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 the perturbative (weak field) regime is characterized by a rapid decline in efficiency as the harmonic order increases. This phenomenon can be explained by imagining an atom absorbing n photons and emitting one high-energy photon. The likelihood of absorbing n photons diminishes as n grows, which accounts for the swift reduction in initial harmonic intensities.