Design Principles for Sensitivity Optimization in Plasmonic Hydrogen Sensors

Topological Learning for the Classification of Disorder: An Application to the Design of Metasurfaces

Structural disorder can improve the optical properties of metasurfaces, whether it is emerging from some large-scale fabrication methods or explicitly designed and built lithographically. For example, correlated disorder, induced by a minimum inter-nanostructure distance or by hyperuniformity properties, is particularly beneficial for light extraction. Inspired by topology, we introduce numerical descriptors to provide quantitative measures of disorder with universal properties, suitable to treat both uncorrelated and correlated disorder at all length scales. The accuracy of these topological descriptors is illustrated both theoretically and experimentally by using them to design plasmonic metasurfaces with controlled disorder that we then correlate to the strength of their surface lattice resonances. These descriptors are an example of topological tools that can be used for the fast and accurate design of disordered structures or as aid in improving their fabrication methods.

Rapid detection of hydrogen using narrow bandwidth fiber-optic spectral combs with a low limit of detection

The all-fiber detection solutions are the key technology to detecting hydrogen leakage in time because of the low explosive limit of the hydrogen-air mixture gas. However, most of the fiber-optic-based hydrogen sensing platforms must disrupt their structure to achieve a special architecture for interacting with the hydrogen. Here, we report a promising non-damaged structure of fiber-optic narrow bandwidth spectral combs, that can be developed to determine the refractive change as low as 10-5 using its cut-off cladding resonance mode. Such high performance of response for the refractive index induces a rapid detection of hydrogen after a proper thickness of palladium was deposited on the device. An average response time of hydrogen of 4 min with a low limit of detection of 348 ppm was achieved. It is demonstrated that these narrow bandwidth fiber-optic resonance combs can be used for gas detection after being combined with functional materials.

Tunable Fabry-Pérot Resonator with Dynamic Structural Color: A Visual and Ultrasensitive Hydrogen Sensor

Hydrogen detection is crucial for the forthcoming hydrogen economy. Here, we present a visual, ultrasensitive, optical hydrogen sensor based on a tunable Fabry-Pérot (FP) resonator, which can fully release the volume expansion of palladium during hydrogenation and transfer this volume expansion into an optical signal. The FP resonator consists of a suspended polymethylmethacrylate/palladium (PMMA/Pd) bilayer on a gold (Au) square-hole array. The bottom of the gold square hole and hydrogen-sensitive PMMA/Pd bilayer form a dynamically tunable FP resonator. When hydrogen gas (H₂) is loaded, the hydrogen-induced lateral expanding stress concavely deforms the suspended bilayer downward to the substrate, narrowing the metal-air-metal gap at the center of the hole, and finally leading to a spectral blue shift. Our experimental results show a giant spectral shift of 279 nm with a reflectance variation of 57% on exposure to 0.6% H₂ mixed with air. Such an ultrahigh optical response results in a significant color change, enabling visual hydrogen detection. In addition, the sensor has a H₂ detection limit down to 0.1% and good recyclability. These advantages indicate that the sensor has excellent potential for hydrogen sensing applications.

Dielectric Mie voids: confining light in air

Manipulating light on the nanoscale has become a central challenge in metadevices, resonant surfaces, nanoscale optical sensors, and many more, and it is largely based on resonant light confinement in dispersive and lossy metals and dielectrics. Here, we experimentally implement a novel strategy for dielectric nanophotonics: Resonant subwavelength localized confinement of light in air. We demonstrate that voids created in high-index dielectric host materials support localized resonant modes with exceptional optical properties. Due to the confinement in air, the modes do not suffer from the loss and dispersion of the dielectric host medium. We experimentally realize these resonant Mie voids by focused ion beam milling into bulk silicon wafers and experimentally demonstrate resonant light confinement down to the UV spectral range at 265 nm (4.68 eV). Furthermore, we utilize the bright, intense, and naturalistic colours for nanoscale colour printing. Mie voids will thus push the operation of functional high-index metasurfaces into the blue and UV spectral range. The combination of resonant dielectric Mie voids with dielectric nanoparticles will more than double the parameter space for the future design of metasurfaces and other micro- and nanoscale optical elements. In particular, this extension will enable novel antenna and structure designs which benefit from the full access to the modal field inside the void as well as the nearly free choice of the high-index material for novel sensing and active manipulation strategies.

