Dual-band perfect absorber for multispectral plasmon-enhanced infrared spectroscopy

Tailoring Light-Matter Interactions in Overcoupled Resonator for Biomolecule Recognition and Detection

Plasmonic nanoantennas provide unique opportunities for precise control of light-matter coupling in surface-enhanced infrared absorption (SEIRA) spectroscopy, but most of the resonant systems realized so far suffer from the obstacles of low sensitivity, narrow bandwidth, and asymmetric Fano resonance perturbations. Here, we demonstrated an overcoupled resonator with a high plasmon-molecule coupling coefficient (μ) (OC-Hμ resonator) by precisely controlling the radiation loss channel, the resonator-oscillator coupling channel, and the frequency detuning channel. We observed a strong dependence of the sensing performance on the coupling state, and demonstrated that OC-Hμ resonator has excellent sensing properties of ultra-sensitive (7.25% nm-1), ultra-broadband (3-10 μm), and immune asymmetric Fano lineshapes. These characteristics represent a breakthrough in SEIRA technology and lay the foundation for specific recognition of biomolecules, trace detection, and protein secondary structure analysis using a single array (array size is 100 × 100 µm2). In addition, with the assistance of machine learning, mixture classification, concentration prediction and spectral reconstruction were achieved with the highest accuracy of 100%. Finally, we demonstrated the potential of OC-Hμ resonator for SARS-CoV-2 detection. These findings will promote the wider application of SEIRA technology, while providing new ideas for other enhanced spectroscopy technologies, quantum photonics and studying light-matter interactions.

Highly sensitive and label-free protein immunoassay-based biosensor comprising infrared metamaterial absorber inducing strong coupling

A mid-infrared label-free immunoassay-based biosensor is an effective device to help identify and quantify biomolecules. This biosensor employs a surface-enhanced infrared absorption spectroscopy, which is a highly potent sensing technique for detecting minute quantities of analytes. In this study, a biosensor was constructed using a metamaterial absorber, which facilitated strong coupling effects. For maximum coupling effect, it is necessary to enhance the near-field intensity and the spatial and spectral overlap between the optical cavity resonance and the vibrational mode of the analyte. Due to significant peak splitting, conventional baseline correction methods fail to adequately analyze such a coupling system. Therefore, we employed a coupled harmonic oscillation model to analyze the spectral distortion resulting from the peak splitting induced by the strong coupling effect. The proposed biosensor with a thrombin-binding aptamer-based immunoassay could achieve a limit of detection of 267.4 pM, paving the way for more efficient protein detection in clinical practice.

Continuous spectral and coupling-strength encoding with dual-gradient metasurfaces

To control and enhance light-matter interactions at the nanoscale, two parameters are central: the spectral overlap between an optical cavity mode and the material's spectral features (for example, excitonic or molecular absorption lines), and the quality factor of the cavity. Controlling both parameters simultaneously would enable the investigation of systems with complex spectral features, such as multicomponent molecular mixtures or heterogeneous solid-state materials. So far, it has been possible only to sample a limited set of data points within this two-dimensional parameter space. Here we introduce a nanophotonic approach that can simultaneously and continuously encode the spectral and quality-factor parameter space within a compact spatial area. We use a dual-gradient metasurface design composed of a two-dimensional array of smoothly varying subwavelength nanoresonators, each supporting a unique mode based on symmetry-protected bound states in the continuum. This results in 27,500 distinct modes and a mode density approaching the theoretical upper limit for metasurfaces. By applying our platform to surface-enhanced molecular spectroscopy, we find that the optimal quality factor for maximum sensitivity depends on the amount of analyte, enabling effective molecular detection regardless of analyte concentration within a single dual-gradient metasurface. Our design provides a method to analyse the complete spectral and coupling-strength parameter space of complex material systems for applications such as photocatalysis, chemical sensing and entangled photon generation.

Surface-Enhanced Infrared Absorption Spectroscopy by Resonant Vibrational Coupling with Plasmonic Metal Oxide Nanocrystals

Coupling between plasmonic resonances and molecular vibrations in nanocrystals (NCs) offers a promising approach for detecting molecules at low concentrations and discerning their chemical identities. Metallic NC superlattices can enhance vibrational signals under far-field detection by generating a myriad of intensified electric field hot spots between the NCs. Yet, their effectiveness is limited by the fixed electron concentration dictated by the metal composition and inefficient hot spot creation due to the large mode volume. Doped metal oxide NCs, such as tin-doped indium oxide (ITO), could overcome these limitations by enabling broad tunability of resonance frequencies in the mid-infrared range through independent variation of size and doping concentration. This study investigates the potential of close-packed ITO NC monolayers for surface-enhanced infrared absorption by quantifying trends in the coupling between their plasmon modes and various molecular vibrations. We show that maximum vibrational signal intensity occurs in monolayers composed of larger, more highly doped NCs, where the plasmon resonance peak lies at higher frequency than the molecular vibration. Using finite element and mutual polarization methods, we establish that near-field enhancement is stronger on the low-frequency side of the plasmon resonance and for more strongly coupled plasmonic NCs, thus rationalizing the design rules we experimentally uncovered. Our results can guide the development of optimal metal oxide NC-based superstructures for sensing target molecules or modifying their chemical properties through vibrational coupling.

Surface-enhanced optical-mid-infrared photothermal microscopy using shortened colloidal silver nanowires: a noble approach for mid-infrared surface sensing

We propose surface-enhanced optical-mid-infrared photothermal (MIP) microscopy using highly crystalline silver nanowires, acting as a Fabry-Perot resonator, and demonstrate its applicability to enhanced mid-infrared surface sensing of thin polymer layers as thin as 20 nm.

Dual-band high-Q near-perfect absorption by tilted Si-assisted metasurfaces

All-dielectric metasurface perfect absorbers (MPAs) based on quasibound states in the continuum (QBICs) play a crucial role in optical and photonic devices as they can excite high-Q resonances. These structures require adding back reflectors or placing at least two asymmetric elements in each unit to break the absorption limit of 50%, which will increase the design complexity. In this work, we propose a high-Q monolayer MPA (MMPA) composed of a tilted Si nanocube array. By tuning the tilted angle of the nanocube, dual-QBIC modes at two different wavelengths are excited, which corresponds to magnetic quadrupole (MQ) and toroidal dipole (TD) modes, respectively. The high-reflection but low-Q magnetic dipole (MD) background mode excited by such a dual-band structure can decrease the radiative loss of transmission of MQ and TD modes, enabling the structure to break the absorption limit of 50%. The maximum absorption achieves 94% simultaneously at the wavelength of 933 and 961 nm, with the Q factors of 759 and 986, respectively. Our work provides a simple paradigm for designing dual-band high-Q MMPAs, which would greatly expand their range of applications, such as multiplexed optical nanodevices.

