Current compact emitter and receiver technologies are generally inefficient and impractical at terahertz (THz) frequencies between 0.1 and 10 THz. Hence, a gap exists between mature microwave and developed optical technologies. On-chip, integrated broadly tunable and powerful quantum sources that coherently radiate THz waves between 0.1 and 11 THz (potentially extendable to 15 THz) and with potential output power of >1 mW can be achieved based on quantum tunneling of electron pairs across the stack of intrinsic Josephson junctions (IJJs) naturally present in a single crystal of the layered high-Tc superconducting Bi2Sr2CaCu2O8+δ (BSCCO). Such devices have been found to be especially promising solid-state THz sources capable of bridging the entire THz gap, as their wide-frequency tunability range is superior to that obtained from their semiconducting-based rivals, either single resonant-tunneling diodes (RTDs) or THz-quantum cascade lasers (QCLs). Due to the unique electrodynamics of BSCCO, they can also be operated as switching current detectors, paving the way for the realization of on-chip THz-integrated circuits for applications in ultrahigh-speed telecommunications, quantum information, on-chip spectroscopy, and nondestructive sensing, testing, and imaging. This article reviews the history and recent advances in THz sources and detectors based on IJJs with a focus on the application of IJJ THz devices in THz spectroscopy and various types of THz imaging systems such as reflection, transmission, and computed tomography. We show that compact IJJ THz devices with sub-centimeter-sized modules are easy to use in many applications, as they can be regarded as pocket quantum THz torches.

We study the transverse magnetic (TM) electromagnetic cavity mode wave functions for an ideal equilateral triangular microstrip antenna (MSA) exhibiting C 3v point group symmetry. When the C 3v operations are imposed upon the antenna, the TM(m,n) modes with wave vectors [Formula: see text] are much less dense than commonly thought. The R 3 operations restrict the integral n and m to satisfy [Formula: see text], where [Formula: see text] and [Formula: see text] for the modes even and odd under reflections about the three mirror planes, respectively. We calculate the forms of representative wave functions and the angular dependence of the output power when these modes are excited by the uniform and non-uniform ac Josephson current sources in thin, ideally equilateral triangular MSAs employing the intrinsic Josephson junctions in the high transition temperature T c superconductor Bi2Sr2CaCu2 [Formula: see text], and fit the emissions data from an earlier sample for which the C 3v symmetry was apparently broken.

Takano et al. [Phys. Rev. B 65, 140513 (2002); J. Low Temp. Phys. 131, 533 (2003)] made Josephson junctions from single crystal whiskers of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ crossed an angle ${\phi }_0$ about the c axis. From the mesa structures that formed at the cross-whisker interface, they inferred a critical current density $J_c\left({\phi }_0\right).\mathrm{}$ As with the single crystal results of Li et al. [Phys. Rev. Lett. 83, 4160 (1999)], we show that the whisker data are unlikely to result from a predominantly d-wave order parameter. However, unlike the single crystals, these results, if correct, require the whisker c-axis transport to be coherent.

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.

Using our recent design of thermally managed sandwich device structures, we studied the radiation frequency characteristics of three such devices of the same rectangular dimensions made from the same single crystal of the high-T_c superconductor Bi₂Sr₂CaCu₂O_8+δ, and all three devices exhibit similar characteristics. Their observed radiation intensities appear to be enhanced at many transverse magnetic TM_n,m cavity mode frequencies, possibly including some higher TM_0,m modes with waves solely along the rectangular length, none of which have previously been reported. In addition, the temperature dependences of the radiation frequencies correspond strongly to the temperature dependences of the maximum bias voltages applied to the devices. The excitations of many cavity modes higher in frequency than that of the usually observed TM_1,0 mode and the high reproducibility of the radiation frequency characteristics both appear to originate from the reduction in the Joule self-heating of the thermally managed sandwich structures. The information provided here should aid in the design of future devices to obtain the desired emission frequency ranges.

We calculate the angular and temperature T dependencies of the upper critical field H-c2(theta, phi, T) for the C-4v point group helical p-wave states, assuming a single uniaxial ellipsoidal Fermi surface, Pauli limiting and strong spin-orbit coupling that locks the spin-triplet d-vectors onto the layers. Good fits to the Sr2RuO4 H-c2, a(theta, T) data of Kittaka et al (2009 Phys. Rev. B 80 174514) are obtained. Helical states with d(k)= k(x)x - k(y)y and k(y)x + k(x)y (or k(x)x + k(y)y and k(y)x - k(x)y) produce H-c2(90 degrees, phi, T) that greatly exceed (or do not exhibit) the fourfold azimuthal anisotropy magnitudes observed in Sr2RuO4.by Kittaka et al and by Mao et al (2000 Phys. Rev. Lett. 84 991), respectively.