Ultrafast optical nonlinearity in nanostructured selenium allotropes

In this letter we investigate nonlinear optical properties of amorphous selenium (a-Se) nanoparticles and trigonal selenium (t-Se) nanowires using the open aperture Z-scan, employing 100 fs and 5 ns laser pulses, at 800 nm and 532 nm, respectively. In the former case (ultrafast excitation) optical nonlinearity arises largely from absorption saturation and three-photon absorption, whereas in the latter (short-pulse excitation) it occurs mostly due to excited state absorption. Nonlinearity parameters for both regimes have been numerically evaluated from experimental data. Both allotropes are found to be efficient optical limiters, particularly for short-pulse excitation, with potential applications in laser safety devices.