Nanostrip-Induced High Tunability Multipolar Fano Resonances in a Au Ring-Strip Nanosystem

Terahertz Vibrational Fingerprints Detection of Molecules with Particularly Designed Graphene Biosensors

In this research, an arc I-shaped graphene sensing structure with multi-resonance characteristics is proposed for the simultaneous detection of vibrational fingerprints with spectral separation in the terahertz range. The resonant frequencies of the sensor can be dynamically tuned by changing the gate voltage applied to the graphene arrays. The two vibrational fingerprints of lactose molecules (0.53 THz and 1.37 THz) in the transmission spectrum can be enhanced simultaneously by strictly optimizing the geometrical parameters of the sensor. More importantly, these two resonant frequencies can be tuned precisely to coincide with the two standard resonances of the lactose molecule. The physical mechanism of the sensor is revealed by inspection of the electric field intensity distribution, and the advantage of the sensor, which is its ability to operate at a wide range of incident angles, has been demonstrated. The sensing performance of the structure as a refractive index sensor has also been studied. Finally, a double arc I-shaped graphene sensor is further designed to overcome the polarization sensitivity, which demonstrates excellent molecular detection performance under different polarization conditions. This study may serve as a reference for designing graphene biosensors for molecular detection.

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 WGM_1,1 (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.

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.

Planar metamaterial sensor with graphene elliptical rings in transmission mode

A periodic planar metamaterial sensor in the terahertz band based on surface plasmon polariton resonances is proposed and studied. The unit cell includes four half-elliptical graphene rings located on a three-layer substrate including a SiO₂ layer, an air gap, and another SiO₂ layer. The embedded air gap between the two layers of SiO₂ improves the sensitivity of the sensor. Parametric study is performed, and the effects of the dimensions of the elliptical rings, the air gap thickness, and the Fermi energy of graphene on resonant frequency, sensitivity, and figure of merit (FoM) are investigated and graphically illustrated. The parameters of the sensor are optimized to provide a high sensitivity with a suitable FoM. By changing the refractive index of the sensing environment from 1.2 to 2, maximum sensitivity of 21.1 µm/RIU with FoM 5.14 is provided. The performance of the sensor is compared with previous works, and it is shown that a considerable improvement in sensitivity is achieved. The proposed sensor is suitable for biosensing applications.

Graphene Multiple Fano Resonances Based on Asymmetric Hybrid Metamaterial

We theoretically investigate multiple Fano resonances in an asymmetric hybrid graphene-metal metamaterial. The multiple Fano resonances emerge from the coupling of the plasmonic narrow bonding and antibonding modes supported by an in-plane graphene nanoribbon dimer with the broad magnetic resonance mode supported by a gold split-ring resonator. It is found that the Fano resonant mode with its corresponding dark mode of the antibonding mode in the in-plane graphene nanoribbon dimer is only achieved by structural symmetry breaking. The multiple Fano resonances can be tailored by tuning the structural parameters and Fermi levels. Active control of the multiple Fano resonances enables the proposed metamaterial to be widely applied in optoelectronic devices such as tunable sensors, switches, and filters.

Realization of Tunable Localized Surface Plasmon Resonance of Cu@Cu2O Core-Shell Nanoparticles by the Pulse Laser Deposition Method

Cu@Cu₂O core-shell nanoparticles (NPs) not only possess a stabilized structure but also exhibit better photocatalytic performance as compared to pure Cu₂O. Therefore, preparation of Cu@Cu₂O core-shell NPs is key toward efficient photocatalysis applications. In this paper, the fabrication of Cu@Cu₂O core-shell NPs on single-crystal MgO(100) substrates has been studied systematically by pulse laser deposition. Scanning electron microscopy (SEM) images show that the average diameter of NPs is enlarged from 89.9 to 150.3 nm with the increasing of oxygen pressure. Transmission electron microscopy (TEM) images indicate that the NPs have elongated hexagons and a core-shell structure with a shell thickness of about 10 nm. UV-vis absorption spectra show that the position of the localized surface plasmon resonance (LSPR) peaks shifts from 648 to 858 nm and the full width at half-maximum (fwhm) of the LSPR peaks broadens from 226.7 to 436.5 nm with increasing average diameter of NPs. According to the analysis, the red shift of the LSPR peaks is caused by enlargement of the core diameter; higher fwhm is a result of broadened particle size distribution and the elongated morphology of NPs. Therefore, the width and range of LSPR peaks of the absorption spectrum can be tuned using this method, which is beneficial for enhancing the light absorption and improving the photocatalytic efficiency of Cu@Cu₂O core-shell NPs.

Spin-Selected Dual-Wavelength Plasmonic Metalenses

Several novel spin-selected dual-wavelength metalenses have been proposed and investigated based on the plasmonic metasurface consisting of two kinds of rotary rectangle gap nanoantennas (RGN), which are designed based on merging two or four polarity-inverse lenses corresponding to different wavelengths (765 and 1300 nm). The spin-selected dual-wavelength metalenses with two similar and two different vertical and lateral focal points have also been proposed respectively, which can focus two wavelengths with inverse spin states to arbitrary special positions. The three-dimensional metalens with four focal points have also been proposed, which can focus four beams with inverse spin states and different wavelengths to preset positions. Moreover, a spin-dependent achromatic metalens has also been proposed, which can focus left circularly polarized (LCP) incidence with different wavelengths to the same position. Our work opens up new avenues toward establishing novel spin-selected and wavelength-selected metadevices, and is significant for the development of spin-controlled photonics and particles manipulation. In addition, it provides a new idea for solving the problem of data transmission from optical fiber communication to visible light communication.

