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.

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((R)) 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.

Ultrafast nonradiative decay rates on metallic surfaces by comparing surface-enhanced Raman and fluorescence signals of single molecules.

By the simultaneous observation of surface-enhanced Raman scattering and surface-enhanced fluorescence signals from a single molecule, we can measure and quantify the modification of the total decay rate of emitters in very close proximity to metals, even down to adsorbed molecules. This modified decay rate is shown to be largely dominated by its nonradiative component, which would be extremely difficult to estimate with conventional approaches. The method provides an indirect measurement of ultrafast (approximately 25 fs) mechanisms, which would be impossible to gain with time-resolved spectroscopy of a single molecule.

Single-molecule vibrational pumping in SERS.

Single-molecule vibrational pumping in surface-enhanced Raman scattering (SERS) is demonstrated rigorously using the bi-analyte SERS method at low temperatures. These experiments reveal a systematic difference between the radiative SERS cross section estimated from the Stokes intensity and that obtained by pumping itself (from the anti-Stokes-to-Stokes ratio), the latter being always larger. This difference can only be reliably demonstrated in the single-molecule SERS regime, for it is otherwise affected by complications of the averaging (over the enhancement distribution) of the signals of several molecules. The findings in this paper highlight the limitations of the pumping cross-section, which cannot (in general) be taken as a reliable measure of the SERS cross-section itself. We provide a discussion of the main possible explanations for the systematic difference of the two estimates.

Plasmon-dispersion corrections and constraints for surface selection rules of single molecule SERS spectra.

The problem of extracting information from relative intensities of Raman peaks in surface-enhanced-Raman-scattering (SERS) is intimately related to several important topics in the technique. Among them: (i) the possibility (or sometimes impossibility) of observing surface selection rules in different situations, (ii) the role of analyte resonance conditions, (iii) the crucial inclusion of plasmon-resonance dispersion corrections in the analysis of relative Raman intensities among peaks, and (iv) the connection of these phenomena with (broader) issues like surface-enhanced fluorescence (SEF). This paper deals with the underlying connections among these (apparently disconnected at first sight) topics. The technique is now at a mature stage to review the aforementioned phenomena from a unified point of view; thus pinpointing the most important issues, clarifying concepts that have been historically confusing (or treated in isolation), and paving the road for future developments.

Phenomenological local field enhancement factor distributions around electromagnetic hot spots.

We propose a general phenomenological description of the enhancement factor distribution for surface-enhanced Raman scattering (SERS) and other related phenomena exploiting large local field enhancements at hot spots. This description extends naturally the particular case of a single (fixed) hot spot, and it is expected to be "universal" for many classes of common SERS substrates containing a collection of electromagnetic hot spots with varying geometrical parameters. We further justify it from calculations with generalized Mie theory. The description studied here provides a useful starting point for a qualitative (and semiquantitative) understanding of experimental data and, in particular, the analysis of the statistics of single-molecule SERS events.

Guiding molecules with electrostatic forces in surface enhanced Raman spectroscopy.

In surface enhanced raman scattering (SERS) the problem of drawing molecules to the places where surface plasmon resonance enhancements will produce signals is one of the most basic ones, and the initial obstacle to every application of the effect. We explore the possibility of using electrostatic forces as a means to "guide" charged molecules in solution toward SERS active substrates. We also show explicitly the possibility of selectively enhancing different types of dyes according to their charge, and we discuss briefly possible extensions for other applications where "electrostatic guiding" could be an option.

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).

A perspective on single molecule SERS: current status and future challenges.

We present an overview of a some of the basic principles underlying current research in single-molecule surface-enhanced Raman scattering (SM-SERS). We summarize, by the same token, a series of conditions and characteristics that are common to most SM-SERS conditions, and discuss their implications for the understanding of data and for the comparison among different methods. We try to emphasize aspects of the problem that are not conventionally discussed in detail in the literature. In particular, we provide a full length discussion on the topics of: (i) the minimum SERS enhancement necessary to observe a single molecule, and (ii) the spatial distribution of the enhancement factor (EF) around hot-spots (which affects the statistics of SM-SERS events). A brief outlook into future perspectives of the different techniques used in SM-SERS and a few outstanding questions are also provided.

Bi-analyte SERS with isotopically edited dyes.