Molecular Plasmonics with Metamaterials

Molecular plasmonics, the area which deals with the interactions between surface plasmons and molecules, has received enormous interest in fundamental research and found numerous technological applications. Plasmonic metamaterials, which offer rich opportunities to control the light intensity, field polarization, and local density of electromagnetic states on subwavelength scales, provide a versatile platform to enhance and tune light-molecule interactions. A variety of applications, including spontaneous emission enhancement, optical modulation, optical sensing, and photoactuated nanochemistry, have been reported by exploiting molecular interactions with plasmonic metamaterials. In this paper, we provide a comprehensive overview of the developments of molecular plasmonics with metamaterials. After a brief introduction to the optical properties of plasmonic metamaterials and relevant fabrication approaches, we discuss light-molecule interactions in plasmonic metamaterials in both weak and strong coupling regimes. We then highlight the exploitation of molecules in metamaterials for applications ranging from emission control and optical modulation to optical sensing. The role of hot carriers generated in metamaterials for nanochemistry is also discussed. Perspectives on the future development of molecular plasmonics with metamaterials conclude the review. The use of molecules in combination with designer metamaterials provides a rich playground both to actively control metamaterials using molecular interactions and, in turn, to use metamaterials to control molecular processes.

Inverse designed plasmonic metasurface with parts per billion optical hydrogen detection

Plasmonic sensors rely on optical resonances in metal nanoparticles and are typically limited by their broad spectral features. This constraint is particularly taxing for optical hydrogen sensors, in which hydrogen is absorbed inside optically-lossy Pd nanostructures and for which state-of-the-art detection limits are only at the low parts-per-million (ppm) range. Here, we overcome this limitation by inversely designing a plasmonic metasurface based on a periodic array of Pd nanoparticles. Guided by a particle swarm optimization algorithm, we numerically identify and experimentally demonstrate a sensor with an optimal balance between a narrow spectral linewidth and a large field enhancement inside the nanoparticles, enabling a measured hydrogen detection limit of 250 parts-per-billion (ppb). Our work significantly improves current plasmonic hydrogen sensor capabilities and, in a broader context, highlights the power of inverse design of plasmonic metasurfaces for ultrasensitive optical (gas) detection.

Plasmonic hydrogen sensors have limited sensitivity due to broad spectral features. Here, the authors use a particle swarm optimization algorithm to inversely design a plasmonic metasurface based on a periodic array of Pd nanoparticles, and demonstrate hydrogen detection limit of 250 ppb.

Predicting Laser-Induced Colors of Random Plasmonic Metasurfaces and Optimizing Image Multiplexing Using Deep Learning

Structural colors of plasmonic metasurfaces have been promised to a strong technological impact thanks to their high brightness, durability, and dichroic properties. However, fabricating metasurfaces whose spatial distribution must be customized at each implementation and over large areas is still a challenge. Since the demonstration of printed image multiplexing on quasi-random plasmonic metasurfaces, laser processing appears as a promising technology to reach the right level of accuracy and versatility. The main limit comes from the absence of physical models to predict the optical properties that can emerge from the laser processing of metasurfaces in which random metallic nanostructures are characterized by their statistical properties. Here, we demonstrate that deep neural networks trained from experimental data can predict the spectra and colors of laser-induced plasmonic metasurfaces in various observation modes. With thousands of experimental data, produced in a rapid and efficient way, the training accuracy is better than the perceptual just noticeable change. This accuracy enables the use of the predicted continuous color charts to find solutions for printing multiplexed images. Our deep learning approach is validated by an experimental demonstration of laser-induced two-image multiplexing. This approach greatly improves the performance of the laser-processing technology for both printing color images and finding optimized parameters for multiplexing. The article also provides a simple mining algorithm for implementing multiplexing with multiple observation modes and colors from any printing technology. This study can improve the optimization of laser processes for high-end applications in security, entertainment, or data storage.

