Optical Interrogation of Subsurface Nanogap Thermal Transport

Nanoscale thermal transport has received significant attention for next-generation energy
conversion and thermal management applications. In particular, state-of-the-art experiments have
shown for vacuum gap distances down to 10 nm that thermal transport is governed by near-field
thermal radiation, and can be enhanced by orders of magnitude above the Planck black body limit.
Moreover, radiative thermal transport is bridged to heat conduction by so-called acoustic phonon
tunneling at contact, which can be controlled via external forces. However, to perform such
complex experiments, precision measurement of heat flow and isolation from parasitic conduction
are required. Here, we demonstrate the utility of hyperspectral frequency-domain
thermoreflectance (FDTR) for the interrogation of thermal transport across subsurface nanogaps,
alleviating the extreme challenges faced by nanoscale thermal transport experiments.

FDTR is a pump-probe optical technique sensitive to the thermal properties of an underlying
material system. Here, a periodically modulated pump beam deposits energy into the sample,
where subsequent temperature oscillations are monitored by a continuous wave probe beam via
the thermoreflectance effect. Measurement of the phase lag between the thermal oscillations and
periodic heating enables quantification of thermal properties though parametric fitting with the
heat diffusion equation. In particular, FDTR at low pump frequencies can quantify and image
interfaces up to 200 um below the surface. We demonstrate FDTR’s ability to interrogate
subsurface nanogaps by looking at heterogeneously integrated gallium nitride (GaN) with diamond
substrates and silicon with silicon substrates. We show that FDTR can precisely quantify the
thermal conductivity of air trapped in sub-200 nm gap that is 15 um below the GaN surface.
Similarly, we use hyperspectral FDTR imaging to map a mechanical standing wave in a silicon
membrane supported by a silicon substrate, which spatially modulates the gap thermal
conductance. Thus, FDTR eliminates the need for microfabricated precision calorimeters and
thermally insulated gap supports through spatial mapping of subsurface thermal boundary
conductances.

Acknowledgements: Sandia National Laboratories is a multi-mission laboratory managed and
operated by the National Technology & Engineering Solutions of Sandia, LLC, a wholly owned
subsidiary of Honeywell International Inc., for the U.S. Department of Energy's National Nuclear
Security Administration under contract No. DE-NA0003525. SAND2023-11105A