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Student contribution: Plasmonic systems are efficient in converting optical energy into heat hence show technological significance in solar thermophotovoltaics, nanoparticle manipulation, and photocatalysis, etc. Conventional techniques to characterize plasmonic heaters are mostly thermal camera- and thermographic phosphor (TGP)- based. In this work, we present our results of characterizing a plasmonic heater using thermoreflectance imaging (TRI). The TRI technique presented here outperforms thermal camera-based technique in spatial resolution due to the visible light utilized for illumination, and does not require special sample preparation as in TGP-based technique. We chose to use a gap plasmon structure to maximize the optical absorption, and fabricated structures with various dimensions that exhibit varying optical absorptions at a fixed wavelength of 825 nm, which is the wavelength of pump light used in the TRI measurement. The TRI setup uses a millisecond-modulated continuous-wave pump laser to induce local temperature fluctuation on the sample surface, a 530 nm LED probe light then senses the change in the temperature-dependent material reflectance between high and low temperatures, which combined with a pre-calibrated thermoreflectance coefficient can be used to calculate the temperature rise on each image pixel. This technique grants us a resolution of ~200 nm. The experimentally obtained temperature rise on various gap plasmon structures correlates well with their optical absorption, and we compare the results against a finite element heat transfer model. Using a separate pump-probe thermoreflectance technique, we experimentally obtain the heat transfer dynamics of such gap plasmon structure under laser irradiation with picosecond resolution.
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