Spatial and Temporal Nanoscale Plasmonic Heating Quantified by Thermoreflectance.

The field of thermoplasmonics has thrived in the past decades because it uniquely provides remotely controllable nanometer-scale heat sources that have augmented numerous technologies. Despite the extensive studies on steady-state plasmonic heating, the dynamic behavior of the plasmonic heaters in the nanosecond regime has remained largely unexplored, yet such a time scale is indeed essential for a broad range of applications such as photocatalysis, optical modulators, and detectors. Here, we use two distinct techniques based on the temperature-dependent surface reflectivity of materials, optical thermoreflectance imaging (OTI) and time-domain thermoreflectance (TDTR), to comprehensively investigate plasmonic heating in both spatial and temporal domains. Specifically, OTI enables the rapid visualization of plasmonic heating with sub-micron resolution, outperforming a standard thermal camera, and allows us to establish the connection between the optical absorptance and heating efficiency as well as to analyze plasmonic heating dynamics on the millisecond scale. Using the TDTR technique, we, for the first time, study the optical resonance-dependent heat-transfer dynamics of a nanometer-scale plasmonic structure in the nanosecond regime and use a detailed computational model to extract the impulse response and thermal interface conductance of a multilayer plasmonic structure. The study reveals a quantitative relationship between the dimensions of the nanopatterned structure and its spatiotemporal thermal response to the light pulse excitation, a thermoplasmonic effect resulting from the spatial distribution of the absorbed electromagnetic energy. We also conclude that the two thermoreflectance techniques provide necessary feedback to nanoscale thermoplasmonic heat management, for which optimization in either heating power or temperature decay speed is needed.