Linear and Nonlinear Optical Response of Silver Nanoprisms:  Local Electric Fields of Dipole and Quadrupole Plasmon Resonances

By means of femtosecond pump and probe spectroscopy, linear and nonlinear optical properties of silver nanoprisms have been investigated, focusing on enhancement of local electric fields due to dipole and quadrupole surface plasmons. Ag nanoprsims were prepared by direct reduction of AgNO3 by the solvent N,N-dimethylformamide in the presence of poly(vinylpyrrolidone). The average edge length and thickness of the nanoprisms (triangles and truncated triangles) were 67 and 35 nm, respectively. In the absorption spectra, dipole and quadrupole plasmon resonance bands have been observed. The values of the imaginary part of nonlinear susceptibility Imχ(3) measured at the dipole and quadrupole plasmon bands are −5.7 × 10-15 and −3.0 × 10-15 esu, respectively. The local electric field factors fQ and fD of quadrupole and dipole plasmon resonances were obtained from the observed dispersion curve of Imχ(3) and absorption spectra, yielding a ratio fQ /fD of ∼0.7. The nonlinear response times of both resonances were found to be ∼2 ps, which is close to the value for spherical nanoparticles, indicating that the relaxation process via electron−phonon interaction is governed by bulk crystal properties.