Focal spot of femtosecond laser pulse under tight focusing condition

摘要

A tight focusing scheme using a low f-number focusing optic is frequently considered as an effort to efficiently increase a peak intensity of a high power laser. In this paper, we present a method for describing the focal spot of a femtosecond laser pulse which is formed in the spatio-temporal region under low f-number (f-number ≤ 1) focusing conditions. In the method, transverse and longitudinal electromagnetic (EM) fields for a monochromatic wave are calculated in the focal plane and its vicinity, and then, in order to precisely describe the femtosecond focal spot in the spatio-temporal domain, the calculated monochromatic EM fields are coherently superposed with a given amount of spectral bandwidth and phase. The accuracy and validity of the method are tested and compared to results obtained with Fourier transform method under high f-number conditions. The single electron trajectory under a strong longitudinal field formed by a low f-number optic is presented to emphasize the importance of the tight focusing scheme.