Electromagnetic wave generation by mesoscopic intrinsic Josephson junctions of single crystal $\mathrm{Bi_2Sr_2CaCu_2O_{8+\delta}}$

It is known that the junction resistance $R_c$ of mesoscopic intrinsic Josephson junctions of single crystal $\mathrm{Bi_2Sr_2CaCu_2O_{8+\delta}}$ in a sweeping magnetic field parallel to the $ab$-plane exhibits a strong quantum oscillating behavior with periods of a unit of magnetic quantum flux $\phi_0$ or $\phi_0/2$, which penetrate through each insulating layer between superconducting $\mathrm{CuO_2}$ layers of $\mathrm{Bi_2Sr_2CaCu_2O_{8+\delta}}$ above or below a certain magnetic field $H^*$, respectively. This happens only at low level of currents, whereas at high currents this oscilating behavior fades away and $R_c$ becomes rather smooth saturation behavior. In such a condition it is expected that the collective motion of Josephson vortices would generate Josephson plasma in a junction, which may continuously emit the coherent THz electromagnetic waves. In reaching a current levels at $\sim$0.3-0.7$J_c$ in rather low fields, we were indeed able to detect electromagnetic radiation emitted from the junctions by a bolometer detecter located near the junction. The power observed is very large, reaching a few 100 W/cm$^2$, and is extremely efficient, showing 3-7\% of the total input current energies. This is compared with the other methds such as cascade lasers using quantum dots and parametric oscillators using laser mixing in a non-linear optical materials.