Three-dimensional numerical analysis of terahertz radiation emitted from intrinsic Josephson junctions with hot spots

Abstract

In this study, we numerically investigate the terahertz radiation from mesa-structured intrinsic Josephson junctions (IJJs) using a three-dimensional calculation model. We assume an in-phase mode of the phase differences and calculate electromagnetic fields inside and outside of the IJJs simultaneously. We consider the appearance of a hot spot in the mesa where j c locally decreases and investigate the change of the radiation power with varying hot-spot positions. The radiation powers for three different hot-spot positions are calculated as functions of voltage. We observe strong radiation when the ac Josephson frequency satisfies the cavity resonance condition. Transverse-magnetic modes TM m,n whose indices m and n are even appear regardless of the positions of hot spots. Meanwhile, the TM m,n cavity modes whose m or n are odd appear only when the hot spots break the reflectional symmetry of the mesa structure. Moreover, we calculate the radiation patterns emitted by the IJJs at these cavity resonance conditions. The radiation patterns reflect the existence of two types of internal modes, that is, a uniform background mode and a cavity resonance mode.

Control of circularly polarized THz wave from intrinsic Josephson junctions by local heating

“…Secondly, the local increase of the mesa's temperature by the laser heating results in a local change in the critical current density j c . Theoretical studies have pointed out that such an inhomogeneous j c distribution affects the excitation of cavity modes in the mesa [16,22]. Moreover, the ability of laser heating to change the THz emission power dramatically has been predicted theoretically and shown experimentally [13][14][15]25].…”

“…Stacks of these natural Josephson junctions, which are referred to as intrinsic Josephson junctions (IJJs), can generate an AC Josephson current in the THz frequency range by application of a voltage, and the THz wave emission is attributed to the generation of this current. Intense THz emissions have been reported for IJJs fabricated in a mesa geometry, which itself behaves as a cavity resonator, and thus, a number of studies on such IJJ mesas have been carried out both experimentally [4][5][6][7][8][9][10][11][12][13][14][15] and theoretically [16][17][18][19][20][21][22][23][24][25][26]. However, in the previous studies, little attention was given to the state of the polarization of the THz waves.…”

“…Hence, the j c distribution must be antisymmetric with respect to both axes in order to excite both cavity modes. [16,22] Thus, to begin with, we examined the THz emissions from the mesa for heating position A, which is at the corner of the mesa. We calculated the THz emission power and far field from the mesa around the voltages at which the frequencies of the emission waves satisfy the cavity resonant conditions for the TM(1,0) and the TM(0,1) modes.…”

“…Moreover, we set ǫ c = 17.64, D = 1.2 nm, σ c = 10 S/m, and j c = 2.18 · 10 3 A/cm 2 . For simplicity, the laser heating was simulated by decreasing j c in a square area (20 µm × 20 µm) in the x-y plane, [22] ; the heating positions A ∼ G are indicated in Fig. 2.…”

“…We set W = 78 µm, L = 80 µm, and set the thicknesses of the electrode and mesa to 2 µm. We used an in-phase approximation in which all phase differences are equal to a common phase difference φ, [16,19,22] and the dynamics of φ is given by, hǫ c 2eD…”