Ultrasensitive Plasmon-Enhanced Infrared Spectroelectrochemistry.

IR spectroelectrochemistry (EC-IR) is a cutting-edge operando method for exploring electrochemical reaction mechanisms. However, detection of interfacial molecules is challenged by the limited sensitivity of existing EC-IR platforms due to the lack of high-enhancement substrates. Here, we propose an innovative plasmon-enhanced infrared spectroelectrochemistry (EC-PEIRS) platform to overcome this sensitivity limitation. Plasmonic antennae with ultrahigh IR signal enhancement are electrically connected via monolayer graphene while preserving optical path integrity, serving as both the electrode and IR substrate. The [Fe(CN)6 ]3- /[Fe(CN)6 ]4- redox reaction and electrochemical CO2 reduction reaction (CO2 RR) are investigated on the EC-PEIRS platform with a remarkable signal enhancement. Notably, the enhanced IR signals enable a reconstruction of the electrochemical curve of the redox reactions and unveil the CO2 RR mechanism. This study presents a promising technique for boosting the in-depth understanding of interfacial events across diverse applications.

Ultra-broadband and completely modulated absorption enhancement of monolayer graphene in a near-infrared region.

Achieving ultra-broadband and completely modulated absorption enhancement of monolayer graphene in near-infrared region is practically important to design graphene-based optoelectronic devices, however, which remains a challenge. In this work, by spectrally designing multiple magnetic plasmon resonance modes in metamaterials to be adjacent to each other, near-infrared light absorption in monolayer graphene is greatly improved to have an averaged absorption efficiency exceeding 50% in a very broad absorption bandwidth of about 800 nm. Moreover, by exerting an external bias voltage on graphene to change Fermi energy of graphene, the ultra-broadband absorption enhancement of monolayer graphene exhibits an excellent tunability, which has a nearly 100% modulation depth and an electrical switching property. This work is promising for applications in near-infrared photodetectors, amplitude modulators of electromagnetic waves, etc.

Ultraviolet graphene ultranarrow absorption engineered by lattice plasmon resonance.

We numerically demonstrate an ultraviolet graphene ultranarrow absorption in a hybrid graphene-metal structure. The full-width at half maximum of the absorption band being 9 nm in ultraviolet range is achieved based on the coupling of lattice plasmon resonances of the metallic nanostructure to the optical dissipation of graphene. The position, absorbance and linewidth of the hybridized narrow resonant mode tuned by controlling geometrical parameters and materials are systematically investigated. The proposed structure possesses high refractive index sensitivity of 288 nm/RIU and figure of merit of 72, and can also be used to detect small molecules layer of sub-nanometer thickness and refractive index with small changes, providing promising applications in ultra-compact efficient biosensors.

Graphene ultraviolet ultrahigh-Q perfect absorption for nanoscale optical sensing.

We propose an ultraviolet perfect ultranarrow band absorber by coating a dielectric grating on the monolayer graphene-dielectric-metal structure. The absorber presents an ultranarrow Fano lineshape with quality (Q) factor of 70 and a nearly perfect absorption of over 99.9% in the ultraviolet region, which is ascribed to the near field coupling of the optical dissipation of graphene and guide mode resonance of the dielectric grating. Structure parameters to the influence of the performance are investigated. The structure exhibits the high optical sensitivity (S = 150 nm/RIU, S* = 48/RIU) and figure of merit (FOM = 50, FOM* = 25374) and can also be used to detect the nanoscale analyte layer of sub-nanometer thickness, suggesting great potential applications in ultra-compact efficient biosensors for a much more sensitive detection of small refractive index changes.

Ultra-narrowband light absorption enhancement of monolayer graphene from waveguide mode.

Greatly improving the light absorption efficiency of graphene and simultaneously manipulating the corresponding absorption bandwidth (broadband or narrowband) is practically important to design graphene-based optoelectronic devices. In this work, we will theoretically show how to largely enhance the absorption in graphene and efficiently control the absorption bandwidth in the visible region, by the excitation of the waveguide mode for the graphene monolayer to be sandwiched between the gold sphere array and dielectric waveguide structure composed of indium tin oxide (ITO) film on a quartz substrate. It is found that the maximum absorption efficiency can reach as high as about 45% and the full-width at half-maximum (FWHM) of the absorption peak can be tuned from about 1 to 10 nanometers, when the array period of gold spheres or the thickness of ITO film is changed.

