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After the in-situ characterization of the intrinsic nanoparticle properties and of its dielectric environment using quantitative single-particle spectroscopy, we developed as setup to subsequently perform time-resolved spectroscopy on the same particle with subpicosecond time resolution. This allowed us to measure the ultrafast electronic relaxation of the individual nanoparticles.
Figure 1 (Left) xy-images of a single 40-nm silver nanoparticle taken with (1.) linear absorption method, (2.) time-resolved spectroscopy at t = 0.5 ps (peak in electronic response). (Right) Time-resolved transmission change of the particle due to heating by a femtosecond laser pulse. Inset shows same curve on logarithmic scale, indicating an electron-phonon decay time of 1.1 ps.
Figure 1(right) shows the time-resolved ultrafast response of a 40-nm silver particle after excitation by a femtosecond laser pulse. The typical response curve is due to equilibration and relaxation of the electron gas to the lattice phonons with a time constant of approximately one picosecond. We can use the pump-probe signal as a contrast mechanism for 2D imaging of the single particle by scanning the sample under the laser focus, as shown in Figure 1(left). From the ratio between linear absorption and the transient signal amplitudes we can derive the change in absolute cross-section due to the pump-induced heating.
Correlation of the single-particle ultrafast dynamics to the linear optical spectra allows resolving the dependence of the nanoparticle relaxation on its size, shape, and environment. Our first results in this direction have shown the electron-phonon relaxation time to increase with particle temperature, as could be derived from the deposited energy in particles with calibrated sizes.
Reference:
O. L. Muskens, N. Del Fatti, F. Vallée, Femtosecond response of a single small metal nanoparticle, Nano. Lett. 6(3), 552:556 (2006)
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