![]() ![]() ![]() The steep initial warming up of the nanoparticle, which is homogeneous as the thermal energy is spread rapidly in the metal by electron transport, results in the sudden lattice expansion which launches a set of coherent acoustic vibration modes. The thermal energy is further released into the surrounding medium through the interface. All these processes last for less than a few picoseconds. The energy is then redistributed within the electron gas and transferred to the lattice, through electron–electron and electron–phonon collisions, respectively, and the nanoparticle heats up. The light energy is initially absorbed by the electron gas, which is driven out of thermodynamical equilibrium. While continuous-wave spectroscopy probes the stationary optical properties of the NPs, ultrashort and intense pulses of laser light can excite a series of energy exchange processes, each of them being characterized by a specific timescale. The last two decades have witnessed unprecedented development in the synthesis of noble metal nanoparticles (NPs), in their applications, and in the techniques used to characterize them. Using the combined simulations of the NP elastic and optical properties, we elucidate this influence by analyzing the effect of the mechanisms involved in the acousto-plasmonic coupling. Moreover, the contributions of the vibrational modes to the experimental TA spectra are shown to vary with the probe laser wavelength at which the signal is monitored. The amplitude and frequency of these modes are shown to depend on the Ag shell thickness, as the silver load modifies the NP aspect ratio and mass. While bare Au nanorods exhibit extensional and breathing modes, the bimetallic NPs undergo more complex motions, involving the displacement of facets, edges and corners. The fast Fourier transform analysis of the TA dynamics reveals specific vibration modes in the frequency range 15–150 GHz, further studied by numerical simulations based on the finite element method. In this paper, we take advantage of the strong signature of these modes in the NP ultrafast transient optical response, measured by pump-probe transient absorption (TA) spectroscopy, to explore the NP vibrational landscape. Bimetallic Au/Ag core–shell cuboid nanoparticles (NPs) exhibit a complex plasmonic response dominated by a dipolar longitudinal mode and higher-order transverse modes in the near-UV, which may be exploited for a range of applications. ![]()
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