We discuss the cavity modes and radiation pattern in solid state terahertz (THz) sources based on layered high-temperature superconducting Bi ₂ Sr ₂ CaCu ₂ O _8+δ (Bi-2212). We experimentally measure the emission spectra using the Fourier transform infrared spectrometer in order to elucidate the radiation mechanism of the mesa samples. Moreover, we experimentally study the angular dependence of the emission intensity obtained in one mesa's plane at the frequency f = 0.61 THz, at various detection angles θ by rotating the sample holder relative to the detector, to identify the excited EM cavity modes within the mesa that participate in the device output power enhancement. We modelled the mesa cavity and find a relatively good agreement between the experiment and theory. Our results show that compact, coherent and continuous-wave Bi-2212 THz devices are one of the most promising THz sources, capable of bridging the microwaves to photonics gap.

We studied the etching of small crystals of the high-T_c superconductor Bi₂Sr₂CaCu₂O_8+δ (Bi2212) with various dilute compositions of hydrochloric and nitric acids. A particular mixture of those acids was chosen to simultaneously fabricate multiple rectangular stand-alone Bi2212 mesa structures from a large, doubly-cleaved and doubly metallic-coated single crystal. The radiation characteristics of these devices were found to be very similar to stand-alone devices fabricated previously using dry-etching techniques. The greatly reduced time and cost of fabrication of stand-alone Bi2212 devices using our wet-etching technique should facilitate the mass production of a large number of identical stand-alone devices from a large single-crystalline Bi2212 substrate.

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.