Observation of saturable and reverse-saturable absorption at longitudinal surface plasmon resonance in gold nanorods

Saturable and reverse saturable absorption at longitudinal surface plasmon resonance (SPR) in gold nanorods have been observed using $Z$ scan and transient absorption techniques with femtosecond laser pulses. At lower excitation irradiances, the wavelength dispersion of saturable absorption has been determined near the longitudinal mode of SPR with a recovery time determined to be a few ten picoseconds on the SPR resonance. With higher excitation irradiances, reverse saturable absorption occurs and becomes dominant. The underlying mechanisms are discussed. Such reversible saturable absorption makes Au NRs an ideal candidate for optical limiting applications.