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Cascaded nonlinearities have attracted much interest, but ultrafastapplications have been seriously hampered by the simultaneous requirements ofbeing near phase-matching and having ultrafast femtosecond response times. Herewe show that in strongly phase-mismatched nonlinear frequency conversioncrystals the pump pulse can experience a large and extremely broadbandself-defocusing cascaded Kerr-like nonlinearity. The large cascadednonlinearity is ensured through interaction with the largest quadratic tensorelement in the crystal, and the strong phase-mismatch ensures an ultrafastnonlinear response with an octave-spanning bandwidth. We verify thisexperimentally by showing few-cycle soliton compression with noncriticalcascaded second-harmonic generation: Energetic 47 fs infrared pulses arecompressed in a just 1-mm long bulk lithium niobate crystal to 17 fs (under 4optical cycles) with 80% efficiency, and upon further propagation anoctave-spanning supercontinuum is observed. Such ultrafast cascading isexpected to occur for a broad range of pump wavelengths spanning the near- andmid-IR using standard nonlinear crystals.

Ultrafast and octave-spanning optical nonlinearities from strongly phase-mismatched cascaded interactions