Highly stable and efficient spatiotemporal filtering in cross-polarized wave generation via nonlinearity tailoring in multi-plate crystals

We propose a simple and effective method for achieving highly stable and efficient cross-polarized wave (XPW) generation in a multi-crystal setup with excellent beam quality. Unlike previous approaches optimized for highly nonlinear regimes, our method increases the number of crystals to achieve moderate nonlinear evolution, suppressing higher-order modulation effects. Thin nonlinear crystals are precisely spaced and fixed at specific rotational angles to maximize spatial mode matching via the Kerr lensing effect. Using three uncoated BaF₂ crystals, we achieved an XPW conversion efficiency of 33%, with an estimated internal efficiency of up to 40% when accounting for Fresnel losses. A mode selector reduced the XPW signal by 20% but resulted in nearly diffraction-limited beam quality (<i>M</i>²=1.1) and exceptional pointing stability below 5 µrad. The XPW spectral broadening factor of 4.5 led to a pulse shortening factor of 3.2, while energy fluctuations (1.5%) were lower than those of the input (2.4%). Furthermore, a temporal contrast in the range of 10^-11 to 10^-12 was achieved after further amplification. These characteristics make the proposed approach highly suitable for seed injection in advanced lasers for strong-field physics applications.