The Last Step in Converting the Surface Plasmonic Energy into Heat by Nanocages and Nanocubes on Substrates

Abstract

The phonons of the vibrationally excited nanoparticle lattice couple to the environment on a time scale of tens to hundreds of picoseconds. The dynamics of electron–phonon, phonon–phonon, and vibrational processes in plasmonic nanoparticles may be followed optically through time-resolved pump–probe spectroscopy. [ 3–7 ] Vibrational motion of the lattice causes instantaneous modulation of the nanoparticle size, which modulates the electron density and results in periodic variations of the localized surface plasmon resonance (LSPR) spectrum at the lattice vibration frequency. The relaxation of these oscillations by coupling the nanoparticle lattice phonons to the phonons of the surrounding medium could lead to observed coherent oscillation of the medium phonons. The oscillations thus modulate the dielectric function of the medium around the nanoparticle resulting in modulations in the LSPR spectrum. In this manner, the excited medium phonons can be optically determined. The strength of the signal depends, among other factors, on the coupling between the nanoparticle and the medium as well as the coherence lifetime of the medium phonons produced. We recently reported on the lattice oscillations of gold nanocages having single and double shells [ 8 ] and showed that the value of the frequencies of the totally symmetric coherent oscillations observed refl ected the mechanical strength of these hollow nanoparticles. In the present study, we extend our time scale to that beyond the phonon–phonon relaxation