The change of the surface plasmon resonance is sketched in Fig. 1.11A
for the electron heating in the first picosecond, and in Fig. 1.11B for the lat-
tice heating. This change can be detected by pump-probe spectroscopy. This
technique is based on two pulses, a pump pulse and a probe pulse, and ex-
ploits the changes in spectral response that are induced by a pump pulse. In
other words, a metal nanoparticle is excited by a pump pulse at a time t = 0,
which induces a series of heating and cooling steps as described in this sec-
tion. These steps will, at different time intervals, lead to spectral changes as
shown in Fig. 1.11. A probe pulse (weak enough not to change the state of
the particle) that arrives at the sample an interval ∆t later than the pump, will
sense the particle in the state as produced by the pump pulse. If the intensity
of the pump beam is modulated, and the detected signal is demodulated with
a lock-in amplifier, the difference between a state perturbed by the pump and
an unperturbed state is measured, which largely enhances the sensitivity of