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

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_cubic^II₁ where n_cubic^I = n_SHG^I +n_Kerr^I. It includes contributions from the material Kerr nonlinearity n_Kerr^I >; 0 and the cascaded quadratic nonlinearities n_SHG^I ≃ - 4πd_eff²/cε₀λ₁n₁²n₂Δk. The goal is to achieve a self-defocusing nonlinearity n_cubic^I <; 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.