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Optimal Condition for Strong Terahertz Radiation from Intrinsic Josephson Junctions

In order to enhance the radiation power in terahertz band based on the intrinsic Josephson junctions of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ single crystal, we investigate a long cylindrical sample embedded in a dielectric material. Tuning the dielectric constant, the radiation power has a maximum which is achieved when it equals the dissipation caused by Josephson plasma. This yields the optimal dielectric constant of wrapping material in terms of the properties of BSCCO single crystal. The maximal radiation power is found proportional to the product of the typical superconducting current squared and the typical normal resistance, or the gap energy squared divided by the typical normal resistance, which offers a guideline for choosing superconductor as a source of strong radiation. By introducing an anti-reflection layer, we can build a compact device with the BSCCO cylinder and two wrapping dielectric layers with finite thicknesses.