Emission of Terahertz Radiation by Optically Excited Infrared-Active Phonons in Layered Compounds MRe(MoO4)2

Abstract

The generation of monochromatic terahertz (THz) electromagnetic radiation remains a critical task for many current and future THz applications. Recently, we have demonstrated the emission of monochromatic sub-THz radiation by optical phonons in the dielectric material KY(MoO4)2. We are now extending our study to other materials in the MRe(MoO4)2 family, where Re stands for rare earth ions and M for an alkali ion. The layered crystallographic structure of these materials causes infrared-active lattice vibrations with the energies below 3.7 meV, corresponding to frequencies below 900 GHz, where solid-state monochromatic radiation sources are rare. Changing the Re3+ in the chemical composition enables more than 10 percent tunability of the optical phonon frequencies. After broadband excitations by 5 ps THz pulses, infrared active optical phonons in MRe(MoO4)2 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 the detection of more than 50 periods of radiation at frequencies of 568 and 860 GHz. The remarkably long decay time, together with the chemical stability of materials used, suggest a variety of possible applications in THz technology.