Tunable nonlinear absorption of gold nanostars and application as a saturable absorber

Surface plasmon resonance (SPR) of metal nanostructures can significantly strengthen the light-matter interaction at the nanoscale. We investigate the plasmon characteristics and strong field enhancement near the sharp tips of gold nanostars and demonstrate surface-enhanced nonlinear absorption properties. Gold nanostars show saturable absorption (SA) under laser excitation at 700 nm and 750 nm, while reverse saturable absorption (RSA) is found in the range of 500 nm to 650 nm. The saturation intensity reaches the minimum value (0.4 MW/cm2) when the incident wavelength resonates to the SPR peak. In comparison with other plasmon nanostructures, gold nanostars have extraordinarily higher modulation depth up to 53%, which means that they have outstanding SA characteristic. Based on the theoretical calculation of the compressed Gaussian optical pulse width of gold nanostars, it was successfully applied to compress the pulse width of nanosecond laser pulses.