Advancements and Perspectives in Optical Biosensors

Optical biosensors exhibit immense potential, offering extraordinary possibilities for biosensing due to their high sensitivity, reusability, and ultrafast sensing capabilities. This review provides a concise overview of optical biosensors, encompassing various platforms, operational mechanisms, and underlying physics, and it summarizes recent advancements in the field. Special attention is given to plasmonic biosensors and metasurface-based biosensors, emphasizing their significant performance in bioassays and, thus, their increasing attraction in biosensing research, positioning them as excellent candidates for lab-on-chip and point-of-care devices. For plasmonic biosensors, we emphasize surface plasmon resonance (SPR) and its subcategories, along with localized surface plasmon resonance (LSPR) devices and surface enhance Raman spectroscopy (SERS), highlighting their ability to perform diverse bioassays. Additionally, we discuss recently emerged metasurface-based biosensors. Toward the conclusion of this review, we address current challenges, opportunities, and prospects in optical biosensing. Considering the advancements and advantages presented by optical biosensors, it is foreseeable that they will become a robust and widespread platform for early disease diagnostics.

Surface plasmons-phonons for mid-infrared hyperspectral imaging

Surface plasmons have proven their ability to boost the sensitivity of mid-infrared hyperspectral imaging by enhancing light-matter interactions. Surface phonons, a counterpart technology to plasmons, present unclear contributions to hyperspectral imaging. Here, we investigate this by developing a plasmon-phonon hyperspectral imaging system that uses asymmetric cross-shaped nanoantennas composed of stacked plasmon-phonon materials. The phonon modes within this system, controlled by light polarization, capture molecular refractive index intensity and lineshape features, distinct from those observed with plasmons, enabling more precise and sensitive molecule identification. In a deep learning-assisted imaging demonstration of severe acute respiratory syndrome coronavirus (SARS-CoV), phonons exhibit enhanced identification capabilities (230,400 spectra/s), facilitating the de-overlapping and observation of the spatial distribution of two mixed SARS-CoV spike proteins. In addition, the plasmon-phonon system demonstrates increased identification accuracy (93%), heightened sensitivity, and enhanced detection limits (down to molecule monolayers). These findings extend phonon polaritonics to hyperspectral imaging, promising applications in imaging-guided molecule screening and pharmaceutical analysis.

Advanced Terahertz Refractive Sensing And Fingerprint Recognition Through Metasurface-Excited Surface Waves

High-sensitive metasurface-based sensors are essential for effective substance detection and insightful bio-interaction studies, which compress light in subwavelength volumes to enhance light-matter interactions. However, current methods to improve sensing performance always focus on optimizing near-field response of individual meta-atom, and fingerprint recognition for bio-substances necessitates several pixelated metasurfaces to establish a quasi-continuous spectrum. Here, a novel sensing strategy is proposed to achieve Terahertz (THz) refractive sensing, and fingerprint recognition based on surface waves (SWs). Leveraging the long-range transmission, strong confinement, and interface sensitivity of SWs, a metasurface-supporting SWs excitation and propagation is experimentally verified to achieve sensing integrations. Through wide-band information collection of SWs, the proposed sensor not only facilitates refractive sensing up to 215.5°/RIU, but also enables the simultaneous resolution of multiple fingerprint information within a continuous spectrum. By covering 5 µm thickness of polyimide, quartz and silicon nitride layers, the maximum phase change of 91.1°, 101.8°, and 126.4° is experimentally obtained within THz band, respectively. Thus, this strategy broadens the research scope of metasurface-excited SWs and introduces a novel paradigm for ultrasensitive sensing functions.

Multi-band perfect absorber based on an elliptical cavity coupled with an elliptical metal nanorod

We proposed a triple-band narrowband device based on a metal-insulator-metal (MIM) structure in visible and near-infrared regions. The finite difference time domain (FDTD) simulated results illustrated that the absorber possessed three perfect absorption peaks under TM polarization, and the absorption efficiencies were about 99.76%, 99.99%, and 99.92% at 785 nm, 975 nm, and 1132 nm, respectively. Simulation results matched well with the results of coupled-mode theory (CMT). Analyses of the distributions of the electric field indicated the "perfect" absorption was due to localized surface plasmon polaritons resonance (LSPPR) and Fabry-Perot resonance. We developed a multi-band absorber with more ellipsoid pillars. The four band-absorbing device presented perfect absorption at 767 nm, 1046 nm, 1122 nm, and 1303 nm, and the absorption rates were 99.45%, 99.41%, 99.99%, and 99.94%, respectively. By changing the refractive index of the surrounding medium, the resonant wavelengths could be tuned linearly. The maximum sensitivity and Figure of Merit were 230 nm RIU-1 and 10.84 RIU-1, respectively. The elliptical structural design provides more tuning degrees of freedom. The absorber possessed several satisfactory performances: excellent absorption behavior, multiple bands, tunability, incident insensitivity, and simple structure. Therefore, the designed absorbing device has enormous potential in optoelectronic detection, optical switching, and imaging.

A super asymmetric cross antenna structure with tunable dual-frequency resonances

The detection performance of traditional infrared spectroscopy can be very limited in the case of molecular vibrational modes with low absorption cross-sections. On account of its electric field enhancement, plasmonic antenna can be combined with infrared spectroscopy to realize surface enhanced infrared detection and characterization of molecules. In this work, a super asymmetric cross antenna structure with tunable dual-frequency resonance and a high enhancement factor is designed. By systematically studying the transmission spectrum and charge distribution of this super asymmetric cross antenna structure, the physical origin of the dual-frequency resonance and its tunability are characterized in detail. In addition, in order to target desired molecular ensembles, the relationship between the resonance frequency and electric-field intensity of the two resonance modes and the parameters of structure and incident light are examined, yielding an enhancement factor close to 100 in the desired frequency region. Finally, the experimental results show that the proposed super asymmetric cross antenna structure can indeed generate dual-frequency resonances, agreeing reasonably with the theoretical results. It is believed that the super asymmetric cross antenna structure can be widely used to sensitively detect trace molecules, and in monolayered chemistry and bio-molecules, allowing their structures and dynamics to be studied using nonlinear infrared spectroscopy.

