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

TL;DR

This paper demonstrates that strongly phase-mismatched nonlinear crystals can produce ultrafast, broadband nonlinearities, enabling efficient few-cycle pulse compression and supercontinuum generation in a compact setup.

Contribution

It introduces a method to achieve ultrafast, octave-spanning nonlinearities via strongly phase-mismatched cascaded interactions in standard nonlinear crystals.

Findings

Achieved 47 fs pulses compressed to 17 fs in lithium niobate

Observed octave-spanning supercontinuum generation

Demonstrated broadband ultrafast nonlinear response in bulk crystals

Abstract

Cascaded nonlinearities have attracted much interest, but ultrafast applications have been seriously hampered by the simultaneous requirements of being near phase-matching and having ultrafast femtosecond response times. Here we show that in strongly phase-mismatched nonlinear frequency conversion crystals the pump pulse can experience a large and extremely broadband self-defocusing cascaded Kerr-like nonlinearity. The large cascaded nonlinearity is ensured through interaction with the largest quadratic tensor element in the crystal, and the strong phase-mismatch ensures an ultrafast nonlinear response with an octave-spanning bandwidth. We verify this experimentally by showing few-cycle soliton compression with noncritical cascaded second-harmonic generation: Energetic 47 fs infrared pulses are compressed in a just 1-mm long bulk lithium niobate crystal to 17 fs (under 4 optical cycles) with 80% efficiency, and upon further propagation an octave-spanning supercontinuum is observed. Such ultrafast cascading is expected to occur for a broad range of pump wavelengths spanning the near- and mid-IR using standard nonlinear crystals.