  Photothermal spectroscopy and microscopy provides a route to measure the
spectral and spatial properties of individual nanoscopic absorbers, independent
from scattering, extinction, and emission. The approach relies upon use of two
light sources, one that resonantly excites and heats the target and its
surrounding environment and a second off-resonant probe that scatters from the
resulting volume of thermally modified refractive index. Over the past twenty
years, considerable effort has been extended to apply photothermal methods to
detect, spatially resolve, and perform absorption spectroscopy on single
non-emissive molecules and other absorbers like plasmonic nanoparticles at room
temperature conditions. Companion theoretical models have been developed to
interpret these experimental advances, yet it is not clear how they are related
to each other nor how the effects of lock-in detection modify the theory. For
larger target systems that host their own intrinsic scattering resonances as
well as for background media that do not instantaneously thermalize with the
absorbing target, additional dependencies arise that are yet to be explored
theoretically. The aim of this Perspective is to overview the theory of
photothermal spectroscopy and microscopy and present a unifying theoretical
approach that recovers past models in certain limits while explicitly including
the effects of target scattering resonances, thermal and optical retardation,
and lock-in detection. Focus is made on plasmonic particles to interpret the
photothermal signal, yet all results are applicable equally to individual
molecules or nanoparticle absorbers. Consequently, we expect this review to
provide a useful foundation for the understanding of photothermal measurements
independent of target identity.