Dual-band chirality-selective absorbing by plasmonic metasurfaces with breaking mirror and rotational symmetry

In this work, we proposed a state-of-the-art metasurface model that breaks the mirror symmetry and rotation symmetry of the structure. It consists of two-layer rotating gold split rings, and has the capability of chirality-selective absorption for circularly polarized light (CPL) in two bands. The absorption peaks for left- and right- circularly polarized (LCP&RCP) light appeared at 989 nm and 1404 nm, respectively, with the maximum absorptivity of 98.5% and 96.3%, respectively. By changing the rotation angle of the two-layer gold split rings, it could also be designed as a single-band chiral metasurface absorber, which only absorbed RCP light but not LCP light, and the absorptivity of RCP light could be up to 97.4%. Furthermore, we found our designed absorbers had the characteristics of great circular dichroism (CD) and symmetric absorption. The physical mechanism of the selective absorption of CPL by the absorbers may be explained by the current vector analysis. In addition, the absorption peak could be tuned with the changing of the geometrical parameters of the structure. The proposed chirality-selective metasurface absorbers could be used in CD spectral detection, optical communication, optical filtering, and other fields.

Electromagnetically Induced Absorption Overcomes the Upper Limit of Light Absorption: Dipole-Dipole Coupling with Phase Retardation in Plasmonic-Dielectric Dimers

Electromagnetically induced absorption (EIA) by a phase-retarded coupling is theoretically investigated using a dimer composed of a plasmonic and dielectric particle. This phase-retarded coupling originates from the particles interacting with each other through their scattered intermediate fields (in between near and far fields). Our analysis based on the coupled-dipole method and an extended coupled-oscillator model indicates that EIA by the phase-retarded coupling occurs due to constructive interference in the scattered fields of the particles. By employing the finite element method, we demonstrate that the absorption of the plasmonic particle is dramatically enhanced by tuning the interparticle distance and achieving constructive interference. In contrast to EIA by near-field coupling, which has been intensively researched using coupled plasmonic systems, EIA by a phase-retarded coupling enables us to strengthen the absorption of plasmonic systems more significantly. This significant absorption enhancement is expected to be beneficial to advancing various applications, such as energy harvesting and radiative cooling.

Dual-band complementary metamaterial perfect absorber for multispectral molecular sensing

Metamaterial perfect absorbers (MPAs) show great potential in achieving exceptional sensing performance, particularly in the realm of surface-enhanced infrared absorption (SEIRA) spectroscopy. To this aim, it is highly desirable for the localized hotspots to be readily exposed and accessible to analyte with strong mode confinement to enhance absorption. Here, we propose a quasi-three-dimensional MPA based on cross-shaped coupled complementary plasmonic arrays for highly sensitive refractive index sensing and molecular vibrational sensing. Dual-band perfect absorption can be approached with the two plasmonic resonances corresponding to the electric dipole-like mode of cross antenna array and the magnetic dipole-like mode of cross hole array, respectively. Large portions of the electric field of the hotspots are exposed and concentrated in the gap between the elevated cross antenna and its complementary structure on the substrate, leading to improved sensing sensitivities. An ultrathin polymethyl methacrylate (PMMA) film induces a significant redshift of the magnetic dipole-like mode with an 11.8 nm resonance shift per each nanometer polymer thickness. The value is comparable to the reported sensitivity of single molecule layer sensors. Additionally, the simultaneous detection of the C = O and C-H vibrations of PMMA molecules is enabled with the two plasmonic resonances adjusted by changing the lengths of the two cross branches. Remarkably, the observed mode splitting and anti-crossing behavior imply the strong interaction between plasmonic resonance and molecular vibration. Our dual-band MPA based on coupled complementary plasmonic arrays opens a new avenue for developing highly sensitive sensors for the detection of refractive index and multispectral molecular vibrations.

A hybrid graphene metamaterial absorber for enhanced modulation and molecular fingerprint retrieval

Surface-enhanced infrared absorption (SEIRA) has proven its ability to improve the detection performance of traditional infrared spectroscopy at unprecedented levels. However, the resonant frequency of the metamaterial absorber (MA) lacks tunability once the structure is fabricated, which poses a challenge for broadband fingerprint retrieval of molecules. Here, we propose a pixelated and electric tunable hybrid graphene MA with a broadband response for molecular fingerprint retrieval. Loss engineering is employed to optimize the sensing sensitivity of MA. The resonant frequency of MA is approximately linearly modulated with a change in the graphene Fermi level. This design allows a meta-pixel to match multiple characteristic absorption spectra, thereby establishing a one-to-many mapping relationship between spatial and spectral information. The one-to-many mapping relationship greatly reduces the number of meta-pixels. As a concept demonstration, we integrate 9 meta-pixels to achieve full spectral coverage from 1000 cm-1 to 2000 cm-1. Based on the broadband spectral properties of the sensor, we demonstrate its potential for multi-fingerprint detection, quantitative detection, chemical identification, and compositional analysis. Our proposed hybrid graphene MA can be easily integrated with other on-chip devices, providing a potential platform for optical sensing, infrared spectroscopy, and photodetection.

Bidirectional planar absorber with polarization-selective absorption and reflection capabilities

In this study, we developed a novel planar bidirectional perfect metamaterial absorber (PMA) with polarization-selective absorption and reflection capabilities. The proposed bidirectional PMA has near-perfect absorption for y- and x-polarized waves propagating in the -z and + z directions. It also reflects x- and y-polarized waves propagating in the -z and + z directions. We used full-wave simulations and Fabry-Perot cavity models to evaluate the performance of the proposed bidirectional PMA. We also used a free-space method to measure the fabricated sample. To demonstrate the potential of the proposed PMA in multiband systems, we extended our PMA design to a dual-band bidirectional absorber.

Research Progress in Surface-Enhanced Infrared Absorption Spectroscopy: From Performance Optimization, Sensing Applications, to System Integration

Infrared absorption spectroscopy is an effective tool for the detection and identification of molecules. However, its application is limited by the low infrared absorption cross-section of the molecule, resulting in low sensitivity and a poor signal-to-noise ratio. Surface-Enhanced Infrared Absorption (SEIRA) spectroscopy is a breakthrough technique that exploits the field-enhancing properties of periodic nanostructures to amplify the vibrational signals of trace molecules. The fascinating properties of SEIRA technology have aroused great interest, driving diverse sensing applications. In this review, we first discuss three ways for SEIRA performance optimization, including material selection, sensitivity enhancement, and bandwidth improvement. Subsequently, we discuss the potential applications of SEIRA technology in fields such as biomedicine and environmental monitoring. In recent years, we have ushered in a new era characterized by the Internet of Things, sensor networks, and wearable devices. These new demands spurred the pursuit of miniaturized and consolidated infrared spectroscopy systems and chips. In addition, the rise of machine learning has injected new vitality into SEIRA, bringing smart device design and data analysis to the foreground. The final section of this review explores the anticipated trajectory that SEIRA technology might take, highlighting future trends and possibilities.

