Extraordinary Optical Transmission Property of X-Shaped Plasmonic Nanohole Arrays

Optical transmission properties of periodic X-shaped plasmonic nanohole arrays in a silver film are investigated by performing the finite element method. Obvious peaks appear in the transmission spectra due to surface plasmon polaritons (SPPs) on the top surface of the silver film, to the Fabry–Ferot resonance effect of SPPs in the nanohole, and to the localized surface plasmon resonance of the nanohole. Besides the topologic shape parameters of the X-shaped nanohole, transmission properties strongly depend on incident polarization. The results of this study not only present a tunable plasmonic filter, but also aid in the understanding of the mechanisms of the extraordinary optical transmission phenomenon.     

Multiple Surface Plasmon Resonances in Compound Structure with Metallic Nanoparticle and Nanohole Arrays

The optical properties of a compound structure with metallic nanoparticle and nanohole arrays are numerically investigated by the means of finite-difference time domain method. We report on the observation of multi-valleys in the reflection spectra due to the excitation of surface plasmon (SP) resonant modes of the compound structure. Simulation results show that multiple SP resonances consist of surface plasmon polaritons on the gold film, localized surface plasmons on the nanoparticles, and coupling mode between them. These findings are important for applications utilizing multiple surface plasmon resonances.     

Near-Field Optical Experimental Investigation of Gold Nanohole Array

Gold nanohole arrays are fabricated with focused ion beam irradiating gold thin film supported on quartz substrate. The topography of the nanohole arrays is characterized using an atomic force microscope, and the near-field optical transmission properties of the nanohole array are investigated with a near-field scanning optical microscope. Our experimental results verify the near-optical transmission performance and further demonstrate that they are in agreement with the theoretical calculation results. The enhanced optical transmission of the nanohole arrays are expected to be used for a variety of applications in sensor and photonics devices.     

A Three-Dimensional Plasmonic Nanostructure with Extraordinary Optical Transmission

We report a 3D plasmonic nanostructure having an extraordinary optical transmission due to localized surface plasmon (LSP) coupling between nanoholes and nanodisks. The nanostructure contains a free-standing gold nanohole array (NHA) film above a cavity and an array of nanodisks at the bottom of the cavity that is aligned with the NHA. For the device, the LSP-mediated resonance position was dependent on the hole and nanodisk diameter as well as the separation distance. Also, the effect of LSP coupling between each hole and corresponding nanodisk became negligible for cavities deeper than 200 nm as observed as a disappearance of the LSP resonance. The greatest LSP resonance transmission and the highest electric field intensity were observed for the structure with the shallowest cavity. In addition, the structure had high surface plasmon resonance sensitivity and may have potential for surface-enhanced Raman spectroscopy and optical trapping applications.     

Subradiant,Superradiant Plasmon Modes and Fano Resonance in a Multilayer Nanocylinder Array Standing on a Thin Metal Film

The optical properties of a novel nanostructure consisting of a hexagonal array of aligned vertically three-layered metal-dielectric-metal nanodisks on a silver film are theoretically studied through the finite-difference time-domain method. The novel nanostructure exhibits three obvious optical transmission bands due to the excitation of subradiant plasmon modes, superradiant plasmon modes, and Fano resonances. Surface plasmon polaritons of the underlying Ag film also play a significant role on these three optical transmission bands via coupling with localized surface plasmons of nanodisk pairs. Moreover, the nanostructure also exhibits a good tunability of optical response by modifying the sizes of cylinders, the thickness of underlying metal film, and the dielectric constant of middle layer. These results demonstrate the nanostructure with great advantages in optical sensors and filters.

     

Extraordinary Transmission Characteristics of Subwavelength Nanoholes with Rectangular Lattice

We investigate the extraordinary optical transmission (EOT) properties of nanohole arrays with a rectangular lattice for label-free refractive index sensing applications. We show that the deviation within the periodicities along the two axes at the nanohole plane leads to more advantageous spectral quality of EOT signal compared to the conventional square lattice geometries. We introduce a way to further improve the sensitivity of the aperture system by carefully choosing the periodicities. We introduce nanohole arrays with a rectangular lattice supporting EOT signals with larger figure-of-merit values as well as enabling much stronger light transmission. We also model a nanohole system covered with a thin dielectric layer, mimicking biomolecules captured on the gold surface, in order to show its biosensing capability. We also show that certain deviation amounts between periodicities create spectral splitting within the EOT signal leading to larger spectral shifts in the presence of a thin dielectric film.     