Nanophotonic Chiral Sensing: How Does It Actually Work?

Nanophotonic chiral sensing has recently attracted a lot of attention. The idea is to exploit the strong light-matter interaction in nanophotonic resonators to determine the concentration of chiral molecules at ultralow thresholds, which is highly attractive for numerous applications in life science and chemistry. However, a thorough understanding of the underlying interactions is still missing. The theoretical description relies on either simple approximations or on purely numerical approaches. We close this gap and present a general theory of chiral light-matter interactions in arbitrary resonators. Our theory describes the chiral interaction as a perturbation of the resonator modes, also known as resonant states or quasi-normal modes. We observe two dominant contributions: A chirality-induced resonance shift and changes in the modes' excitation and emission efficiencies. Our theory brings deep insights for tailoring and enhancing chiral light-matter interactions. Furthermore, it allows us to predict spectra much more efficiently in comparison to conventional approaches. This is particularly true, as chiral interactions are inherently weak and therefore perturbation theory fits extremely well for this problem.

Metallic Plasmonic Array Structures: Principles, Fabrications, Properties, and Applications

The vast development of nanofabrication has spurred recent progress for the manipulation of light down to a region much smaller than the wavelength. Metallic plasmonic array structures are demonstrated to be the most powerful platform to realize controllable light-matter interactions and have found wide applications due to their rich and tunable optical performance through the morphology and parameter engineering. Here, various light-management mechanisms that may exist on metallic plasmonic array structures are described. Then, the typical techniques for fabrication of metallic plasmonic arrays are summarized. Next, some recent applications of plasmonic arrays are reviewed, including plasmonic sensing, surface-enhanced spectroscopies, plasmonic nanolasing, and perfect light absorption. Lastly, the existing challenges and perspectives for metallic plasmonic arrays are discussed. The aim is to provide guidance for future development of metallic plasmonic array structures.

Thin Film and Nanostructured Pd-Based Materials for Optical H2 Sensors: A Review

In this review paper, we provide an overview of state-of-the-art Pd-based materials for optical H2 sensors. The first part of the manuscript introduces the operating principles, providing background information on the thermodynamics and the primary mechanisms of optical detection. Optical H2 sensors using thin films (i.e., films without any nanostructuring) are discussed first, followed by those employing nanostructured materials based on aggregated or isolated nanoparticles (ANPs and INPs, respectively), as well as complex nanostructured (CN) architectures. The different material types are discussed on the basis of the properties they can attribute to the resulting sensors, including their limit of detection, sensitivity, and response time. Limitations induced by cracking and the hysteresis effect, which reduce the repeatability and reliability of the sensors, as well as by CO poisoning that deteriorates their performance in the long run, are also discussed together with an overview of manufacturing approaches (e.g., tailoring the composition and/or applying functionalizing coatings) for addressing these issues.

Gallium Plasmonic Nanoantennas Unveiling Multiple Kinetics of Hydrogen Sensing, Storage, and Spillover

