Disentangling the Ultrafast Nonlinear Optical Behavior of Plasmonic Resonances Near the Interband Transition

Abstract

The photoexcitation of plasmonic nanostructures with ultrashort laser pulses allows for elucidating the mechanisms underlying the ultrafast nonlinear optical response of such systems, gaining insight into the fundamental processes triggered by light absorption at the nanoscale. To date, the complex temporal and spectral features of the photoinduced response are not fully understood, especially when the photon energies are close to the interband transitions of the metallic medium. Herein, the effects of photoexcitation of plasmonic nanostructures are studied by resorting to a combinaion of broadband transient absorption spectroscopy and semiclassical nonlinear simulations of the energy relaxation processes following illumination. The proposed approach enables an in‐depth disentanglement of all the contributions to the ultrafast transient optical response of supported gold nanocrystals. From these methods, the apparent transient blue shift of the localized plasmon resonance observed in the pump–probe signals is rationalized as an interplay between different and spectrally dispersed permittivity modulations, instead of a simple negative permittivity change, as it could be concluded based on the Fröhlich condition. The results provide a comprehensive understanding of the thermo‐modulational nonlinearities of plasmonic nanostructures exhibiting resonances close to the interband transition threshold.

Coherent and Incoherent Ultrafast Dynamics in Colloidal Gold Nanorods

“…The electron–phonon time constants extracted from Δ ω i ( t d ) scale linearly with the pump fluence, in agreement with the prediction of the TTM model, while the ones extracted from the Δ γ i dynamics show a slight nonlinearity at higher fluences and assume consistently greater values for LSPR than for TSPR. An analogous nonlinear behavior of τ e –ph with pump fluence was already noticed in the literature at comparable fluence values, suggesting the establishment of a nonperturbative regime. ,,, While the experimental error does not allow for definitive identification of the saturation behavior, the comparison between the trends reported in Figure c and d seems to suggest a higher sensitivity of the photoinduced broadening of the LSPR to the pulse fluence, as discussed also below. The trends reported in Figure c and d have then been used to estimate the intrinsic electron–phonon relaxation time τ e –ph , 0 .…”

“…An analogous nonlinear behavior of τ e –ph with pump fluence was already noticed in the literature at comparable fluence values, suggesting the establishment of a nonperturbative regime. 30 , 38 , 40 , 48 While the experimental error does not allow for definitive identification of the saturation behavior, the comparison between the trends reported in Figure 4 c and d seems to suggest a higher sensitivity of the photoinduced broadening of the LSPR to the pulse fluence, as discussed also below. The trends reported in Figure 4 c and d have then been used to estimate the intrinsic electron–phonon relaxation time τ e –ph , 0 .…”

“…The femtosecond dynamics of the NRs were first investigated by pump–probe spectroscopy, one of the most commonly used techniques to study the ultrafast dynamics of noble metal nanoparticles. ,,− In our experiments, the relaxation of the systems was investigated in the first nanosecond after photoexcitation, with a time resolution of about 150 fs. The pump pulse was centered at 3.1 eV (Figure c), while the probe was a white light supercontinuum (1.8–2.6 eV).…”

“…This distribution of excited electrons rapidly thermalizes via electron–electron scattering ( τ e –e ∼ 100 fs), losing information about the initial excitation and increasing the electronic temperature T e . This increase of electron temperature results in a transient broadening, as it accelerates the total dephasing rate, and a transient shift of the SPR. − ,,,− Then, electron–phonon coupling ( τ e –ph ∼ 1–6 ps) equilibrates the electron and lattice temperature T l , thereby decreasing T e and raising the temperature of the nanoparticle. In addition, if the deposited thermal energy is high enough, the rapid thermal expansion of the metallic nanoparticles induced by electron–phonon coupling can coherently excite its breathing vibrational mode. ,, Eventually, the hot particle equilibrates with the environment through phonon–phonon interactions ( τ ph –ph ∼ 100–200 ps).…”

“…This increase of electron temperature results in a transient broadening, as it accelerates the total dephasing rate, and a transient shift of the SPR. 30 − 32 , 35 , 36 , 38 − 40 Then, electron–phonon coupling ( τ e –ph ∼ 1–6 ps) equilibrates the electron and lattice temperature T l , thereby decreasing T e and raising the temperature of the nanoparticle. In addition, if the deposited thermal energy is high enough, the rapid thermal expansion of the metallic nanoparticles induced by electron–phonon coupling can coherently excite its breathing vibrational mode.…”