Defects play a fundamental role in the energy relaxation of hot photoexcited carriers in graphene, thus a complete understanding of these processes are vital for improving the development of graphene devices. Recently, it has been theoretically predicted and experimentally demonstrated that defect-assisted acoustic phonon supercollision, the collision between a carrier and both an acoustic phonon and a defect, is an important energy relaxation process for carriers with excess energy below the optical phonon emission. Here, we studied samples with defects optically generated in a controlled manner to experimentally probe the supercollision model as a function of the defect density. We present pump and probe transient absorption measurements showing that the decay time decreases as the density of defect increases as predicted by the supercollision model.
We investigate, with a combination of ultrafast optical spectroscopy and semiclassical modeling, the photothermal properties of various water-soluble nanocrystal assemblies. Broadband pump-probe experiments with ∼100-fs time resolution in the visible and near infrared reveal a complex scenario for their transient optical response that is dictated by their hybrid composition at the nanoscale, comprising metallic (Au) or semiconducting (Fe 3 O 4 ) nanostructures and a matrix of organic ligands. We track the whole chain of energy flow that starts from light absorption by the individual nanocrystals and subsequent excitation of out-ofequilibrium carriers followed by the electron-phonon equilibration, occurring in a few picoseconds, and then by the heat release to the matrix on the 100-ps timescale. Two-dimensional finiteelement method electromagnetic simulations of the composite nanostructure and multitemperature modeling of the energy flow dynamics enable us to identify the key mechanism presiding over the light-heat conversion in these kinds of nanomaterials. We demonstrate that hybrid (organic-inorganic) nanocrystal assemblies can operate as efficient nanoheaters by exploiting the high absorption from the individual nanocrystals, enabled by the dilution of the inorganic phase that is followed by a relatively fast heating of the embedding organic matrix, occurring on the 100-ps timescale.
The transient optical
response of plasmonic nanostructures has
recently been the focus of extensive research. Accurate prediction
of the ultrafast dynamics following excitation of hot electrons by
ultrashort laser pulses is of major relevance in a variety of contexts
from the study of light harvesting and photocatalytic processes to
nonlinear nanophotonics and the all-optical modulation of light. So
far, all studies have assumed the correspondence between the temporal
evolution of the dynamic optical signal, retrieved by transient absorption
spectroscopy, and that of the photoexcited hot electrons, described
in terms of their temperature. Here, we show both theoretically and
experimentally that this correspondence does not hold under a nonperturbative
excitation regime. Our results indicate that the main mechanism responsible
for the breaking of the correspondence between electronic and optical
dynamics is universal in plasmonics, being dominated by the nonlinear
smearing of the Fermi–Dirac occupation probability at high
hot-electron temperatures.
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.
The transient optical response of gold nanorods is investigated beyond the perturbative regime. Ultrafast pump-probe spectroscopy and semiclassical modeling of hot electrons reveal a universal mechanism presiding over the saturation of nonlinear plasmonic effects.
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