A Maier and A Tittl and D Brida and R Colombelli and M Ortolani and C Cirac\`{i}},

url = {\<Go to ISI\>://WOS:001495020200001},

doi = {10.1038/s41377-025-01783-4},

issn = {2095-5545},

year  = {2025},

date = {2025-05-13},

journal = {Light-Science \& Applications},

volume = {14},

number = {1},

abstract = {The efficiency of nanoscale nonlinear elements in photonic integrated circuits is hindered by the physical limits to the nonlinear optical response of dielectrics, which cannot be engineered as it is a fundamental material property. Here, we experimentally demonstrate that ultrafast optical nonlinearities in doped semiconductors can be engineered and can easily exceed those of conventional undoped dielectrics. The electron response of heavily doped semiconductors acquires in fact a hydrodynamic character that introduces nonlocal effects as well as additional nonlinear sources. Our experimental findings are supported by a comprehensive computational analysis based on the hydrodynamic model. In particular, by studying third-harmonic generation from plasmonic nanoantenna arrays made out of heavily n-doped InGaAs with increasing levels of free-carrier density, we discriminate between hydrodynamic and dielectric nonlinearities. Most importantly, we demonstrate that the maximum nonlinear efficiency as well as its spectral location can be engineered by tuning the doping level. Crucially, the maximum efficiency can be increased by almost two orders of magnitude with respect to the classical dielectric nonlinearity. Having employed the common material platform InGaAs/InP that supports integrated waveguides, our findings pave the way for future exploitation of plasmonic nonlinearities in all-semiconductor photonic integrated circuits.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {All-optical permittivity-asymmetric quasi-bound states in the continuum},

author = {R Bert\'{e} and T Possmayer and A Tittl and L D S Menezes and S A Maier},

url = {https://doi.org/10.1038/s41377-025-01843-9},

doi = {10.1038/s41377-025-01843-9},

issn = {2047-7538},

year  = {2025},

date = {2025-05-07},

journal = {Light: Science \& Applications},

volume = {14},

number = {1},

pages = {185},

abstract = {Resonances are usually associated with finite systems\textemdashthe vibrations of clamped strings in a guitar or the optical modes in a cavity defined by mirrors. In optics, resonances may be induced in infinite continuous media via periodic modulations of their optical properties. Here we demonstrate that periodic modulations of the permittivity of a featureless thin film can also act as a symmetry-breaking mechanism, allowing the excitation of photonic quasi-bound states in the continuum (qBICs). By interfering two ultrashort laser pulses in the unbounded film, transient resonances can be tailored through different parameters of the pump beams. We show that the system offers resonances tunable in wavelength and quality-factor, and spectrally selective enhancement of third-harmonic generation. Due to a fast decay of the permittivity asymmetry, we observe ultrafast dynamics, enabling time-selective near-field enhancement with picosecond precision. Optically induced permittivity asymmetries may be exploited in on-demand weak to ultrastrong light-matter interaction regimes and light manipulation at dynamically chosen wavelengths in lithography-free metasurfaces.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Polarization-independent metasurfaces based on bound states in the continuum with high Q-factor and resonance modulation},

author = {X Y Yang and A Antonov and A Aigner and T Weber and Y H Lee and T Jiang and H Y Hu and A Tittl},

url = {\<Go to ISI\>://WOS:001503744200007},

doi = {10.1364/oe.547467},

issn = {1094-4087},

year  = {2025},

date = {2025-04-07},

journal = {Optics Express},

volume = {33},

number = {7},

pages = {15682-15689},

abstract = {Metasurfaces offer a powerful platform for effective light manipulation, which is crucial for advanced optical technologies. While designs of polarization-independent structures have reduced the need for polarized illumination, they are often limited by either low Q factors or low resonance modulation. Here, we design and experimentally demonstrate a metasurface with polarization-independent quasi-bound state in the continuum (quasi-BIC), where the unit cell consists of four silicon squares arranged in a two-dimensional array and the resonance properties can be controlled by adjusting the edge length difference between different squares. Our metasurface experimentally achieves a Q factor of approximately 100 and a resonance modulation of around 50%. This work addresses a common limitation in previous designs, which either achieved high Q factors exceeding 200 with a resonance modulation of less than 10%, leading to challenging signal-to-noise ratio requirements, or achieved strong resonance modulation with Q factors of only around 10, limiting light confinement and fine-tuning capabilities. In contrast, our metasurface ensures that the polarization-independent signal is sharp and distinct within the system, reducing the demands on signal-to-noise ratio and improving robustness. Experiments show the consistent performance across different polarization angles. This work contributes to the development of versatile optical devices, enhancing the potential for the practical application of BIC-based designs in areas such as optical filtering and sensing. (c) 2025 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Engineering of Active and Passive Loss in High-Quality-Factor Vanadium Dioxide-Based BIC Metasurfaces},

author = {A Aigner and F Ligmajer and K Rovensk\'{a} and J Holobr\'{a}dek and B Idesov\'{a} and S A Maier and A Tittl and L De S. Menezes},

url = {https://doi.org/10.1021/acs.nanolett.4c01703},

doi = {10.1021/acs.nanolett.4c01703},

issn = {1530-6984},

year  = {2024},

date = {2024-09-04},

journal = {Nano Letters},

volume = {24},

number = {35},

pages = {10742-10749},

abstract = {Active functionalities of metasurfaces are of growing interest in nanophotonics. The main strategy employed to date is spectral resonance tuning affecting predominantly the far-field response. However, this barely influences other essential resonance properties like near-field enhancement, signal modulation, quality factor, and absorbance, which are all vital for numerous applications. Here we introduce an active metasurface approach that combines temperature-tunable losses in vanadium dioxide with far-field coupling tunable symmetry-protected bound states in the continuum. This method enables exceptional precision in independently controlling both radiative and nonradiative losses. Consequently, it allows for the adjustment of both the far-field response and, notably, the near-field characteristics like local field enhancement and absorbance. We experimentally demonstrate continuous tuning from under- through critical- to overcoupling, achieving quality factors of 200 and a relative switching contrast of 78%. Our research marks a significant step toward highly tunable metasurfaces, controlling both near- and far-field properties.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Nanoscale Mechanical Manipulation of Ultrathin SiN Membranes Enabling Infrared Near-Field Microscopy of Liquid-Immersed samples},

author = {E Ba\`{u} and T G\"{o}lz and M Benoit and A Tittl and F Keilmann},

url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/smll.202402568},

doi = {https://doi.org/10.1002/smll.202402568},

issn = {1613-6810},

year  = {2024},

date = {2024-08-25},

journal = {Small},

volume = {20},

number = {47},

pages = {2402568},

abstract = {Abstract Scattering scanning near-field optical microscopy (s-SNOM) is a powerful technique for mid-infrared spectroscopy at nanometer length scales. By investigating objects in aqueous environments through ultrathin membranes, s-SNOM has recently been extended toward label-free nanoscopy of the dynamics of living cells and nanoparticles, assessing both the optical and the mechanical interactions between the tip, the membrane and the liquid suspension underneath. Here, the study reports that the tapping AFM tip induces a reversible nanometric deformation of the membrane manifested as either an indentation or protrusion. This mechanism depends on the driving force of the tapping cantilever, which is exploited to minimize topographical deformations of the membrane to improve optical measurements. Furthermore, it is shown that the tapping phase delay between driving signal and tip oscillation is a highly sensitive observable to study the mechanics of adhering objects, exhibiting highest contrast at low tapping amplitudes where the membrane remains nearly flat. Mechanical responses are correlated with simultaneously recorded spectroscopy data to reveal the thickness of nanometric water layers between membrane and adhering objects. Besides a general applicability of depth profiling, the technique holds great promise for studying mechano-active biopolymers and living cells, biomaterials that exhibit complex behaviors when under a mechanical load.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Environmental permittivity-asymmetric BIC metasurfaces with electrical reconfigurability},

