We study by femtosecond pump-probe microscopy the transient plasmonic response of individual gold nanoantennas fabricated by electron-beam lithography on a glass substrate. By exploiting the capability of the fabrication technique to control geometrical parameters at the nanoscale, we tuned the plasmonic resonance in a broad wavelength range, from the visible to the infrared. Numerical simulations based on a three-temperature model (3TM) for the electrons and lattice dynamics, combined with * To whom correspondence should be addressed full-wave numerical analysis and semiclassical theory of optical transitions in the solid state, are compared with the measurements on a single gold nanoantenna probed at different wavelengths. The agreement between the experiment and the prediction of the 3TM turns out to be comparable to that achievable with the more sophisticated Boltzmann equation formalism. We also investigate the influence of the plasmon detuning with respect to the pump and probe wavelengths on the nonlinear optical response using different nanoantennas. Quantitative comparison of the experimental data with the theoretical model also provides a disentanglement of the different contributions to the optical nonlinearity of gold giving rise to the complex features observed in the transient optical response. Our study provides a complete analysis of the physical mechanisms dominating the nonlinear plasmon dynamics of an individual nanoobject taking place on a few ps time scale.
When circularly polarized light interacts with a nanostructure, the optical response depends on the geometry of the structure. If the nanostructure is chiral (i.e. it cannot be superimposed on its mirror image) then its optical response, both in near-field and far-field, depends on the handedness of the incident light. In contrast, achiral structures exhibit identical farfield responses for left-and right-circular polarization. Here, we show that a perfectly achiral nanostructure, a plasmonic metamolecule with trigonal D3h symmetry, exhibits a near-field response that is sensitive to the handedness of light. This effect stems from the near-field interference between the different plasmonic modes sustained by the plasmonic metamolecule under circularly polarized light excitation. The local chirality in a plasmonic trimer is then experimentally evidenced with nanoscale resolution using a molecular probe. Our experiments demonstrate that the optical near-field chirality can be imprinted into the photosensitive polymer, turning the optical chirality into a geometrical chirality that can be imaged using atomic force microscopy. These results are of interest for the field of polarization-sensitive photochemistry.
Hybrid plasmonic nanoemitters based on the combination of quantum dot emitters (QD) and plasmonic nanoantennas open up new perspectives in the control of light. However, precise positioning of any active medium at the nanoscale constitutes a challenge. Here, we report on the optimal overlap of antenna's near-field and active medium whose spatial distribution is controlled via a plasmon-triggered 2-photon polymerization of a photosensitive formulation containing QDs. Au nanoparticles of various geometries are considered. The response of these hybrid nano-emitters is shown to be highly sensitive to the light polarization. Different light emission states are evidenced by photoluminescence measurements. These states correspond to polarization-sensitive nanoscale overlap between the exciting local field and the active medium distribution. The decrease of the QD concentration within the monomer formulation allows trapping of a single quantum dot in the vicinity of the Au particle. The latter objects show polarization-dependent switching in the single-photon regime.
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