Plasmonic Nanopillars Research Articles

Morphological Design of Gold Nanopillar Arrays and Their Optical Properties

The fabrication of large areas of gold nanopillar arrays by combining nanosphere lithography and electrodeposition of metals is demonstrated here for the first time. Both the morphology and the surface density of the nanopillars can be finely tuned by controlling the size of the colloidal beads used in the template, the sintering time of the template and the electrodeposition time of metals. The colloidal templates are removed after electrodeposition, and the nanopillar arrays can be transferred onto a transparent polydimethylsiloxane substrate and optically characterized. Our focus here is on the ease and versatility of this technique to prepare large two-dimensional arrays of plasmonic gold nanopillars and their resulting optical properties. By making use of finite difference time domain numerical simulations, the excitation of localized surface plasmon resonances of the individual nanopillar with a strong dipolar character is observed, which is characteristic of such uncoupled metallic nanostructures. ...

Directional Light Extinction and Emission in a Metasurface of Tilted Plasmonic Nanopillars.

Plasmonic optical antennas and metamaterials with an ability to boost light-matter interactions for particular incidence or emission angles could find widespread use in solar harvesting, biophotonics, and in improving photon source performance at optical frequencies. However, directional plasmonic structures have generally large footprints or require complicated geometries and costly nanofabrication technologies. Here, we present a directional metasurface realized by breaking the out-of-plane symmetry of its individual elements: tilted subwavelength plasmonic gold nanopillars. Directionality is caused by the complex charge oscillation induced in each individual nanopillar, which essentially acts as a tilted dipole above a dielectric interface. The metasurface is homogeneous over a macroscopic area and it is fabricated by a combination of facile colloidal lithography and off-normal metal deposition. Fluorescence excitation and emission from dye molecules deposited on the metasurface is enhanced in specific directions determined by the tilt angle of the nanopillars. We envisage that these directional metasurfaces can be used as cost-effective substrates for surface-enhanced spectroscopies and a variety of nanophotonic applications.

Plasmonic nanopillar coupled two-dimensional random medium for broadband light trapping and harvesting

Random textures have proven to be a better option for light localization and energy harvesting in solar cells. On the other hand, plasmonic structures are significant in localizing the fields into submicron domains. We propose a layered structure design that contains the random dielectric medium with a plasmonic nanopillars array as a back reflector, followed by demon- strating its efficient light trapping ability through simulation means. This structure has shown significant enhancement in the broadband absorption of the light spectrum in the wavelength range UV-IR and a higher extinction in the near-infrared wavelengths. The structure also shows the dependence of reflection on the nanopillar height as well as localization in the nano- pillar region. The broadened and red shifted plasmonic nanopillar resonances (transverse and longitudinal) in a high-index medium are shown as the reasons for enhanced broadband absorption. © 2015 Society of Photo-Optical Instrumentation Engineers (SPIE) (DOI: 10.1117/1. JNP.9.093061)

High Aspect-Ratio Iridium-Coated Nanopillars for Highly Reproducible Surface-Enhanced Raman Scattering (SERS).

A variety of different gold and silver nanostructures have been proposed over the years as high sensitivity surface-enhanced Raman scattering (SERS) sensors. However, efficient use of SERS has been hindered by the difficulty of realizing SERS substrates that provide reproducible SERS response over the whole active area. Here, we show that atomic layer deposition (ALD) grown iridium can be used to produce highly reliable SERS substrates. The substrates are based on a periodic array of high aspect-ratio iridium coated nanopillars that feature efficient and symmetrically distributed hot spots within the interpillar gaps (gap width<10 nm). We show that the enhancement with the iridium based nanostructures is of significant magnitude and it equals the enhancement of silver based reference substrates. Most notably, we demonstrate that the ordered and well-defined plasmonic nanopillars offer a measurement-to-measurement variability of 5%, which paves the way for truly quantitative SERS measurements.

Fabrication and characterization of well-aligned plasmonic nanopillars with ultrasmall separations.

We show the fabrication of well-aligned gold and silver nanopillars with various array parameters via interference lithography followed by ion beam milling and compare the etching rates of these two metallic materials. Silver is suitable for fabricating ultrafine arrays with ultrasmall separations due to high milling rates. The optical properties of the fabricated nanopillars are specifically characterized from both normal incidence and oblique incident angles. Tunable surface plasmon resonances are achieved with varying structural parameters. Strong coupling effects are enabled when the separation between adjacent nanopillars is dramatically reduced, leading to useful applications in sensing and waveguiding.

Highly tunable self-assembled plasmonic lattices through nanosphere lithography

This Letter reports a method to produce two-dimensional self-assembled plasmonic nanopillar (NP) arrays with independent control of the diameter (d), spacing (s), and height (h) of the NPs. A plasmonic lattice was designed and optimized for maximum plasmonic activity at 980 nm using three-dimensional finite-difference time-domain simulations. The optimized lattice with d=365 nm, s=410 nm, and h=70 nm was fabricated utilizing a self-assembled nanosphere lithography approach. Outstanding agreement between the observed and predicted results confirms the validity of the design process and the controllability and repeatability of the fabrication process. The excellent short-range order in the lattice structure suggests that this method can replace the electron-beam lithography approach in a scalable and cost-effective manner.

The Role of Propagating and Localized Surface Plasmons for SERS Enhancement in Periodic Nanostructures

Periodic arrays of plasmonic nanopillars have been shown to provide large, uniform surface-enhanced Raman scattering (SERS) enhancements. We show that these enhancements are the result of the combined impact of localized and propagating surface plasmon modes within the plasmonic architecture. Here, arrays of periodically arranged silicon nanopillars of varying sizes and interpillar gaps were fabricated to enable the exploration of the SERS response from two different structures; one featuring only localized surface plasmon (LSP) modes and the other featuring LSP and propagating (PSP) modes. It is shown that the LSP modes determine the optimal architecture, and thereby determine the optimum diameter for the structures at a given incident. However, the increase in the SERS enhancement factor for a system in which LSP and PSP cooperatively interact was measured to be over an order of magnitude higher and the peak in the diameter dependence was significantly broadened, thus, such structures not only provide larger enhancement factors but are also more forgiving of lithographic variations.