Narrow-band terahertz (THz) Cherenkov radiation can be excited as a relativistic electron bunch passes through the dielectric capillary with sub-millimeter radius. However, due to the diffraction effect, the radiation will enter free space with a large divergence angle, which makes it difficult to collect the radiation energy efficiently. In this Letter, to deal with this challenge, we propose to add a new dielectric layer, which satisfies a special relationship with the electron velocity, between the metal coating and original dielectric layer in the capillary. According to numerical simulation and theoretical analysis results, the divergence angle of radiation is significantly suppressed, and the peak power density is also enhanced by over two orders. As a result, the transmission efficiency from the radiation source to the optical system can be increased to over 90%. We expect that this method will provide a new way to generate THz Cherenkov radiation.

中文翻译：

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本质上减小介电毛细管引起的切伦科夫辐射的发散角

当相对论电子束穿过半径小于毫米的介电毛细管时，可以激发窄带太赫兹（THz）的契伦科夫辐射。然而，由于衍射效应，辐射将以大的发散角进入自由空间，这使得难以有效地收集辐射能量。在这封信中，为了应对这一挑战，我们建议在毛细管中的金属涂层和原始电介质层之间增加一个新的电介质层，该层满足电子速度的特殊关系。根据数值模拟和理论分析结果，辐射的发散角得到明显抑制，峰值功率密度也提高了两个数量级。结果是，从辐射源到光学系统的传输效率可以提高到90％以上。我们希望这种方法将提供一种产生THz Cherenkov辐射的新方法。