Mechanism of Terahertz Electromagnetic Wave Emission from Intrinsic Josephson Junctions

Using a 3-D parallelepiped model of intrinsic Josephson junctions, we calculate the cavity resonance modes of Josephson plasma waves excited by external electric currents. The electromagnetic (EM) wave of the excited Josephson plasma is converted to a THz EM wave at the sample surfaces. The cavity modes accompanied by static phase kinks of the superconducting order parameter have been intensively investigated. The phase kinks induce a spatial modulation of the amplitude of the order parameter around the kinks and decrease the superconducting condensation energy. The Josephson plasma produces a magnetic field in the vacuum in addition to the emitted EM wave. This magnetic energy detemines the orientation of the cavity mode. Taking account of the facts mentioned above, we obtained sharp resonance peaks in the I-V curves and sizable powers of continuous and coherent terahertz wave emission at the cavity resonance. The emission frequencies are inversely proportional to the length of the shorter side of the samples in agreement with experiments.