Through systematic tailoring of the optical properties of lithographically patterned plasmonic nanostructures it is possible to optimize solar absorption and thermal reemission for photo-thermal heating to temperatures well above ambient. We outline a method to take advantage of such resonant photothermal heating in addition to photo-excited hot electrons to promote electron emission from the metal with high efficiency. Due to the close relation to purely thermionic emission this process is termed Hot-Electron Enhanced Thermionic Emission (HEETE). This dual mechanism of electron emission may provide a technique to more efficiently utilize optical power and can theoretically out-perform traditional semiconductor based solar cells.
To address design of such nanostructures, we have developed a simple model of the photo-thermal response of a plasmonic absorber with allows us to explore features such as spectral width of absorbance and emittance as well as angular dependence of emission. Additionally, it allows us to examine the roll of non-radiative thermal loss pathways such as conduction and convection. While these pathways normally dominate, placing the structure in vacuum is a simple way to minimize this loss. In such conditions temperature increases of well over 900 K are achievable without additional optical concentration. The nanostructures that reach these temperatures have high absorption, greater than 90%, in the visible up to 1100 nm and emissivity of approximately 2% through the infrared as well as minimized emission at oblique angles.
Using full wave optical simulations (FDTD method) and particle swarm optimization algorithms, where we were able to use temperature as calculated by our model as the figure of merit to identify possible nanostructures. We found a range of structures that will have the desired absorbance and emittance properties which are made of a variety of noble metals such as gold, silver, and copper. When coated with a dielectric material such as aluminum oxide to increase the thermal tolerance of the nanostructures while minimally impacting the emission characteristics, our designs takes advantage of highly absorbing plasmon resonances in the visible as well as the naturally low emissivity in the infrared which are both characteristic to metals without losing the thermal stability of higher melting point refractory materials. The dielectric coating also allows for accurate temperature measurements of the structure via in-situ anti-stokes Raman thermometry. Test HEETE devices have been nanofabricated on thermally isolated Si3N4 membranes to minimize thermal conduction to the surrounding substrate. Initial temperature measurements demonstrate that these plasmonic arrays greatly exceed the temperatures of ideal blackbodies under solar fluence.