Superconducting Emitter Powered at 1.5 Terahertz by an External Resonator

A different type of terahertz emitter is made by coupling a source mesa fabricated from the intrinsic Josephson junctions present in single-crystalline high-$T_c$ superconducting ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Ca}\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$ ($\mathrm{Bi}2212$) to a much larger and impedance-matched external resonator with resonance frequencies up to 1.5 THz. The lateral dimensions of the source mesa are only $20\times 20\mu {\mathrm{m}}^2$, about 0.5%–3% of those of conventional $\mathrm{Bi}2212$ mesas, but its thickness of $2\mu \mathrm{m}$ is comparable to them. The observed emission frequencies that lie near to 1.5 THz match very well the resonance frequencies of the external resonator. This result ensures that the functions of resonator and antenna are separated from that of the THz current source in the $\mathrm{Bi}2212$ mesa structure. These facts allow for much greater flexibility in the emitter design to achieve higher power and higher frequency THz wave sources.

Figure 1

Design of the THz emitter. (a) Paradigm shift of the device construction from the previous mesa source cavity-mode self-resonator (left) to the present mesa source external resonator (right). (b) A photograph of two source mesas and four spacer mesas fabricated on a single-crystalline $\mathrm{Bi}2212$ surface with sketches of an external square $\mathrm{Au}$ patch resonator with electrodes. (c) Three-dimensional device overview.

Figure 2

The $c$-axis $R$-$T$ curves of Devices A, B, and C.

Figure 3

Schematic view of the experimental setup (a) for radiation directivity measurements and (b) for frequency spectra and polarization measurements.

Figure 4

Characteristics of Devices A, B, and C. (a) IVCs and (b) the corresponding HEB detector outputs for Devices A, B, and C. The data from Devices A and C are offset for clarity. (c) The set of IVCs at various $T$ values from 40 to 60 K of Device B. (d) The base temperature $T_b$ dependencies of the HEB detector outputs for Devices A, B, and C.

Figure 5

Emission characteristics. (a) Observed THz emission spectra for Device A (blue) and Device B (red) at 55 K. (b) Observed THz emission spectra for Device C at 26 K (green) and 40 K (purple). Inset shows the $I$-$V$ curve at 40 K and the arrow indicates the position where THz emission is observed. (c) Observed radiation pattern (directivity) for Device A.

Figure 6

The comsol simulation results. (a) Directivity and (b) electric field distribution for Device A. (c) Directivity and (d) electric field distribution for Device B.