Effect of Bias Electrode Position on Terahertz Radiation From Pentagonal Mesas of Superconducting ${\hbox {Bi}}_{2}{\hbox {Sr}}_{2}{\hbox {CaCu}}_{2}{\hbox {O}}_{8+\delta }$

We examined the effect of the bias current position on the radiation of intense and coherent terahertz electromagnetic waves from the intrinsic Josephson junctions (IJJs) in regular pentagonal mesas of single crystalline high-T_c superconducting Bi₂Sr₂CaCu₂O_8+δ. The emission from Sample 1, a mesa with a current bias feed on its middle, is strong and sharply peaked at 0.47 THz, whereas that from Sample 2, a mesa with its feed along an edge, shows a comparably strong, sharp peak at 0.43 THz. The common backbending in the current-voltage characteristics due to Joule heating was seen for Sample 1 but almost not for Sample 2. The experimental results are in good agreement with numerical simulations and calculations, both for the cavity resonances and the angular emission patterns. The resonance peaks obtained from these pentagonal mesas are both stronger and narrower than those found previously from differently shaped mesas fabricated by focused ion beam milling. The overall devices are small enough to be cooled to their operational temperatures by Stirling refrigerators, so that properly designed devices could be held in one hand as for laser pointers. Therefore, our IJJ-based emitter devices are promising candidates to fill the THz gap with compact, continuous-wave quantum solid-state sources.