= Second-harmonic intensity autocorrelation
{title2=$C(\tau)\propto\int I(t)I(t-\tau)\,dt$}
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.
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