We report on measurements of the linewidth $\Delta f$ of terahertz radiation emitted from intrinsic Josephson junction stacks, using a Nb/AlN/NbN integrated receiver for detection. Previous resolution-limited measurements indicated that $\Delta f$ may be below 1 GHz—much smaller than expected from a purely cavity-induced synchronization. While at low bias we found $\Delta f$ to be not smaller than $\sim$500 MHz, at high bias, where a hot spot coexists with regions which are still superconducting, $\Delta f$ turned out to be as narrow as 23 MHz. We attribute this to the hot spot acting as a synchronizing element. $\Delta f$ decreases with increasing bath temperature, a behavior reminiscent of motional narrowing in NMR or electron spin resonance (ESR), but hard to explain in standard electrodynamic models of Josephson junctions.

We have combined a stand-alone Bi₂Sr₂CaCu₂O₈ intrinsic Josephson junction stack, emitting terahertz radiation, with a YBa₂Cu₃O₇ grain boundary Josephson junction acting as detector. The detector is mounted on a lens, positioned 1.2 cm away from the emitter on a similar lens. With the emitter radiating at 0.5 THz, we observed up to 7 Shapiro steps on the current-voltage characteristic of the detector. The ac current induced in this junction was 0.9 mA, and the dissipated power was 1.8 μW. The setup, although far from being optimized, may be considered as a first step towards an integrated high-T_c receiver.

We report on Bi₂Sr₂CaCu₂O₈ (BSCCO) intrinsic Josephson junction stacks with improved cooling, allowing for a remarkable increase in emission frequency compared to the previous designs. We started with a BSCCO stack embedded between two gold layers. When mounted in the standard way to a single substrate, the stack emits in the range of 0.43–0.82 THz. We then glued a second, thermally anchored substrate onto the sample surface. The maximum voltage of this better cooled and dimension-unchanged sample was increased and, accordingly, both the emission frequencies and the tunable frequency range were significantly increased up to 1.05 THz and to 0.71 THz, respectively. This double sided cooling may also be useful for other “hot” devices, e.g., quantum cascade lasers.

We report on a liquid-nitrogen-cooled compact source for continuous terahertz (THz) emission. The emitter is a ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$ intrinsic Josephson-junction stack embedded between two gold layers and sandwiched between two MgO substrates. The radiation is emitted to free space through a hollow metallic tube acting as a waveguide. The maximum emission power is $1.17\mu \mathrm{W}$. The tunable emission frequency bandwidth is up to 100 GHz with a maximum emission power at 0.311 THz. Since the operation voltage is about 1 V and the current is less than 30 mA, we are able to drive this terahertz source at 77 K with only one commercial 1.5-V battery, just like a torch. This convenient and economical setup may find applications in fields like tracer-gas detection or nondestructive evaluation.

Recently, it has been found that, when operated at large input power, the linewidth $\Delta f$ of terahertz radiation emitted from intrinsic Josephson junction stacks can be as narrow as some megahertz. In this high-bias regime, a hot spot coexists with regions which are still superconducting. Surprisingly, $\Delta f$ was found to decrease with increasing bath temperature. We present a simple model describing the dynamics of the stack in the presence of a hot spot by two parallel arrays of pointlike Josephson junctions and an additional shunt resistor in parallel. Heat diffusion is taken into account by thermally coupling all elements to a bath at temperature $T_b$. We present current-voltage characteristics of the coupled system and calculations of the linewidth of the radiation as a function of $T_b$. In the presence of a spatial gradient of the junction parameters' critical current and resistance, $\Delta f$ deceases with increasing $T_b$, similar to the experimental observation.