Modeling Surface-Enhanced Spectroscopy With Perturbation Theory

Theoretical modeling of surface-enhanced Raman scattering (SERS) is of central importance for unraveling the interplay of underlying processes and a predictive design of SERS substrates. In this work we model the plasmonic enhancement mechanism of SERS with perturbation theory. We consider the excitation of plasmonic modes as an integral part of the Raman process and model SERS as higher-order Raman scattering. Additional resonances appear in the Raman cross section which correspond to the excitation of plasmons at the wavelengths of the incident and the Raman-scattered light. The analytic expression for the Raman cross section can be used to explain the outcome of resonance Raman measurements on SERS analytes as we demonstrate by comparison to experimental data. We also implement the theory to calculate the optical absorption cross section of plasmonic nanoparticles. From a comparison to experimental cross sections, we show that the coupling matrix elements need to be renormalized by a factor that accounts for the depolarization by the bound electrons and interband transitions in order to obtain the correct magnitude. With model calculations we demonstrate that interference of different scattering channels is key to understand the excitation energy dependence of the SERS enhancement for enhancement factors below 103

Band-edge Bilayer Plasmonic Nanostructure for Surface Enhanced Raman Spectroscopy

Spectroscopic analysis of large biomolecules is critical in a number of applications, including medical diagnostics and label-free biosensing. Recently, it has been shown that Raman spectroscopy of proteins can be used to diagnose some diseases, including a few types of cancer. These experiments have however been performed using traditional Raman spectroscopy and the development of the Surface enhanced Raman spectroscopy (SERS) assays suitable for large biomolecules could lead to a substantial decrease in the amount of specimen necessary for these experiments. We present a new method to achieve high local field enhancement in surface enhanced Raman spectroscopy through the simultaneous adjustment of the lattice plasmons and localized surface plasmon polaritons, in a periodic bilayer nanoantenna array resulting in a high enhancement factor over the sensing area, with relatively high uniformity. The proposed plasmonic nanostructure is comprised of two interacting nanoantenna layers, providing a sharp band-edge lattice plasmon mode and a wide-band localized surface plasmon for the separate enhancement of the pump and emitted Raman signals. We demonstrate the application of the proposed nanostructure for the spectral analysis of large biomolecules by binding a protein (streptavidin) selectively on the hot-spots between the two stacked layers, using a low concentration solution (100 nM) and we successfully acquire its SERS spectrum

Estimating the plasmonic field enhancement using high-order harmonic generation: The role of inhomogeneity of the fields

In strong field laser physics it is a common practice to use the high-order harmonic cutoff to estimate the laser intensity of the pulse that generates the harmonic radiation. Based on the semiclassical arguments it is possible to find a direct relationship between the maximum value of the photon energy and the laser intensity. This approach is only valid if the electric field driving HHG is spatially homogenous. In laser-matter processes driven by plasmonics fields, the enhanced fields present a spatial dependence that strongly modifies the electron motion and consequently the laser driven phenomena. As a result, this method should be revised in order to more realistically estimate the field. In this work, we demonstrate how the inhomogeneity of the fields will effect this estimation. Furthermore, by employing both quantum mechanical and classical calculations, we show how one can obtain a better estimation for the intensity of the enhanced field in plasmonic nanostructure.Comment: 7 pages and 2 figure

A novel plasmonic nanostructure for localized near-field light enhancement

The combination of plasmonic structures with dielectric materials, metallic oxides and in particular, with two dimensional (2D) materials has been a subject of great interest in the scientific community for a wide range of applications, such as optoelectronic devices, solar cells, or photochemistry. Plasmonic properties have the potential to enhance the capabilities of 2D materials for harvesting light, which alone have a low efficiency due to low absorption (approximately only 11%). This work presents a new plasmonic structure, “nanohippo” with a perspective to integrate a monolayer material inside the cavity, being this nanostructure directly excited by the incident light taking advantage of localized surface plasmonic resonance (LSPR). The samples were prepared by colloidal lithography and material deposition was made through an electron beam assisted evaporation system. A fabrication method was developed to create a cavity by using a sacrificial material and etching it later on. The sample’s characterization consisted in a morphologic analysis by Scanning Electron Microscopy (SEM), the optical response was studied both theoretically and experimentally by Finite-difference-time-domain (FDTD) as well as experimentally by spectrophotometry. Finally, an elemental analysis was performed via X-ray photoelectron microscopy (XPS). The diameter and height of the structures were studied (different sizes nanoparticles and thicker layers of bottom layer gold) reaching to a structure that presented a plasmonic cavity. This nanostructure, with a new geometry, presented a visible plasmonic nanocavity with up to sixty times more enhancement of the electrical field inside it

Dual-tip-enhanced ultrafast CARS nanoscopy

Coherent anti-Stokes Raman scattering (CARS) and, in particular, femtosecond adaptive spectroscopic techniques (FAST CARS) have been successfully used for molecular spectroscopy and microscopic imaging. Recent progress in ultrafast nanooptics provides flexibility in generation and control of optical near fields, and holds promise to extend CARS techniques to the nanoscale. In this theoretical study, we demonstrate ultrafast subwavelentgh control of coherent Raman spectra of molecules in the vicinity of a plasmonic nanostructure excited by ultrashort laser pulses. The simulated nanostructure design provides localized excitation sources for CARS by focusing incident laser pulses into subwavelength hot spots via two self-similar nanolens antennas connected by a waveguide. Hot-spot-selective dual-tip-enhanced CARS (2TECARS) nanospectra of DNA nucleobases are obtained by simulating optimized pump, Stokes and probe near fields using tips, laser polarization- and pulse-shaping. This technique may be used to explore ultrafast energy and electron transfer dynamics in real space with nanometre resolution and to develop novel approaches to DNA sequencing.Comment: 11 pages, 6 figure