Few-cycle nonlinear mid-IR pulse generated with cascaded quadratic nonlinearities

Generating few-cycle energetic and broadband mid-IR pulses is an urgent current challenge in nonlinear optics. Cascaded second-harmonic generation (SHG) gives access to an ultrafast and octave-spanning self-defocusing nonlinearity: when ΔkL >> 2π the pump experiences a Kerr-like nonlinear index change Δn = n_cascI, where n_case ∝ -d²_eff/Δk, and d_eff is the effective quadratic nonlinearity. Due to competing material nonlinearities n_Kerr the total nonlinear refractive is n_cubic = n_casc + n_Kerr. Interestingly n_cubic can become negative (self-defocusing), elegantly avoiding self-focusing problems, and making it possible to excite solitons with normal dispersion [1].