Harmonic generation in the perturbative (weak field) regime is characterised by rapidly decreasing efficiency with increasing harmonic order. This behaviour can be understood by considering an atom absorbing n photons then emitting a single high energy photon. The probability of absorbing n photons decreases as n increases, explaining the rapid decrease in the initial harmonic intensities.
Harmonic generation in weak field perturbation exhibits a rapid decline in efficiency with higher harmonic orders. This can be understood by imagining an atom that absorbs multiple photons and emits one high-energy photon. The probability of absorbing more photons decreases as the number increases, resulting in a sharp reduction in initial harmonic intensities.