author = {H Hu and W Lu and A Antonov and R Bert\'{e} and S A Maier and A Tittl},

url = {https://doi.org/10.1038/s41467-024-51340-7},

doi = {10.1038/s41467-024-51340-7},

issn = {2041-1723},

year  = {2024},

date = {2024-08-15},

journal = {Nature Communications},

volume = {15},

number = {1},

pages = {7050},

abstract = {Achieving precise spectral and temporal light manipulation at the nanoscale remains a critical challenge in nanophotonics. While photonic bound states in the continuum (BICs) have emerged as a powerful means of controlling light, their reliance on geometrical symmetry breaking for obtaining tailored resonances makes them highly susceptible to fabrication imperfections, and their generally fixed asymmetry factor fundamentally limits applications in reconfigurable metasurfaces. Here, we introduce the concept of environmental symmetry breaking by embedding identical resonators into a surrounding medium with carefully placed regions of contrasting refractive indexes, activating permittivity-driven quasi-BIC resonances (ε-qBICs) without altering the underlying resonator geometry and unlocking an additional degree of freedom for light manipulation through active tuning of the surrounding dielectric environment. We demonstrate this concept by integrating polyaniline (PANI), an electro-optically active polymer, to achieve electrically reconfigurable ε-qBICs. This integration not only demonstrates rapid switching speeds and exceptional durability but also boosts the system’s optical response to environmental perturbations. Our strategy significantly expands the capabilities of resonant light manipulation through permittivity modulation, opening avenues for on-chip optical devices, advanced sensing, and beyond.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Van der Waals heterostructure metasurfaces: atomic-layer assembly of ultrathin optical cavities},

author = {L Sortino and J Biechteler and L Lafeta and L K\"{u}hner and A Hartschuh and L D S Menezes and S A Maier and A Tittl},

year  = {2024},

date = {2024-07-23},

journal = {arXiv preprint arXiv:2407.16480},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Si metasurface supporting multiple quasi-BICs for degenerate four-wave mixing},

author = {G Q Moretti and T Weber and T Possmayer and E Cort\'{e}s and L D S Menezes and A V Bragas and S A Maier and A Tittl and G Grinblat},

url = {https://doi.org/10.1515/nanoph-2024-0128},

doi = {doi:10.1515/nanoph-2024-0128},

year  = {2024},

date = {2024-06-05},

journal = {Nanophotonics},

volume = {13},

number = {18},

pages = {3421-3428},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Pixelated High-Q Metasurfaces for in Situ Biospectroscopy and Artificial Intelligence-Enabled Classification of Lipid Membrane Photoswitching Dynamics},

author = {M Barkey and R B\"{u}chner and A Wester and S D Pritzl and M Makarenko and Q Wang and T Weber and D Trauner and S A Maier and A Fratalocchi and T Lohm\"{u}ller and A Tittl},

url = {https://doi.org/10.1021/acsnano.3c09798},

doi = {10.1021/acsnano.3c09798},

issn = {1936-0851},

year  = {2024},

date = {2024-04-23},

urldate = {2024-04-23},

journal = {ACS Nano},

volume = {18},

number = {18},

pages = {11644-11654},

abstract = {Nanophotonic devices excel at confining light into intense hot spots of electromagnetic near fields, creating exceptional opportunities for light\textendashmatter coupling and surface-enhanced sensing. Recently, all-dielectric metasurfaces with ultrasharp resonances enabled by photonic bound states in the continuum (BICs) have unlocked additional functionalities for surface-enhanced biospectroscopy by precisely targeting and reading out the molecular absorption signatures of diverse molecular systems. However, BIC-driven molecular spectroscopy has so far focused on end point measurements in dry conditions, neglecting the crucial interaction dynamics of biological systems. Here, we combine the advantages of pixelated all-dielectric metasurfaces with deep learning-enabled feature extraction and prediction to realize an integrated optofluidic platform for time-resolved in situ biospectroscopy. Our approach harnesses high-Q metasurfaces specifically designed for operation in a lossy aqueous environment together with advanced spectral sampling techniques to temporally resolve the dynamic behavior of photoswitchable lipid membranes. Enabled by a software convolutional neural network, we further demonstrate the real-time classification of the characteristic cis and trans membrane conformations with 98% accuracy. Our synergistic sensing platform incorporating metasurfaces, optofluidics, and deep learning reveals exciting possibilities for studying multimolecular biological systems, ranging from the behavior of transmembrane proteins to the dynamic processes associated with cellular communication.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Highly confined incident-angle-robust surface phonon polariton bound states in the continuum metasurfaces},

author = {L Nan and A Mancini and T Weber and G L Seah and E Cort\'{e}s and A Tittl and S A Maier},

year  = {2024},

date = {2024-03-27},

journal = {arXiv preprint arXiv:2403.18743},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Emergent resonances in a thin film tailored by optically-induced small permittivity asymmetries},

author = {R Bert\'{e} and T Possmayer and A Tittl and L D S Menezes and S A Maier},

year  = {2024},

date = {2024-03-08},

journal = {arXiv preprint arXiv:2403.05730},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Surface-Enhanced Raman Scattering in BIC-Driven Semiconductor Metasurfaces},

author = {H Hu and A K Pal and A Berestennikov and T Weber and A Stefancu and E Cort\'{e}s and S A Maier and A Tittl},

url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.202302812},

doi = {https://doi.org/10.1002/adom.202302812},

issn = {2195-1071},

year  = {2024},

date = {2024-02-05},

journal = {Advanced Optical Materials},

volume = {12},

number = {14},

pages = {2302812},

abstract = {Abstract Semiconductor-based surface-enhanced Raman spectroscopy (SERS) substrates, as a new frontier in the field of SERS, are hindered by their poor electromagnetic field confinement and weak light-matter interaction. Metasurfaces, a class of 2D artificial materials based on the electromagnetic design of nanophotonic resonators, enable strong electromagnetic field enhancement and optical absorption engineering for a wide range of semiconductors. However, the engineering of semiconductor substrates into metasurfaces for improving SERS activity remains underexplored. Here, an improved SERS metasurface platform is developed that leverages the combination of titanium oxide (TiO2) and the emerging physical concept of optical bound states in the continuum (BICs) to boost the Raman emission. Moreover, fine-tuning of BIC-assisted resonant absorption offers a pathway for maximizing the photoinduced charge transfer effect (PICT) in SERS. High values of BIC-assisted electric field enhancement (|E/E0|2 ≈103) are achieved, challenging the preconception of weak electromagnetic (EM) field enhancement on semiconductor SERS substrates. The BIC-assisted TiO2 metasurface platform offers a new dimension in spectrally-tunable SERS with earth-abundant and bio-compatible semiconductor materials, beyond the traditional plasmonic ones.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Continuous spectral and coupling-strength encoding with dual-gradient metasurfaces},

