Xiang-nan Zhang

λ3/20000 plasmonic nanocavities with multispectral ultra-narrowband absorption for high-quality sensing

We present a method to theoretically achieve multispectral ultra-narrowband near-unity absorption in metal structure with deep-subwavelength plasmonic nanocavities (i.e., volumes < λ3/20 000) for high-quality sensing. The concept is based on the combined excitation of multiple plasmon resonances and high optical field confinement by an array of hexagonal non-close-packed cross-shape nanocavities on an opaque metal film. A multispectral plasmonic nano-sensor with high refractive index sensitivity (538 nm/RIU) and figure of merit (58) in the optical region is obtained with the minimum bandwidth of 8 nm, which offers new perspectives for achieving ultra-compact efficient biosensors.

Robust multispectral transparency in continuous metal film structures via multiple near-field plasmon coupling by a finite-difference time-domain method

We propose a robust multispectral transparent plasmonic structure and calculate its transparency response by using the three-dimensional finite-difference time-domain (FDTD) method. The proposed structure is composed of a continuous ultrathin metal film sandwiched by double two-dimensional (2D) hexagonal non-close-packed metal-dielectric multilayer core-shell nanoparticle arrays. The top and bottom plasmonic arrays in such a structure, respectively, act as the light input and output couplers to carry out the efficient trapping and release of light. Near-perfect multispectral optical transparency in the visible and near-infrared regions is achieved theoretically. The calculated electric field distribution patterns show that the near-perfect multispectral optical transparency mainly originates from the excitation and hybridization of shell and core plasmon modes, strong near-field coupling of dipole plasmon modes between adjacent nanoparticles as well as the excitation of surface plasmon waves of the metal film. The robust transparency bands can be efficiently tuned in a large range by varying the structural parameters and the surrounding dielectric environment. The proposed structure also shows additional merits such as a deep sub-wavelength size and fully retained electrical and mechanical properties of the natural metal. These features might provide promising applications in highly integrated optoelectronic devices including plasmonic filters, nanoscale multiplexers, and non-linear optics.

Narrowband Light Total Antireflection and Absorption in Metal Film–Array Structures by Plasmonic Near-Field Coupling

We study the cooperative effects between plasmon gap modes and optical cavity modes of a novel triple-layer structure consisting of double continuous gold films separated by a gold nanosphere array. Narrowband near-perfect antireflection of optical field is achieved for the first time due to the strong near-field light–matter interaction within the deep sub-wavelength gaps between adjacent nanospheres combined with the spatial field confinement effects of the optical cavity built by the double gold films. The coexistence cooperation of near-field dipole plasmon resonances and spatial optical field confinement presents more efficient light modification than that of the individual subsystem and may open a new approach to manage light flow. By varying the period of nanosphere array, the diameter of nanospheres, and the distance between the array and the film, optical behaviors of the proposed structure can be tuned in a wide range. High environmental sensitivity and large figure of merit factor are obtained using this structure as the detecting substrate. Furthermore, ultra-compact structure and high conduction suggest the proposed structure being a good candidate for potential applications in highly integrated optoelectronic devices, such as plasmonic filters and sensors.

Improved Broadband Near-Unity Light Transparency of a Metal Layer With Film-Coupled Dual Plasmonic Arrays

We report improved light transparency over a broad bandwidth in a metal-layered structure with two film-coupled subwavelength non-close-packed plasmonic arrays. Through the introduction of dual ultrathin dielectric spacing layers between the metal layer and the double plasmonic disk arrays, coupling of the input and output effects of light is efficiently enhanced through strong near-field localized plasmon resonances between adjacent plasmonic disks and the near-field plasmon cavity mode in the gap between the double plasmonic arrays and the metal layer. A broad bandwidth of 300 nm with near-unity light transmittance (above 90%) in the optical regime is achieved through the localized plasmon resonances and the symmetrical structure used here. The transparency of this structure is polarization independent and incident angle insensitive, and can be tuned by varying the structure parameters and the dielectric environment. In addition, the period of the plasmonic arrays and the thickness of the nanometer-separated plasmonic structure are less than λ /20 and λ/8, respectively. These values suggest that the proposed structure may have potential applications in deep subwavelength optoelectronic devices, including broadband optically transparent electrodes, highly integrated light input and output components, and plasmonic filters.

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.

Effects of Compound Rectangular Subwavelength Hole Arrays on Enhancing Optical Transmission

The optical transmission properties of the metallic film with an array of compound rectangular nanoholes are numerically investigated by the finite-difference time-domain (FDTD) method. The compound rectangular nanohole (unit cell) in such a structure consists of a large square hole with two small rectangular holes symmetrically distributed at its both sides. Extraordinary optical transmission (EOT) of more than 85% is obtained in this structure, which is larger than that found in the metal film perforated only with the large hole array (55%) or the small hole array (18%). The EOT in the optical regime mainly results from the excitation and coupling of localized surface plasmon resonances and surface plasmon polaritons. The EOT properties can be efficiently tailored in both wavelength and transmission intensity by varying the size and shape of nanoholes. Our structure also shows the sensitivity to environmental dielectric constant. These results indicate that our structure has potential applications in plasmonic filters and sensors.

Perfect superbroadband optical transparency from visible to near-infrared in a thin metallic film perforated with a cubic hole periodic array

Abstract. We propose a thin metallic film perforated with a hexagonal periodic array of cubic holes and calculate its optical properties through the three-dimensional finite-difference time-domain method. Perfect superbroadband optical transparency from visible to near-infrared is achieved with the transmittance up to 99% due to the excitation of surface plasmon polaritons on the nanopatterned metal surface, localized surface plasmons at the edges of the cubic holes, and their cooperative interaction. The perfect superbroadband optical transparency of the proposed structure mainly depends on the size of holes and the period of the cubic hole array, and the proposed structure with perfect superbroadband optical transparency can resist to the interference of surrounding dielectric environment, which would provide fascinating potential applications in absorbers, solar cells, and transparent electrodes.

Near-field plasmon effects in extraordinary optical transmission through periodic triangular hole arrays

Abstract. We present a theoretical investigation of the transmission properties of light through a metallic film perforated with different arrays of compound triangular holes. The extraordinary optical transmission (EOT) in the optical region is obtained by employing the finite-difference time-domain method. The excitation of localized surface plasmon resonances (LSPRs) at the top corners and surface plasmon polaritons (SPPs) on the metal surface, plasmon coupling effects between adjacent apertures, and the waveguide modes for delivering light mainly contribute to the EOT in such structures. The optical characteristics can be effectively tailored by changing the arrangement of triangular holes and the structural parameters. This study may be helpful for plasmonic nanostrucutres based on EOT, and has potential applications in optoelectronic devices.

Double-spectral enhanced optical transmission via the hybridization of plasmon modes in nanohole and nanocube arrays

We propose a novel plasmonic nanostructure composed of a silver (Ag) nanocube array coated on an Ag film perforated with a cubic nanohole array, and theoretically study its optical properties by the finite-difference time-domain method. Double greatly enhanced optical transmission bands in the visible and near-infrared regions are achieved via strong plasmon resonance coupling effects of the nanohole and nanocube arrays. The double-spectral enhanced optical transmission behaviors can be efficiently tailored by varying the widths, periods and heights of nanocubes and cubic nanoholes. The proposed structure may provide potential applications in deep subwavelength optoelectronic devices, including highly integrated light input and output components, sensors and plasmonic filters.