Zigzag quasi-parametric amplification in a crystal slab

Achieving efficient, broadband, and high-average-power amplification for ultrashort pulses across wide spectral ranges remains a significant challenge. Here, we introduce a novel, to the best of our knowledge, nonlinear amplification scheme, zigzag quasi-parametric amplification (ZQPA), based on a crystal slab with high-reflectivity surfaces for both pump and signal beams. Inside the crystal, the pump and signal travel in a zigzag path, with partial emptying of the idler at each interface reflection. This enables continuous signal amplification while suppressing the efficiency limitation imposed by back-conversion. The zigzag geometry also aligns the beam propagation with the direction of heat conduction, effectively mitigating thermal effects during high-power operation. Moreover, introducing a wedge angle between the crystal interfaces creates multiple phase-matching angles within a single crystal, enabling broadband amplification in materials previously limited by narrow gain bandwidths. The ZQPA scheme features a compact structure and can be cascaded for use, making it competent to realize high-performance amplification of broadband chirped pulses spanning from the near-infrared to the long-wave mid-infrared region.