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.

     

Remote Excitation Surface Plasmon and Consequent Enhancement of Surface-Enhanced Raman Scattering Using Evanescent Wave Propagating in Quasi-One-Dimensional MoO3 Ribbon Dielectric Waveguide

We report experimentally the remote excitation surface plasmon and consequent enhancement of surface-enhanced Raman scattering (SERS) using evanescent wave propagating in quasi-one-dimensional (Q1D) MoO₃ ribbon dielectric waveguide. The propagating dielectric waveguide along Q1D MoO₃ ribbon is realized experimentally, when the 632.8 nm laser radiates on the one side edge of Q1D MoO₃ ribbon. The remote excitation SERS spectra-enhanced by chemical and electromagnetic field mechanisms are measured, respectively, where silver (Ag) nanoparticles are excited by electromagnetic field after propagating 7.3 μm in the ribbon. The chemical mechanism for the remote excitation SERS is contributed from the charge transfer between the analyte molecule and MoO₃ ribbon. The electromagnetic field mechanism for the remote excitation SERS arises from the energy conversion from the propagating dielectric waveguide to the surface plasmon of Ag nanoparticles on the Q1D MoO₃ ribbon. It is important to reveal the mechanism of energy conversion from the propagating dielectric waveguide to the surface plasmon for potential applications in micro- and nanoscale devices.     

FDTD Study on Evolution of Trimer Silver@Silica Nanospheres to Dimer for SERS Characteristics

Light enhancement occurs strongly within the plasmonic clusters by interaction with surface plasmons. Surface-enhanced Raman spectroscopic (SERS) characteristics of a series of silver@silica trimer core–shell (CS) nanosphere (NS) clusters are investigated in this paper. It is significant to understand the electric field (EF) enhancement mechanism behind the SERS technique. The effect of symmetry breaking is studied for the series starting from the highly symmetric trimer cluster and transformed to linear dimer geometry which progresses through the gradual reduction in the vertex NS. The optical activity such as the evolution of LSPR peak is discussed, the formation of hot spots is demonstrated and the strength of the local EF enhancement is calculated and correlated with the plasmon dipolar modes by using plasmon hybridization theory to understand the underlying physical concepts.

     

Individual Split Au Square Nanorings for Surface-Enhanced Raman and Hyper-Raman Scattering

In this paper, individual split Au square nanorings were numerically proposed as novel substrates for surface-enhanced Raman and hyper-Raman scattering (SERS and SEHRS) simultaneously. The peak wavelengths of their localized surface plasmon resonance (LSPR) fall in the near-infrared and visible light regions, respectively, which are able to be finely tuned to match well with the wavelengths of the incident laser and hyper-Raman scattered light beams. Their SEHRS and SERS performances along with electromagnetic (EM) field distributions are numerically investigated by finite element method. With the enhancement of near electric-fields generated by LSPRs, the maximum SEHRS and SERS enhancement factors are demonstrated to reach 1.22?×?10¹² and 10⁸, respectively. Meanwhile, the corresponding SERS-based refractive index (RI) sensitivity factor reaches as high as 258 nm/RIU and 893 nm/RIU, at visible and near-infrared wavelengths, respectively. The proposed structure holds great promise both for developing SEHRS- and SERS-based RI sensing substrates, which shows strong potential applications in nanosensing and enhanced Raman scattering.

     

Remote Excitation Polarization-Dependent Surface Photochemical Reaction by Plasmonic Waveguide

For the first time, we report remote excitation polarization-dependent surface photochemical reaction by plasmonic waveguide. Remote excitation polarization-dependent surface-enhanced Raman scattering (SERS) spectra indicate a surface photochemical reaction that p-aminothiophenol is converted to p,p′-dimercaptoazobenzene (DMAB) induced by the plasmonic waveguide. Surface plasmon polaritons generated at the end of a silver nanowire can propagate efficiently along the nanowire, and be coupled by nanoparticles near the nanowire as a nanoantenna. Massive electromagnetic enhancement is generated in the nanogap between the nanowire and the nanoparticles. The remote excitation polarization-dependent SERS spectra can be obtained experimentally in the nanogaps; furthermore, the remote excitation polarization-dependent SERS spectra of DMAB reveal the occurrence of this surface catalytic reaction. Theoretical simulations using finite-difference time-domain methods strongly support our experimental results.     

