Harmonic generation for  (5HG) or more is theoretically possible, but the interaction requires a very high number of photons to interact and has therefore a low probability to happen: the signal at higher harmonics will be very low, and requires very intense lasers to be generated.
Theoretically, harmonic generation for  (5HG) or more is feasible, but it requires an enormous number of photons to interact with each other, resulting in a very low probability of occurrence. This makes the signal at higher harmonics virtually undetectable and necessitates extremely powerful lasers.