Recent studies have established that the anti-Stokes Raman signal from plasmonic metal nanostructures can be used to determine the two separate temperatures that characterize carriers inside the metal-the temperature of photoexcited "hot carriers" and carriers that are thermalized with the metal lattice. However, the related signal in the Stokes spectral region has historically impeded surface enhanced Raman spectroscopy (SERS), as the vibrational peaks of adsorbed molecules are always accompanied by the broad background of the metal substrate. The fundamental source of the metal signal, and hence its contribution to the spectrum, has been unclear. Here, we outline a unified theoretical model that describes both the temperature dependent behavior and the broad spectral distribution. We suggest that the majority of the Raman signal is from inelastic scattering directly with non-thermal carriers that have been excited via damping of the surface plasmon. In addition, a significant spectral component (~ 1%) is due to a subpopulation of hot carriers in an elevated thermal distribution. We have performed temperature and power-dependent Raman experiments to show how a simple fitting procedure reveals the plasmon dephasing time, as well as the temperatures of the hot carriers and the metal lattice, in order to correlate these parameters with quantitative Raman analysis of chemical species adsorbed on metal surface.
Circularly polarized optical excitation of plasmonic nanostructures causes coherent circulating motion of their electrons, which in turn gives rise to strong optically induced magnetization, a phenomenon known as the inverse Faraday effect (IFE). In this study we report how the IFE also significantly decreases plasmon damping. By modulating the optical polarization state incident on achiral plasmonic nanostructures from linear to circular, we observe reversible increases of reflectance by up to 8% and simultaneous increases of optical field concentration by 35.7% under 109 W/m2 continuous wave (CW) optical excitation. These signatures of decreased plasmon damping were also monitored in the presence of an external magnetic field (0.2 T). We rationalize the observed decreases in plasmon damping in terms of the Lorentz forces acting on the circulating electron trajectories. Our results outline strategies for actively modulating intrinsic losses in the metal via optomagnetic effects encoded in the polarization state of incident light.
Colloidal lead halide perovskite nanorods have recently emerged as promising optoelectronic materials. However, more information about how shape anisotropy impacts their complex dielectric function is required to aid the development of applications that take advantage of the strongly polarized absorption and emission. Here, we have determined the anisotropy of the complex dielectric function of CsPbBr 3 nanorods by analyzing the ensemble absorption spectra in conjunction with the ensemble spectral fluorescence anisotropy. This strategy allows us to distinguish the absorption of light parallel and perpendicular to the main axis so that the real and imaginary components of the dielectric function along each direction can be determined by the use of an iterative matrix inversion (IMI) methodology. We find that quantum confinement gives rise to unique axis-dependent electronic features in the dielectric function that increase the overall fluorescence anisotropy in addition to the optical anisotropy that results from particle shape, even in the absence of quantum confinement. Further, the procedure outlined here provides a strategy for obtaining anisotropic complex dielectric functions of colloidal materials of varying composition and aspect ratios using ensemble solution-phase spectroscopy.
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