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.
This article is devoted to the exploration of the benefits of a new ultrafast confocal pump-probe technique, able to study the photophysics of different structured materials with nanoscale resolution. This tool offers many advantages over standard stationary microscopy techniques because it directly interrogates excited state dynamics in molecules, providing access to both radiative and non-radiative deactivation processes at a local scale. In this paper we present a few different examples of its application to organic semiconductor systems. The first two are focussed on the study of the photophysics of phase-separated polymer blends: (i) a blue-emitting polyfluorene (PFO) in an inert matrix of PMMA and (ii) an electron donor polythiophene (P3HT) mixed with an electron acceptor fullerene derivative (PCBM). The experimental results on these samples demonstrate the capability of the technique to unveil peculiar interfacial dynamics at the border region between phase-segregated domains, which would be otherwise averaged out using conventional pump-probe spectroscopy. The third example is the study of the photophysics of isolated mesoscopic crystals of the PCBM molecule. Our ultrafast microscope could evidence the presence of two distinctive regions within the crystals. In particular, we could pinpoint for the first time areas within the crystals showing photobleaching/stimulated emission signals from a charge-transfer state.
We introduce balanced-detection (BD) Raman-induced Kerr-effect (RIKE) as a powerful coherent Raman scattering (CRS) technique. RIKE relies on the Raman-induced birefringence that occurs when the pump-Stokes frequency detuning is in resonance with a vibrational transition. We show how a balanced-detection configuration, inspired by that applied in electro-optic sampling, suppresses both the linear and nonlinear backgrounds that affect other CRS techniques. BD-RIKE enables homodyne amplification of the signal and linear dependence on the concentration of the target oscillators, as well as cancellation of laser intensity noise. In addition, it allows straightforward selection of either the imaginary or the real part of the nonlinear vibrational response
We present a new coherent Raman scattering technique, which we call balanced-detection Raman-induced Kerr effect. The technique relies on a balanced detection architecture, inspired to that applied for electro-optic sampling in the terahertz domain, which allows one to sensitively measure the Raman-induced Kerr effect-induced polarization rotation of the Stokes field. Balanced detection allows for an intrinsic rejection of laser noise, thus making possible to approach shot-noise limited conditions even when using fiber laser sources and relatively low modulation frequencies. Balanced-detection Raman-induced Kerr effect removes both linear and nonlinear background, provides self-heterodyne amplification of the nonlinear Raman signal, and scales linearly the sample concentration. Furthermore, by properly changing the detection conditions, it allows the reconstruction of the full complex vibrational response, both in amplitude and phase, greatly increasing chemical selectivity in single-color excitation
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