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Photothermal spectroscopy and microscopy provides a route to measure thespectral and spatial properties of individual nanoscopic absorbers, independentfrom scattering, extinction, and emission. The approach relies upon use of twolight sources, one that resonantly excites and heats the target and itssurrounding environment and a second off-resonant probe that scatters from theresulting volume of thermally modified refractive index. Over the past twentyyears, considerable effort has been extended to apply photothermal methods todetect, spatially resolve, and perform absorption spectroscopy on singlenon-emissive molecules and other absorbers like plasmonic nanoparticles at roomtemperature conditions. Companion theoretical models have been developed tointerpret these experimental advances, yet it is not clear how they are relatedto each other nor how the effects of lock-in detection modify the theory. Forlarger target systems that host their own intrinsic scattering resonances aswell as for background media that do not instantaneously thermalize with theabsorbing target, additional dependencies arise that are yet to be exploredtheoretically. The aim of this Perspective is to overview the theory ofphotothermal spectroscopy and microscopy and present a unifying theoreticalapproach that recovers past models in certain limits while explicitly includingthe effects of target scattering resonances, thermal and optical retardation,and lock-in detection. Focus is made on plasmonic particles to interpret thephotothermal signal, yet all results are applicable equally to individualmolecules or nanoparticle absorbers. Consequently, we expect this review toprovide a useful foundation for the understanding of photothermal measurementsindependent of target identity.

Resolving resonance effects in the theory of single particle photothermal imaging