Optical Interrogation of Subsurface Nanogap Thermal Transport

Nanoscale thermal transport has received significant attention for next-generation energy<br/>conversion and thermal management applications. In particular, state-of-the-art experiments have<br/>shown for vacuum gap distances down to 10 nm that thermal transport is governed by near-field<br/>thermal radiation, and can be enhanced by orders of magnitude above the Planck black body limit.<br/>Moreover, radiative thermal transport is bridged to heat conduction by so-called acoustic phonon<br/>tunneling at contact, which can be controlled via external forces. However, to perform such<br/>complex experiments, precision measurement of heat flow and isolation from parasitic conduction<br/>are required. Here, we demonstrate the utility of hyperspectral frequency-domain<br/>thermoreflectance (FDTR) for the interrogation of thermal transport across subsurface nanogaps,<br/>alleviating the extreme challenges faced by nanoscale thermal transport experiments.<br/><br/>FDTR is a pump-probe optical technique sensitive to the thermal properties of an underlying<br/>material system. Here, a periodically modulated pump beam deposits energy into the sample,<br/>where subsequent temperature oscillations are monitored by a continuous wave probe beam via<br/>the thermoreflectance effect. Measurement of the phase lag between the thermal oscillations and<br/>periodic heating enables quantification of thermal properties though parametric fitting with the<br/>heat diffusion equation. In particular, FDTR at low pump frequencies can quantify and image<br/>interfaces up to 200 um below the surface. We demonstrate FDTR’s ability to interrogate<br/>subsurface nanogaps by looking at heterogeneously integrated gallium nitride (GaN) with diamond<br/>substrates and silicon with silicon substrates. We show that FDTR can precisely quantify the<br/>thermal conductivity of air trapped in sub-200 nm gap that is 15 um below the GaN surface.<br/>Similarly, we use hyperspectral FDTR imaging to map a mechanical standing wave in a silicon<br/>membrane supported by a silicon substrate, which spatially modulates the gap thermal<br/>conductance. Thus, FDTR eliminates the need for microfabricated precision calorimeters and<br/>thermally insulated gap supports through spatial mapping of subsurface thermal boundary<br/>conductances.<br/><br/>Acknowledgements: Sandia National Laboratories is a multi-mission laboratory managed and<br/>operated by the National Technology & Engineering Solutions of Sandia, LLC, a wholly owned<br/>subsidiary of Honeywell International Inc., for the U.S. Department of Energy's National Nuclear<br/>Security Administration under contract No. DE-NA0003525. SAND2023-11105A