Perspectives on studying phonon transport in the gigahertz to terahertz range with Brillouin light scattering and frequency-domain thermoreflectance.Transport of heat and hypersound with gigahertz (GHz) to terahertz (THz) phonons is crucial for heat management in electronics, mediating signal processing with microwave radiation, thermoelectrics, and various types of sensors based on nanomechanical resonators. Efficient control of heat and sound transport requires new materials, novel experimental techniques, and a detailed knowledge of the interaction of phonons with other elementary excitations. Wave-like heat transport, also known as second sound, has recently attracted renewed attention since it provides several opportunities for overcoming some of the limitations imposed by diffusive transport (Fourier’s regime). The frequency-domain detection of GHz-to-THz phonons can be carried out in a remote, non-destructive, and all-optical manner. The ongoing development of nanodevices and metamaterials made of low-dimensional nanostructures will require spatially resolved, time-resolved, and anisotropic measurements of phonon-related properties. These tasks can be accomplished with Brillouin light scattering (BLS) and various newly developed variants of this method, such as pumped-BLS. In the near future, pumped-BLS is expected to become useful for characterizing GHz topological nanophononics. Finally, second-sound phenomena can be observed with all-optical methods like frequency-domain thermoreflectance.This project has received funding from the European Union’s Horizon 2020 Research and Innovation Programme (No. 101003436) and the Polish National Science Centre (No. UMO-2018/31/D/ST3/03882). J.S.R. acknowledges financial support from the Spanish Ministerio de Economıa, Industria y Competitividad for its support through Grant No. CEX2019-000917-S (FUNFUTURE) in the framework of the Spanish Severo Ochoa Centre of Excellence program and Grant No. PID2020-119777GB-I0016 (THERM2MAIN)
