Photothermal induced resonance (PTIR), also known as
AFM-IR, enables
nanoscale infrared (IR) imaging and spectroscopy by using the tip
of an atomic force microscope to transduce the local photothermal
expansion and contraction of a sample. The signal transduction efficiency
and spatial resolution of PTIR depend on a multitude of sample, cantilever,
and illumination source parameters in ways that are not yet well understood.
Here, we elucidate and separate the effects of laser pulse length,
pulse shape, sample thermalization time (τ), interfacial thermal
conductance, and cantilever detection frequency by devising analytical
and numerical models that link a sample’s photothermal excitations
to the cantilever dynamics over a broad bandwidth (10 MHz). The models
indicate that shorter laser pulses excite probe oscillations over
broader bandwidths and should be preferred for measuring samples with
shorter thermalization times. Furthermore, we show that the spatial
resolution critically depends on the interfacial thermal conductance
between dissimilar materials and improves monotonically, but not linearly,
with increasing cantilever detection frequencies. The resolution can
be enhanced for samples that do not fully thermalize between pulses
(i.e., laser repetition rates ≳ 1/3τ) as the probed depth
becomes smaller than the film thickness. We believe that the insights
presented here will accelerate the adoption and impact of PTIR analyses
across a wide range of applications by informing experimental designs
and measurement strategies as well as by guiding future technical
advances.