Metasurfaces with broadband optical absorption and engineered thermal emissivity have gained significant interest for applications that require precise control over optical and thermal energy pathways, for example, as selective absorbers in solar heating schemes, or as thermal emitters in thermophotovoltaics. Our laboratory has also recently explored the use of such metasurface absorbers in thermionic power convertor applications. In contrast with traditional methods of photothermalization, plasmonic metasurfaces can also resonantly promote a large population of photo-excited non-thermal ‘hot’ electrons, so that the photo-induced effective temperature of absorbers is a complex combination of separate phononic and electronic contributions, even under steady-state solar excitation.
Here we show how systematic analysis of the photo-induced surface temperature of metasurface absorbers using anti-stokes raman thermometry can be used to separately analyze the extent of vibrational (phononic) heating versus the effective temperature of the electron gas, as well as provide more detailed insight into the non-equilibrium electron distribution under steady-state CW illumination. The spectral dependence of the luminescent up-conversion of anti-stokes scattered photons reflects contributions from both phonon interactions as well as direct electron scattering, and these contributions can be decoupled by analyzing the dependence on excitation wavelength, intensity, substrate temperature, and other systematic variations of structural features of the metasurface.
Matthew T. Sheldon, "Decoupling electronic and phononic temperature in plasmonic absorbers (Conference Presentation)," Proc. SPIE 10719, Metamaterials, Metadevices, and Metasystems 2018, 1071926 (Presented at SPIE Nanoscience + Engineering: August 23, 2018; Published: 17 September 2018); https://doi.org/10.1117/12.2321522.5836027094001.
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