Broadband and Spectrally Selective Photothermal Conversion through Nanocluster Assembly of Disordered Plasmonic Metasurfaces

Plasmonic metasurfaces have been realized for efficient light absorption, thereby leading to photothermal conversion through nonradiative decay of plasmonic modes. However, current plasmonic metasurfaces suffer from inaccessible spectral ranges, costly and time-consuming nanolithographic top-down techniques for fabrication, and difficulty of scale-up. Here, we demonstrate a new type of disordered metasurface created by densely packing plasmonic nanoclusters of ultrasmall size on a planar optical cavity. The system either operates as a broadband absorber or offers a reconfigurable absorption band right across the visible region, resulting in continuous wavelength-tunable photothermal conversion. We further present a method to measure the temperature of plasmonic metasurfaces via surface-enhanced Raman spectroscopy (SERS), by incorporating single-walled carbon nanotubes (SWCNTs) as an SERS probe within the metasurfaces. Our disordered plasmonic system, generated by a bottom-up process, offers excellent performance and compatibility with efficient photothermal conversion. Moreover, it also provides a novel platform for various hot-electron and energy-harvesting functionalities.

Over-coupled resonator for broadband surface enhanced infrared absorption (SEIRA)

Detection of molecules is a key issue for many applications. Surface enhanced infrared absorption (SEIRA) uses arrays of resonant nanoantennas with good quality factors which can be used to locally enhance the illumination of molecules. The technique has proved to be an effective tool to detect small amount of material. However, nanoresonators can detect molecules on a narrow bandwidth so that a set of resonators is necessary to identify a molecule fingerprint. Here, we introduce an alternative paradigm and use low quality factor resonators with large radiative losses (over-coupled resonators). The bandwidth enables to detect all absorption lines between 5 and 10 μm, reproducing the molecular absorption spectrum. Counterintuitively, despite a lower quality factor, the system sensitivity is improved and we report a reflectivity variation as large as one percent per nanometer of molecular layer of PMMA. This paves the way to specific identification of molecules. We illustrate the potential of the technique with the detection of the explosive precursor 2,4-dinitrotoluene (DNT). There is a fair agreement with electromagnetic simulations and we also introduce an analytic model of the SEIRA signal obtained in the over-coupling regime.

Ultrasensitive Molecular Fingerprint Retrieval Using Strongly Detuned Overcoupled Plasmonic Nanoantennas

Tailoring light-matter interactions via plasmonic nanoantennas (PNAs) has emerged as a breakthrough technology for spectroscopic applications. The detuning between molecular vibrations and plasmonic resonances, as a fundamental and inevitable optical phenomenon in light-matter interactions, reduces the interaction efficiency, resulting in a weak molecule sensing signal at the strong detuning state. Here, it is demonstrated that the low interaction efficiency from detuning can be tackled by overcoupled PNAs (OC-PNAs) with a high ratio of the radiative to intrinsic loss rates, which can be used for ultrasensitive spectroscopy at strong plasmonic-molecular detuning. In OC-PNAs, the ultrasensitive molecule signals are achieved within a wavelength detuning range of 248 cm-1 , which is 173 cm-1 wider than previous works. Meanwhile, the OC-PNAs are immune to the distortion of molecular signals and maintain a lineshape consistent with the molecular signature fingerprint. This strategy allows a single device to enhance and capture the full and complex fingerprint vibrations in the mid-infrared range. In the proof-of-concept demonstration, 13 kinds of molecules with some vibration fingerprints strongly detuning by the OC-PNAs are identified with 100% accuracy with the assistance of machine-learning algorithms. This work gains new insights into detuning-state nanophotonics for potential applications including spectroscopy and sensors.

Metasurface-Enhanced Infrared Spectroscopy: An Abundance of Materials and Functionalities

Infrared 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, noninvasive, and real-time detection, infrared spectroscopy approaches have unlocked a plethora of breakthrough applications 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 infrared spectroscopy, a broad palette of resonator geometries, materials, and arrangements for realizing highly sensitive metadevices is showcased, 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, advanced sensor functionalities of metasurface-based infrared spectroscopy, including multiresonance, tunability, dielectrophoresis, live cell sensing, and machine-learning-aided analysis are highlighted.

Electrically controllable optical switch metasurface based on vanadium dioxide

We report a voltage-tunable reflective gold wire grid metasurface on vanadium dioxide thin film, which consists of a metal-insulator-metal (MIM) structure. We excite surface plasmon polariton (SPP) modes on the gold surface by fabricating a one-dimensional structured gold wire grid. Joule heating of laser-induced graphene (LIG) can be controlled by the voltage at the bottom, allowing vanadium dioxide in the structure to complete the transition from the insulating state to the metallic state. The phase transition of vanadium dioxide strongly disrupts the plasmon modes excited by the gold wire grid above, thereby realizing a huge change in the reflection spectrum. This acts as a tunable metasurface optical switch with a maximum modulation depth (MD) of over 20 dB. We provide a more effective and simple method for creating tunable metasurfaces in the near-infrared band, which can allow metasurfaces to have wider applications in optical signal processing, optical storage, and holography.

Bidirectional planar absorber with polarization-selective absorption and transmission capabilities

In this study, we developed a novel planar bidirectional perfect metamaterial absorber (PMA) with polarization-selective absorption and transmission capabilities. The proposed structure can bidirectionally absorb x-polarized incident waves almost perfectly while functioning as a transparent surface for y-polarized incident waves at the same frequency. We discussed the performance and properties of the proposed PMA through simulation results and a theoretical model. We also used the free-space method in experimental tests of a fabricated sample. The results indicated fair consistency between the simulated and measured results, thereby validating the quality of our PMA design.

Uncooled two-microbolometer stack for long wavelength infrared detection

We have investigated an uncooled infrared (IR) detector utilizing a dual level architecture. This was achieved by combining two-microbolometer stack in the vertical direction to achieve high IR absorption over two distinct spectral windows across the long wavelength infrared region (LWIR). In addition, we have studied amorphous silicon germanium oxide (Si_xGe_yO_1-x-y) as an IR sensitive material, and metasurface to control IR absorption/reflection in interaction with standard Fabry-Perot cavity. The bottom microbolometer uses a metasurface to selectively absorbs a portion of the spectrum and reflects radiation outside this window range. At the same time, the top microbolometer uses a conventional Fabry-Perot resonant cavity to absorb a different portion of the spectrum and transmit any unabsorbed radiation outside this window. This device can be used to measure the absolute temperature of an object by comparing the relative signals in the two spectral bands. The spectral responsivity and detectivity, and thermal response time were > 10⁵ V/W, > 10⁸ cm Hz^1/2/W, and 1.13 ms to filtered blackbody infrared radiation between (2-16) µm. The microbolometer voltage noise power spectral density was reduced by annealing the microbolometers in vacuum at 300 °C.

