Universal saturation behavior in the transient optical response of plasmonic structures

We perform ultrafast pump-probe spectroscopy of plasmonic structures beyond the perturbative excitation regime. As a prototypical system we choose gold nanorods dispersed in water, which display both longitudinal and transverse plasmon resonances. A broadband investigation reveals a complex scenario for the saturation of the transient optical response as a function of the pump fluence, with strong dependence on the probe wavelength. In particular, we observed stronger saturation effects for the high energy transverse plasmonic resonance as compared to the low energy longitudinal one. This behavior is well captured by a three-temperature model and is understood in terms of the nonlinearity of the Fermi-smearing mechanism, presiding over the optical nonlinearity of noble metal nanomaterials. Our results highlight a universal saturation dynamics in the transient optical response of plasmon-enhanced photonic structures excited by intense light beams, with potential impact on many applications, from the all-optical modulation of light to photovoltaics and photocatalysis.