  Searching multiple types of terahertz (THz) irradiation source is crucial for
the THz technology. Here, by utilizing a two-dimensional (2D) ferromagnetic
Cr2Ge2Te6 crystal, we firstly demonstrate a magneto-tunable monochromatic THz
irradiation source. With a low-photonic-energy broadband THz pump, a strong THz
irradiation with frequency ~0.9 THz and bandwidth ~0.25 THz can be generated
and its conversion efficiency could even reach 2.1% at 160 K. Moreover, it is
intriguing to find that such monochromatic THz irradiation can be efficiently
modulated by the magnetic field below 160 K. According to both experimental and
theoretical analyses, the emergent THz irradiation is identified as the
emission from the phonon-polariton and its temperature and magnetic field
dependent behaviors confirmed the large spin-lattice coupling in this 2D
ferromagnetic crystal. These observations provide a new route for the creation
of tunable monochromatic THz source which may have great practical interests in
future applications in photonic and spintronic devices.
