Flat-top vortex pumping enables high-contrast amplification of femtosecond vortex pulse

Strong-field laser physics and laser-plasma interactions require femtosecond vortex pulses that simultaneously deliver high pulse energy and high spatial intensity contrast (SIC). An optical parametric amplifier (OPA) is a promising energy scaling approach for femtosecond vortex pulses. However, vortex amplification is typically pumped with Gaussian or flat-top beams that retain nonzero intensity at the vortex center. Consequently, the intensity null is partially filled and the annular profile is distorted, resulting in a marked degradation of both the SIC and the ring-shaped structure. We introduce vortex-mode-matched pumping and flat-top vortex pumping as SIC-preserving strategies. To our knowledge, this is the first systematic SIC-centered study of ultrafast vortex-pulse OPA under different pump spatial modes. Numerical results show that vortex pumping effectively mitigates SIC degradation, while flat-top vortex pumping further improves energy extraction and achieves the best overall performance. We also establish experimentally actionable design rules by matching the pump and signal topological charges and optimizing the pump-to-signal beam-waist ratio. These research results offer valuable insights for the development of high energy and high-contrast vortex sources for supporting structured-light strong-field applications.