Tunable Plasmon–Induced Transparency Based on Dirac Semimetals

Numerical and theoretical studies were conducted on the plasmon induced transparency (PIT) of the symmetrical structure of Dirac semi-metal films (DSFS). The films have a parallel strip and split resonant ring structure. After analysing the surface current intensity and distribution, it was found that the electromagnetically induced transparency is as a result of destructive interference between these two structures, with the amplitude modulation depth of the frequency of the transmission window reaching as high as 99.09%. Moreover, by adjusting the Fermi level of the DSFS, the Fermi level changed from 50 to 90 meV, and the transmission window blue-shifted from 0.529 to 0.799 THz. The transmission peak frequency was found to have a linear relationship with the Fermi level. Similarly, the transmission phase and group delay under different Fermi levels was investigated. The positive group delay of the film reaches 7.026 ps, which provides a direction for new applications of terahertz, such as optical storage and slow optical devices.

     

Plasmonic, Low-Frequency Raman, and Nonlinear Optical-Limiting Studies in Copper–Silica Nanocomposites

Nanocomposite thin films consisting of Cu nanoparticles embedded in silica matrix were synthesized by atom beam co-sputtering technique. Plasmonic, optical, and structural properties of the nanocomposite films were investigated by using ultraviolet (UV)–visible absorption spectroscopy, nonlinear optical transmission, X-ray diffraction (XRD), and low-frequency Raman scattering. UV–visible absorption studies revealed the surface plasmon resonance absorption at 564 nm which showed a red shift with increase in Cu fraction. XRD results together with surface plasmon resonance absorption confirmed the presence of Cu nanoparticles of different size. Low-frequency Raman studies of nanocomposite films revealed breathing modes in Cu nanoparticles. Nanocomposites with lower metal fractions were found to behave like optical limiters. The possibility of controllably tuning the optical nonlinearity of these nanocomposites could enable them to be the potential candidates for applications in nanophotonics.     

Novel Covellite CuS Single-Crystal Thin Films for Optoelectronic Applications

Copper sulfide (CuS) thin films have been used in many applications such as solar cells, photo-thermal, electro-conductive, and microwave shielding. In this work, copper sulfide thin films were deposited on glass and silicon substrates by thermal evaporation of in situ synthesized CuS powder. XRD analysis of these films revealed a single-crystal structure, AFM measurements indicated the films have a surface roughness (14.1 nm) and agglomerates of multiple monocrystalline particles with average size (66 nm), and the optical properties were investigated by UV-Vis spectrophotometer showing the films have high transmission (>80%) in the visible region and low absorbance with wide energy gap (3.813 eV). This novel structure with outstanding optical properties makes it very promising optical materials in optoelectronics.

     

Preparation of Nanostructured Film Arrays for Transmission Localized Surface Plasmon Sensing

This article presents a concise review of preparation methods for transparent nanostructured films, with an emphasis on their current applications in transmission-localized surface plasmon resonance (T-LSPR) sensing. One of the first methods used for the fabrication of transparent nanostructured metal films is a direct vacuum evaporation of thin gold films. Self-induced formations of small gold islands result in transparent nanostructured gold arrays. The most well-established method is a nanosphere lithography developed by Van Duyne. Nanotriangular island arrays with controlled size and optical properties can be fabricated by this protocol. A different nanolithography method known as focused ion beam milling is reported and used for the fabrication of nanohole arrays. Simple assembly of solution-phase synthesized nanoparticles has also been utilized for the preparation of nanoparticle arrays capable of T-LSPR sensing. Lastly, this article also describes a new preparation strategy, in which self-assembly/thermolysis of nanoparticle multilayers is employed to obtain transparent nanoisland architectures on glass substrates.     

Numerical Simulation of Extinction Spectra of Plasmonically Coupled Nanospheres Using Discrete Dipole Approximation: Influence of Compositional Asymmetry

We demonstrate plasmon coupling phenomenon between equivalent (homodimer) and non-equivalent (heterodimer) spherical shape noble metal nanoparticle (Ag, Au and Al). A systematic comparison of surface plasmon resonance (SPR) and extinction properties of various configurations (monomer, homodimer and heterodimer) has been investigated to observe the effect of compositional asymmetry. Numerical simulation has been done by using discrete dipole approximation method to study the optical properties of plasmonically coupled metal nanoparticles (MNPs). Plasmon coupling between similar nanoparticles allows only higher wavelength bonding plasmon mode while both the plasmon modes lower wavelength antibonding mode as well as higher wavelength bonding mode in the case of heterodimer. Au monomer of radius 50 nm shows resonance peak at 518 nm while plasmon coupling between Au-Au homodimer results in a spectral red shift around 609 nm. Au-Ag plasmonic heterodimer (radius 50 nm) reveals two resonant modes corresponding to higher energy antibonding mode (422 nm) as well as lower energy bonding mode (533 nm). Further, we have shown that interparticle edge-to-edge separation is the most significant parameter affecting the surface plasmon resonances of MNPs. As the inter particle separation decreases, resonance wavelength shows red spectral shift which is maximum for the touching condition. It is shown that plasmon coupling is a reliable strategy to tune the SPR.

