Surface plasmon resonance damping

Optical properties of copper nanoparticles are quite remarkable because the energy of the dipolar mode of surface collective electron plasma oscillations (surface plasmon resonance or SPR) coincides with the onset of interband transition. Therefore, optical spectroscopy gives an opportunity to study the particle-size dependence of both valence and conduction electrons. The intrinsic size effect in metal nanoparticles, caused by size and interface damping of the SPR, is revealed experimentally by two prominent effects a red shift of the surface plasmon band and its broadening. 

The localized surface plasmon resonance of individual plasmonic nanoparticles depends heavily on the size and shape of each nanoparticle. For instance, the wavelength of the dipolar surface plasmon red shifts with the increase of particle size. However, for much larger nanoparticles new bands for some multipolar modes will appear in the short-wavelength range, while the dipolar band at long-wavelength will be damped. Typically, the size of Au or Ag nanoparticles synthesized for SERS should be less than 150 nm, and larger than 20 nm. 

Grigorchuk NI. Radiative damping of surface plasmon resonance in spheroidal metalhc nanoparticle embedded in a dielectric medium. / Opt Soc Am B 2012 29 (12) 3404-3411. 

Pemer M, Bost P, Lemmer U, Plessen, G. von Feldmann J, Becker U, Mennig M, Schmitt M, Schmidt H. Optically induced damping of the surface plasmon resonance in gold colloids. Phys. Rev. Lett. 1997 78 (11) S2192-S2195... 

Schmidt, Optically induced damping of the surface plasmon resonance in gold colloids, Phys. Rev. Lett. 78,2192-2195 (1997). 

An additional effect to consider for SERRS is that of the fluorescence which can occur simultaneously with a RRS process. This is the so-called delayed fluorescence, " which is a radiative decay process for molecules which have relaxed from their initial excited vibrational state of the excited electronic state to the lowest vibrational level of the excited state. Delayed fluorescence can be additionally damped on a rough Ag surface for the same reasons that the RRS process is additionally damped, i.e., by coupling to the surface plasmon resonances. On the other hand, the electromagnetic enhancement factors L (o)L (Oip), will also cause an enhancement of the fluorescence in a similar manner to the Raman process. However, the decay of excited molecules by the surface channel will tend to mitigate the EM fluorescence enhancement effect. Two cases have been discussed in the literature " (a) the case of a molecule with a fluorescence quantum efficiency near unity in the free state, i.e., QE 1, and (b) the case with QE 1. If QE is low (<0.01), the fluorescent emission is estimated to be enhanced by ca. 10 whereas, for QE 1, a surface quenching of fluorescent emission by ca. 10" has been... 

In the present paper, we report on observation of the pronounced enhancement of photoluminescence of semiconductor nanocrystals near nanostructured metal surfaces which is shown to depend essentially on nanocrystal-metal spacing. Unlike conventional SERS, the surface enhanced PL should exhibit non-monotonous character with distance between emitting dipole (QD) and metal surface (Au colloid). The reason is that at smallest distances when QDs and colloidal particles are in close contact, the QD emission should be damped due to resonant energy transfer (RET) from photoexcited QDs to metal colloidal nanoparticles. Enhancement of photoluminescence (PL) is possibly promoted by surface plasmons excited in the metal. So, at a certain distance the enhanced QD emission would exhibit a maximum. We use a polyelectrolyte multilayers as the most appropriate... 

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