Highly efficient fourth harmonic generation of broadband laser pulses retaining 3D pulse shape

A method for highly efficient fourth harmonic generation (FHG) retaining spatiotemporal intensity distribution of broadband chirped laser pulses is proposed. It is based on the use of pulses with linear frequency chirps of equal absolute magnitudes and opposite signs at the oo-e type noncollinear second harmonic generation. Fourth harmonic generation can by obtained by the classical method, since the second harmonic pulse is narrowband and spectrally limited. A possibility to retain the three-dimensional (3D) structure of the field at FHG with a conversion efficiency ∼ 60 % is demonstrated by the numerical simulation of 3D pulses with a quasi-ellipsoidal intensity distribution.