Study of coherent and continuous terahertz wave emission in equilateral triangular mesas of superconducting Bi2Sr2CaCu2O8+δ intrinsic Josephson junctions

► We observed intense and coherent THz waves in the equilateral triangular mesas. ► We numerically investigated EM modes of the mesas which emitted intense THz waves. ► Irreversible (IR)-type THz radiation is observed in the equilateral triangular mesas.