author = {A Aigner and T Weber and A Wester and S A Maier and A Tittl},

url = {https://arxiv.org/abs/2312.05600},

doi = {https://doi.org/10.48550/arXiv.2312.05600},

year  = {2023},

date = {2023-12-09},

journal = {arXiv preprint arXiv:2312.05600},

abstract = {Optical metasurfaces excel at enhancing and controlling light-matter interactions, which are primarily dictated by two factors: the spectral overlap of the resonances with target excitations in the material and the coupling-strength between them, where resonance linewidth and localized field enhancement are the governing influences. Current metasurface designs are limited to sampling a few discrete points within this vast 2D interaction parameter space or have varied only a single parameter. Symmetry-protected bound states in the continuum (BICs) allow precise control over the wavelength and linewidth of individual resonances, but rely on large arrangements of identical unit cells, limiting the continuous mapping of the parameter space. Therefore, optical platforms that concurrently probe the spectral and coupling parameters, so far, remained elusive. Here, we introduce the concept of dual-gradient metasurfaces for the continuous and simultaneous encoding of the spectral and coupling-strength of light-matter interactions, enabled by smooth local variations of the unit cell parameters. Contrary to conventional understanding, we demonstrate that BICs can be excited in such non-periodic systems provided the parameter variations are sufficiently small. Our dual-gradient metasurface exhibits an extraordinary resonance density, with each unit cell supporting a unique mode. This results in up to 27,500 distinct modes, all contained within a compact footprint. We apply this technology to surface-enhanced molecular sensing, capturing not only the spectral fingerprint of molecules but also unveiling an additional coupling-based dimension of spectroscopic data. This advancement in metasurface design paves the way for generalized light-matter coupling with metasurfaces, with applications ranging from on-chip spectrometer, to chirality encoding and AI-driven biochemical spectroscopy.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Active Loss Engineering in Vanadium Dioxide Based BIC Metasurfaces},

author = {A Aigner and F Ligmajer and K Rovensk\'{a} and J Holobr\'{a}dek and B Idesov\'{a} and S A Maier and A Tittl and L D S Menezes},

url = {https://arxiv.org/abs/2312.00547},

doi = {https://doi.org/10.48550/arXiv.2312.00547},

year  = {2023},

date = {2023-12-01},

journal = {arXiv preprint arXiv:2312.00547},

abstract = {Metasurfaces have unlocked significant advancements across photonics, yet their efficient active control remains challenging. The active materials required often lack continuous tunability, exhibit inadequate refractive index (RI) changes, or suffer from high losses. These aspects pose an inherent limitation for resonance-shifting based switching: when RI changes are small, the resulting shift is also minor. Conversely, high RI changes typically come with high intrinsic losses necessitating broad modes because narrow ones cannot tolerate such losses. Therefore, larger spectral shifts are required to effectively detune the modes. This paper introduces a novel active metasurface approach that converts the constraint of high intrinsic losses into a beneficial feature. This is achieved by controlling the losses in a hybrid vanadium dioxide (VO2) - silicon metasurface, supporting symmetry-protected bound states in the continuum (BICs) within the infrared spectrum. By leveraging the temperature-controlled losses in VO2 and combining them with the inherent far-field-coupling tunability of BICs, we gain unprecedented precision in independently controlling both the radiative and nonradiative losses of the resonant system. Our dual-control mechanism allows us to optimize our metasurfaces and we experimentally demonstrate quality factors above 200, a maximum reflectance amplitude of 90%, a relative switching contrast of 78%, and continuous tuning from under- to over-coupling within the infrared spectral range. This study provides a foundation for experimentally and technologically simple, fine-tunable, active metasurfaces for applications ranging from molecular sensors to filters and optical modulators.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {All-Dielectric Structural Coloration Empowered by Bound States in the Continuum},

author = {H Zheng and H Hu and T Weber and J Wang and L Nan and B Zou and S A Maier and A Tittl},

url = {https://arxiv.org/abs/2311.13315},

doi = {https://doi.org/10.48550/arXiv.2311.13315},

year  = {2023},

date = {2023-11-28},

journal = {arXiv preprint arXiv:2311.13315},

abstract = {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 coloration techniques, which overcome previous constraints related to insufficient resolution and color fading. In this context, resonant dielectric nanostructures have emerged as a promising paradigm, showing great potential for high efficiency, remarkably high color saturation, wide gamut palette, and realistic 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 titanium dioxide (TiO2) ellipses with carefully controlled sizes and geometrical asymmetry, allowing versatile and on-demand variation of the brightness and hue of the output colors, respectively.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Semiconductor Metasurfaces for Surface-enhanced Raman Scattering},

author = {H Hu and A K Pal and A Berestennikov and T Weber and A Stefancu and E Cortes and S A Maier and A Tittl},

url = {https://arxiv.org/abs/2309.10732},

doi = {https://doi.org/10.48550/arXiv.2309.10732},

year  = {2023},

date = {2023-09-19},

journal = {arXiv preprint arXiv:2309.10732},

abstract = {Semiconductor-based surface-enhanced Raman spectroscopy (SERS) substrates, as a new frontier in the field of SERS, are hindered by their poor electromagnetic field confinement, and weak light-matter interaction. Metasurfaces, a class of 2D artificial materials based on the electromagnetic design of nanophotonic resonators, enable strong electromagnetic field enhancement and optical absorption engineering for a wide range of semiconductor materials. However, the engineering of semiconductor substrates into metasurfaces for improving SERS activity remains underexplored. Here, we develop an improved SERS metasurface platform that leverages the combination of titanium oxide (TiO2) and the emerging physical concept of optical bound states in the continuum (BICs) to boost the Raman emission. Moreover, fine-tuning of BIC-assisted resonant absorption offers a pathway for maximizing the photoinduced charge transfer effect (PICT) in SERS. We achieve ultrahigh values of BIC-assisted electric field enhancement (|E/E0|^2 ~ 10^3), challenging the preconception of weak electromagnetic (EM) field enhancement on semiconductor SERS substrates. Our BIC-assisted TiO2 metasurface platform offers a new dimension in spectrally-tunable SERS with earth-abundant and bio-compatible semiconductor materials, beyond the traditional plasmonic ones.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Multi-band metasurface-driven surface-enhanced infrared absorption spectroscopy for improved characterization of in-situ electrochemical reactions},

author = {M Duportal and L M Berger and S A Maier and A Tittl and K Krischer},

url = {https://arxiv.org/abs/2307.10951},

doi = {https://doi.org/10.48550/arXiv.2307.10951},

year  = {2023},

date = {2023-07-20},

journal = {arXiv preprint arXiv:2307.10951},

abstract = {Surface-enhanced spectroscopy techniques are the method-of-choice to characterize adsorbed intermediates occurring during electrochemical reactions, which are crucial in realizing a green sustainable future. Characterizing species with low coverages or short lifetimes have so far been limited by low signal enhancement. Recently, metasurface-driven surface-enhanced infrared absorption spectroscopy (SEIRAS) has been pioneered as a promising narrowband technology to study single vibrational modes of electrochemical interfaces during CO oxidation. However, many reactions involve several species or configurations of adsorption that need to be monitored simultaneously requiring reproducible and broadband sensing platforms to provide a clear understanding of the underlying electrochemical processes. Here, we experimentally realize multi-band metasurface-driven SEIRAS for the in-situ study of electrochemical CO2 reduction on a Pt surface. We develop an easily reproducible and spectrally-tunable platinum nano-slot metasurface. Two CO adsorption configurations at 2030 cm-1 and 1840 cm-1 are locally enhanced as a proof of concept that can be extended to more vibrational bands. Our platform provides a 41-fold enhancement in the detection of characteristic absorption signals compared to conventional broadband electrochemically roughened platinum films. A straightforward methodology is outlined starting by baselining our system in CO saturated environment and clearly detecting both configurations of adsorption, in particular the hitherto hardly detectable CO bridge configuration. Then, thanks to the signal enhancement provided by our platform, we find that the CO bridge configuration on platinum does not play a significant role during CO2 reduction in an alkaline environment. We anticipate that our technology will guide researchers in developing similar sensing platforms.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Probing the micro- and nanoscopic properties of dental materials using infrared spectroscopy: A proof-of-principle study},

author = {M Beddoe and T G\"{o}lz and M Barkey and E Bau and M Godejohann and S A Maier and F Keilmann and M Moldovan and D Prodan and N Ilie and A Tittl},

