Tóth György, Pálfalvi László, Fülöp József A, Krizsán Gergő, Matlis Nicholas H, Almási Gábor, Hebling János
Opt Express. 2019 Mar 4;27(5):7762-7775. doi: 10.1364/OE.27.007762.
Recently a hybrid-type terahertz (THz) pulse source was proposed for high energy terahertz pulse generation. It is the combination of the conventional tilted-pulse-front setup and a nonlinear crystal with a transmission stair-step echelon of period in the hundred-micrometer range etched into the front face. The tilt angle introduced by the conventional tilted-pulse-front setup (pre-tilt) was chosen to be equal to the tilt-angle needed inside the nonlinear crystal (62° for lithium niobate (LN)) in order to fulfill velocity-matching. In this case, plane-parallel nonlinear optical crystals can be used. The possibility of using a plane-parallel nonlinear optical crystal for producing good-quality, symmetric THz beams was considered the most important advantage of this setup. In the present paper, a thorough numerical investigation of a modified version of that setup is presented. In the new version, the tilted pulse-front is created by a transmission grating without any imaging optics, and a wedged nonlinear optical crystal with a small wedge angle is supposed. According to a 1D numerical code, significantly higher THz generation efficiency can be achieved with a transmission stair-step echelon-faced nonlinear crystal having a 5 - 15-degree wedge angle than with a plane-parallel one or with the conventional tilted-pulse-front setup. Because of the spatially-dependent group-delay dispersion introduced by the transmission grating, a small wedge in the nonlinear crystal improves the spatial homogeneity of the THz-generation process, resulting in higher efficiencies and better beam profiles. At 100 K temperature, and by using 800 nm pump pulses with 20 mJ pulse energy, 100 fs pulse length and 8 mm beam spot radius, approximately 4.5% conversion efficiency and close to 1 mJ terahertz pulse energy can be reached with the newly-proposed setup.
最近，一种混合型太赫兹（THz）脉冲源被提出用于产生高能量太赫兹脉冲。它是传统倾斜脉冲前沿装置与一种非线性晶体的结合，该非线性晶体的正面蚀刻有周期在百微米范围内的透射阶梯式梯形光栅。传统倾斜脉冲前沿装置引入的倾斜角（预倾斜角）被选择为等于非线性晶体内所需的倾斜角（铌酸锂（LN）为62°），以实现速度匹配。在这种情况下，可以使用平行平面非线性光学晶体。使用平行平面非线性光学晶体来产生高质量、对称太赫兹光束的可能性被认为是该装置最重要的优点。在本文中，对该装置的一个改进版本进行了全面的数值研究。在新版本中，倾斜脉冲前沿由透射光栅产生，无需任何成像光学元件，并且设想使用具有小楔角的楔形非线性光学晶体。根据一维数值代码，与平行平面晶体或传统倾斜脉冲前沿装置相比，具有5 - 15度楔角的透射阶梯式梯形面非线性晶体能够实现显著更高的太赫兹产生效率。由于透射光栅引入的空间相关群延迟色散，非线性晶体中的小楔角改善了太赫兹产生过程的空间均匀性，从而提高了效率并改善了光束轮廓。在100 K温度下，使用脉冲能量为20 mJ、脉冲长度为100 fs且光斑半径为8 mm的800 nm泵浦脉冲，新提出的装置可以达到约4.5%的转换效率和接近1 mJ的太赫兹脉冲能量。
