Green light stimulates terahertz emission from mesocrystal microspheres

The discovery of efficient sources of terahertz radiation has been exploited in imaging applications¹, and developing a nanoscale terahertz source could lead to additional applications. High-frequency mechanical vibrations of charged nanostructures can lead to radiative emission, and vibrations at frequencies of hundreds of kilohertz have been observed from a ZnO nanobelt under the influence of an alternating electric field². Here, we observe mechanical resonance and radiative emission at ∼0.36 THz from core–shell ZnO mesocrystal microspheres excited by a continuous green-wavelength laser. We find that ∼0.016% of the incident power is converted into terahertz radiation, which corresponds to a quantum efficiency of ∼33%, making the ZnO microspheres competitive with existing terahertz-emitting materials^1,3. The mechanical resonance and radiation stem from the coherent photo-induced vibration of the hexagonal ZnO nanoplates that make up the microsphere shells. The ZnO microspheres are formed by means of a nonclassical, self-organized crystallization process^4,5,6, and represent a straightforward route to terahertz radiation at the nanoscale.