Electron and Lattice Heating Contributions to the Transient Optical Response of a Single Plasmonic Nano-Object

Abstract

The sudden absorption of light by a metal nanoparticle launches a series of relaxation processes (internal thermalization, acoustic vibrations and cooling) which induce a transient modification of its optical response. In this work, the transient optical response associated to the internal thermalization of a single gold nanodisk (occurring on a few picoseconds timescale) was quantitatively investigated by time-resolved spectroscopy experiments, and the measured signals were compared with a model accounting for the effects of both electron and ionic lattice heating. We show that experimental timeresolved signals at delays posterior to nanodisk excitation and electron gas thermalization can be simply interpreted as a combination of electron and lattice temperature evolutions, with probe wavelength-dependent weights. This demonstrates the possibility to selectively probe the electronic or lattice dynamics, through the choice of specific probe wavelengths. Additionally, the timedependent spectral shape of transient extinction cross-section changes is shown to be successively dominated by the effect of electron and lattice heating, which present distinct spectral signatures.

How to Measure Hot Electron and Phonon Temperatures with Time Domain Thermoreflectance Spectroscopy?

“…Equation can then be reduced to γ T normale ∂ T normale ∂ t = p ( t ) In that case, the electron–phonon time constant C ph g is far bigger than the excitation laser pulse duration. Then, the maximum electron temperature increase is given by normalΔ T normale max = T 0 ( 2 α F e γ T 0 2 + 1 − 1 ) In Figure b,d, the experimental data fit on the blue line corresponding to the temperature increase of eq . This is the evidence that, at this specific wavelength for gold, the TDTR signal is proportional to the hot electron temperature and the thermoreflectance coefficient is constant with respect to the fluence.…”

Energy-Resolved Femtosecond Hot Electron Dynamics in Single Plasmonic Nanoparticles

“…Energy-averaged hot carrier thermalization kinetics have been well studied in metal nanoparticles using ultrafast optical pump–probe measurements of the transient dielectric function response. − Such studies have been central to understanding picosecond electron–lattice thermalization and impulsive acoustic excitations, − also offering insight into hundreds-of-femtosecond electron–electron thermalization times . However, the overall energy-averaged response probed in these studies is disproportionately influenced by the longer lifetimes (>100 fs) of lower-energy hot carriers (<1 eV), which mask the much faster tens-of-femtosecond decay times of the higher-energy nascent carriers.…”

Cooling Dynamics of Individual Gold Nanodisks Deposited on Thick Substrates and Nanometric Membranes

“…Moreover, they also allow for a much more quantitative connection between thermal dynamics and their induced transient optical response, as the nanoparticle initial temperature rise and transient extinction changes can both be precisely quantified in these experiments. 38,39 In this work, we apply single-particle time-resolved optical spectroscopy to the study of the cooling dynamics of individual gold nanodisks (NDs) supported on various dielectric substrates differing by their composition, thermal properties, and thickness and thus allowing a more or less efficient evacuation of heat away from the NDs. We investigate here the two extreme cases of NDs deposited on a thick crystalline substrate with high thermal conductivity (sapphire) and on nanometric, amorphous membranes with much lower thermal conductivity (silica and silicon nitride), the latter one corresponding to the original situation of a system where both the heater and its close environment have nanometric dimensions.…”

“…This simplification was supported by the absence of significant difference between time-resolved signals measured on a given ND using different modulation frequencies. On the other hand, with the duration of the used laser pulses (≈150 fs) and that of internal ND thermalization by energy exchanges between electrons and the ionic lattice (about 1 ps) being both much smaller than the characteristic times of ND cooling, excitation by each pump pulse was supposed to be instantaneous. We therefore considered an ND excitation process described in the time domain by a periodic train of delta pulses: q false( t false) ∝ ∑ n = − ∞ + ∞ δ ( t − n T r e p ) , with which is associated the Fourier transform q̃ false( f false) ∝ ∑ n = − ∞ + ∞ δ ( f − n f r e p ) .…”

“…The absence of averaging effects in single-particle experiments first allows the establishment of a finer connection between the composition and morphological features of the investigated systems and their thermal dynamics, as well as to characterize the reproducibility of these dynamics from one nanoparticle to another. Moreover, they also allow for a much more quantitative connection between thermal dynamics and their induced transient optical response, as the nanoparticle initial temperature rise and transient extinction changes can both be precisely quantified in these experiments. , …”