Ultrafast hot electron dynamics in plasmonic nanostructures: experiments, modelling, design

Abstract Metallic nanostructures exhibit localized surface plasmons (LSPs), which offer unprecedented opportunities for advanced photonic materials and devices. Following resonant photoexcitation, LSPs quickly dephase, giving rise to a distribution of energetic 'hot' electrons in the metal. These out-of-equilibrium carriers undergo ultrafast internal relaxation processes, nowadays pivotal in a variety of applications, from photodetection and sensing to the driving of photochemical reactions and ultrafast all-optical modulation of light. Despite the intense research activity, exploitation of hot carriers for real-world nanophotonic devices remains extremely challenging. This is due to the complexity inherent to hot carrier relaxation phenomena at the nanoscale, involving short-lived out-of-equilibrium electronic states over a very broad range of energies, in interaction with thermal electronic and phononic baths. These issues call for a comprehensive understanding of ultrafast hot electron dynamics in plasmonic nanostructures. This paper aims to review our contribution to the field: starting from the fundamental physics of plasmonic nanostructures, we first describe the experimental techniques used to probe hot electrons; we then introduce a numerical model of ultrafast nanoscale relaxation processes, and present examples in which experiments and modelling are combined, with the aim of designing novel optical functionalities enabled by ultrafast hot-electron dynamics

All-Optical Reconfiguration of Ultrafast Dichroism in Gold Metasurfaces

Optical metasurfaces have come into the spotlight as a promising platform for light manipulation at the nanoscale, including ultrafast all-optical control via excitation with femtosecond laser pulses. Recently, dichroic metasurfaces have been exploited to modulate the polarization state of light with unprecedented speed. This work theoretically predicts and experimentally demonstrates by pump–probe spectroscopy the capability to reconfigure the ultrafast dichroic signal of a gold metasurface by simply acting on the polarization of the pump pulse, which is shown to reshape the spatio-temporal distribution of the optical perturbation. The photoinduced anisotropic response, driven by out-of-equilibrium carriers and extinguished in a sub-picosecond temporal window, is readily controlled in intensity by tuning the polarization direction of the excitation up to a full sign reversal. Hence, nonlinear metasurfaces are here demonstrated to offer the flexibility to tailor their ultrafast optical response in a fully all-optically reconfigurable platform

Interplay between Strong Coupling and Radiative Damping of Excitons and Surface Plasmon Polaritons in Hybrid Nanostructures

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Ultra-broadband 2D electronic spectroscopy of carotenoid-bacteriochlorophyll interactions in the LH1 complex of a purple bacterium

International audienc

Coherent ultrafast charge transfer in an organic photovoltaic blend

Blends of conjugated polymers and fullerene derivatives are prototype systems for organic photovoltaic devices. The primary charge-generation mechanism involves a light-induced ultrafast electron transfer from the light-absorbing and electron-donating polymer to the fullerene electron acceptor. Here, we elucidate the initial quantum dynamics of this process. Experimentally, we observed coherent vibrational motion of the fullerene moiety after impulsive optical excitation of the polymer donor. Comparison with first-principle theoretical simulations evidences coherent electron transfer between donor and acceptor and oscillations of the transferred charge with a 25-femtosecond period matching that of the observed vibrational modes. Our results show that coherent vibronic coupling between electronic and nuclear degrees of freedom is of key importance in triggering charge delocalization and transfer in a noncovalently bound reference system

Tracking the coherent generation of polaron pairs in conjugated polymers

The optical excitation of organic semiconductors not only generates charge-neutral electron-hole pairs (excitons), but also charge-separated polaron pairs with high yield. The microscopic mechanisms underlying this charge separation have been debated for many years. Here we use ultrafast two-dimensional electronic spectroscopy to study the dynamics of polaron pair formation in a prototypical polymer thin film on a sub-20-fs time scale. We observe multi-period peak oscillations persisting for up to about 1 ps as distinct signatures of vibronic quantum coherence at room temperature. The measured two-dimensional spectra show pronounced peak splittings revealing that the elementary optical excitations of this polymer are hybridized exciton-polaron-pairs, strongly coupled to a dominant underdamped vibrational mode. Coherent vibronic coupling induces ultrafast polaron pair formation, accelerates the charge separation dynamics and makes it insensitive to disorder. These findings open up new perspectives for tailoring light-to-current conversion in organic materials

Solvent-dependent photo-induced dynamics in a non-rigidly linked zinc phthalocyanine-perylenediimide dyad probed using ultrafast spectroscopy

In this work, we characterize the energy and electron transfer kinetics of a zinc phthalocyanine-perylenediimide dyad (ZnPc-PDI) in various solvents using steady-state and tunable narrowband pump-probe spectroscopy. We fit the ultrafast data with global analysis techniques and find that upon excitation of the PDI moiety (pump pulse at 540 nm), the excitation energy transfer (EET) rate to the ZnPc moiety displays a solvent sensitivity that we attribute to changes in the relative equilibrium moiety orientation. We rationalize these observations by considering the nature of the non-rigid bridge used to link the two moieties as well as the degenerate nature of the Q band transitions in the ZnPc species. By tuning the pulse into resonance with the ZnPc Q band (685 nm) we can directly photo-induce an electron transfer (ET) process back to the PDI moiety. Employing the same global analysis, we find that the dynamics of the ultrafast electron transfer are completely kinetically controlled according to the Bixon-Jortner model of barrierless solvent-controlled curve crossing, while the recombination to reform the ground state is well-described using the static energetic picture according to Marcus theory