Second-harmonic intensity autocorrelation ($C(\tau )\propto \int I(t)I(t-\tau )dt$)

Overlap delayed replicas in a sufficiently thin phase-matched second-order crystal and collect their cross-generated sum-frequency signal. Its integrated energy gives the displayed autocorrelation under undepleted, broadband-response assumptions. Temporal resolution depends on gate overlap, group-velocity mismatch and spectral acceptance. Duration inference requires a pulse-shape assumption, and arbitrary temporal phase is not recovered.