Diagonally Aligned Squared Metal Nano-pillar with Increased Hotspot Density as a Highly Reproducible SERS Substrate

Metal nanostructure on dielectric substrate with increased hotspot density has drawn considerable research interest in recent years toward the study of surface-enhanced Raman spectroscopy (SERS). In this paper, we report the fabrication of a diagonally aligned squared metal nano-pillar (SMNP) on a dielectric substrate and revealed it as an efficient SERS substrate with increased hotspot density for sensing of Raman active materials. Due to dipolar coupling and lightening rod effect between the neighboring nano-pillars, the localized surface plasmon resonance (LSPR) field intensity increased significantly in the space between two neighboring SMNP which would lead to the enhancement of SERS signal. The SMNP has been fabricated using electron beam lithographic (EBL) technique with hotspot density of 2.45 × 10⁷/mm². With the designed SERS substrate an average enhancement factor (EF) of 3.27 × 10⁸ has been observed with relative standard deviation of ～13 %.     

Direct Fabrication Route to Plastic-Supported Gold Nanoparticles for Flexible NIR-SERS

Stable gold nanoparticles with surface plasmon resonance tunable from visible (Vis) to near-infrared (NIR) are deposited via a direct sputtering methodology on large area polyethylene terephthalate (PET) to be used as effective, flexible NIR surface-enhanced Raman scattering (SERS) substrates. An O₂ plasma treatment of PET is used to tailor growth dynamics, geometry, and plasmonic properties of nanoparticles. The O₂ plasma treatment of PET results also in effective improvement of nanoparticle anchoring on the plastic substrate, providing more stable, flexible SERS systems. The functionality of fabricated SERS substrates has been tested using benzylthiol, and SERS enhancement factors in the range 10⁴ have been achieved, which are comparable with those reported in literature for gold nanostructures fabricated on silicon substrate. These results attest the great potentiality of this methodology for the production of cost-effective flexible and reusable large-scale SERS substrates.     

Self-Organized Ag Nanorings Antenna Substrates for Surface-Enhanced Raman Spectroscopy

We investigate the surface-enhanced Raman spectroscopy of Ag nanorings antenna in both experiment and simulation. Self-organized Ag nanorings antenna were formed on quartz glass wafers by a simple chemistry reaction without any template. The three-dimensional finite-difference time-domain simulation calculations indicate that the electric field enhancement of Ag nanoring antenna is strongly dependent on the gap distance. A very strong surface plasmon coupling in the gap region of Ag nanoring antenna is observed, whose field intensity is enhanced four times compared to that for Ag nanodomes antenna with the same gap distance. Surface-enhanced Raman scattering (SERS) measurements have shown that the SERS intensity acquired from the Ag nanoring antenna is about 16 times stronger than that obtained from Ag nanodomes antenna. These results pave the way to design plasmonic nanostructures for practical applications that require coupled metallic nanoparticles with enhanced electric fields.     

Optical Properties of Noncontinuous Gold Shell Engineered on Silica Mesosphere

The optical properties of individual noncontinuous shells with different gold coverage are investigated by the single-particle dark field scattering measurements and single-particle surface-enhanced Raman scattering (SERS) measurements at different excitation wavelengths. By controlling the growth of gold seeds, multi-metallic nanogaps/crevices with different optical responses are assembled on silica mesospheres forming noncontinuous shells that can be confirmed through the transmission electron microscope images. We find the surface plasmon resonance of single shell red-shifts from 510 to 680 nm with the increase of gold coverage. At the excitation of 532 and 785 nm, the best enhancements about 2.0?×?10⁵ and 1.1?×?10⁷ are obtained on spheres with ～60 and 83 % gold coverage, respectively. The weak polarization-dependent SERS indicates that the enhancement is from the multi-gaps on single noncontinuous shell. This optical tunable and SERS active noncontinuous gold shell can be applied in biosensing, ultra trace detection, and molecule analysis needing multi-wavelengths excitation.     

Surface-Enhanced Raman Scattering in Tunable Bimetallic Core-Shell

In this paper, optical properties of multilayer spherical core-shell nanoparticles based on quasi-static approach and plasmon hybridization theory are investigated. Calculations show that light absorption spectrum of bimetallic multilayer core-shell has three intense plasmon resonance peaks, which are more suitable for multiplex biosensing based on surface-enhanced Raman scattering (SERS) and localized surface plasmon resonance (LSPR). The plasmon resonance peaks in bimetal nanshells are optimized by tuning the geometrical parameters. In addition, the optimal geometry is discussed to obtain the Raman enhancement factor in bimetallic multilayer nanoshell. SERS enhancement factor is calculated with consideration of dampings due to both the electron scattering and the radiation at the boundary and modified Drude model in dielectric function of bimetallic nanoshell. It is shown that bimetallic nanoshell with the small size exhibits strong SERS enhancement factor (~6.63 × 10⁵) with additional collision dampings and ~2.9 × 10⁹ with modified Drude model which are suitable for biosensing applications. In addition, any variation in blood concentration and oxygen level can be detected by this bimetallic core-shell nanoparticle with sensitivity of Δλ/Δn = 264.91 nm/RIU.     

