Measurement of Mechanical Forces Acting on Optically Trapped Dielectric Spheres Induced by Surface-Enhanced Raman Scattering

Surface enhanced Raman scattering (SERS) is studied from optically trapped dielectric spheres partially covered with silver colloids in a solution with SERS active molecules. The Raman scattering and Brownian motion of the sphere are simultaneously measured to reveal correlations between the enhancement of the Raman signal and average position of the sphere. The correlations are due to the momenta transfer of the emitted Raman photons from the probe molecules. The addition of a mechanical force measurement provides a different dimension to the study of Raman processes.

Figure 1

Contour plot (a) of the successive acquisition Raman spectra of CV. The plot is correlated with a plot of the $z$ bead position fluctuations (b). Raman spectra (c) recorded at the time $t_1$ and $t_2$ when a high and low enhancement were observed. An SEM image of the metal covered bead (inset).

Figure 2

SERS power ($1145–1567{\mathrm{cm}}^{-1}$ integration range) compared with the bead position fluctuations for the $x$, $y$, and $z$ directions. Histograms shown on the right illustrate the average position of the bead during SERS on or off events.

Figure 3

The $z$ bead position fluctuation (a) and SERS power (b) data for emodin ($950–1315{\mathrm{cm}}^{-1}$ integration range).

Figure 4

Comparison of SERS power (dotted line) with $z$ position fluctuations data (solid line) of the trapped bead for CV (a) and emodin (b).