Highly tunable multiple narrow emissions of dyed dielectric-metal core-shell resonators: towards efficient fluorescent labels.

We report a potential efficient fluorescent label based on the dyed dielectric-metal core-shell resonators (DMCSRs). By utilizing the near-field coupling between the dyes and the multipolar sharp cavity plasmon resonances, the dyed DMCSRs with diameter of 1.02 µm are demonstrated to be capable of supporting multiple spontaneous emission peaks with the linewidths as narrow as âˆ¼ 10 nm in visible range, and these reshaped fluorescent emissions are insensitive to the surrounding dielectric environment. Furthermore, these multiple narrow emission peaks show a precise tunability on the spectrum by simply separating a nanometric dielectric layer between the dielectric core and the metallic shell, which may provide an attractive spectral multiplexing strategy in the fields of cell biology and medical sciences.

Double Fano resonances in individual metallic nanostructure for high sensing sensitivity.

In this paper, we report the design and observation of double Fano resonances (DFRs) in an individual symmetry reduced nanostructure and the induced high sensing sensitivity. Such a plasmonic nanostructure consists of a partially overlapped double metallic nanotriangles with unequal sizes fabricated by using the fast and low-cost angle-resolved nanosphere lithography. Symmetry breaking generates two narrow quadrupolar dark modes, which further enhance the coupling with fundamental bright dipole modes within the same structure, manifesting the effect of DFRs. The resonance wavelength and line shape of DFRs can be tailored by changing the degree of asymmetry as well as the size of the designed nanostructure. Based on DFRs, a high sensitivity to dielectric environment with a maximum figure of merit of 35 is measured. Due to fast manufacturing process with high reproducibility and high structural tunability, the fabricated individual metallic nanostructure provides an opportunity to significant potential applications in localized surface plasmon resonance (LSPR) based single or double-wavelength sensors in the near-infrared region.

Double Fano resonances in an individual metallic nanostructure for high sensing sensitivity.

In this paper, we report on the design and observation of double Fano resonances (DFRs) in an individual symmetry-reduced nanostructure and the induced high sensing sensitivity. Such a plasmonic nanostructure consists of a partially overlapped double-metallic nanotriangles with unequal sizes fabricated by using fast and low-cost angle-resolved nanosphere lithography. Symmetry breaking generates two narrow quadrupolar dark modes, which further enhance the coupling with fundamental bright dipole modes within the same structure, manifesting the effect of DFRs. The resonance wavelength and line shape of DFRs can be tailored by changing the degree of asymmetry as well as the size of the designed nanostructure. Based on DFRs, a high sensitivity to dielectric environment with a maximum figure of merit of 35 is measured. Due to a fast manufacturing process with high reproducibility and high structural tunability, the fabricated individual metallic nanostructure provides an opportunity for significant potential applications in localized surface plasmon resonance based single or double-wavelength sensors in the near-infrared region.

Dual broadband near-infrared perfect absorber based on a hybrid plasmonic-photonic microstructure.

High performance light absorber with a broad bandwidth is particularly desirable for near-infrared photodetection and optical interconnects. Here we demonstrate a dual broadband perfect absorber in the near-infrared regime, which is based on a hybrid plasmonic-photonic microstructure. Such a microstructure is fabricated by self-assembling a monolayer colloidal crystal on an optically opaque metal film followed by depositing a thin metallic half-shell on the top of the colloidal particles. Both experimental and numerical simulation results show that the simply designed absorbers have dual broadband with absorption exceeding 90% in the near-infrared region with the absorption bands being scalable by tuning the size of the colloidal particles. Moreover, the absorption efficiency shows an extremely slight dispersion for the incident angles up to 50 degrees, benefit from the high symmetry as well as the highly modulated plasmonic microstructures that lead to a weak polarization dependence of these two absorption bands. The relative ease of growing high-quality colloidal crystals and the low cost of fabricating such plasmonic-photonic microstructures with high reproducibility could promise applicability of the light absorber in the field of photodetectors, thermal emitters and photovoltaics.