url = {https://www.sciencedirect.com/science/article/pii/S1742706123004026},

doi = {https://doi.org/10.1016/j.actbio.2023.07.017},

issn = {1742-7061},

year  = {2023},

date = {2023-07-19},

journal = {Acta Biomaterialia},

volume = {168},

pages = {309-322},

abstract = {The preservation of oral health over a person's lifespan is a key factor for a high quality of life. Sustaining oral health requires high-end dental materials with a plethora of attributes such as durability, non-toxicity and ease of application. The combination of different requirements leads to increasing miniaturization and complexity of the material components such as the composite and adhesives, which makes the precise characterization of the material blend challenging. Here, we demonstrate how modern IR spectroscopy and imaging from the micro- to the nanoscale can provide insights on the chemical composition of the different material sections of a dental filling. We show how the recorded IR-images can be used for a fast and non-destructive porosity determination of the studied adhesive. Furthermore, the nanoscale study allows precise assessment of glass cluster structures and distribution within their characteristic organically modified ceramic (ORMOCER) matrix and an assessment of the interface between the composite and adhesive material. For the study we used a Fourier-Transform-IR (FTIR) microscope and a quantum cascade laser-based IR-microscope (QCL-IR) for the microscale analysis and a scattering-type scanning near-field optical microscopy (s-SNOM) for the nanoscale analysis. The paper ends with an in-depth discussion of the strengths and weaknesses of the different imaging methods to give the reader a clear picture for which scientific question the microscopes are best suited for. Statement of significance Modern resin-based composites for dental restoration are complex multi-compound materials. In order to improve these high-end materials, it is important to investigate the molecular composition and morphology of the different parts. An emergent method to characterize these materials is infrared spectroscopic imaging, which combines the strength of infrared spectroscopy and an imaging approach known from optical microscopy. In this work, three state of the art methods are compared for investigating a dental filling including FTIR- and quantum cascade laser IR-imaging microscopy for the microscale and scattering-type scanning near-field optical microscopy for the nanoscale.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Intrinsic strong light-matter coupling with self-hybridized bound states in the continuum in van der Waals metasurfaces},

author = {T Weber and L K\"{u}hner and L Sortino and A Ben Mhenni and N P Wilson and J K\"{u}hne and J J Finley and S A Maier and A Tittl},

url = {https://doi.org/10.1038/s41563-023-01580-7},

doi = {10.1038/s41563-023-01580-7},

issn = {1476-4660},

year  = {2023},

date = {2023-06-22},

journal = {Nature Materials},

volume = {22},

number = {8},

pages = {970-976},

abstract = {Photonic bound states in the continuum (BICs) provide a standout platform for strong light-matter coupling with transition metal dichalcogenides (TMDCs) but have so far mostly been implemented as traditional all-dielectric metasurfaces with adjacent TMDC layers, incurring limitations related to strain, mode overlap and material integration. Here, we demonstrate intrinsic strong coupling in BIC-driven metasurfaces composed of nanostructured bulk tungsten disulfide (WS2) and exhibiting resonances with sharp, tailored linewidths and selective enhancement of light-matter interactions. Tuning of the BIC resonances across the exciton resonance in bulk WS2 is achieved by varying the metasurface unit cells, enabling strong coupling with an anticrossing pattern and a Rabi splitting of 116 meV. Crucially, the coupling strength itself can be controlled and is shown to be independent of material-intrinsic losses. Our self-hybridized metasurface platform can readily incorporate other TMDCs or excitonic materials to deliver fundamental insights and practical device concepts for polaritonic applications.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Intrinsic strong light-matter coupling with self-hybridized bound states in the continuum in van der Waals metasurfaces},

author = {T Weber and L K\"{u}hner and L Sortino and A Ben Mhenni and N P Wilson and J K\"{u}hne and J J Finley and S A Maier and A Tittl},

url = {https://doi.org/10.1038/s41563-023-01580-7},

doi = {10.1038/s41563-023-01580-7},

issn = {1476-4660},

year  = {2023},

date = {2023-06-22},

journal = {Nature Materials},

volume = {22},

number = {8},

pages = {970-976},

abstract = {Photonic bound states in the continuum (BICs) provide a standout platform for strong light-matter coupling with transition metal dichalcogenides (TMDCs) but have so far mostly been implemented as traditional all-dielectric metasurfaces with adjacent TMDC layers, incurring limitations related to strain, mode overlap and material integration. Here, we demonstrate intrinsic strong coupling in BIC-driven metasurfaces composed of nanostructured bulk tungsten disulfide (WS2) and exhibiting resonances with sharp, tailored linewidths and selective enhancement of light-matter interactions. Tuning of the BIC resonances across the exciton resonance in bulk WS2 is achieved by varying the metasurface unit cells, enabling strong coupling with an anticrossing pattern and a Rabi splitting of 116 meV. Crucially, the coupling strength itself can be controlled and is shown to be independent of material-intrinsic losses. Our self-hybridized metasurface platform can readily incorporate other TMDCs or excitonic materials to deliver fundamental insights and practical device concepts for polaritonic applications.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Metallic and All-Dielectric Metasurfaces Sustaining Displacement-Mediated Bound States in the Continuum},

author = {L M Berger and M Barkey and S A Maier and A Tittl},

url = {https://arxiv.org/abs/2306.00591},

doi = {https://doi.org/10.48550/arXiv.2306.00591},

year  = {2023},

date = {2023-06-01},

journal = {arXiv preprint arXiv:2306.00591},

abstract = {Bound states in the continuum (BICs) are localized electromagnetic modes within the continuous spectrum of radiating waves. Due to their infinite lifetimes without radiation losses, BICs are driving research directions in lasing, non-linear optical processes, and sensing. However, conventional methods for converting BICs into leaky resonances, or quasi-BICs, with high-quality factors typically rely on breaking the in-plane inversion symmetry of the metasurface and often result in resonances that are strongly dependent on the angle of the incident light, making them unsuitable for many practical applications. Here, we numerically analyze and experimentally demonstrate an emerging class of BIC-driven metasurfaces, where the coupling to the far field is controlled by the displacement of individual resonators. In particular, we investigate both all-dielectric and metallic as well as positive and inverse displacement-mediated metasurfaces sustaining angular-robust quasi-BICs in the mid-infrared spectral region. We explore their behavior with changes in the incidence angle of illumination and experimentally show their superior performance compared to two conventional alternatives: silicon-based tilted ellipses and cylindrical nanoholes in gold. We anticipate our findings to open exciting perspectives for bio-sensing, conformal optical devices, and photonic devices using focused light.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Active Huygens' metasurface based on in-situ grown conductive polymer},

author = {W Lu and L D S Menezes and A Tittl and H Ren and S A Maier},

url = {https://arxiv.org/abs/2305.07356},

doi = {https://doi.org/10.48550/arXiv.2305.07356},

year  = {2023},

date = {2023-05-12},

journal = {arXiv preprint arXiv:2305.07356},

abstract = {Active metasurfaces provide unique advantages for on-demand light manipulation at a subwavelength scale for emerging applications of 3D displays, augmented/virtual reality (AR/VR) glasses, holographic projectors and light detection and ranging (LiDAR). These applications put stringent requirements on switching speed, cycling duration, controllability over intermediate states, modulation contrast, optical efficiency and operation voltages. However, previous demonstrations focus only on particular subsets of these key performance requirements for device implementation, while the other performance metrics have remained too low for any practical use. Here, we demonstrate an active Huygens' metasurface based on in-situ grown conductive polymer with holistic switching performance, including switching speed of 60 frames per second (fps), switching duration of more than 2000 switching cycles without noticeable degradation, hysteresis-free controllability over intermediate states, modulation contrast of over 1400%, optical efficiency of 28% and operation voltage range within 1 V. Our active metasurface design meets all foundational requirements for display applications and can be readily incorporated into other metasurface concepts to deliver high-reliability electrical control over its optical response, paving the way for compact and robust electro-optic metadevices.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Improved In Situ Characterization of Electrochemical Interfaces Using Metasurface-Driven Surface-Enhanced IR Absorption Spectroscopy},