Hydrogen is the key element to accomplish a carbon-free based economy. Here, the first evidence of plasmonic gallium (Ga) nanoantennas is provided as nanoreactors supported on sapphire (α-Al₂ O₃ ) acting as direct plasmon-enhanced photocatalyst for hydrogen sensing, storage, and spillover. The role of plasmon-catalyzed electron transfer between hydrogen and plasmonic Ga nanoparticle in the activation of those processes is highlighted, as opposed to conventional refractive index-change-based sensing. This study reveals that, while temperature selectively operates those various processes, longitudinal (LO-LSPR) and transverse (TO-LSPR) localized surface plasmon resonances of supported Ga nanoparticles open selectivity of localized reaction pathways at specific sites corresponding to the electromagnetic hot-spots. Specifically, the TO-LSPR couples light into the surface dissociative adsorption of hydrogen and formation of hydrides, whereas the LO-LSPR activates heterogeneous reactions at the interface with the support, that is, hydrogen spillover into α-Al₂ O₃ and reverse-oxygen spillover from α-Al₂ O_3. This Ga-based plasmon-catalytic platform expands the application of supported plasmon-catalysis to hydrogen technologies, including reversible fast hydrogen sensing in a timescale of a few seconds with a limit of detection as low as 5 ppm and in a broad temperature range from room-temperature up to 600 °C while remaining stable and reusable over an extended period of time.

Shaping the Color and Angular Appearance of Plasmonic Metasurfaces with Tailored Disorder

The optical properties of plasmonic nanoparticle ensembles are determined not only by the particle shape and size but also by the nanoantenna arrangement. To investigate the influence of the spatial ordering on the far-field optical properties of nanoparticle ensembles, we introduce a disorder model that encompasses both "frozen-phonon" and correlated disorder. We present experimental as well as computational approaches to gain a better understanding of the impact of disorder. A designated Fourier microscopy setup allows us to record the real- and Fourier-space images of plasmonic metasurfaces as either RGB images or fully wavelength-resolved data sets. Furthermore, by treating the nanoparticles as dipoles, we calculate the electric field based on dipole-dipole interaction, extract the far-field response, and convert it to RGB images. Our results reveal how the different disorder parameters shape the optical far field and thus define the optical appearance of a disordered metasurface and show that the relatively simple dipole approximation is able to reproduce the far-field behavior accurately. These insights can be used for engineering metasurfaces with tailored disorder to produce a desired bidirectional reflectance distribution function.

Niobium nitride plasmonic perfect absorbers for tunable infrared superconducting nanowire photodetection

Quantum technologies such as quantum computing and quantum cryptography exhibit rapid progress. This requires the provision of high-quality photodetectors and the ability to efficiently detect single photons. Hence, conventional avalanche photodiodes for single photon detection are not the first choice anymore. A better alternative are superconducting nanowire single photon detectors, which use the superconducting to normal conductance phase transition. One big challenge is to reduce the product between recovery time and detection efficiency. To address this problem, we enhance the absorption using resonant plasmonic perfect absorber effects, to reach near-100% absorption over small areas. This is aided by the high resonant absorption cross section and the angle insensitivity of plasmonic resonances. In this work we present a superconducting niobium nitride plasmonic perfect absorber structure and use its tunable plasmonic resonance to create a polarization dependent photodetector with near-100% absorption efficiency in the infrared spectral range. Further we fabricated a detector and investigated its response to an external light source. We also demonstrate the resonant plasmonic behavior which manifests itself through a polarization dependence detector response.

Optical properties of tungsten trioxide, palladium, and platinum thin films for functional nanostructures engineering

In recent years, we have been witnessing the intensive development of optical gas sensors. Thin palladium and platinum films as well as tungsten trioxide films with palladium or platinum catalysts are widely used for hydrogen detection, and the optical constants of these materials are required for sensor development. We report the optical parameters retrieved from a set of ellipsometric and transmission spectra for electron-beam evaporated palladium, platinum, and tungsten trioxide films. The tungsten trioxide films were 81 nm, 162 nm, and 515 nm thick and the metal films were as thin as 5-7 nm. Ultrathin palladium and platinum films were shown to be successfully described by local and isotropic permittivity, which is quite different from known bulk values. However, this permittivity showed a strong dependence on adjacent materials, thus illustrating that the ultrathin metallic films can be considered composites characterized by effective permittivity. With the obtained refractive indices and permittivities, the optical spectra of fabricated WO₃/Pd and WO₃/Pt nanostructures incorporating 1D grating of Al₂O₃ were in an excellent agreement with the calculated ones without requiring any additional fitting procedures or inclusion of surface roughness layers in numerical models.