     

The Focusing and Talbot Effect of Periodic Arrays of Metallic Nanoapertures in High-Index Medium

Three finite-sized two-dimensional (2D) periodic arrays of metallic nanoapertures with the shape of nanowave, nanohole, and nanodot have been developed. Using water as an output medium, although the operating wavelengths are larger than the array period, both the focusing and far-field plasmon Talbot effect are experimentally observed, showing a good agreement with the 2D finite-difference time-domain (FDTD) simulation results. The focusing performance in both cases, with the output medium of air and of water, is compared. A detailed investigation of the plasmon Talbot revivals reveals that they are composed of subwavelength hotspots with the size of ～0.5λ distributed in the same array period as the original device. Three-dimensional FDTD simulations prove that the existence of surface plasmons (SPs) exhibits an enhanced optical transmission at some SP resonant wavelengths dependent on the output medium. Additionally, it is demonstrated that the Talbot revivals provide a high-resolution mean to distinguish the slight geometric nonuniformity in periodic nanostructures.     

Mode Controlling of Surface Plasmon Polaritons by Geometric Phases

Metasurfaces are made of two-dimensional arrays of subwavelength nanostructures that form a spatially varying optical response, to control the wave fronts of optical waves. As the feature size of its constituent materials is nanoscale, investigation of the light-nanostructure interactions in the near field is critical for understanding the novel properties of metasurfaces. Here, we used a scanning near-field optical microscope (SNOM) to observe the near-field distribution of surface plasmon polaritons (SPPs) from a ring-shaped metasurface under illumination of circularly polarized light. It was found that with an additional degree of freedom of the geometric phase provided by the regularly arranged metamolecules, control over the near-field interference of the SPPs can be achieved, which is governed by the metasurface geometric symmetry that can be tuned by its topological charge. Meanwhile, the planar chiral character of the metamolecules exerts a deep influence on the near-field interference patterns. Our results can pave the way for active control of SPP propagation in near fields and have potential applications in highly integrated optical communication systems.

     

Surface-Enhanced Raman Scattering (SERS) on 1D Nano-gratings

Surface-enhanced Raman scattering (SERS) enhancement factor (EF) is among the major applications of surface plasmon polaritons (SPP’s). In this work, the SERS EF of 1D rectangular and sinusoidal-shaped gold (Au) grating structures has been designed and optimized on Au film using COMSOL multiphysics (5.3a) RF module taking glass as substrate. The 1D grating models are simulated by variation in slit width ranging 200–600 nm while other parameters including periodicity of 700 nm and Au film thickness of 50 nm remained fixed. In order to study the several phenomena including enhanced optical transmission and SERS EF, the transmission and electric field spectra have been obtained from both types of grating structures. In agreement with fundamental plasmonic mode, the slit width of two-thirds of the periodicity found to be optimum for SERS EF. Remarkable value of SERS EF is obtained in the case of a sinusoidal Au grating device (6.4 × 10⁹) which is calculated to be five times that of the rectangular grating (1.2 × 10⁹). These devices are also the fingerprints of molecules, hence find applications in biosensing, pollution control, and chemical and food industry.

     

Study of Design Tunable Optical Sensor and Monochromatic Filter of the One-Dimensional Periodic Structure of TiO2/MgF2 with Defect Layer of Liquid Crystal (LC) Sandwiched with Two Silver Layers

In this paper, we have inspected the optical characteristics of one-dimensional periodic structure (1DPS) of TiO₂ and MgF₂ dielectric materials with defect layer of liquid crystal (LC) sandwiched with two silver layers, i.e., (TiO₂|MgF₂)³|Ag|LC|Ag|(TiO₂/MgF₂)³ using transfer matrix method (TMM). The optical tunable properties of considered periodic structures investigated at different incident angles and temperatures for TE and TM modes. Our study shows that absorption peak of 1DPS varies with incident angle and temperature. The defect layer (Ag-LC-Ag), sandwiched LC within two metallic (Ag) layers, exhibits the surface plasmon waves at the metal LC interfaces. The effect of surface plasmon waves can be better understand through the optical sensing property of such defect periodic structure. The detailed study concludes that such a type of one-dimensional periodic structure (1DPS) may be useful to design a tunable sensor and monochromatic filter.

