Terahertz broadband anti-reflection moth-eye structures fabricated by femtosecond laser processing

Anti-reflection (AR) coatings aiming at the reduction of Fresnel reflection losses has come into demand in the terahertz (THz) region. Implementation of such a coating in practice is a difficult task, partially because the broad spectrum of the THz signal is difficult to control. Here, we propose and demonstrate a moth-eye AR structure capable of suppressing reflection losses in the range of 0.3 to 2.5 THz for high-resistivity silicon, resulting in a maximum transmission of 91%. The structure comprises of pyramid-like structures with a height of about 100 µm created on the material surface by femtosecond laser processing. We demonstrate experimentally and theoretically that such micromachining considerably increases transmittance of the silicon in the spectral range of 0.3-2.5 THz. We also demonstrate experimentally that such a structure allows one to improve performance of the THz source based on the LiNbO₃ crystal.