author = {L M Berger and M Duportal and L D S Menezes and E Cort\'{e}s and S A Maier and A Tittl and K Krischer},

url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/adfm.202300411},

doi = {https://doi.org/10.1002/adfm.202300411},

issn = {1616-301X},

year  = {2023},

date = {2023-03-20},

urldate = {2023-03-20},

journal = {Advanced Functional Materials},

volume = {33},

issue = {25},

pages = {2300411},

abstract = {Abstract Electrocatalysis plays a crucial role in realizing the transition toward a zero-carbon future, driving research directions from green hydrogen generation to carbon dioxide reduction. Surface-enhanced infrared absorption spectroscopy (SEIRAS) is a suitable method for investigating electrocatalytic processes because it can monitor with chemical specificity the mechanisms of the reactions. However, it remains difficult to detect many relevant aspects of electrochemical reactions such as short-lived intermediates. Herein, an integrated nanophotonic-electrochemical SEIRAS platform is developed and experimentally realized for the in situ investigation of molecular signal traces emerging during electrochemical experiments. A platinum nano-slot metasurface featuring strongly enhanced electromagnetic near fields is implemented and spectrally targets the weak vibrational mode of the adsorbed carbon monoxide at ≈2033 cm−1. The metasurface-driven resonances can be tuned over a broad range in the mid-infrared spectrum and provide high molecular sensitivity. Compared to conventional unstructured platinum films, this nanophotonic-electrochemical platform delivers a 27-fold improvement of the experimentally detected characteristic absorption signals, enabling the detection of new species with weak signals, fast conversions, or low surface concentrations. By providing a deeper understanding of catalytic reactions, the nanophotonic-electrochemical platform is anticipated to open exciting perspectives for electrochemical SEIRAS, surface-enhanced Raman spectroscopy, and other fields of chemistry such as photoelectrocatalysis.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Plasmonic bound states in the continuum to tailor light-matter coupling},

author = {A Aigner and A Tittl and J Wang and T Weber and Y Kivshar and S A Maier and H Ren},

url = {https://doi.org/10.1126/sciadv.add4816},

doi = {10.1126/sciadv.add4816},

year  = {2022},

date = {2022-12-09},

journal = {Science Advances},

volume = {8},

number = {49},

pages = {eadd4816},

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. Here, we demonstrate the design and 3D laser nanoprinting of plasmonic nanofin metasurfaces, which support symmetry-protected bound states in the continuum up to the fourth order. By breaking the nanofins? out-of-plane symmetry in parameter space, we achieve high-quality factor (up to 180) modes under normal incidence. The out-of-plane symmetry breaking can be fine-tuned by the nanofins? triangle angle, 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. We observe a strong dependence of the sensing performance on the coupling regime, demonstrating the importance of judicious tailoring of light-matter interactions. Our demonstration provides a metasurface platform for enhanced light-matter interaction with a wide range of applications. 3D-nanoprinted plasmonic nanofin metasurfaces with high-quality factor resonances can achieve enhanced light-matter interaction.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Mirror-coupled plasmonic bound states in the continuum for tunable perfect absorption},

author = {J Wang and T Weber and A Aigner and S A Maier and A Tittl},

url = {https://arxiv.org/abs/2211.03673},

doi = {https://doi.org/10.48550/arXiv.2211.03673},

year  = {2022},

date = {2022-11-07},

journal = {arXiv preprint arXiv:2211.03673},

abstract = {Tailoring critical light-matter coupling is a fundamental challenge of nanophotonics, impacting diverse fields from higher harmonic generation and energy conversion to surface-enhanced spectroscopy. Plasmonic perfect absorbers (PAs), where resonant antennas couple to their mirror images in adjacent metal films, have been instrumental for obtaining different coupling regimes by tuning the antenna-film distance. However, for on-chip uses, the ideal PA gap size can only match one wavelength, and wide range multispectral approaches remain challenging. Here, we introduce a new paradigm for plasmonic PAs by combining mirror-coupled resonances with the unique loss engineering capabilities of plasmonic bound states in the continuum (BICs). Our BIC-driven PA platform leverages the asymmetry of the constituent meta-atoms as an additional degree of freedom for reaching the critical coupling (CC) condition, delivering resonances with unity absorbance and high quality factors approaching 100 in the mid-infrared. Such a platform holds flexible tuning knobs including asymmetry parameter, dielectric gap, and geometrical scaling factor to precisely control the coupling condition, resonance frequency, and selective enhancement of magnetic and electric fields while maintaining CC. We demonstrate a pixelated PA metasurface with optimal absorption over a broad range of mid-infrared frequencies (950 ~ 2000 1/cm) using only a single spacer layer thickness and apply it for multispectral surface-enhanced molecular spectroscopy in tailored coupling regimes. Our concept greatly expands the capabilities and flexibility of traditional gap-tuned PAs, opening new perspectives for miniaturized sensing platforms towards on-chip and in-situ detection.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Permittivity-asymmetric quasi-bound states in the continuum},

author = {R Bert\'{e} and T Weber and L D S Menezes and L K\"{u}hner and A Aigner and M Barkey and F J Wendisch and Y S Kivshar and A Tittl and S A Maier},

url = {https://arxiv.org/abs/2211.01176},

doi = {https://doi.org/10.48550/arXiv.2211.01176},

year  = {2022},

date = {2022-11-02},

journal = {arXiv preprint arXiv:2211.01176},

abstract = {Broken symmetries lie at the heart of nontrivial physical phenomena. Breaking the in-plane geometrical symmetry of optical systems allows to access a set of electromagnetic states termed symmetry-protected quasi-bound states in the continuum (qBICs). Here we demonstrate, theoretically, numerically and experimentally, that such optical states can also be accessed in metasurfaces by breaking the in-plane symmetry in the permittivity of the comprising materials, showing a remarkable equivalence to their geometrically-asymmetric counterparts. However, while the physical size of atoms imposes a limit on the lowest achievable geometrical asymmetry, weak permittivity modulations due to carrier doping and electro-optical Pockels and Kerr effects, usually considered insignificant, open up the possibility of infinitesimal permittivity asymmetries for on-demand, and dynamically tuneable optical resonances of extremely high quality factors. We probe the excitation of permittivity-asymmetric qBICs (ε-qBICs) using a prototype Si/TiO2 metasurface, in which the asymmetry in the unit cell is provided by the refractive index contrast of the dissimilar materials, surpassing any unwanted asymmetries from nanofabrication defects or angular deviations of light from normal incidence. ε-qBICs can also be excited in 1D gratings, where quality-factor enhancement and tailored interference phenomena via the interplay of geometrical and permittivity asymmetries are numerically demonstrated. The emergence of ε-qBICs in systems with broken symmetries in their permittivity may enable to test time-energy uncertainties in quantum mechanics, and lead to a whole new class of low-footprint optical and optoelectronic devices, from arbitrarily narrow filters and topological sources, biosensing and ultrastrong light-matter interaction platforms, to tuneable optical switches.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {High-Q nanophotonics over the full visible spectrum enabled by hexagonal boron nitride metasurfaces},

author = {L K\"{u}hner and L Sortino and B Tilmann and T Weber and K Watanabe and T Taniguchi and S A Maier and A Tittl},