Photothermal-Driven Polymer for a Smart Window with Adaptive Solar Modulation.

Although the thermochromic smart windows with adjustable sunlight transmittance to achieve energy savings are gradually improving, they are still difficult to use, limited by their unreasonable thermal response temperature, slow switching time, and poor durability. Here, we demonstrate a dual-function hybrid thermoresponsive smart window device (CPH) by trapping the phase-change polyHEA-HDA polymer (HEA = hydroxyethyl acrylate, HDA = hexadecyl acrylate) and polydopamine@CsxWO3 (PDA@CWO) core-shell nanoparticles within glasses. The introduced PDA@CWO nanoparticles substantially increase the energy transformation efficiency of solar energy to heat due to their outstanding photothermal conversion. When the temperature increases above the phase-transition temperature of polyHEA-HDA polymer, the copolymer components in the composite material undergo a reversible crystalline-amorphous transition, which enables the transformation of the whole smart window from transparency to opaque in a low ambient temperature. The light transmittance in the solar range can be dynamically modulated between 54.8 and 22.9% with a low ambient temperature while maintaining acceptable visible light transparency and effective UV shielding. A model house testing proves an indoor temperature cooling of 7.1 °C. This study offers a new approach to designing an energy-saving smart window system with multifunctionality.

Optical transmission of corrugated metal films on a two-dimensional hetero-colloidal crystal.

The near infrared transmission of corrugated metal films deposited on hetero-colloidal crystals is investigated. The transmission response of the quasi-three-dimensional (quasi-3D) metal film is modified by controlling the nominal thickness of a dielectric layer pre-deposited on the top surface of the colloidal crystal to form a new hetero-colloidal crystal. An extraordinary optical transmission (EOT) phenomenon could be presented in such metallodielectric (MD) architectures. We have found that the main transmission peak is suppressed as the thickness of the intercalated dielectric layer is increased. We propose that the observed EOT is a result of constructive interference between a localized sphere-like plasmon mode and an index-guided eigen mode mainly confined in the colloidal crystal, which is confirmed by our numerical simulations. Based on the MD microstructures, a distinct plasmon sensitivity response difference is achieved, which indicates potential applications for biochemical sensing.

The Light Absorption Enhancement in Graphene Monolayer Resulting from the Diffraction Coupling of Surface Plasmon Polariton Resonance.

In this study, we investigate a physical mechanism to improve the light absorption efficiency of graphene monolayer from the universal value of 2.3% to about 30% in the visible and near-infrared wavelength range. The physical mechanism is based on the diffraction coupling of surface plasmon polariton resonances in the periodic array of metal nanoparticles. Through the physical mechanism, the electric fields on the surface of graphene monolayer are considerably enhanced. Therefore, the light absorption efficiency of graphene monolayer is greatly improved. To further confirm the physical mechanism, we use an interaction model of double oscillators to explain the positions of the absorption peaks for different array periods. Furthermore, we discuss in detail the emerging conditions of the diffraction coupling of surface plasmon polariton resonances. The results will be beneficial for the design of graphene-based photoelectric devices.

Flexible core-shell Cs x WO3-based films with high UV/NIR filtration efficiency and stability.

Cesium-doped tungsten bronze Cs x WO3 (CWO) is an ideal near infrared (NIR) shielding material for solar filters. However, the NIR shielding ability of CWO-dispersed films easily deteriorates in hot humid environments, which severely hinders the commercial application of CWO. In this paper, UV/NIR shielding nanocomposite films were prepared by dispersing core-shell structured CWO@polydopamine (CWO@PDA) in a poly(vinyl alcohol) matrix. Because of the strong ultraviolet light absorption ability of PDA, it can shield ultraviolet light, which is generally detrimental to our health. The prepared nanocomposite films can efficiently shield 88.3% UV and 85.5% NIR radiation even though they show relatively high transparency in the visible range. Importantly, the good protection of the continuous PDA shells played an important role in enhancing the stability of CWO nanoparticles. The nanocomposite films also exhibit excellent stability in hot humid environments. Therefore, core-shell structured CWO@PDA nanoparticles have great potential as a novel UV/NIR shielding material for the development of efficient energy-saving windows.

