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.
Harmonic generation for  (5HG) or more is theoretically possible, but it requires an immense number of photons to interact with each other, leading to a very low likelihood of success. As a result, the signal at higher harmonics will be extremely weak and demands incredibly high-powered lasers.