url = {https://arxiv.org/abs/2210.11314},

doi = {https://doi.org/10.48550/arXiv.2210.11314},

year  = {2022},

date = {2022-10-20},

journal = {arXiv preprint arXiv:2210.11314},

abstract = {All-dielectric optical metasurfaces with high quality (Q) factors have so far been hampered by the lack of simultaneously lossless and high refractive index (RI) materials over the full visible spectrum. To achieve broad spectral coverage, the use of low-index materials is, in fact, unavoidable due to the inverse correlation between the band-gap energy (and therefore the optical losses) and the RI. However, for Mie resonant photonics, smaller RIs are associated with reduced Q factors and mode volume confinement. In this work, we leverage symmetry-broken bound states in the continuum (BICs) to efficiently suppress radiation losses from the low-index (n~2) van der Waals material hexagonal boron nitride (hBN), realizing metasurfaces with high-Q resonances over the complete visible spectrum. In particular, we analyze the rational use of low and high RI materials as resonator components and harness our insights to experimentally demonstrate sharp BIC resonances with Q factors above 300, spanning wavelengths between 400 nm and 1000 nm from a single hBN flake. Moreover, we utilize the enhanced electric near-fields to demonstrate second harmonic generation (SHG) with enhancement factors above 102. Our results provide a theoretical and experimental framework for the implementation of low RI materials as photonic media for metaoptics.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Unlocking the out-of-plane dimension for photonic bound states in the continuum to achieve maximum optical chirality},

author = {L K\"{u}hner and F J Wendisch and A A Antonov and J B\"{u}rger and L H\"{u}ttenhofer and L D S Menezes and S A Maier and M V Gorkunov and Y Kivshar and A Tittl},

url = {https://arxiv.org/abs/2210.05339},

doi = {https://doi.org/10.48550/arXiv.2210.05339},

year  = {2022},

date = {2022-10-11},

journal = {arXiv preprint arXiv:2210.05339},

abstract = {The realization of lossless metasurfaces with true chirality crucially requires the fabrication of three-dimensional structures, constraining their feasibility for experiments and hampering practical implementations. Even though the three-dimensional assembly of metallic nanostructures has been demonstrated previously, the resulting plasmonic resonances suffer from high intrinsic and radiative losses. The concept of photonic bound states in the continuum (BICs) is instrumental for tailoring radiative losses in diverse geometries, especially when implemented using lossless dielectrics, but applications have so far been limited to planar and intrinsically achiral structures. Here, we introduce a novel nanofabrication approach to unlock the height of generally flat all-dielectric metasurfaces as an accessible parameter for efficient resonance and functionality control. In particular, we realize out-of-plane symmetry breaking in quasi-BIC metasurfaces and leverage this design degree of freedom to demonstrate, for the first time, an optical all-dielectric quasi-BIC metasurface with maximum intrinsic chirality that responds selectively to light of a particular circular polarization depending on the structural handedness. Our experimental results not only open a new paradigm for all-dielectric BICs and chiral nanophotonics but also promise advances in the realization of efficient generation of optical angular momentum, holographic metasurfaces, and parity-time symmetry-broken optical systems.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Metasurface Measuring Twisted Light in Turbulence},

author = {T Dinter and C Li and L K\"{u}hner and T Weber and A Tittl and S A Maier and J M Dawes and H Ren},

url = {https://doi.org/10.1021/acsphotonics.2c00800},

doi = {10.1021/acsphotonics.2c00800},

year  = {2022},

date = {2022-09-09},

journal = {ACS Photonics},

volume = {9},

number = {9},

pages = {3043-3051},

abstract = {Orbital angular momentum (OAM) of light represents an independent degree of freedom using orthogonal helical modes for optical and quantum multiplexing, offering great potential to transform future ultrahigh-bandwidth information systems. Practical OAM communication systems suffer from turbulence-induced phase distortions to the propagating beams, decreasing the orthogonality of OAM modes through introduced modal crosstalk. To date, optical systems used for measuring OAM orthogonality breakdown in different turbulence conditions are too bulky and slow (e.g., one OAM mode at a time) for any practical use. Here, we demonstrate the use of an ultrathin OAM mode-sorting metasurface for characterizing the OAM orthogonality breakdown under different turbulence conditions. Our approach allows the measurement of the whole OAM spectrum at the same time. This metasurface exhibits strong OAM selectivity with an average modal crosstalk below −42.4 dB for OAM modes with topological charges ranging from −15 to +15. Our results suggest that higher-order OAM modes are as robust as lower-order modes in particular turbulence environments, paving the way for future practical free-space OAM communications harnessing high-dimensional OAM multiplexing. We demonstrated that a flat optical device with a small form factor can be integrated with practical communication systems for compact, fast, and efficient generation and detection of twisted light.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Strong light-matter interaction with self-hybridized bound states in the continuum in monolithic van der Waals metasurfaces},

author = {T Weber and L K\"{u}hner and L Sortino and A B Mhenni and N P Wilson and J K\"{u}hne and J J Finley and S A Maier and A Tittl},

url = {https://arxiv.org/abs/2209.01944},

doi = {https://doi.org/10.48550/arXiv.2209.01944},

year  = {2022},

date = {2022-09-05},

journal = {arXiv preprint arXiv:2209.01944},

abstract = {Photonic bound states in the continuum (BICs) are a standout nanophotonic platform for strong light-matter coupling with transition metal dichalcogenides (TMDCs), but have so far mostly been employed as all-dielectric metasurfaces with adjacent TMDC layers, incurring limitations related to strain, mode overlap, and material integration. In this work, we experimentally demonstrate for the first time asymmetry-dependent BIC resonances in 2D arrays of monolithic metasurfaces composed solely of the nanostructured bulk TMDC WS2 with BIC modes exhibiting sharp and tailored linewidths, ideal for selectively enhancing light-matter interactions. Geometrical variation enables the tuning of the BIC resonances across the exciton resonance in bulk WS2, revealing the strong-coupling regime with an anti-crossing pattern and a Rabi splitting of 116 meV. The precise control over the radiative loss channel provided by the BIC concept is harnessed to tailor the Rabi splitting via a geometrical asymmetry parameter of the metasurface. Crucially, the coupling strength itself can be controlled and is shown to be independent of material-intrinsic losses. Our BIC-driven monolithic metasurface platform can readily incorporate other TMDCs or excitonic materials to deliver previously unavailable fundamental insights and practical device concepts for polaritonic applications.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Introducing a Symmetry-Breaking Coupler into a Dielectric Metasurface Enables Robust High-Q Quasi-BICs},

author = {G Q Moretti and A Tittl and E Cort\'{e}s and S A Maier and A V Bragas and G Grinblat},

url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/adpr.202200111},

doi = {https://doi.org/10.1002/adpr.202200111},

issn = {2699-9293},

year  = {2022},

date = {2022-08-21},

journal = {Advanced Photonics Research},

volume = {n/a},

number = {n/a},

pages = {2200111},

abstract = {Dielectric metasurfaces supporting quasibound states in the continuum (quasi-BICs) exhibit very high-quality factor resonances and electric field confinement. However, accessing the high-Q end of the quasi-BIC regime usually requires marginally distorting the metasurface design from a BIC condition, pushing the needed nanoscale fabrication precision to the limit. This work introduces a novel concept for generating high-Q quasi-BICs, which strongly relaxes this requirement by incorporating a relatively large perturbative element close to high-symmetry points of an undistorted BIC metasurface, acting as a coupler to the radiation continuum. This approach is validated by adding a ≈100 nm diameter cylinder between two reflection-symmetry points separated by a 300 nm gap in an elliptical disk metasurface unit cell, using gallium phosphide as the dielectric. It is found that high-Q resonances emerge when the cylindrical coupler is placed at any position between such symmetry points. This metasurface's second harmonic generation capability in the optical range is further explored. Displacing the coupler as much as a full diameter from a BIC condition produces record-breaking normalized conversion efficiencies \>102 W−1. The strategy of enclosing a disruptive element between multiple high-symmetry points in a BIC metasurface can be applied to construct robust high-Q quasi-BICs in many geometrical designs.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Catalytic Metasurfaces Empowered by Bound States in the Continuum},

author = {H Hu and T Weber and O Bienek and A Wester and L H\"{u}ttenhofer and I D Sharp and S A Maier and A Tittl and E Cort\'{e}s},

