Nonlinear absorption in silicon nanocrystals

The nonlinear absorption of light in silicon nanocrystals suspended in glycerol is studied by the Z-scan method. The experimental data are used for calculating the nonlinear absorption coefficient {beta}{sub Si-gl} for silicon nanocrystals in glycerol (with a volume filling factor f=2x10{sup -4}), and the coefficient {beta}{sub Si} for pure silicon with a hypothetical volume filling factor f{approx}1. For the laser radiation wavelength {lambda}=497 nm and the pulse duration {tau}=0.5 ns, these coefficients are {beta}{sub Si-gl}=1.2x10{sup -8} cm W{sup -1} and {beta}{sub Si} =7.36x10{sup -5} cm W{sup -1}, while the corresponding values for {lambda}=532 nm and {tau}=10 ns are {beta}{sub Si-gl}=5.36x10{sup -5} cm W{sup -1} and {beta}{sub Si} =0.25 cm W{sup -1}. Experiments with 540-nm, 20-ps laser pulses performed for two different filling factors equal to 2x10{sup -4} and 3x10{sup -3} gave nonlinear absorption coefficients {beta}{sub Si-gl}=2x10{sup -7} and 3.6x10{sup -6} cm W{sup -1}, respectively. Optical absorption and Raman scattering spectra of silicon nanocrystals are also studied. A theoretical analysis of the experimental results shows that optical absorption can be related to the localisation of photoexcited carriers in the conduction band. The localisation is caused by the action of strong static electric fields on an electron in a nanoparticle. (nonlinear optical phenomena)