Revealing the Hydrogen Bonding Interaction of DNA with Unnatural Bases via Plasmonic Antenna Enhanced Infrared Spectroscopy.

The incorporation of unnatural bases in DNA programming can break through the limits of Watson-Crick and Hoogsteen base pairing to expand the diversity of DNA structures. Thus, understanding the interaction between DNA and unnatural bases is of great importance in DNA nanotechnology. Here, we propose an approach of plasmonic antenna enhanced infrared spectroscopy to study the hydrogen bonding interaction between poly(thymine) DNA (poly T DNA) and melamine. The formation of multiple hydrogen bonds between melamine and thymine of poly T DNA is revealed by the appearance of a new infrared (IR) feature of the NH2 deformation vibration at 1680 cm-1. The binding rate constant (kb) and the dissociation rate constant (kd) of the affinity reaction reach 39.70 M-1·s-1 and 4.49 × 10-5 s-1, respectively. This work offers a valuable IR technique to study DNA nanostructures at the molecular level, providing unique physicochemical views of the interaction mechanism between DNA and unnatural bases in DNA programming.

Pronounced Linewidth Narrowing of Vertical Metallic Split-Ring Resonators via Strong Coupling with Metal Surface.

We theoretically study the plasmonic coupling between magnetic plasmon resonances (MPRs) and propagating surface plasmon polaritons (SPPs) in a three-dimensional (3D) metamaterial consisting of vertical Au split-ring resonators (VSRRs) array on Au substrate. By placing the VSRRs directly onto the Au substrate to remove the dielectric substrates effect, the interaction between MPRs of VSRRs and the SPP mode on the Au substrate can generate an ultranarrow-band hybrid mode with full width at half maximum (FWHM) of 2.2 nm and significantly enhanced magnetic fields, compared to that of VSRRs on dielectric substrates. Owing to the strong coupling, an anti-crossing effect similar to Rabi splitting in atomic physics is also obtained. Our proposed 3D metamaterial on a metal substrate shows high sensitivity (S = 830 nm/RIU) and figure of merit (FOM = 377), which could pave way for the label-free biomedical sensing.

Multiple Sharp Fano Resonances in a Deep-Subwavelength Spherical Hyperbolic Metamaterial Cavity.

We theoretically study the multiple sharp Fano resonances produced by the near-field coupling between the multipolar narrow plasmonic whispering-gallery modes (WGMs) and the broad-sphere plasmon modes supported by a deep-subwavelength spherical hyperbolic metamaterial (HMM) cavity, which is constructed by five alternating silver/dielectric layers wrapping a dielectric nanosphere core. We find that the linewidths of WGMs-induced Fano resonances are as narrow as 7.4-21.7 nm due to the highly localized feature of the electric fields. The near-field coupling strength determined by the resonant energy difference between WGMs and corresponding sphere plasmon modes can lead to the formation of the symmetric-, asymmetric-, and typical Fano lineshapes in the far-field extinction efficiency spectrum. The deep-subwavelength feature of the proposed HMM cavity is verified by the large ratio (~5.5) of the longest resonant wavelength of WGM1,1 (1202.1 nm) to the cavity size (diameter: 220 nm). In addition, the resonant wavelengths of multiple Fano resonances can be easily tuned by adjusting the structural/material parameters (the dielectric core radius, the thickness and refractive index of the dielectric layers) of the HMM cavity. The narrow linewidth, multiple, and tunability of the observed Fano resonances, together with the deep-subwavelength feature of the proposed HMM cavity may create potential applications in nanosensors and nanolasers.

Theoretical Study on Metasurfaces for Transverse Magneto-Optical Kerr Effect Enhancement of Ultra-Thin Magnetic Dielectric Films.