Multi-wavelength lock-in spectroscopy for extracting perturbed spectral responses: molecular signatures in nanocavities

Detecting small changes in spectral fingerprints at multiple wavelength bands simultaneously is challenging for many spectroscopic techniques. Because power variations, drift, and thermal fluctuations can affect such measurements on different timescales, high speed lock-in detection is the preferred method, however this is typically a single channel (wavelength) technique. Here, a way to achieve multichannel (multi-wavelength) lock-in vibrational spectroscopy is reported, using acousto-optic modulators to convert nanosecond periodic temporal perturbations into spatially distinct spectra. This simultaneously resolves perturbed and reference spectra, by projecting them onto different locations of the spectrometer image. As an example, we apply this multichannel time-resolved methodology to detect molecular frequency upconversion in plasmonic nanocavities from the perturbed Raman scattering at different wavelengths. Our phase-sensitive detection scheme can be applied to any spectroscopy throughout the visible and near-infrared wavelength ranges. Extracting perturbed spectra for measurements on nanosecond timescales allows for capturing many processes, such as semiconductor optoelectronics, high-speed spectro-electrochemistry, catalysis, redox chemistry, molecular electronics, or atomic diffusion across materials.

Visible transparent mid-infrared broadband absorbers based on gradient refractive indexes and multi-size cavity resonances

We proposed a multi-layered nanorod structure with the same tilt angle and different diameters, which has high visible transmittance and strong 3-5 µm absorption based on the principles of the gradient of the refractive index and the multi-size cavity resonances. The indium tin oxide (ITO) was selected as the target material to fabricate the structure by using a glancing angle deposition method. The experimental results show that when the deposition angle θ is 80°, swing deposition is successively done with the rotation angle φ of ±8°, ± 5°, ± 3°, and 0° on the surface of the substrate, and the quartz crystal microbalance thicknesses of ITO nanorods are 220 nm for each deposition, the average transmittance is 80.5% in the range of 400-800 nm and the integrated absorption is 86% in the 3-5 µm band. Such a simple, low-cost, and easy-to-fabricate device has potential applications in window stealth materials and other related fields.

Menezes L, Tittl A, Ren H, Maier SA. Optical Metasurfaces for Energy Conversion

Nanostructured surfaces with designed optical functionalities, such as metasurfaces, allow efficient harvesting of light at the nanoscale, enhancing light-matter 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.

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.

An Infrared Ultra-Broadband Absorber Based on MIM Structure

We designed an infrared ultra-broadband metal-insulator-metal (MIM)-based absorber which is composed of a top layer with four different chromium (Cr) nano-rings, an intermediate media of aluminum trioxide (Al₂O₃), and a bottom layer of tungsten (W). By using the finite-difference time-domain (FDTD), the absorption performance of the absorber was studied theoretically. The results indicate that the average absorption of the absorber can reach 94.84% in the wavelength range of 800-3000 nm. The analysis of the electric and magnetic field indicates that the ultra-broadband absorption rate results from the effect of local surface plasmon resonance (LSPR). After that, the effect of structural parameters, metal and dielectric materials on the absorptivity of the absorber was also discussed. Finally, the effect of incidence angle on absorption was investigated. It was found that it is not sensitive to incidence angle; even when incidence angle is 30°, average absorptivity can reach 90%. The absorber is easy to manufacture and simple in structure, and can be applied in infrared detection and optical imaging.

Near-perfect (>99%) dual-band absorption in the visible using ultrathin semiconducting gratings

Electromagnetic perfect absorption entails impedance-matching between two adjacent media, which is often achieved through the excitation of photonic/plasmonic resonances in structures such as metamaterials. Recently, super absorption was achieved using a simple bi-layer configuration consisting of ultrathin lossy films. These structures have drawn rising interest due to the structural simplicity and mechanical stability; however, the relatively broadband absorption and weak angular dependence can limit its versatility in many technologies. In this work, we describe an alternative structure based on an ultrathin semiconducting (Ge) grating that features a dual-band near-perfect resonant absorption (99.4%) in the visible regime. An angular-insensitive resonance is attributed to strong interference inside the ultrathin grating layer, akin to the resonance obtained with a single ultrathin planar film, while an angular-sensitive resonance shows a much narrower linewidth and results from the diffraction-induced surface mode coupling. With an appropriately designed grating period and thickness, strong coherent coupling between the two modes can give rise to an avoided-crossing in the absorption spectra. Further, the angular-insensitive resonance can be tuned separately from the angularly sensitive one, yielding a single narrow-banded absorption in the visible regime and a broadband absorption resonance that is pushed into the near-infrared (NIR). Our design creates new opportunities for ultra-thin and ultra-compact photonic devices for application in technologies including image sensing, structural color-filtering and coherent thermal light-emission.

Multispectral plasmonic biosensors based on a Penta-supercell metamaterial for detection of prostate-specific antigen: Ultrasensitive in LC resonance mode

Here, the detection of prostate-specific antigen (PSA) was done using a refractive index sensor based on a plasmonic Penta-supercells metamaterial array. The proposed Penta-supercells metamaterial array consists of four split ring-shaped supercells and a multiplication-shaped supercell in the middle. The results were validated using the three-dimensional Finite-difference time-domain (FDTD) numerical method. The detection of low-concentration biomolecules is a typical drawback of conventional plasmonic biosensors. Our designed Penta-supercells shape biosensor benefits from an ultra-sensitivity. In our designed biosensor, simultaneous excitations of three main plasmon (inductance-capacitance (LC), quadrupole, and dipole) modes can occur, and the LC mode shows a superior sensitivity. The structure was designed optimally, and three different types of metals (Au, Ag, and Al) were examined. LC mode appeiers in Ag and Au and this mode is not seen in Al. Also, the results of this study show the superiority of Ag to Au and Al. Based on the results of this study, the proposed structure achieves a record high sensitivity of 2256 nm/RIU in LC mode and high sensitivity of 1022 nm/RIU in quadrupole mode, and 494 nm/RIU in dipole mode. As another result, the proposed structure is insensitive to orthogonal polarization. The full utilization of these three resonance plasmon modes shows bright prospects for multi-spectral application. In the case of biosensor application, the designed Penta-supercells-based biosensor and its ultra-high sensitivity of 2256 nm/RIU (4.5 times larger than the sensitivity of conventional plasmonic structures) can help the medical to detect low concentrations.

