Sub-20 fs energetic near-IR pulses generated with cascaded soliton compression in short lithium niobate crystals

摘要

Access to energetic µJ-multi-mJ ultra-short fs laser pulses is crucial for many research topics today and in the future. Typically, laser amplifier pulses are an order of magnitude longer that the single-cycle limit making pulse compression desirable. We use soliton compression using cascaded quadratic nonlinearities to compress energetic pulses towards few-cycle duration. Specifically, we obtain sub-20 fs solitons by launching 0.25 mJ 55 fs pulses at 1300 nm in a lithium niobate (LN) crystal as short as 1 mm. The solitons are formed by phase-mismatched (cascaded) second-harmonic generation (SHG): upon propagation little second harmonic (SH) is generated, but the fundamental wave (FW) will instead experience a strong Kerr-like nonlinear phase shift. The total FW refractive index is n1+n^IcubicI1 where n^Icubic = n^ISHG +n^IKerr. It includes contributions from the material Kerr nonlinearity n^IKerr > 0 and the cascaded quadratic nonlinearities n^ISHG ≃ − 4πd²eff/cε0λ1n²1n2Δk. The goal is to achieve a self-defocusing nonlinearity n^Icubic < 0 through a large phase mismatch Δk ≫ 0. In this case, solitons can be formed with normal dispersion, which means anywhere in the visible and near-IR. Moreover it is possible to compress multi-mJ pulse-energies because there are no self-focusing problems [1].