url = {https://doi.org/10.1021/acsnano.2c05680},

doi = {10.1021/acsnano.2c05680},

issn = {1936-0851},

year  = {2022},

date = {2022-08-11},

journal = {ACS Nano},

volume = {16},

number = {8},

pages = {13057-13068},

abstract = {Photocatalytic platforms based on ultrathin reactive materials facilitate carrier transport and extraction but are typically restricted to a narrow set of materials and spectral operating ranges due to limited absorption and poor energy-tuning possibilities. Metasurfaces, a class of 2D artificial materials based on the electromagnetic design of nanophotonic resonators, allow optical absorption engineering for a wide range of materials. Moreover, tailored resonances in nanostructured materials enable strong absorption enhancement and thus carrier multiplication. Here, we develop an ultrathin catalytic metasurface platform that leverages the combination of loss-engineered substoichiometric titanium oxide (TiO2\textendashx) and the emerging physical concept of optical bound states in the continuum (BICs) to boost photocatalytic activity and provide broad spectral tunability. We demonstrate that our platform reaches the condition of critical light coupling in a TiO2\textendashx BIC metasurface, thus providing a general framework for maximizing light\textendashmatter interactions in diverse photocatalytic materials. This approach can avoid the long-standing drawbacks of many naturally occurring semiconductor-based ultrathin films applied in photocatalysis, such as poor spectral tunability and limited absorption manipulation. Our results are broadly applicable to fields beyond photocatalysis, including photovoltaics and photodetectors.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Optical Metasurfaces for Energy Conversion},

author = {E Cort\'{e}s and F J Wendisch and L Sortino and A Mancini and S Ezendam and S Saris and L De S. Menezes and A Tittl and H Ren and S A Maier},

url = {https://doi.org/10.1021/acs.chemrev.2c00078},

doi = {10.1021/acs.chemrev.2c00078},

issn = {0009-2665},

year  = {2022},

date = {2022-06-21},

journal = {Chemical Reviews},

volume = {122},

number = {19},

pages = {15082-15176},

abstract = {Nanostructured surfaces with designed optical functionalities, such as metasurfaces, allow efficient harvesting of light at the nanoscale, enhancing light\textendashmatter interactions for a wide variety of material combinations. Exploiting light-driven matter excitations in these artificial materials opens up a new dimension in the conversion and management of energy at the nanoscale. In this review, we outline the impact, opportunities, applications, and challenges of optical metasurfaces in converting the energy of incoming photons into frequency-shifted photons, phonons, and energetic charge carriers. A myriad of opportunities await for the utilization of the converted energy. Here we cover the most pertinent aspects from a fundamental nanoscopic viewpoint all the way to applications.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Radial bound states in the continuum for polarization-invariant nanophotonics},

author = {L K\"{u}hner and L Sortino and R Bert\'{e} and J Wang and H Ren and S A Maier and Y S Kivshar and A Tittl},

url = {https://arxiv.org/abs/2206.05206},

doi = {https://doi.org/10.48550/arXiv.2206.05206},

year  = {2022},

date = {2022-06-10},

journal = {arXiv preprint arXiv:2206.05206},

abstract = {All-dielectric nanophotonics underpinned by bound states in the continuum (BICs) have demonstrated breakthrough applications in nanoscale light manipulation, frequency conversion and optical sensing. Leading BIC implementations range from isolated nanoantennas with localized electromagnetic fields to symmetry-protected metasurfaces with controllable resonance quality (Q) factors. However, they either require structured light illumination with complex beamshaping optics or large, fabrication-intense arrays of polarization-sensitive unit cells, hindering tailored nanophotonic applications and on-chip integration. Here, we introduce radial quasi bound states in the continuum (rBICs) as a new class of radially distributed electromagnetic modes controlled by structural asymmetry in a ring of dielectric rod pair resonators. The rBIC platform provides polarization-invariant and tunable high-Q resonances with strongly enhanced near-fields in an ultracompact footprint as low as 2 μm2. We demonstrate rBIC realizations in the visible for sensitive biomolecular detection and enhanced second-harmonic generation from monolayers of transition metal dichalcogenides, opening new perspectives for compact, spectrally selective, and polarization-invariant metadevices for multi-functional light-matter coupling, multiplexed sensing, and high-density on-chip photonics.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Metasurface-Enhanced Infrared Spectroscopy: An Abundance of Materials and Functionalities},

author = {A John-Herpin and A Tittl and L K\"{u}hner and F Richter and S H Huang and G Shvets and S-H Oh and H Altug},

url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/adma.202110163},

doi = {https://doi.org/10.1002/adma.202110163},

issn = {0935-9648},

year  = {2022},

date = {2022-05-31},

journal = {Advanced Materials},

volume = {n/a},

number = {n/a},

pages = {2110163},

abstract = {Abstract Infrared (IR) spectroscopy provides unique information on the composition and dynamics of biochemical systems by resolving the characteristic absorption fingerprints of their constituent molecules. Based on this inherent chemical specificity and the capability for label-free, non-invasive, and real-time detection, IR spectroscopy approaches have unlocked a plethora of breakthrough application perspectives for fields ranging from environmental monitoring and defense to chemical analysis and medical diagnostics. Nanophotonics has played a crucial role for pushing the sensitivity limits of traditional far-field spectroscopy by using resonant nanostructures to focus the incident light into nanoscale hot-spots of the electromagnetic field, greatly enhancing light-matter interaction. Metasurfaces composed of regular arrangements of such resonators further increase the design space for tailoring this nanoscale light control both spectrally and spatially, which has established them as an invaluable toolkit for surface-enhanced spectroscopy. Starting from the fundamental concepts of metasurface-enhanced IR spectroscopy, we showcase a broad palette of resonator geometries, materials and arrangements for realizing highly sensitive metadevices, with a special focus on emerging systems such as phononic and 2D van der Waals materials, and integration with waveguides for lab-on-a-chip devices. Furthermore, we will highlight some advanced sensor functionalities of metasurface-based IR spectroscopy, including multiresonance, tunability, dielectrophoresis, live cell sensing, and machine-learning-aided analysis. This article is protected by copyright. All rights reserved},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Introducing a Symmetry-Breaking Coupler into a Dielectric Metasurface Enables Robust High-Q Quasibound States in the Continuum and Efficient Nonlinear Frequency Conversion},

author = {G Q Moretti and A Tittl and E Cort\'{e}s and S A Maier and A V Bragas and G Grinblat},

url = {https://doi.org/10.48550/arXiv.2204.07097},

doi = {https://doi.org/10.48550/arXiv.2204.07097},

year  = {2022},

date = {2022-04-14},

journal = {arXiv preprint arXiv:2204.07097},

abstract = {Dielectric metasurfaces supporting quasi-bound states in the continuum (quasi-BICs) exhibit very high quality factor resonances and electric field confinement. However, accessing the high-Q end of the quasi-BIC regime usually requires marginally distorting the metasurface design from a BIC condition, pushing the needed nanoscale fabrication precision to the limit. This work introduces a novel concept for generating high-Q quasi-BICs, which strongly relaxes this requirement by incorporating a relatively large perturbative element close to high-symmetry points of an undistorted BIC metasurface, acting as a coupler to the radiation continuum. We validate this approach by adding a ∼100 nm diameter cylinder between two reflection-symmetry points separated by a 300 nm gap in an elliptical disk metasurface unit cell, using gallium phosphide as the dielectric. We find that high-Q resonances emerge when the cylindrical coupler is placed at any position between such symmetry points. We further explore this metasurface's second harmonic generation capability in the optical range. Displacing the coupler as much as a full diameter from a BIC condition produces record-breaking normalized conversion efficiencies \>102 W−1. The strategy of enclosing a disruptive element between multiple high-symmetry points in a BIC metasurface could be applied to construct robust high-Q quasi-BICs in many geometrical designs.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Trends in Nanophotonics-Enabled Optofluidic Biosensors},

