E. C. Le Ru

An analytic model for the optical properties of gold

A simple analytic model for the optical properties of gold is proposed. The model includes a minimum set of parameters necessary to represent the complex dielectric function of gold in the visible and near-uv regions. Explicit values for the parameters to reproduce the Johnson and Christy data [Phys. Rev. B 6, 4370 (1972)] on the optical properties of gold are provided.

Polarization-dependent effects in surface-enhanced Raman scattering (SERS)

A few key examples of polarization effects in surface-enhanced Raman scattering (SERS) are highlighted and discussed. It is argued that the polarization of the local field, which is felt by an analyte molecule in a location of high electromagnetic field enhancement (hot-spot), can be very different from that of the incident exciting beam. The polarization dependence of the SERS signal is, therefore, mostly dictated by the coupling of the laser to the plasmons rather than by the symmetry of the Raman tensor of the analyte. This sets serious restrictions for the interpretation of both single-molecule SERS polarization studies and for the use of circularly polarized light in techniques like surface-enhanced Raman optical activity.

Experimental demonstration of surface selection rules for SERS on flat metallic surfaces

We have measured the polarization and incident angle dependence of the Surface-Enhanced Raman Scattering (SERS) signal of a nile blue monolayer adsorbed on a flat gold surface. Comparisons with predictions of electromagnetic (EM) theory indicate that the molecules are predominantly adsorbed flat on the surface. These results provide the most direct demonstration of the concept of surface selection rules in SERS, and further confirm the validity of the SERS-EM model beyond the |E|(4)-approximation.

Influence of Photostability on Single-Molecule Surface Enhanced Raman Scattering Enhancement Factors

Experimental determinations of enhancement factors in Surface Enhanced Raman Scattering (SERS) are intimately intertwined with the photostability of the probes. We study the effect of the limited photostability in single-molecule SERS (SM-SERS) events and show explicitly how this may result in a large under-estimation of the SERS enhancement factors (EFs) obtained experimentally. To this end, we use the bianalyte technique with isotopically edited probes to provide the best-case scenario for the isolation of single molecule events, and study the statistics of EFs at different incident laser powers. When photobleaching stops playing an important role within the integration time used to capture the spectra, SM-SERS EFs approach an upper bound, which is in agreement with estimations of the EFs within the electromagnetic theory of SERS enhancements. Our results reinforce, in addition, the fact that the highest SM-SERS EFs observed experimentally are typically of the order of approximately 10(10).

An Iterative Algorithm for Background Removal in Spectroscopy by Wavelet Transforms

Wavelet transforms are an extremely powerful tool when it comes to processing signals that have very “low frequency” components or non-periodic events. Our particular interest here is in the ability of wavelet transforms to remove backgrounds of spectroscopic signals. We will discuss the case of surface-enhanced Raman spectroscopy (SERS) for illustration, but the situation it depicts is widespread throughout a myriad of different types of spectroscopies (IR, NMR, etc.). We outline a purpose-built algorithm that we have developed to perform an iterative wavelet transform. In this algorithm, the effect of the signal peaks above the background is reduced after each iteration until the fit converges close to the real background. Experimental examples of two different SERS applications are given: one involving broad backgrounds (that do not vary much among spectra), and another that involves single molecule SERS (SM-SERS) measurements with narrower (and varying) backgrounds. In both cases, we will show that wavelet transforms can be used to fit the background with a great deal of accuracy, thus providing the framework for automatic background removal of large sets of data (typically obtained in time-series or spatial mappings). A MATLAB® based application that utilizes the iterative algorithm developed here is freely available to download from http://www.victoria.ac.nz/raman/publis/codes/cobra.aspx.

Physics of single molecule fluctuations in surface enhanced Raman spectroscopy active liquids.

Surface enhanced Raman spectroscopy (SERS) of dyes in solution allows the study of the differences between ensemble averaged spectroscopic signals and single molecular events. We address several outstanding issues in single molecule detection via SERS; in particular, evidence for single molecule vibrational pumping and/or single molecule laser heating, the statistics of hotspots in the liquid, and anti-Stokes/Stokes anomalies. We demonstrate that anti-Stokes/Stokes ratios are a very unreliable measure of temperature, because the two processes are affected differently by the underlying frequency-dependent plasmon resonances. Subtle hints of vibrational pumping and/or heating in single molecules can only obtained via careful cross correlations between the parameters (frequency position, width, and intensity) of the Stokes signals for different excitation lasers. We introduce the use of single-peak parameter cross correlations for the study of these phenomena.

Multipolar emission in the vicinity of metallic nanostructures

Electromagnetic emission from finite-size multipoles very close to metal surfaces exhibiting plasmon resonances can have profound consequences on the characteristics of the emitted radiation in the far field in both its integrated intensity and its spatial distribution. The problem is relevant for many types of spectroscopies that use metals to enhance optical signals, but it also represents a complementary aspect to the recent interest in near-field imaging via surface plasmon resonances. The breakdown of selection rules for multipolar emission and the phenomenon of radiation funnelling in the far field are explicitly discussed. The relevance of these concepts for certain types of spectroscopy, like surface enhanced Raman scattering (SERS), is also highlighted.

A study of local heating of molecules under Surface Enhanced Raman Scattering (SERS) conditions using the anti-Stokes/Stokes ratio

We make systematic measurements of the anti-Stokes/Stokes (aS/S) ratios using low power lasers (0.5 mW at 514 and 633 nm) of rhodamine 6G (RH6G) on dried silver colloids over a wide range of temperatures from 140 to 350 K. We show that a scan in temperature allows the extraction of the contributions to the anti-Stokes/Stokes ratio from resonance effects and heating independently, thus decoupling the two aspects of the problem.

Resonance contributions to anti-Stokes/Stokes ratios under surface enhanced Raman scattering conditions

Anti-Stokes/Stokes asymmetries under surface enhanced Raman scattering (SERS) conditions are studied for a wide variety of SERS-active media and different analytes. Evidence is provided for the existence of underlying resonances that create these asymmetries. We show here that these resonances are associated with the electromagnetic coupling between the analyte (probe) and the metal. The work demonstrates the use of the anti-Stokes/Stokes ratio as a tool to understand the hierarchy of resonances in the SERS problem, which is essential for quantification purposes.

Enhancement factor averaging and the photostability of probes in SERS vibrational pumping

The technique of temperature dependent vibrational pumping in surface enhanced Raman scattering (SERS) has been recently demonstrated as a promising new tool to estimate SERS cross-sections. In this paper we expand on the previous developments and study several details around the implementation and physics of the vibrational pumping technique in SERS. Here we concentrate on two specific aspects related to: (i) the different averaging properties (over the distribution of enhancements) of the Stokes and anti-Stokes signals in the pumping regime; and (ii) the role of the finite photostability of the probes. The fact that the anti-Stokes signal is averaged differently from the Stokes counterpart leads to some unique phenomena in Raman spectroscopy that can only be observed under the conditions of vibrational pumping in SERS.