We propose a simple analytical formula that can quantitatively predict resonant light scattering from metal nanoparticles of arbitrary shape, whose sizes are too large for Rayleigh approximation to be applicable. The formula has been derived as an empirical extension of Mie’s rigorous calculation for light scattering from spheres. It can very well reproduce the experimental characteristics of light scattering from Au nanorods.
Light scattering by individual Ag nanoparticles and structures have been studied spectroscopically. Individual particles were selected and manipulated with a micromanipulator installed inside a scanning electron microscope (SEM). With typical particle dimensions of some 100 nm, the plasma resonances of particles and the coupled modes of particle pairs were observed in the visible region. The polarization dependence of the resonance frequencies strongly reflects the shape anisotropy; the effect that would be averaged out for experiments on ensembles. With a simple approximation to take the glass substrate into account, the results are in good agreement with the analytical calculations by Mie scattering, and with numerical calculations by the finite-difference time-domain method, both of which are performed with the morphological parameters obtained from the SEM observation for the corresponding particle or particle pair.
The electromagnetic ͑EM͒ enhancement in surface-enhanced resonance Raman scattering ͑SERRS͒ is quantitatively evaluated for rhodamine molecules adsorbed on Ag nanostructures. Polarization dependence of the plasma resonance ͑plasmon resonance͒ and the SERRS spectra from single isolated Ag nanostructures was evaluated to determine one-to-one relationship between optical anisotropy of plasma resonance, that of SERRS, and the morphology of the nanostructures. Experimental observations were compared with finitedifference time-domain calculations of the EM field induced by plasma resonance using individual morphology of the nanostructures. The experimental enhancement factor of SERRS ϳ10 9 was consistent with that of the calculations within a factor of ϳ2 for three excitation wavelengths. We conclusively fortify the indispensible importance of SERRS-EM theory with our results to design metal nanostructures generating strong EM enhancement.
We analyze blinking in surface enhanced resonance Raman scattering (SERRS) and surface enhanced fluorescence (SEF) of rhodamine 6G molecules as intensity and spectral instability by electromagnetic (EM) mechanism. We find that irradiation of intense NIR laser pulses induces blinking in SERRS and SEF. Thanks to the finding, we systematically analyze SERRS and SEF from stable to unstable using single Ag nanoparticle (NP) dimers. The analysis reveals two physical insights into blinking as follows. (1) The intensity instability is inversely proportional to the enhancement factors of decay rate of molecules. The estimation using the proportionality suggests that separation of the molecules from Ag NP surfaces is several angstroms. (2) The spectral instability is induced by blueshifts in EM enhancement factors, which have spectral shapes similar to the plasmon resonance. This analysis provides us with a quantitative picture for intensity and spectral instability in SERRS and SEF within the framework of EM mechanism.
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