minescent [52–54]. For example, gold nanorods exhibit an enhancement of 10
6
in the photoemission yield as compared to the bulk metal [53]. Many interesting
nonlinear optical phenomenon [55, 56] as well as surface enhanced Raman scattering effects [57–64] have been observed with silver and gold nanoparticles. Examples discussed in the following sections highlight laser induced processes in
gold and silver nanoparticles.
19.3.1
Transient Bleaching of the Surface Plasmon Band
The visible absorption bands of the gold (@530 nm) and silver (@380 nm) colloids
make them ideal candidates to probe the optical effects using transient absorption
[65–67] and transient grating spectroscopy [68]. Optical limiting and nonlinear
optical properties of metal colloids have also been explored recently [55, 56, 68, 69].
When the metal colloids are excited with a short laser pulse, a prompt bleaching
of the plasmon band is observed within the laser pulse duration. The difference
absorption spectrum of gold colloids shown in Figure 19.3 exhibits an intense
bleaching of the surface plasmon band at 520 nm. The plasmon band of metal
nanoparticles, as explained on the basis of Mie theory, involves dipolar oscillations
of the free electrons in the conduction band that occupy energy states immediately
above the Fermi level [50, 70]. Once these electrons are excited by a laser pulse,
they do not oscillate at the same frequency as that of the unexcited electrons, thus
resulting in a decrease of the plasmon absorption band. These aspects have been
addressed in recent spectroscopic investigations [65, 68, 71, 72]. The recovery of
the plasmon band on the picosecond timescale arises from the electron–phonon
and phonon–phonon relaxation. Similar transient bleaching has also been observed for silver nanoparticles [66, 73] and bimetallic particles [74–76].
500
525
550
575
600
625
650
-0.20
-0.16
-0.12
-0.08
-0.04
0.00
0.04 Ex. 355 nm
a. 0 ps
b. 200 ps
c. 1000 ps
c
b
a
Absorbance
Wavelength, nm
Au
Fig. 19.3. Time-resolved difference absorption spectra
recorded following 355 nm laser pulse (pulse width 20 ps)
excitation of 0.12 mM Au colloidal suspension. (From [24].)
19.3 Photoinduced Transformations of Metal Nanoparticles 623
6
in the photoemission yield as compared to the bulk metal [53]. Many interesting
nonlinear optical phenomenon [55, 56] as well as surface enhanced Raman scattering effects [57–64] have been observed with silver and gold nanoparticles. Examples discussed in the following sections highlight laser induced processes in
gold and silver nanoparticles.
19.3.1
Transient Bleaching of the Surface Plasmon Band
The visible absorption bands of the gold (@530 nm) and silver (@380 nm) colloids
make them ideal candidates to probe the optical effects using transient absorption
[65–67] and transient grating spectroscopy [68]. Optical limiting and nonlinear
optical properties of metal colloids have also been explored recently [55, 56, 68, 69].
When the metal colloids are excited with a short laser pulse, a prompt bleaching
of the plasmon band is observed within the laser pulse duration. The difference
absorption spectrum of gold colloids shown in Figure 19.3 exhibits an intense
bleaching of the surface plasmon band at 520 nm. The plasmon band of metal
nanoparticles, as explained on the basis of Mie theory, involves dipolar oscillations
of the free electrons in the conduction band that occupy energy states immediately
above the Fermi level [50, 70]. Once these electrons are excited by a laser pulse,
they do not oscillate at the same frequency as that of the unexcited electrons, thus
resulting in a decrease of the plasmon absorption band. These aspects have been
addressed in recent spectroscopic investigations [65, 68, 71, 72]. The recovery of
the plasmon band on the picosecond timescale arises from the electron–phonon
and phonon–phonon relaxation. Similar transient bleaching has also been observed for silver nanoparticles [66, 73] and bimetallic particles [74–76].
500
525
550
575
600
625
650
-0.20
-0.16
-0.12
-0.08
-0.04
0.00
0.04 Ex. 355 nm
a. 0 ps
b. 200 ps
c. 1000 ps
c
b
a
Absorbance
Wavelength, nm
Au
Fig. 19.3. Time-resolved difference absorption spectra
recorded following 355 nm laser pulse (pulse width 20 ps)
excitation of 0.12 mM Au colloidal suspension. (From [24].)
19.3 Photoinduced Transformations of Metal Nanoparticles 623