Wafer-scale ultra-broadband perfect absorber based on ultrathin Al-SiO2 stack metasurfaces

Broadband absorbers with high absorption, ultrathin thickness, and lithography-free planar structure have a wide range of potential applications, such as clocking and solar energy harvesting. For plasmonic metal materials, achieving perfect ultra-broadband absorption remains a challenge owing to the intrinsically narrow bandwidth. In this study, wafer-scale Al-SiO₂ stack metasurfaces were experimentally fabricated to realize perfect ultra-broadband absorption. The experimental results show that the absorption for Al-SiO₂ stack metasurfaces can reach up to 98% for the wavelength range from the ultraviolet to the near-infrared (350-1400 nm). It was experimentally verified that the absorption performance of Al-SiO₂ stack metasurfaces is dependent on the layer number and is superior to that of other metal-based stack metasurfaces. This study will pave the way for development of plasmonic metal-based ultra-broadband absorbers as in low cost and high performance robust solar energy devices.

Colorful Conductive Threads for Wearable Electronics: Transparent Cu-Ag Nanonets

Electronic textiles have been regarded as the basic building blocks for constructing a new generation of wearable electronics. However, the electronization of textiles often changes their original properties such as color, softness, glossiness, or flexibility. Here a rapid room-temperature fabrication method toward conductive colorful threads and fabrics with Ag-coated Cu (Cu-Ag) nanonets is demonstrated. Cu-Ag core-shell nanowires are produced through a one-pot synthesis followed by electroless deposition. According to the balance of draining and entraining forces, a fast dip-withdraw process in a volatile solution is developed to tightly wrap Cu-Ag nanonets onto the fibers of thread. The modified threads are not only conductive, but they also retain their original features with enhanced mechanical stability and dry-wash durability. Furthermore, various e-textile devices are fabricated such as a fabric heater, touch screen gloves, a wearable real-time temperature sensor, and warm fabrics against infrared thermal dissipation. These high quality and colorful conductive textiles will provide powerful materials for promoting next-generation applications in wearable electronics.

Nanoparticle-on-Mirror Metamaterials for Full-Spectrum Selective Solar Energy Harvesting

Most broadband metamaterial absorbers are realized by patterning periodic arrays of plasmonic nanoparticles (>100 nm) on dielectric/metallic substrates to enable both electric and magnetic resonances. These metamaterials, however, require costly nanolithographic top-down techniques for fabrication. Here, we demonstrate new-concept nanoparticle-on-mirror (NoM) metamaterial absorbers by densely packing plasmonic nanoparticles of much smaller size (∼30 nm) on metal films directly. Such a simple but rational design enables the use of all-solution-based bottom-up processes. Because of the decoupling of electric and magnetic polarizations in these ultrasmall nanoparticles, excellent impedance match and near-perfect light absorption can be achieved in a broad band over the solar spectrum with weak thermal emission. Proof-of-concept large-area NoM metamaterial absorbers that offer a solar absorptance of 94% but a low IR emittance of 2% are experimentally demonstrated. The outstanding performance, bottom-up process, and great compatibility render the design promising for efficient and large-scale solar energy harvesting.

Experimental Study of a Quad-Band Metamaterial-Based Plasmonic Perfect Absorber as a Biosensor

We present a metamaterial-based perfect absorber (PA) that strongly supports four resonances covering a wide spectral range from 1.8 µm to 10 µm of the electromagnetic spectrum. The designed perfect absorber has metal–dielectric–metal layers where a MgF₂ spacer is sandwiched between an optically thick gold film and patterned gold nanoantennas. The spectral tuning of PA is achieved by calibrating the geometrical parameters numerically and experimentally. The manufactured quad-band plasmonic PA absorbs light close to the unity. Moreover, the biosensing capacity of the PA is tested using a 14 kDa S100A9 antibody, which is a clinically relevant biomarker for brain metastatic cancer cells. We utilize a UV-based photochemical immobilization technique for patterning of the antibody monolayer on a gold surface. Our results reveal that the presented PA is eligible for ultrasensitive detection of such small biomarkers in a point-of-care device to potentially personalize radiotherapy for patients with brain metastases.

Engineering van der Waals Materials for Advanced Metaphotonics

The outstanding chemical and physical properties of 2D materials, together with their atomically thin nature, make them ideal candidates for metaphotonic device integration and construction, which requires deep subwavelength light-matter interaction to achieve optical functionalities beyond conventional optical phenomena observed in naturally available materials. In addition to their intrinsic properties, the possibility to further manipulate the properties of 2D materials via chemical or physical engineering dramatically enhances their capability, evoking new science on light-matter interaction, leading to leaped performance of existing functional devices and giving birth to new metaphotonic devices that were unattainable previously. Comprehensive understanding of the intrinsic properties of 2D materials, approaches and capabilities for chemical and physical engineering methods, the resulting property modifications and novel functionalities, and applications of metaphotonic devices are provided in this review. Through reviewing the detailed progress in each aspect and the state-of-the-art achievement, insightful analyses of the outstanding challenges and future directions are elucidated in this cross-disciplinary comprehensive review with the aim to provide an overall development picture in the field of 2D material metaphotonics and promote rapid progress in this fast emerging and prosperous field.

Predictable infrared dual-band narrow-band absorber for infrared detection

Dual-band infrared absorbers have received a great deal of attention for their potential applications in the field of sensing and detection. In this paper, we proposed a composite model consisting of Platinum nano-cylinder and micro-ring column stacked on top of Si₃N₄and Platinum films. The effect of geometrical parameters on spectral absorption was explored by finite difference in time domain methods, and the results revealed that there were narrow perfect absorption peaks in each of the two atmospheric window bands due to the magnetic polaritons. Meanwhile, the quantitative relationship of resonance wavelength and geometrical parameters were predicted by LC equivalent circuits. In addition, graphene was added to the structure to dynamically adjust the resonance wavelength by varying the Fermi level. The combination of graphene and microstructure achieved full coverage detection of wavelengths in the atmospheric window range. This dual-band absorber has potential applications in infrared detection because of its good absorption properties and its tunability.

Large-area long-wave infrared broadband all-dielectric metasurface absorber based on markless laser direct writing lithography

Scalable and low-cost manufacturing of broadband absorbers for use in the long-wave infrared region are of enormous importance in various applications, such as infrared thermal imaging, radiative cooling, thermal photovoltaics and infrared sensor. In recent years, a plethora of broadband absorption metasurfaces made of metal nano-resonators with plasmon resonance have been synthesized. Still, their disadvantages in terms of complex structure, production equipment, and fabrication throughput, limit their future commercial applications. Here, we propose and experimentally demonstrate a broadband large-area all-dielectric metasurface absorber comprised of silicon (Si) arrys of square resonators and a silicon nitride (Si₃N₄) film in the long-wave infrared region. The multiple Mie resonance modes generated in a single-size Si resonator are utilized to enhance the absorption of the Si₃N₄ film to achieve broadband absorption. At the same time, the transversal optical (TO) phonon resonance of Si₃N₄ and the Si resonator's magnetic dipole resonance are coupled to achieve a resonator size-insensitive absorption peak. The metasurface absorber prepared by using maskless laser direct writing technology displays an average absorption of 90.36% and a peak absorption of 97.55% in the infrared region of 8 to 14 µm, and still maintains an average absorption of 88.27% at a inciedent angle of 40°. The experimentally prepared 2 cm × 3 cm patterned metasurface absorber by markless laser direct writing lithography (MLDWL) exhibits spatially selective absorption and the thermal imaging of the sample shows that the maximum temperature difference of 17.3 °C can exist at the boundary.