Directional Excitation of Surface Plasmon Polaritons by Circularly Polarized Vortex Beams

We investigate the excitation of surface plasmon polaritons (SPPs) using a metallic nanoaperture array illuminated by circularly polarized Laguerre-Gaussian (LG) vortex beams. The direction of SPP excitation is tunable by changing the circular polarization and topological charge of LG beams. The left- or right-handed circular polarization determines SPP propagation on either side of the nanoaperture array. Furthermore, varying the topological charge of LG beam will result in beam splitting of SPPs. We also utilize a composite nanoaperture array with different periods to achieve unidirectional excitation of SPPs. The study provides an interesting approach to control the excitation direction of SPPs and may find great applications in SPP generators and optical switches.

     

Characterization of multilayer-enhanced surface-enhanced raman scattering (SERS) substrates and their potential for SERS nanoimaging

Multilayer gold surface-enhanced Raman scattering (SERS) substrates, which consist of continuous gold films that are separated by self-assembled monolayers (SAMs) and cast over 430-nm diameter silica nanospheres on a glass slide, have been evaluated as a means of further enhancing the SERS signals produced from conventional metal film over nanostructure substrates. Evaluation of the effect of various SAMs, with different terminal functional groups, on the SERS enhancement factor were measured and compared to conventional single-layer gold film over nanostructure substrates, revealing relative enhancements as great as 22.4-fold in the case of 2-mercapto-ethanol spacer layers. In addition to evaluation of the effect of different terminal functionalities, the effect of spacer length was also investigated, revealing that the shorter chain length alcohols provided the greatest signals. Employing the optimal SERS multilayer geometry, SERS nanoimaging probes were fabricated and the SERS enhancement factor and variability in enhancement factor were measured over the SERS active imaging area, providing absolute enhancements similar to previous silver-based SERS nanoimaging probes (i.e., 1.2 × 10⁸). Varying the size of the multilayer gold islands that were deposited on the tip of the SERS active nanoimaging probe, it is possible to tune the optimal SERS excitation wavelength accurately and predictably over the range of approximately 450 to 600 nm, without coating the entire surface of the probe and significantly reducing the transmission and resulting signal-to-noise ratio of the images obtained.     

Double-Layered Metal Nano-Strip Antennas for Sensing Applications

A coupled plasmonic system based on double-layered metal nano-strips for sensing applications is investigated by means of mode analysis and two-dimensional finite-difference time-domain simulations. The nano-strips act as optical antennas through constructive interference of short-range surface plasmon polaritons, thus increasing their scattering cross-section and optical field enhancement. Near-field modulation by optical trapped metal nanoparticles (NPs) is also demonstrated. Our results reveal that the device exhibits a refractive index sensitivity of ～200 nm/RIU, and a maximum surface-enhanced Raman scattering (SERS) factor of 10⁹–10¹⁰ from metal NPs trapped in the near-field region. The proposed device shows reasonable figure-of-merit and is ready for integration with common optofluidic biosensors.     

Trace-Level Detection of Explosive Molecules with Triangular Silver Nanoplates-Based SERS Substrates

We report a simple route to design highly sensitive triangular silver nanoplates (TSNPs)-based SERS substrate for the trace-level detection of explosive molecules. The size-dependent localized surface plasmon resonance (LSPR) tunability for the synthesis of TSNPs is achieved by controlling reaction kinetics and seed volume in a modified seed-mediated approach. The computed extinction spectra of TSNP, using the finite-difference time-domain (FDTD) method, are in excellent agreement with the experimental results, therefore assisting further in the investigation of the plasmonic properties of TSNP. The higher electric field enhancement offered by TSNP is systematically investigated by performing the FDTD simulations for various sizes and corner rounding of TSNP. The FDTD results show that the dipolar plasmon resonance wavelength, size, and corner rounding of TSNP are the principal contributing factors for designing the high-performance SERS substrate. Herein, we have used a portable Raman system for the SERS-based detection of three important explosive molecules: picric acid (PA), ammonium nitrate (AN), and 2, 4-dinitrotoluene (DNT). The TSNP-based SERS substrates display excellent intensity enhancement factors of?~?10⁷ for rhodamine 6G (R6G) and PA and?~?10⁵ for AN. The high sensitivity of SERS substrate with limit-of-detection (LOD) of value 2.3?×?10^?11 M for PA and 3.1?×?10^?8 M for AN and effective batch-to-batch reproducibility for DNT, thus offering its potentials for field detection of explosive molecules at trace-level.

