Dispersion-engineered multipass optical parametric amplification

The amplification of extremely short laser pulses (under 100 fs) presents a fundamental challenge due to the trade-off between amplification bandwidth, efficiency and gain¹. Conventional methods rely on complex optical set-ups with preprocessing and postprocessing steps². Optical parametric amplification³ offers a high optical gain that scales with the length of the nonlinear medium at the expense of bandwidth, limiting its effectiveness for extremely short and intrinsically broadband ultrashort pulses, whose amplification requires a broad gain-bandwidth, high single-pass gain and simultaneously strong nonlinear interaction. Here we introduce a new multipass⁴ optical parametric amplification system that leverages dispersion-engineered dielectric mirrors to repeatedly focus the laser into a nonlinear gain crystal. The coatings simultaneously compensate for the group delay⁵ after each multipass step and suppress the idler wave and, therefore, backconversion. This approach achieves ×1,500 higher gain compared with single-pass amplification, a photon conversion efficiency of up to 81% (52% system conversion efficiency) and near Fourier-limited time-bandwidth products of the amplified pulses, while fully preserving the spatial beam quality. Our concept breaks the gain versus bandwidth barrier and achieves 12 THz at 41 dB gain. As our concept does not require specific gain materials, it is versatile and broadly applicable to ultrafast⁶ laser systems in quantum technologies^7-9, attosecond physics^10-12, material processing and ultrabroadband low-cost bio-imaging systems^13,14. Our concept offers device sizes in the single-digit cubic centimetre range.