Absorption and scattering in perfect thermal radiation absorber-emitter metasurfaces

Detailed spectral analysis of radiation absorption and scattering behaviors of metasurfaces was carried out via finite-difference time-domain (FDTD) photonic simulations. It revealed that, for typical metal-insulator-metal (MIM) nanodisc metasurfaces, absorbance and scattering cross-sections exhibit a ratio of σ_abs/σ_sca = 1 at the absorption peak spectral position. This relationship was likewise found to limit the attainable photo-thermal conversion efficiency in experimental and application contexts. By increasing the absorption due to optical materials, such as Cr metal nano-films typically used as an adhesion layer, it is possible to control the total absorption efficiency η = σ_abs/σ_sca and to make it the dominant extinction mechanism. This guided the design of MIM metasurfaces tailored for near-perfect-absorption and emission of thermal radiation. We present the fabrication as well as the numerical and experimental spectral characterisation of such optical surfaces.

Self-Assembled Metal Nanohole Arrays with Tunable Plasmonic Properties for SERS Single-Molecule Detection

Arrays of metal nano-holes have proved to be among of the most promising structures for applications in the field of nano-photonics and optoelectronics. Supporting both localized and propagating surface plasmons resonances, they are characterized by very high versatility thanks to the tunability of these modes, by means of the change of their periodicity, the size of the holes and metal composition. The interaction between different optical features can be exploited to modulate electromagnetic field distribution leading various hot-spots excitations on the metal surfaces. In this work, long range ordered arrays of nano-holes in thin gold films, with different geometrical characteristics, were fabricated by a modified nano-sphere lithography protocol, which allows precise control on holes' dimensions together with the preservation of the order and of the pristine periodicity of the array. An in-depth analysis of the correlation between surface plasmon modes interference and its effect on electromagnetic field distribution is proposed, both by numerical simulations and experimentally. Finally, metal nano-holes arrays are exploited for surface enhanced Raman experiments, evaluating and comparing their performances by the estimation of the enhancement factor. Values close to the single molecule detection are obtained for most of the samples, proving their potentialities in surface enhanced spectroscopy applications.

Biosensing on a Plasmonic Dual-Band Perfect Absorber Using Intersection Nanostructure

Optical absorbers with multiple absorption channels are required in integrated optical circuits and have always been a challenge in visible and near-infrared (NIR) region. This paper proposes a perfect plasmonic absorber (PPA) that consists of a closed loop and a linked intersection in a unit cell for sensitive biosensing applications. We elucidate the physical nature of finite element method simulations through the absorptance spectrum, electric field intensity, magnetic flux density, and surface charge distribution. The designed PPA achieves triple channels, and the recorded dual-band absorptance reaches 99.64 and 99.00% nm, respectively. Besides, the sensitivity can get 1000.00 and 650 nm/RIU for mode 1 and mode 2, respectively. Our design has a strong electric and magnetic field coupling arising from the mutual inductance and the capacitive coupling in the proposed plasmonic system. Therefore, the designed structure can serve as a promising option for biosensors and other optical devices. Here, we illustrated two examples, i.e., detecting cancerous cells and diabetes cells.

Plasmonic Resonant Nanoantennas Induce Changes in the Shape and the Intensity of Infrared Spectra of Phospholipids

Surface enhanced infrared absorption spectroscopic studies (SEIRAS) as a technique to study biological molecules in extremely low concentrations is greatly evolving. In order to use the technique for identification of the structure and interactions of such biological molecules, it is necessary to identify the effects of the plasmonic electric-field enhancement on the spectral signature. In this study the spectral properties of 1,2-Dipalmitoyl-sn-glycero-3 phosphothioethanol (DPPTE) phospholipid immobilized on gold nanoantennas, specifically designed to enhance the vibrational fingerprints of lipid molecules were studied. An AFM study demonstrates an organization of the DPPTE phospholipid in bilayers on the nanoantenna structure. The spectral data were compared to SEIRAS active gold surfaces based on nanoparticles, plain gold and plain substrate (Si) for different temperatures. The shape of the infrared signals, the peak positions and their relative intensities were found to be sensitive to the type of surface and the presence of an enhancement. The strongest shifts in position and intensity were seen for the nanoantennas, and a smaller effect was seen for the DPPTE immobilized on gold nanoparticles. This information is crucial for interpretation of data obtained for biological molecules measured on such structures, for future application in nanodevices for biologically or medically relevant samples.

Broadband Bi-Directional Polarization-Insensitive Metamaterial Absorber

Conventional metamaterial absorbers eliminate the transmitted electromagnetic wave by attaching the metal plate with the unidirectional absorption performance; these absorbers limit the practical applications to a large extent. In this paper, we present a broadband bi-directional metamaterial absorber by etching chip resistors on the resonators for expanding the bandwidth, and two orthogonal I-shaped structures are pasted on the both sides of the ultra-thin substrate (FR-4) instead of the metal plate for enhancing absorptance of the absorber. Simulated results show that absorptance of the designed absorber is larger than 0.9 in 1.43-2.51 GHz along the forward and backward directions under both TE and TM polarizations. Microwave experiments in the chamber are performed to verify the simulations, and the experimental results exhibit the excellent agreement with the simulations. Additionally, two I-shaped structures are orthogonally pasted on an ultrathin substrate, leading to the impedance-matching of both forward and backward directions, and the absorptance can be tailed dynamically via the middle layer of the substrate. The physics of the absorption are visualized by using a transmission line based on equivalent circuits. We claim that the designed bi-directional metamaterial absorber can be a good candidate for electromagnetic stealth and energy harvesting.

