Ultrafast Coherent Raman Scattering at Plasmonic Nanojunctions

Abstract

Surface-enhanced coherent anti-Stokes Raman scattering (SECARS) measurements carried out on individual nanosphere dimer nantennas are presented. The ν-domain and t-domain CARS measurements in the few-molecule limit are contrasted as vibrational autocorrelation and cross-correlation, respectively. We show that in coherent Raman spectroscopies carried out with ultrashort pulses, the effect of surface enhancement is to saturate stimulated steps at very low incident intensities (100 fJ in 100 fs pulses), and the principal consideration in sensitivity is the effective quadratic enhancement of spontaneous emission cross sections, σ* = (E L /E o ) 2 σ. Through multicolor femtosecond SECARS measurements we show that beside enhancement factors, an effective plasmon mode matching consideration controls the interplay between coherent electronic Raman scattering on the nantenna and vibrational Raman scattering on its molecular load. Through extensive measurements on individual nantennas, we establish the tolerable average and peak intensities that can be used in ultrafast measurements at nanojunctions, and we highlight a variety of plasmon-driven chemical and physical channels of signal and sample degradation.

Nonlinear spectroscopy of plasmonic nanoparticles

“…Investigating a mixture of two Raman-active molecules, they could show that the statistical occurrence of one Raman spectrum alone or a superposition of both is compatible with on average 0.9 molecules per plasmonic structure ( Figure 10). Similar results have been obtained for a molecule near the touching point of two gold nanospheres held together by a silica coating [105,106].…”

Surface‐enhanced micro‐CARS mapping of a nanostructured cerium dioxide/aluminum film surface with gold nanoparticle‐bound organic molecules

“…But as it has been figured out by Crampton et al, the dynamic range of laser power between detection and destruction of a metal particle/reporter molecule complex in case of transient (femto‐ or picosecond) SECARS study is rather small. Hence, searching of new optical field enhancing structures, which demonstrate stable and reproducible single‐molecule SECARS sensitivity, is an actual and challenging task.…”

“…But the field of the most interesting SECARS applications opens when the capability of the technique to be employed in femto‐ and picosecond time‐resolved experiments is realized. This already led to an impressive direct observation of vibrations of a single molecule and of other exciting features of ultrafast SECARS …”

Stimulated Raman Scattering: From Bulk to Nano

“…9 On the other hand, for spontaneous Raman scattering, the rate of emission, W spon , is proportional to n 1 , because n 2 = 0. The effect of the stimulating field is thus to increase the rate of ω 2 emission: 65,66 $$\frac{W_{\text{stim}}}{W_{\text{spon}}}\propto n_2+1$$ Because the number of photons in the stimulating beam is high, the increase of the emission rate in ω 2 is significant. This implies that information about the Raman transition can be retrieved much faster in SRS than in spontaneous Raman scattering by many orders of magnitude.…”