# Ray phononics

In phononics – the science of phonons – researchers invent and develop micro- and nanostructures that can manipulate phonon motion. In nanostructures called phononic crystals, similarly to photonic crystals, phonons reflect from periodic boundaries and generate wave interference, which can guide phonons in certain directions. For such interference to occur, phonons must preserve their phase after scattering from the boundaries, for which phonon wavelength must by far exceed the surface roughness. While this condition is usually satisfied for mechanical waves and sound, phonons that carry heat have wavelengths of a few nanometers and cannot preserve their phase after multiple reflections. Thus, classical phononic structures cannot effectively control heat conduction via phonon wave interference.

However, in our recent experiments and simulations, we found that phononic crystals can still control directions of heat fluxes but using particle instead of wave properties of phonons. Indeed, in crystals phonons can exhibit particle-like behavior and ballistically travel in straight lines for hundreds of nanometers between diffuse scattering events. As diffuse scattering randomizes the direction of the next phonon flight, we can design nanostructures with specific boundaries to maximize the probability of phonon transport in certain directions. Thus, using ballistic phonon motion and nanostructuring we can shape the overall phonon flux and engineer heat transport. We call this concept of heat flux manipulation “ray-phononics” to contrast with classical wave-based phononics. Recently, we measured the limits of this approach at various temperatures by measuring the phonon mean free path in silicon.

Curently, I research all the new possibilities available with the ray-phononics concept. In future, I plan to design various phononic nanostructures that use this concept and demonstrate the application of ray-phononics experimentally.

The project was started under the JSPS postdoctoral schilarship (2016-2018) and is curently continued under PRESTO JSP research grant (2019-2021).