Large-area mid-infrared broadband absorbers based on spiral ITO resulting from the combination of two different broadening absorption methods

A large-area mid-infrared broadband absorber is proposed in this paper. The absorber is a spiral ITO structure grown on a hexagonal lattice arrangement of silicon nanopillars by using a glancing angle deposition method. The experimental results show that when the heights of the silicon nanopillars are 1.7 µm and the number of rotation depositions is n = 5, that is, the rotation angle is 150 degrees, the absorber absorbs more than 81% of electromagnetic waves in the 2.5-6 µm spectral range. In the atmospheric window of 3-5 µm, the integral absorption reaches 96%. The experimental results also show that the absorbing ability of the ITO structure in the mid-infrared atmospheric window is significantly stronger than that of the structure composed of silver under the same preparation conditions. The main reasons for the broadband absorption are that the spiral ITO structure has resonant absorption of electromagnetic waves with different wavelengths in the empty cavity regions with different sizes, and ITO has longer penetration depths than noble metals in the mid-infrared band, which brings about stronger broadband absorption. The combination of the two leads to a broadening of the total absorption spectrum. The higher heights of the silicon nanopillars enhance absorption further. Additionally, the loose spiral ITO distributions indicate lower mean plasma concentration and then increase penetration depths further, resulting in stronger light absorption. Such a large-area mid-infrared absorption structure with a simple preparation method has potential applications in mid-infrared cloaking and sensing.

Multifunctional Chemical Sensing Platform Based on Dual-Resonant Infrared Plasmonic Perfect Absorber for On-Chip Detection of Poly(ethyl cyanoacrylate)

Multifunctional chemical sensing is highly desirable in industry, agriculture, and environmental sciences, but remains challenging due to the diversity of chemical substances and reactions. Surface-enhanced infrared absorption (SEIRA) spectroscopy can potentially address the above problems by ultra-sensitive detection of molecular fingerprint vibrations. Here, a multifunctional chemical sensing platform based on dual-resonant SEIRA device for sensitive and multifunctional on-chip detection of poly(ethyl cyanoacrylate) (PECA) is reported. It is experimentally demonstrated that the SEIRA sensing platform achieves multiple functions required by the PECA glue industry, including vibrational detection, thickness measurement, and in situ observation of polymerization and curing, which are usually realized by separately using a spectrometer, a viscometer, and an ellipsometer in the past. Specifically, the all-in-one sensor offers a dual-band fingerprint vibration identification, sub-nm level detection limit, and ultrahigh sensitivity of 0.76%/nm in thickness measurement, and second-level resolution in real-time observation of polymerization and curing. This work not only provides a valuable toolkit for ultra-sensitive and multifunctional on-chip detection of PECA, but also gives new insights into the SEIRA technology for multi-band, multi-functional, and on-chip chemical sensing.

Multifunctional and reconfigurable graphene/liquid crystal-assisted asymmetrical Fabry-Pérot cavity for reflected light control

Multifunctional and reconfigurable devices are crucial for compact and smart optoelectronic devices. In this paper, we propose a multifunctional and spectrally reconfigurable asymmetric 1D PhC Fabry-Pérot cavity filled with nematic liquid crystal and bounded by two graphene monolayers. Due to the large number of available degrees of freedom, such a structure can behave as either a notch filter, an absorber, an amplitude modulator, or a phase shifter for the reflected electromagnetic waves. The chemical potential of one or both graphene monolayers can be exploited to modulate the amplitude and phase-shift angle of the reflected electromagnetic waves. Furthermore, all functions are narrowband (1 nm linewidth) and are spectrally tunable over a range of about 200 nm around the working wavelength of 1550 nm by controlling the orientation of the elongated molecules of the liquid crystal. This structure may be advantageously exploited for the realization of optical modulators and beamsteering systems.

Ultra-narrow-band metamaterial perfect absorber based on surface lattice resonance in a WS2 nanodisk array

Metamaterial perfect absorbers (MPAs) are attractive platforms for the unique manipulation of electromagnetic waves from near-field to far-field. Narrow-band MPAs are particularly intriguing for their potential applications as thermal emitters or biosensors. In this work, we proposed ultra-narrow-band MPAs based on surface lattice resonance (SLR) modes of WS₂ nanodisk arrays on gold films. The SLR modes stem from the coupling between the magnetic dipole modes of individual nanodisks and the Rayleigh anomaly of the array giving rise to high quality-factor resonances. With proper design of the nanodisk array, an ultra-narrow-band of 15 nm is achieved in the near infrared wavelength range. The underneath gold film provides the loss channel converting the incident light within the narrow band into heat in the gold film, effectively creating a perfect absorber. Systematic numerical simulations were performed to investigate the effects of the geometrical parameters on their optical properties, demonstrating the great tunability of this type of MPAs as well as their potential for engineering light-matter interactions.

Ultrasensitive Molecule Detection Based on Infrared Metamaterial Absorber with Vertical Nanogap

Surface-enhanced infrared absorption (SEIRA) spectroscopy is a powerful methodology for sensing and identifying small quantities of analyte molecules via coupling between molecular vibrations and an enhanced near-field induced in engineered structures. A metamaterial absorber (MA) is proposed as an efficient SEIRA platform; however, its efficiency is limited because it requires the appropriate insulator thickness and has a limited accessible area for sensing. SEIRA spectroscopy is proposed using an MA with a 10 nm thick vertical nanogap, and a record-high reflection difference SEIRA signal of 36% is experimentally achieved using a 1-octadecanethiol monolayer target molecule. Theoretical and experimental comparative studies are conducted using MAs with three different vertical nanogaps. The MAs with a vertical nanogap are processed using nanoimprint lithography and isotropic dry etching, which allow cost-effective large-area patterning and mass production. The proposed structure may provide promising routes for ultrasensitive sensing and detection applications.

Infrared Plasmonic Biosensor with Tetrahedral DNA Nanostructure as Carriers for Label-Free and Ultrasensitive Detection of miR-155

MicroRNAs play an important role in early development, cell proliferation, apoptosis, and cell death, and are aberrantly expressed in many types of cancers. To understand their function and diagnose cancer at an early stage, it is crucial to quantitatively detect microRNA without invasive labels. Here, a plasmonic biosensor based on surface-enhanced infrared absorption (SEIRA) for rapid, label-free, and ultrasensitive detection of miR-155 is reported. This technology leverages metamaterial perfect absorbers stimulating the SEIRA effect to provide up to 1000-fold near-field intensity enhancement over the microRNA fingerprint spectral bands. Additionally, it is discovered that the limit of detection (LOD) of the biosensor can be greatly improved by using tetrahedral DNA nanostructure (TDN) as carriers. By using near-field enhancement of SEIRA and specific binding of TDN, the biosensor achieves label-free detection of miR-155 with a high sensitivity of 1.162% pm^-1 and an excellent LOD of 100 × 10^-15 m. The LOD is about 5000 times lower than that using DNA single strand as probes and about 100 times lower than that of the fluorescence detection method. This work can not only provide a powerful diagnosis tool for the microRNAs detection but also gain new insights into the field of label-free and ultrasensitive SEIRA-based biosensing.