Tunable Graphene-based Plasmonic Perfect Metamaterial Absorber in the THz Region

The optical performance of a periodically tunable plasma perfect metamaterial absorber based on a square-square-circle array we propose in the terahertz region is analyzed in this work by the finite difference time domain (FDTD) method. We not only discuss the impact of various parameters such as period a, length L, radius R, and incident angle θ under transverse magnetic (TM)- and transverse electric (TE)-polarization on the absorption spectra of the absorber but also study the effect of the Fermi energy E_F and relaxation time τ. Finally, we simulate the spectra as the surrounding refractive index n changes to better evaluate the sensing performance of the structure, producing a sensitivity S of the structure of up to 15006 nm/RIU. On account of this research, we find that the absorber is beneficial to sensors and detectors in the terahertz region.

A Novel SERS Substrate Platform: Spatially Stacking Plasmonic Hotspots Films

Surface-enhanced Raman scattering (SERS) technique has presented great potential in medical diagnosis, environment monitoring, and food detection due to its high sensitivity, rapid response, and fingerprint effect. Many efforts have been concentrated on all kinds of strategies to produce efficient SERS platforms. Here, we report a simple and controllable method to produce large-area efficient SERS platforms with spatially stacked plasmonic hotspots. The SERS platforms consist of double-layer metal porous films and are easily fabricated by magnetron sputtering and annealing, assisted by the evaporation of hydrofluoric acid. The stacked dual-layer metal porous films show prominent Raman enhancement and ultrasensitive SERS sensing capability for different target molecules. The detection limit is demonstrated down to 10^-13 M by detecting rhodamine 6G molecules. These superior Raman properties can be mainly ascribed to the highly dense spatially stacked plasmonic hotspots formed in the dual-layer metal porous films. The simple, controllable, and scalable fabrication strategy and superior Raman performance make these platforms promising candidates for the development of inexpensive, efficient, and mass-produced SERS substrates.

Fabriction of ZnO Nanorods with Strong UV Absorption and Different Hydrophobicity on Foamed Nickel under Different Hydrothermal Conditions

ZnO nanorods have been grown on the surface of foamed nickel by a two-step method. Firstly, a layer of ZnO seed is sputtered on the surface of the foamed nickel by magnetron sputtering, and then the hydrothermal method is used to grow ZnO nanorods at different conditions (solution concentration, reaction time and reaction temperature). The results show that the morphology of ZnO nanorods is closely related to the solution concentration, reaction time, and reaction temperature. The energy band structure formed by the foamed nickel and ZnO seed layers and the growth mechanism of ZnO nanorods are discussed. The samples are characterized by Energy dispersive spectrometer (EDS), X-ray diffraction (XRD), and Raman spectroscopy. The absorption characteristics of samples to light are characterized by ultraviolet-to-visible (UV–VIS) absorption. The hydrophilicity of the samples is characterized by the static contact angle. By analyzing the performance characteristics of the samples at different conditions, we finally obtained the optimal growth parameters. At the optimal parameters, the morphology of the grown nanorods is regular, the ultraviolet band has strong absorption, and the surface of the samples forms a superhydrophobic surface.

Seed-Mediated Electroless Deposition of Gold Nanoparticles for Highly Uniform and Efficient SERS Enhancement

A seed-mediated electroless deposition (SMED) approach for fabrication of large-area and uniform gold nanoparticle films as efficient and reproducible as surface-enhanced Raman scattering (SERS) substrates was presented. This approach involved a seeding pretreatment procedure and a subsequent growth step. The former referred to activation of polylysine-coated glass slides in gold seed solution, and the latter required a careful control of the reactant concentration and reaction time. With the aid of gold seeds and appropriate reaction conditions, a large-area and uniform nanofilm with evenly distributed gold nanoparticles (Au NPs) was formed on the surface of the substrates after adding a mixed solution containing ascorbic acid and trisodium citrate. The morphology of the Au nanofilm was examined by scanning electron microscopy. The size evolution of Au NPs on the surface of the substrates was analyzed in detail. The nanofilm substrate was prepared by reaction conditions of the seeded activation process: 10 mL ascorbic acid and trisodium citrate mixture and 30 min of soaking time, which exhibited an excellent uniformity and reproducibility of SERS enhancement with relative standard deviation (RSD) values of less than 8% (particularly, a RSD value of 3% can be reached for the optimized measurement). Compared to the common electroless deposition, the seed-mediated electroless deposition possessed inherent advantages in controllability, reproducibility, and economic benefit.

Tunable Multipolar Fano Resonances and Electric Field Enhancements in Au Ring-Disk Plasmonic Nanostructures

We theoretically research the characteristics of tunable multipolar Fano resonances in novel-designed Au ring-disk plasmonic nanostructures. We systematically study some structural parameters that influence the multipolar Fano resonances of the nanostructures. Adjustment of the radius (R₁ and R₂) of the Au ring, the radius (R₃) of the Au disk and the thickness (H) of the Au ring-disk can effectively adjust the multipolar Fano resonances. The complex field distributions excited by a Au ring-disk can produce dark resonance modes. At the frequency of the multipolar Fano resonances, strong localized field distributions can be obtained. The Fano resonances exhibit strong light-extinction properties in Au ring-disk nanostructures, which can be applied to an optical tunable filter and optical switch.