Terahertz Resonant Emission by Optically Excited Infrared‐Active Shear Phonons in KY(MoO4)2

The generation of monochromatic electromagnetic radiation in the terahertz (THz) frequency range has remained a challenging task for many decades. Here, the emission of monochromatic sub‐THz radiation by optical phonons in the dielectric material KY(MoO4)2 is demonstrated. The layered crystal structure of KY(MoO4)2 causes infrared‐active shear lattice vibrations to have energies below 3.7 meV, corresponding to frequencies lower than 900 GHz where solid‐state‐based monochromatic radiation sources are rare. Directly excited by a 5 ps long broadband THz pulse, infrared‐active optical vibrations in KY(MoO4)2 re‐emit narrowband sub‐THz radiation as a time‐varying dipole for tens of picoseconds, which is exceptionally long for oscillators with frequencies below 1 THz. Such a long coherent emission allows for the detection of more than 50 periods of radiation with frequencies of 568 and 860 GHz. The remarkably long decay time together with the chemical stability of the employed material suggests a variety of possible applications in THz technology.