Design of a contact grating setup for mJ-energy THz pulse generation by optical rectification

Abstract

According to the recent calculations, more efficient THz pulse generation is possible using relief grating on the front surface of the generating LiNbO₃ (LN) crystal for tilted-pulse-front-excitation rather than imaging the pump spot on a reflection grating into the LN crystal. Very recently, it was shown that—compared to a free-standing LN surface-relief grating—significantly higher diffraction efficiency can be reached if the grating profiles are filled with fused silica. Since realisation of such a setup is technically very challenging, in the present paper, we analyse the case where the input side of the LN grating is immersed into a refractive index matching liquid (RIML) instead of a solid material. Our results showed that the diffraction efficiency remains above 90 % for a refractive index ranging 1.45–1.60 of the applied RIML, and it is as high as 99 % for using the RIML for BK7. For this case, we carried out detailed calculations for various grating parameters. We propose a practical setup applying tilted input and slightly tilted output surfaces resulting in low losses and high diffraction efficiency for the pump. We conclude that a contact grating setup using BK7 RIML is suitable for producing THz pulses even in the mJ-energy range.