Generation of higher optical harmonics in focused beams

An analysis is made of the influence of focusing on third, fourth, and fifth harmonic generation allowing for both direct and cascade processes taking place due to lower-order nonlinearities. It is found that as the focusing parameter increases under conditions of optimized wave mismatch, the fourth and fifth harmonic powers increase monotonically. It is also shown that in the case of phase-matched cascade processes, the harmonic power is highest for focusing on one of the faces rather than at the center of the nonlinear medium.