  Experimental data of femtosecond thick-crystal second-harmonic generation
shows that when tuning away from phase matching, a dominating narrow spectral
peak appears in the second harmonic that can be tuned over 100's of nm by
changing the phase-mismatch parameter. Traditional theory explains this as
phase matching between a sideband in the broadband pump to its second-harmonic.
However, our experiment is conducted under high input intensities and instead
shows excellent quantitative agreement with a nonlocal theory describing
cascaded quadratic nonlinearities. This theory explains the detuned peak as a
nonlocal resonance that arises due to phase-matching between the pump and a
detuned second-harmonic frequency, but where in contrast to the traditional
theory the pump is assumed dispersion-free. As a soliton is inherently
dispersion-free, the agreement between our experiment and the nonlocal theory
indirectly proves that we have observed a soliton-induced nonlocal resonance.
The soliton exists in the self-defocusing regime of the cascaded nonlinear
interaction and in the normal dispersion regime of the crystal, and needs high
input intensities to become excited.
