Frequency downshifting stair for ultra-intense femtosecond lasers through a plasma-photonics structure
Wavelength-tunable ultra-intense femtosecond lasers may enable breakthroughs in diverse areas of science spanning attosecond science, particle acceleration and beyond. Conventional crystal-based methods are limited by gain bandwidth and damage thresholds, which restrict their wavelength tunability. Plasma-based frequency conversion, unconstrained by material damage, offers a promising alternative. Here, a novel scheme named Frequency Downshifting Stair (FDS) based on plasma bubble filling control is presented. The FDS enables arbitrary frequency down-conversion of ultra-intense femtosecond pulses and yields chirp-free laser pulses. It can achieve near-100% photon conversion efficiency, approaching the physical limit. This is attributed to the linear control by the FDS of laser chirp evolution during the photon deceleration in the plasma wake bubble. For a laser pulse with an arbitrary wavelength λ_0 (e.g., λ_0=800nm), proof-of-concept PIC simulations demonstrate that a single-stage FDS enables continuous wavelength tuning from λ_0 to {2λ}_0 (800-1600nm). Moreover, a three-stage cascaded FDS achieves more than tenfold frequency (10λ_0) downshifting to a central wavelength of 8.5μm. The FDS scheme thus provides a universal pathway for generating high-energy, few-cycle pulses across the broad infrared regime, offering a powerful new tool for wavelength-dependent ultrafast science.