Wavelength-dependent nonlinear absorption of gold nanobipyramids with large saturable modulation depth

•The process and condition of SA and RSA in gold nanobipyramids were analyzed.•The quantitative energy-level structure of gold nanobipyramids explains the influence of wavelength and incident intensity on the nonlinear absorption.•Gold nanobipyramids exhibit a significantly higher modulation depth up to 47%.•Gold nanobipyramids are applied to pulse width compression of nanosecond laser pulses at multi-wavelengths.