We study how to enhance the transverse magneto-optical Kerr effect (TMOKE) of ultra-thin magnetic dielectric films through the excitation of strong magnetic resonances on metasurface with a metal nanowire array stacked above a metal substrate with an ultra-thin magnetic dielectric film spacer. The plasmonic hybridizations between the Au nanowires and substrate result in magnetic resonances. The periodic arrangement of the Au nanowires can excite propagating surface plasmon polaritons (SPPs) on the metal surface. When the SPPs and the magnetic resonances hybridize, they can strongly couple to form two strong magnetic resonances, which are explained by a coupled oscillator model. Importantly, benefitting from the strong magnetic resonances, we can achieve a large TMOKE signal up to 26% in the ultra-thin magnetic dielectric film with a thickness of only 30 nm, which may find potential applications in nanophotonics, magnonics, and spintronics.

Double Narrow Fano Resonances via Diffraction Coupling of Magnetic Plasmon Resonances in Embedded 3D Metamaterials for High-Quality Sensing.

We theoretically demonstrate an approach to generate the double narrow Fano resonances via diffraction coupling of magnetic plasmon (MP) resonances by embedding 3D metamaterials composed of vertical Au U-shaped split-ring resonators (VSRRs) array into a dielectric substrate. Our strategy offers a homogeneous background allowing strong coupling between the MP resonances of VSRRs and the two surface collective optical modes of a periodic array resulting from Wood anomaly, which leads to two narrow hybridized MP modes from the visible to near-infrared regions. In addition, the interaction effects in the VSRRs with various geometric parameters are also systematically studied. Owing to the narrow hybrid MP mode being highly sensitive to small changes in the surrounding media, the sensitivity and the figure of merit (FoM) of the embedded 3D metamaterials with fabrication feasibility were as high as 590 nm/RIU and 104, respectively, which holds practical applications in label-free biosensing, such as the detection of medical diagnoses and sport doping drugs.

Multipolar Plasmonic Resonances of Aluminum Nanoantenna Tuned by Graphene.

We numerically investigate the multipolar plasmonic resonances of Aluminum nanoantenna tuned by a monolayer graphene from ultraviolet (UV) to visible regime. It is shown that the absorbance of the plasmonic odd modes (l = 1 and l = 3) of graphene-Al nanoribbon structure is enhanced while the absorption at the plasmonic even modes (l = 2) is suppressed, compared to the pure Al nanoribbon structure. With the presence of the monolayer graphene, a change in the resonance strength of the multipolar plasmonic modes results from the near field interactions of the monolayer graphene with the electric fields of the multipolar plasmonic resonances of the Al resonator. In particular, a clear absorption peak with a high quality (Q)-factor of 27 of the plasmonic third-order mode (l = 3) is realized in the graphene-Al nanoribbon structure. The sensitivity and figure of merit of the plasmonic third-order mode of the proposed Graphene-Al nanoribbon structure can reach 25 nm/RIU and 3, respectively, providing potential applications in optical refractive-index sensing.

Ultranarrow and Tunable Fano Resonance in Ag Nanoshells and a Simple Ag Nanomatryushka.

We study theoretically the Fano resonances (FRs) produced by the near-field coupling between the lowest-order (dipolar) sphere plasmon resonance and the dipolar cavity plasmon mode supported by an Ag nanoshell or the hybrid mode in a simple three-layered Ag nanomatryushka constructed by incorporating a solid Ag nanosphere into the center of Ag nanoshell. We find that the linewidth of dipolar cavity plasmon resonance or hybrid mode induced FR is as narrow as 6.8 nm (corresponding to a high Q-factor of ~160 and a long dephasing time of ~200 fs) due to the highly localized feature of the electric-fields. In addition, we attribute the formation mechanisms of typical asymmetrical Fano line profiles in the extinction spectra to the constructive (Fano peak) and the destructive interferences (Fano dip) arising from the symmetric and asymmetric charge distributions between the dipolar sphere and cavity plasmon or hybrid modes. Interestingly, by simply adjusting the structural parameters, the dielectric refractive index required for the strongest FR in the Ag nanomatryushka can be reduced to be as small as 1.4, which largely reduces the restriction on materials, and the positions of FR can also be easily tuned across a broad spectral range. The ultranarrow linewidth, highly tunability together with the huge enhancement of electric fields at the FR may find important applications in sensing, slow light, and plasmon rulers.