Optical Carbon Dioxide Detection in the Visible Down to the Single Digit ppm Range Using Plasmonic Perfect Absorbers

To tackle climate change and reduce CO₂ emissions, it is important to measure CO₂ output precisely. Even though there are many different techniques, no simple and cheap optical method in the visible is available. This work studies plasmonically enhanced optical carbon dioxide sensors in the visible wavelength range. The sensor samples are based on an inert plasmonic perfect absorber, which can be easily and cheaply fabricated by colloidal etching lithography. A CO₂-sensitive polyethylenimine (PEI) layer is then spin-coated on top to complete the samples. The samples are examined continuously by microspectroscopy during different CO₂ exposures to track spectral changes, particularly the position of the resonance centroid wavelength. The samples exhibit a resonance shift of up to 7 nm, depending on the CO₂ concentration and the temperature. The temperature influences the rise time as well as the sensitive concentration range. The concentration dependence of the resonance shift overall follows the shape of a Langmuir isotherm, which includes a nearly linear relation at lower concentrations and elevated temperatures and a saturating behavior at higher concentrations and lower temperatures. The results indicate that a sensitivity in the full range from 100 vol % to below 1 ppm can be achieved. The samples degenerate in a dry inert atmosphere in a matter of days but are useable over multiple weeks when exposed to humidity and CO₂. The PEI reacts very selectively to CO₂, showing no response to CO, NH₃, NO₂, CH₄, H₂, and only a very small response to O₂. Overall, polyethylenimine is very promising as a CO₂-sensitive material for many practical sensing applications over a wide range of concentrations. An adjustment of the temperature is mandatory to control the sensitivity and response time.

Hot-Electron-Activated Peroxidase-Mimicking Activity of Ultrathin Pd Nanozymes

Light-activated nanozymes can provide a wealth of new opportunities for the chemical industry and biotechnology. However, present remote-controlled catalytic systems are still far from satisfactory. Herein, we present an interesting example of applying ultrathin Pd nanosheets (Pd NSs) as a light-controllable peroxidase mimic. Since most of Pd atoms are exposed on their surface, Pd NSs with a thickness of 1.1 nm possess high peroxidase-like activity. More importantly, under light excitation, such intrinsic activity can be further activated by a nearly 2.4- to 3.2-fold. Such a phenomenon can be ascribed to the unique optical property of ultrathin Pd NSs, which can efficiently capture photons to generate hot electrons via surface plasmon resonance effect and thus promote the in situ decomposition of H₂O₂ into reactive oxygen species radicals (O*). This enhanced catalysis can also be used for real-time and highly sensitive colorimetric detection of H2O2. We expect our work can provide valuable insights into the rational design of artificial nanozymes with controllable and efficient activity in biomedical diagnostics, drug delivery, and environmental chemistry.

Low-Cost Hydrogen Sensor in the ppm Range with Purely Optical Readout

Due to the changing global climate, the role of renewable energy sources is of increasing importance. Hydrogen can play an important role as an energy carrier in the transition from fossil fuels. However, to ensure safe operations, a highly reliable and sensitive hydrogen sensor is required for leakage detection. We present a sensor design with purely optical readout that reliably operates between 50 and 100,000 ppm. The building block of the sensor is a reactive sample that consists of a layered structure with palladium nanodisks as the top layer and changes its optical properties depending on the external hydrogen partial pressure. We use a fiber-coupled setup consisting of an LED, a sensor body containing the reactive sample, and a photodiode to probe and read out the reflectance of the sample. This allows separation of the explosive detection area from the operating electronics and thus comes with an inherent protection against hydrogen ignition by electronic malfunctions. Our results prove that this sensor design provides a large detection range, fast response times, and enhanced robustness against aging compared to conventional thin-film technologies. Especially, the simplicity, feasibility, and scalability of the presented approach yield a holistic approach for industrial hydrogen monitoring.