We investigated the emission of subterahertz frequency electromagnetic radiation from high-TC superconducting c-axis NdBa2Cu3O7ÃÂÃÂÃÂÃÂ¢ÃÂÃÂÃÂÃÂ/PrBa2Cu3O7ÃÂÃÂÃÂÃÂ¢NdBa2Cu3O7ÃÂÃÂÃÂÃÂ trilayer thin film tunneling junctions when external electric and magnetic fields are applied. The current-voltage characteristics under applied ab-plane magnetic fields H (up 8 T) exhibit well defined steps, Vn, such that eVnhp/(2n), where the plasma frequency p0.4 THz and n=1,2,3,These steps may be interpreted using Josephson plasma dynamics. The applied voltage creates oscillating currents via the Josephson coupling energy EJ (EJ=hIJ/4e, where IJ is the Josephson current and h is Planck's constant) and the charge energy EC (EC=e2/2C, where C is the junction capacitance). Thus the Josephson plasma becomes excited by the tunneling current, with some of the energy being emitted as subterahertz frequency radiation. Our results provide a new insight into a solid-state quantum system with considerable potential for new solid-state terahertz emission sources.