Metal nanoparticles (NPs) are characterized by unique optical properties that derive from the localized surface plasmon resonance (LSPR). NPs show a sharp spectral absorption for incident photon frequencies resonant with the LSPR. The LSPR is sensitive to the environment of the NPs and to their coupling, making metal NPs of great interest for sensing and biomedical applications.
Understanding the dynamics that occur following photon absorption in metal NPs is fundamental for many applications. The dynamical processes depend on size, shape and composition of the NPs and their interaction with the environment needs to be understood. In this work, we study the ultrafast carrier dynamics in Au and Ag NPs formed on silica nanowire (NW) arrays, using transient absorption (TA) spectroscopy. The silica NW arrays are transparent in the visible to near-UV region and offer a large surface area to attach NPs, providing a macroporous framework for an efficient interaction between the NPs and the environment.
The silica NW arrays were fabricated via thermal oxidation of Si NWs grown on quartz. They were then decorated with Au or Ag NPs by dewetting metallic films evaporated onto the NWs.
Transient absorption spectroscopy was performed using a pump probe configuration in a femtosecond transient absorption spectrometer. The amplified 2nd harmonic of a Ti:Sapphire laser at 400 nm with pulse length of 50 fs and repetition rate of 1 KHz was used as a pump for the measurement on the Au-decorated silica NWs. For the Ag-decorated silica NWs, to avoid overlap between the pump and the LSPR we used a pump at 275 nm obtained from an optical parametric amplifier. A white light supercontinuum generated in the FTAS was used as a probe for both metals in the range between 350 and 800 nm.
The transient absorption spectra show the expected increase of the transmission at the LSPR immediately after the pump excitation when the excited states are occupied and not available for the absorption of the probe (bleaching) . For the Ag NPs both the dipole and quadrupole contributions to the LSPR are observed.
A shift of the LSPR wavelength as a function of delay time has been observed for both Au and Ag. This could indicate the presence of NPs with different shape and size with the resulting overlap of different dynamics. The transient response stems from the modulation of the dielectric function of the NP, following the photo-induced dynamics of the electron distribution. Hence, in order to support the experimental results, we have developed a two-step model, which calculates the transient optical response of the AuNPs-decorated NWs. It provides the spectral dependence of the TA profile in agreement with the experimental one, except that the experimental spectral signature is broader, which can be attributed to the non-uniform shape distribution of the NPs in the real sample. The influence of such a distribution on the theoretical results is demonstrated.