Evolution of spatial modulation of broadband laser pulses with different pulse durations in medium with non-instantaneous Kerr nonlinearity

► Spatial modulation gain spectra affected by pulse length in non-instantaneous media. ► Modulation gain spectra have not cutoff frequencies with short pulse length. ► Growth rate of gain at higher frequency is inverse proportion with pulse length. ► All changes originated from the relaxing character of media's nonlinear response.