Isotopically substituted rhodamine dyes provide ideal probes for the study of single-molecule surface enhanced Raman scattering (SM-SERS) events through multiple-analyte techniques. Isotopic editing should, in principle, provide probes that have identical chemical properties (and surface chemistries); while exhibiting at the same time distinct Raman features which enable us to identify single-molecule SERS events. We present here a specific example of two-analyte SM-SERS based on the isotopic substitution of a methyl ester rhodamine dye. The dyes are carefully characterized (in both standard and SERS conditions) to confirm experimentally their similar chemical properties. We then demonstrate their utility for bi-analyte SERS (BiASERS) experiments and, as an example, highlight the transition from a single, to a few, to many molecules in the statistics of SM-SERS signals.

Vibrational pumping in surface enhanced Raman scattering (SERS).

In this tutorial review, the underlying principles of vibrational pumping in surface enhanced Raman scattering (SERS) are summarized and explained within the framework of their historical development. Some state-of-the-art results in the field are also presented, with the aim of giving an overview on what has been established at this stage, as well as hinting at areas where future developments might take place.

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.

Statistics of single-molecule surface enhanced Raman scattering signals: fluctuation analysis with multiple analyte techniques.

The mathematical background, based on a variation of the principal component analysis (PCA) method, is developed for the understanding of fluctuating multiple analyte single-molecule (SM) surface enhanced Raman scattering (SERS) signals; with emphasis on the bianalyte SERS technique developed recently. The method and its significance are presented to provide a systematic framework with which several aspects of the statistics of SM-SERS signals can be analyzed in general. We also apply the method to a concrete example of bianalyte statistics in silver colloidal solutions and discuss related topics around experimental issues and the interpretation of single-molecule SERS data.

Statistics of single molecule SERS signals: is there a Poisson distribution of intensities?

This paper is aimed at clarifying the statistics of single molecule (SM) surface enhanced Raman scattering (SERS) signals. The argument of the possible existence of a Poisson distribution in the statistics of intensities in SM-SERS has been used many times in the last decade as a proof of single molecule detection. We show theoretically and experimentally that the conditions under which a Poisson distribution would be present are so unlikely to exist in a real system that there is no other option but to attribute the claims to poor statistical sampling. We believe the argument based on Poisson statistics should be dropped as a proof of single molecule detection in SERS.

Enhancement factor distribution around a single surface-enhanced Raman scattering hot spot and its relation to single molecule detection.

We provide the theoretical framework to understand the phenomenology and statistics of single molecule (SM) signals arising in surface enhanced Raman scattering (SERS) under the presence of so-called electromagnetic hot spots. We show that most characteristics of the SM-SERS phenomenon can be tracked down to the presence of a tail-like (power law) distribution of enhancements and we propose a specific model for it. We analyze, in the light of this, the phenomenology of SM-SERS and show how the different experimental manifestations of the effect reported in the literature can be analyzed and understood under a unified "universal" framework with a minimum set of parameters.

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.

On the experimental estimation of surface enhanced raman scattering (SERS) cross sections by vibrational pumping.

We present an in-depth analysis of the experimental estimation of cross-sections in surface enhanced raman scattering (SERS) by vibrational pumping. The paper highlights the advantages and disadvantages of the technique, pinpoints the main aspects and limitations, and provides the underlying physical concepts to interpret the experimental results. Examples for several commonly used SERS probes are given, and a discussion on future possible developments is also presented. Obtaining good estimates of SERS cross-sections is, in general, an extremely hard problem and has been a longstanding ambition of the SERS community for reasons that go from the purely applied (quantification of signals) to the more fundamental (comparisons of theoretical electromagnetic enhancement factors with experiment). Any method that can produce a standard protocol for the estimation of cross-sections is, accordingly, of great interest and an effort to understand its principles and limitations is required.

Vibrational pumping and heating under SERS conditions: fact or myth?

We address in this paper the long debated issue of the possibility of vibrational pumping under Surface Enhanced Raman Scattering (SERS) conditions, both theoretically and experimentally. We revisit with simple theoretical models the mechanisms of vibrational pumping and its relation to heating. This presentation provides a clear classification of the various regimes of heating/pumping, from simple global laser heating to selective pumping of a single vibrational mode. We also propose the possibility of extreme pumping driven by stimulated phonon emission, and we introduce and apply a new experimental technique to study these effects in SERS. Our method relies on correlations between Raman peak parameters, and cross-correlation for two Raman peaks. We find strong evidence for local and dynamical heating, but no convincing evidence for selective pumping under our specific experimental SERS conditions.

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.