Ellipsometry of hot gold nanoparticles reveals deviations of their optical properties from expectations due to temperature-induced surface effects.

Silver is an extremely appealing metal for plasmonics due to its very low optical losses in the visible and near-ultraviolet range and its relatively low reactivity. Within the emerging field of thermoplasmonics, where light-metal interactions are exploited to generate heat on the nanometric scale, knowledge of temperature-dependent complex permittivities of plasmonic materials is indispensable. We extracted the temperature-dependent complex permittivity of silver ${\varepsilon }_{Ag}$ by spectroscopic ellipsometry under high-vacuum conditions. For rising $T$, we observed an increase of the free-electron contribution to the imaginary part of the permittivity $\text{Im}\left[{\varepsilon }_{Ag}\right]$ and a temperature-dependent absorption band splitting off the interband absorption edge in the 320–360-nm range. Around 340 nm the relative increase of $\text{Im}\left[{\varepsilon }_{Ag}\right]$ at 600 K with respect to its room-temperature value is around 500%. In order to understand the implications of this behavior on silver thermoplasmonics, we computed the temperature-dependent extinction efficiency of oblate Ag ellipsoids with localized plasmon resonance within the 320–360-nm range. We predict that dramatic damping of the plasmon resonance occurs for increasing temperature, possibly leading to intriguing self-limiting effects in Ag thermoplasmonics.

Excitons dominate the light absorption and re-emission spectra of monolayer transition-metal dichalcogenides (TMD). Microscopic investigations of the excitonic response in TMD almost invariably extract information from the radiative recombination step, which only constitutes one part of the picture. Here, by exploiting imaging spectroscopic ellipsometry (ISE), we investigate the spatial dependence of the dielectric function of chemical vapor deposition (CVD)-grown WS₂ flakes with a microscopic lateral resolution, thus providing information about the spatially varying, exciton-induced light absorption in the monolayer WS₂. Comparing the ISE results with imaging photoluminescence spectroscopy data, the presence of several correlated features was observed, along with the unexpected existence of a few uncorrelated characteristics. The latter demonstrates that the exciton-induced absorption and emission features are not always proportional at the microscopic scale. Microstructural modulations across the flakes, having a different influence on the absorption and re-emission of light, are deemed responsible for the effect.

Heterostacks composed of two-dimensional WS ₂ and graphene exhibit interesting opto-electronic properties, including ultrafast charge transfer and fast, efficient photodetection. The optical properties of WS ₂ are heavily influenced by the presence of graphene in the heterostack, yet, so far, their characterization in the whole visible range is missing. In this work we report the complex dielectric function of two-dimensional WS ₂ flakes on epitaxial graphene on silicon carbide, obtained by means of spectroscopic ellipsometry (SE). The so-called A, B and C excitonic features are precisely identified, and significant differences with respect to literature data of WS ₂ on fused silica are highlighted. Since this investigation is based on SE, the complex dielectric function of WS ₂ is retrieved without performing Kramers Kronig analysis, which is instead necessary when employing reflectance or transmittance spectroscopy. Furthermore, the approach described in this work can be used in principle to characterize any two-dimensional flakes, both in terms of complex dielectric function and percentage of surface coverage.