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Aggregated gold nanoparticles are widely used in surface-enhanced Raman scattering (SERS), however, gold nanoparticles are excellent light absorbers and its local heating effect should be concerned. The optical properties of plasmonic nanoparticles are strongly dependent on interactions with other nanoparticles, which complicates analysis for systems larger than a few particles. In this work we examine heat dissipation in aggregated nanoparticles and its influence on surface-enhanced Raman scattering (SERS) through correlated photothermal heterodyne imaging (PHI). For dimers the per particle absorption cross sections show evidence of interparticle coupling; however, the effects are much smaller than those for the field enhancements that are important for SERS. For larger aggregates the total absorption was observed to be simply proportional to aggregate volume. This observation allows us to model light absorption and heating in the aggregates by assuming that the particles act as independent heating sources. To push the detection limits in PHI of our system, we use the home-built DC-10MHz low noise large area photodiode amplifier and obtain 7 nV/Hz1/2 noise level which closed to the limitation of SR844 Lock-in amplifier itself. Our work aims to use local heating effects from molecules to improve the spatial resolution and chemical sensitivity of label-free microscopy.
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