Transient Absorption as a Time-Resolved Measurement of Temperature in 2D Gold Nanoparticle Arrays

The dynamics of photoexcited noble metal nanoparticles (NPs) and nanostructures (NSs) takes place on a number of different timescales: electron - electron scattering (100s of femtoseconds), electron - phonon coupling (less than 10 picoseconds) and phonon-phonon coupling (100s of picoseconds) [1]. The purpose of this work is to use the Femtosecond Transient Absorption Spectroscopy (FTAS) to monitor all of the above timescales following interband photoexcitation of 2D arrays of gold nanoparticles. The electronic heating of the NP array leads to a strong change in the dielectric constant of the material and thus to a change of the plasmonic response of the NS in time. In gold spherical particles this typically leads to a bleaching of the central part of the resonance and a simultaneous increase in the wings of the resonance (photoinduced absorption) [2]. In non-spherical particles the transient response is more complicated. In the case of the 2D arrays probed in this work the possibility to control the polarisation of the probing light permits us to selectively probe either the transverse electric field perpendicular to the major axis of the NPs) or the longitudinal (electric field parallel to the major axis of the NPs) plasmon resonances. This makes it possible to separate these contributions to the FTAS and to better understand the transient response of the material. The temporal development of the bleaching/photoinduced absorption of the plasmon reflects the dynamics described above with the rise-time of the signal being related to the electron-electron scattering and the decay occuring on the two timescales related to the electron-phonon and phonon-phonon scattering times. An in-depth analysis of these signals both in transmission and in reflectance has allowed us to gain a detailed insight into these dynamics for 2D AuNP arrays.