  Saturable and reverse saturable absorption at longitudinal surface plasmon
resonance (SPR) in gold nanorods (Au NRs) 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.
