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. The light–matter interaction in noble metal nanoparticles is intrinsically determined by the surface plasmon resonance (SPR) and bandgap. To determine the process of electronic transition and the limitation of energy bands on nonlinearity at different excitation wavelengths, we investigate the broadband nanosecond nonlinear absorption of gold nanobipyramids from 532 nm to 850 nm. The experimental results indicate that with the increase of the incident light intensity, gold nanobipyramids have the characteristic of transforming from saturable absorption (SA) to reverse saturable absorption (RSA) at 532 nm to 690 nm, and exhibit a single SA after 690 nm until 850 nm. Based on the wavelength-dependent response, the SA is attributed to the intraband transition of sp -band, whereas sequential two-photon absorption leads to RSA. Unlike other metal nanoparticles, gold nanobipyramids exhibited a significantly higher modulation depth up to 47%. Lastly, building on the excellent SA characteristic, we achieve pulse width compression of nanosecond laser pulses at multi-wavelengths.