     

Structural,Optical and Micro-Raman Scattering Studies of Nanosized Copper Ion (Cu+) Exchanged Soda Lime Glasses

Copper nanoclusters embedded in soda lime glass are prepared by ion exchanged method. The ion exchanged glasses are annealed in air for 1 h at different temperature. These samples exhibit surface plasmon and surface enhancement effect. UV-visible optical absorption spectroscopic analysis shows the signature of the copper nanocluster in the ion exchanged glasses. Surface-enhanced Raman spectroscopy (SERS) technique is used to characterize the copper ion exchanged and annealed glasses. Additional results are obtained by SERS for optimizing the SERS substrate. The highest Raman enhancement is obtained for substrates with high cluster size. The micro-Raman scattering spectroscopic images of ion exchanged and different temperature annealed glasses support the optical absorption spectroscopy results.     

Nano-patterned SERS substrate: application for protein analysis vs. temperature

We have illustrated the fabrication of nano-structures as a surface enhanced Raman scattering (SERS) substrate using electro-plating and electron-beam lithography techniques to obtain an array of gold nanograin-aggregate structures of diameter ranging between 80 and 100 nm with interstitial gap of 10-30 nm. The nanostructure based SERS substrate permits us to have better control and reproducibility on generation of plasmon polaritons. The calculation shows the possible detection of myoglobin concentration down to attomole. This SERS substrate is used to investigate the structural changes of different proteins; lysozyme, ribonuclease-B, bovin serum albumin and myoglobin in the temperature range between -65 and 90 degrees C. The in-depth analysis even for small conformational changes is performed using 2D Raman correlation analysis and difference Raman analysis in order to gain straightforward understanding of proteins undergoing thermodynamical perturbation.     

Novel and Sensitive Core-Shell Nanoparticles Based on Surface Plasmon Resonance

In this paper, a rough silver core-shell nanoparticle with strong electric field enhancement in the vicinity of a bumpy structure on the silver core-shell surface is reported. A dipolar plasmonic mode of the silver nanoshell is investigated by using the quasi-static approach and plasmon hybridization theory, which analytical results identify the electric field enhancement spectra in which the enhancement is optimized. As the silver shell thickness is small, the hot spots play an important role in the plasmonic field enhancement. In addition, the deposition of a rough silver shell can generate a stronger near-field enhancement near the silver surface which is more desirable than that of a smooth silver shell for sensitive detection based on SPR and surface enhanced Raman scattering (SERS). The plasmonic field enhancement of a bumpy silver core-shell nanoparticle permits the detection and characterization of bovine serum albumin (BSA) protein molecule and hemoglobin solution with a high sensitivity.     

Design of Polarization Independent SERS Substrate with Raman Gain Evaluated Using Purcell Factor

Surface-enhanced Raman scattering (SERS) is a very promising detection/diagnostic technique at trace levels as the molecules exhibit a significant increase in their Raman signals when they are attached or are in proximity to plasmonic structures. In this study, a numerical design of SERS substrate as a probe has been demonstrated for detection and diagnosis of blood, water and urea samples. The proposed nanospiral design is polarization independent, and it offers the enhancement of the electric field strength ~ 10⁹. The substrate design is based on 3D finite difference time domain simulations and is robust, versatile and sensitive even at low concentrations of the analyte. It works equally well when used in the reflection mode. In this study, the cavity quantum electrodynamics (CQED) Purcell factor has also been transposed to plasmonics. The Purcell factor in corroboration with CQED has been used to achieve efficient light–matter interaction at nanoscale by providing a more realistic result. It takes into account the randomness of incident wave polarizations and arbitrary orientations of interacting molecules. This gives a deeper insight into electromagnetic Raman gain in SERS and can be used to design novel SERS substrates.

     

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.     

Scaffold-Based Multi-nanoparticle Clusters as Tunable LSPR Substrates for SERS Applications

Tunable local surface plasmon resonance (LSPR) enhancement properties of scaffold-based multi-nanoparitcle clusters were investigated using finite-difference time-domain (FDTD) method with calculated optical spectra, near-field distribution, and average enhancement of hybrid nanostructures as slab/nanoparticls, cylinder/nanoparticles, and sphere/nanoparticles. Focusing on influence factors including surface curvature, coupling effect, and decorated particle number, several models were built for further understanding on the dominate contribution in complicate multi-particle nanostructure and to explore their potential for plasmonic enhancement applications such as surface-enhanced Raman spectroscopy (SERS), solar cells material, LSPR sensor, and nanoantenna.