  We present an innovative contactless method suitable to study in-plane
thermal transport based on beam-offset frequency-domain thermoreflectance using
a one-dimensional heat source with uniform power distribution. Using a
one-dimensional heat source provides a number of advantages as compared to
point-like heat sources, as typically used in time- and frequency-domain
thermoreflectance experiments, just to name a few: (i) it leads to a slower
spatial decay of the temperature field in the direction perpendicular to the
line-shaped heat source, allowing to probe the temperature field at larger
distances from the heater, hence, enhancing the sensitivity to in-plane thermal
transport; (ii) the frequency range of interest is typically < 100 kHz. This
rather low frequency range is convenient regarding the cost of the required
excitation laser system but, most importantly, it allows the study of materials
without the presence of a metallic transducer with almost no influence of the
finite optical penetration depth of the pump and probe beams on the thermal
phase lag, which arises from the large thermal penetration depth imposed by the
used frequency range. We also show that for the case of a harmonic thermal
excitation source, the phase lag between the thermal excitation and thermal
response of the sample exhibits a linear dependence with their spatial offset,
where the slope is proportional to the inverse of the thermal diffusivity of
the material. We demonstrate the applicability of this method to the cases of:
(i) suspended thin films of Si and PDPP4T, (ii) Bi bulk samples, and (iii) Si,
glass, and highly-oriented pyrollitic graphite (HOPG) bulk samples with a thin
metallic transducer. Finally, we also show that it is possible to study
in-plane heat transport on substrates with rather low thermal diffusivity,
e.g., glass, even using a metallic transducer.