author = {J Wang and S A Maier and A Tittl},

url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.202102366},

doi = {https://doi.org/10.1002/adom.202102366},

issn = {2195-1071},

year  = {2022},

date = {2022-02-22},

journal = {Advanced Optical Materials},

volume = {n/a},

number = {n/a},

pages = {2102366},

abstract = {Abstract Optofluidic sensors integrate photonics with micro/nanofluidics to realize compact devices for the label-free detection of molecules and the real-time monitoring of dynamic surface binding events with high specificity, ultrahigh sensitivity, low detection limit, and multiplexing capability. Nanophotonic structures composed of metallic and/or dielectric building blocks excel at focusing light into ultrasmall volumes, creating enhanced electromagnetic near-fields ideal for amplifying the molecular signal readout. Furthermore, fluidic control on small length scales enables precise tailoring of the spatial overlap between the electromagnetic hotspots and the analytes, boosting light-matter interaction, and can be utilized to integrate advanced functionalities for the pre-treatment of samples in real-world-use cases, such as purification, separation, or dilution. In this review, the authors highlight current trends in nanophotonics-enabled optofluidic biosensors for applications in the life sciences while providing a detailed perspective on how these approaches can synergistically amplify the optical signal readout and achieve real-time dynamic monitoring, which is crucial in biomedical assays and clinical diagnostics.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Tip Coupling and Array Effects of Gold Nanoantennas in Near-Field Microscopy},

author = {R B\"{u}chner and T Weber and L K\"{u}hner and S A Maier and A Tittl},

url = {https://doi.org/10.1021/acsphotonics.1c00744},

doi = {10.1021/acsphotonics.1c00744},

year  = {2021},

date = {2021-10-14},

journal = {ACS Photonics},

abstract = {Scattering-type scanning near-field optical microscopy (s-SNOM) is one of the predominant techniques for the nanoscale characterization of optical properties. The optical response of nanoantennas in s-SNOM is highly sensitive to their environment, including influences of the probing tip or neighboring resonators. Dielectric tips are commonly employed to minimize tip-related perturbations, although they provide a comparatively weak scattering signal. Here we show that when using metallic tips, it is possible to select between distinct weak and strong tip\textendashantenna coupling regimes by careful tailoring of the illumination conditions and resonator orientation. This enables the use of highly scattering metallic instead of dielectric tips for mapping plasmonic modes with comparatively higher signal strengths. This is a particular advantage for the retrieval of near-field spectra, which simultaneously require high near-field signals and unperturbed field patterns. We leverage our approach to analyze the collective effects of nanoantenna arrays, phenomena that are well understood in the optical far-field but have not been extensively studied in the near-field. Probing the dependence of the optical response on the array field size, we identify three regimes: the single rod regime, the intermediate regime, and the array-like regime. We show that these array effects give rise to characteristic spectral features originating from a complex interplay of radiative coupling and plasmon hybridization. These results provide evidence that long-range interactions of antennas also influence the local optical response that is probed in s-SNOM and demonstrate how collective resonances emerge from single building blocks, providing guidelines for optimized array designs for near- and far-field applications.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Fabrication robustness in BIC metasurfaces},

author = {J K\"{u}hne and J Wang and T Weber and L K\"{u}hner and S A Maier and A Tittl},

url = {https://doi.org/10.1515/nanoph-2021-0391},

doi = {doi:10.1515/nanoph-2021-0391},

year  = {2021},

date = {2021-09-06},

journal = {Nanophotonics},

volume = {10},

number = {17},

pages = {4305-4312},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {All‐Dielectric Crescent Metasurface Sensor Driven by Bound States in the Continuum},

author = {J Wang and J K\"{u}hne and T Karamanos and C Rockstuhl and S A Maier and A Tittl},

issn = {1616-301X},

year  = {2021},

date = {2021-08-13},

journal = {Advanced Functional Materials},

pages = {2104652},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{,

title = {Dual Nanoresonators for Ultrasensitive Chiral Detection},

author = {E Mohammadi and A Tittl and K L Tsakmakidis and T V Raziman and A G Curto},

url = {https://doi.org/10.1021/acsphotonics.1c00311},

doi = {10.1021/acsphotonics.1c00311},

year  = {2021},

date = {2021-05-28},

journal = {ACS Photonics},

abstract = {The discrimination of enantiomers is crucial in biochemistry. However, chiral sensing faces significant limitations due to inherently weak chiroptical signals. Nanophotonics is a promising solution to enhance sensitivity thanks to increased optical chirality maximized by strong electric and magnetic fields. Metallic and dielectric nanoparticles can separately provide electric and magnetic resonances. Here we propose their synergistic combination in hybrid metal\textendashdielectric nanostructures to exploit their dual character for superchiral fields beyond the limits of single particles. For optimal optical chirality, in addition to maximization of the resonance strength, the resonances must spectrally coincide. Simultaneously, their electric and magnetic fields must be parallel and π/2 out of phase and spatially overlap. We demonstrate that the interplay between the strength of the resonances and these optimal conditions constrains the attainable optical chirality in resonant systems. Starting from a simple symmetric nanodimer, we derive closed-form expressions elucidating its fundamental limits of optical chirality. Building on the trade-offs of different classes of dimers, we then suggest an asymmetric dual dimer based on realistic materials. These dual nanoresonators provide strong and decoupled electric and magnetic resonances together with optimal conditions for chiral fields. Finally, we introduce more complex dual building blocks for a metasurface with a record 300-fold enhancement of local optical chirality in nanoscale gaps, enabling circular dichroism enhancement by a factor of 20. By combining analytical insight and practical designs, our results put forward hybrid resonators to increase chiral sensitivity, particularly for small molecular quantities.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{,

title = {Anapole-Assisted Absorption Engineering in Arrays of Coupled Amorphous Gallium Phosphide Nanodisks},

author = {L H\"{u}ttenhofer and A Tittl and L K\"{u}hner and E Cort\'{e}s and S A Maier},

url = {https://doi.org/10.1021/acsphotonics.1c00238},

doi = {10.1021/acsphotonics.1c00238},

year  = {2021},

date = {2021-04-26},

journal = {ACS Photonics},

abstract = {Broadband solar light harvesting plays a crucial role for efficient energy conversion. Anapole excitations and associated absorption engineering in dielectric nanoresonators are a focus of nanophotonic research due to the intricate combination of nonradiating modes and strong electromagnetic field confinement in the underlying material. The arising high field strengths are used for enhanced second-harmonic generation and photocatalysis, where devices require large areas with closely spaced nanoresonators for sizable photonic yields. However, most anapole studies have so far been carried out at the single-particle level, neglecting the influence of anapole\textendashanapole interactions. Here, we present a systematic study of coupling mechanisms in rectangular arrays of amorphous GaP nanodisks that support anapole excitations at 600 nm, which is within the lossy spectral regime of the material. Our experimental findings show that maximum visible light extinction by the array and maximum absorption in the GaP are not achieved by the densest packing of resonators. Counterintuitively, increasing the array periodicities such that collective effects spectrally overlap with the anapole excitation of a single particle leads to an absorption enhancement of up to 300% compared to a single disk. An analysis of coupling in one- and two-dimensional arrays with polarization-dependent measurements and numerical simulations allows us to discriminate between coupling interactions parallel and perpendicular to the polarization axis and evaluate their strengths. Utilizing a multipolar decomposition of excitations in single nanodisks embedded in one-dimensional arrays, we can attribute the coupling to enhanced electric and toroidal dipoles under variation of the interparticle spacing. Our results provide a fundamental understanding of tailored light absorption in coupled anapole resonators and reveal important design guidelines for advanced metasurface approaches in a wide range of energy conversion applications.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}