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R C Maher

Publications and source records attributed to R C Maher.

11 recordsLinked to original sources
PubMed2006

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.

Journal Article↗
PubMed2006

A conclusive demonstration of vibrational pumping under surface enhanced Raman scattering conditions.

We provide a conclusive demonstration of vibrational pumping under Surface Enhanced Raman Scattering (SERS) conditions by performing anti-Stokes/Stokes ratio measurements over a large spatial area and low power density, down to 10 K with dried silver colloids, the dye rhodamine 6G, and 676 nm laser excitation. The method we propose allows for the measurement of the cross sections for different modes and also provides the determination of the asymmetry between the anti-Stokes and Stokes SERS cross sections for each mode.

Journal Article↗
PubMed2006

Temperature-dependent anti-stokes/stokes ratios under surface-enhanced Raman scattering conditions.

We make systematic measurements of Raman anti-Stokes/Stokes (aS/S) ratios using two different laser excitations (514 and 633 nm) of rhodamine 6G (RH6G) on dried Ag colloids over a wide range of temperatures (100 to 350 K). We show that a temperature scan allows the separation of the contributions to the aS/S ratios from resonance effects and heating/pumping, thus decoupling the two main aspects of the problem. The temperature rise is found to be larger when employing the 633 nm laser. In addition, we find evidence for mode specific vibrational pumping at higher laser power densities. We analyze our results in the framework of ongoing discussion on laser heating/pumping under surface-enhanced Raman scattering (SERS) conditions.

Journal Article↗
PubMed2006

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.

Journal Article↗
PubMed2005

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.

Journal Article↗
PubMed2004

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.

Journal Article↗
PubMed2004

Stokes/anti-Stokes anomalies under surface enhanced Raman scattering conditions.

The possibility of achieving anti-Stokes stimulation and/or pumping under surface enhanced Raman scattering conditions has been the source of intense controversies in the literature. With the aim of clarifying some of the aspects of this problem, we study theoretically and experimentally the situation in a model system which highlights some of the difficulties in the interpretation of the data. We show that many of the assumptions often presumed in the literature need to be assessed with care in each case. Through careful experiments we show, in particular, that the anti-Stokes/Stokes ratio for a specific mode in the same sample can depend on the chosen laser wavelength. This latter effect is a manifestation of the influence of the internal plasmon resonances (hot spots) in the result. Different possibilities and further research directions are highlighted and discussed.

Journal Article↗
PubMed2003

Electromagnetic optimization of light-harvesting proteins.

The problem of electromagnetic optimization of the structure of light-harvesting proteins is studied within a simplified diffraction scheme. It is suggested that evolutionary pressures for protein antennae has resulted in molecular structures that optimize the absorption in the light-sensitive pigments. Bacteriorhodopsin is shown to be an exceptional case in terms of optimization, a fact that can be connected to its extremely high sensitivity as a light detector down to a single photon level.

Journal Article↗
PubMed1995

Subjective evaluation of four low-complexity audio coding schemes.

In this study the subjective performance of four low-complexity audio data compression methods are compared, operating at nominal bit rates of 2, 3, 4, and 5 bits per sample, applied to four 20-kHz bandwidth, 16-bits per sample digitized musical signals. The simple compression schemes compared were elementary differential pulse-code modulation (DPCM), noise feedback coding DPCM (NFC-DPCM), adaptive quantizer DPCM (DPCM-AQB), and a recently proposed method known as recursively indexed quantizer DPCM (RIQ-DPCM). Pairs consisting of a reconstructed signal and a reference signal were presented in a two-interval preference experiment. The reference signals were processed for specified levels of modulated noise reference unit (MNRU) in order to estimate the equality threshold rating (ETR) of the reconstructed audio stimuli. The subjective MNRU values were found to increase by 2-5 dB for each increment in bits per sample. The DPCM-AQB scores were found to be 8-10 dB higher than for DPCM and NFC-DPCM. RIQ-DPCM was rated highest, exceeding the DPCM-AQB results by 2-5 dB in all tests. Objective measurements of segmental signal-to-noise ratio (SNRSEG) for the reconstructed signals predicted a performance level 2-5 dB lower than was actually found in the subjective results, particularly for SNRSEG values below 25 dB.

Adult↗