Observation of a two-mode resonant state in a Bi2Sr2CaCu2O8+δ mesa device for terahertz emission

Using low-temperature scanning laser microscopy (LTSLM), we have studied the cavity resonance behavior of a rectangular $\mathrm{B}{\mathrm{i}}₂\mathrm{S}{\mathrm{r}}₂\mathrm{CaC}{\mathrm{u}}₂{\mathrm{O}}_{8+\ensuremath{\delta}}$ mesa THz device containing $N=640$ stacked intrinsic Josephson junctions. Our results show that this microscopy technique is an effective means of mapping electromagnetic resonances in this type of device, and we present a detailed analysis of the mechanisms underlying contrast formation in scanning laser microscopy. The LTSLM images reveal that the THz excitation of the stacked junctions contains more than one mode with components along both the long and short axes of the mesa. Thermoluminescent mapping of the same device shows that the mesa temperature is uniform demonstrating that the features seen in LTSLM are electromagnetic in origin, not thermal. Our results also imply that for the purposes of maximizing the THz emission power from $\mathrm{B}{\mathrm{i}}₂\mathrm{S}{\mathrm{r}}₂\mathrm{CaC}{\mathrm{u}}₂{\mathrm{O}}_{8+\ensuremath{\delta}}$ mesas, it is important to design devices that are capable of being excited in a single, purely transverse mode.