Unlike classical heat diffusion at the macroscale, nanoscale heat transportcan occur without energy dissipation because phonons can travel in straightlines for hundreds of nanometres. Despite recent experimental evidence of suchballistic phonon transport, control over its directionality, and thus itspractical use, remains a challenge, as the directions of individual phonons arechaotic. Here, we show a way to control the directionality of ballistic phonontransport using silicon thin-films with arrays of holes. First, we demonstratethe formation of directional heat fluxes in the passages between the holes.Next, we use these nanostructures as a directional source of ballistic phononsand couple the emitted phonons into nanowires. Finally, we introduce ananoscale thermal lens in which the phonons converge at a focal point, thusfocusing heat into a spot of a few hundred nanometres. These results provide abasis for ray-like heat manipulations that enable nanoscale heat guiding,dissipation, localization, confinement and rectification.
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