Abstract
New experiments provide evidence that terahertz thermal phonons undergo specular surface reflections at room temperature and are sensitive to surface imperfections of just a few atoms, a key insight that is needed for novel methods of controlling heat flow.
Highlights
The manner in which waves reflect from rough surfaces is a fundamental problem that plays an important role in fields ranging from optics to acoustics [1,2,3]
We find evidence for specular reflections of thermal phonons even around room temperature for samples with wafer-scale roughness, while predominantly diffuse reflections are observed for samples exhibiting surface imperfections on the scale of just 2–3 atomic planes as determined from transmission electron microscopy
We fit the time traces to a biexponential decay model to extract the apparent thermal conductivity of the membrane at different grating periods as described in Ref. [29]
Summary
The manner in which waves reflect from rough surfaces is a fundamental problem that plays an important role in fields ranging from optics to acoustics [1,2,3]. The problem is important for thermal phonons of terahertz (THz) frequencies that carry heat in crystals [4]. Rough surfaces resist heat flow by disrupting the propagation of lattice waves along the transport direction, and in nanostructures this resistance can dominate over other sources of resistance [5,6,7,8]. Whether phonons reflect specularly from a surface and, preserve their phase is critical for coherent manipulation of heat, which is a topic of intense interest [9,10,11,12]
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