Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

average aspect ratio of 2.9. The excitation wavelength was 400 nm and themonitoring wavelength was 790 nm. The laser pulse energy (9 AJ) was abovethe threshold for a complete rod-to-sphere transformation [30]. The fact thatnanodots were indeed produced in this experiment was verified independentlyby TEM measurements on a nanorod sample that was irradiated until acomplete bleaching of the longitudinal surface plasmon band (see inset).Figure 12 shows the buildup of a permanent bleach signal at the position ofthe longitudinal resonance, which is associated with the shape transformationfrom a rod to a sphere. An exponential fit to the experimental data yields atime constant of 30 F 5 ps, which represents the nanorod-to-nanodottransformation time [30]. No significant dependence of the photoinducedisomerization dynamics on the rod aspect ratio was detected for four samples,with aspect ratios ranging from 1.9 to 3.7 [30]. It is interesting that themeasured transformation time is close to the physical limit imposed on therate of melting by the longitudinal speed of sound [64,65]. For example, itwould take f20 ps for a melting front to travel from one to another end ofa 60 nm rod if we assume that the front speed is equal to the speed of soundin gold.VI.SUMMARY AND CONCLUSIONSColloidal metal nanoparticles show a characteristic surface plasmon absorptionwith extinction coefficients exceeding those for organic laser dyes byseveral orders of magnitude. Large absorption cross sections are combinedwith the ease for spectral tunability over the entire visible and near-infraredspectral ranges achieved through the size, shape, and composition control.Furthermore, collective plasmon oscillations lead to a strong field enhancementin the proximity of metal nanoparticles. All of these properties makemetal nanoparticles very attractive for a wide range of optical and optoelectronicapplications.The surface plasmon absorption is a small-particle effect observed in thesize range from a few nanometers to tens or a few hundreds of nanometers.For particles discussed in this chapter, the energy separation between electroniclevels is much smaller than the thermal energy and, therefore, quantumsize effects are not significant. Quantum confinement becomes important forsizes below f2 nm. For such ‘‘ultrasmall’’ particles, the plasmon absorption isstrongly damped or even completely suppressed.A critical length scale for metal nanoparticles is the electron mean freepath. A reduction of the particle size below this critical length leads to theenhanced electron-surface scattering, which has a direct effect on the dephasingof the coherent electron motion and, hence, the width of the surfaceplasmon resonance. Surprisingly, the electron energy relaxation dynamicsCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.