     

The Effect of Molecular Adsorption on Electro-Optical Properties of Graphene-Based Sensors

Extraordinary electrical and optical features of graphene-based materials attract researchers to improve sensing center of different sensors using them. In this research, the effects of sensing molecules on electro-optical features of graphene-based sensors are modeled. The adsorption effect on the Hamiltonian of the system based on tight-binding model is explored, and also the system band structure is investigated analytically. Then, refractive index deviations based on band gap variations are discovered which are used in response modeling of a graphene-based surface plasmon resonance (SPR) sensor.

     

Tunable Extraordinary Optical Transmission in a Metal Film Perforated with Two-Level Subwavelength Cylindrical Holes

We propose a novel plasmonic metal structure composed of a silver film perforated with a two-dimensional square array of two-level cylindrical holes on a silica substrate. The transmission properties of this structure are theoretically calculated by the finite-difference time-domain (FDTD) method. Double-enhanced transmission peaks are achieved in the visible and infrared regions, which mainly originate from the excitation of localized surface plasmon resonances (LSPRs), the hybridization of plasmon modes, and the optical cavity mode formed in the holes. The enhanced transmission behaviors can be effectively tailored by changing the geometrical parameters and dielectric materials filled in the holes. These findings indicate that our proposed structure has potential applications in highly integrated optoelectronic devices.     

Spectrally Tunable Optical Transmission of Titanium Nitride Split Ring Resonators

In this paper, the optical properties of titanium nitride split ring resonators as an intermetallic metamaterial nanostructure were studied. Our simulation shows the presence of plasmon and LC resonances in the transmission spectrum of a cell consists of four u-shape split ring resonators. The effect of different parameters of resonator such as size, periodic constant, and the material between arms in addition to the polarization of incident beam was examined on the resonance behavior of the system. Also, the optical properties of a cell consist of four complementary split ring resonators within titanium nitride thin film were investigated. An excited mode was observed at λ = 840 nm that was attributed to the plasmon resonance. Changing the arrangement and configuration of the system from C _1v to C _2v symmetry led to the presence of the LC mode beside the plasmon mode in the transmission spectrum. Also, we explored a connection between the complementary split ring resonators and orderly perforated surface plasmon systems. It was determined that a transition occurred from resonator-type to surface plasmon behavior by increasing the size of resonator above 170 nm.     

Numerical Simulation of Broadband Scattering by Coated and Noncoated Metal Nanostructures Using Discrete Dipole Approximation Method

We suggest numerical method to study the optical response of metal nanostructures. The analysis of optical properties such as scattering and absorption by coated and noncoated nanogeometry has been done using discrete dipole approximation (DDA) method. The core-shell nanogeometry supports surface plasmon resonances, which are highly tunable from 400 to 1100 nm. The tunability of surface plasmon resonance (SPR) highly depends on the structural anisotropy and chosen core-shell material. Further, we have observed that aspect ratio is one of the key parameter to decide the nature and position of the plasmonic peaks and magnitude of optical cross section. We have also shown that coated nanospheroid is a more appropriate geometry as compared to coated nanosphere and noncoated nanospheroid in terms of wide tunability of surface plasmon resonance. The wide tunability in SPR is observed for the effective radii 90 nm core-shell (Au@SiO₂) nanospheroid with aspect ratio 0.1.     

Bidirectional Edge Asymmetric Light Transmission in Metal/Dielectric Device Based on Asymmetric Diffraction

Asymmetric light transmission (ALT) or optical diode-like nanodevices have attracted many research interests in recent years for its rosy potential application in all optical computing and information systems. In this work, we propose and numerically demonstrate a bidirectional edge asymmetric light transmission (BE-ALT) device, which is composed by the easy-processing metal/dielectric cylinders arranged periodically on glass substrate. The ALT effect in the proposed BE-ALT device shows a saltation at one critical wavelength, i.e., the asymmetric subtraction owns different signs for the wavelength larger and smaller than the critical wavelength. The asymmetric subtraction designed in this work changes dramatically from − 60% to + 80% at around 600 nm, which can be effectively manipulated by applying different structure parameters. The underlying physical mechanism has been investigated systematically, including the asymmetric diffraction effect, localized surface plasmonic resonance (LSPR), and the waveguide mode (WGM). Our designed BE-ALT device provides a new choice for the practical applications of ALT effect.