Nonlinear absorption in silicon nanocrystals

Abstract

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 βSi-gl for silicon nanocrystals in glycerol (with a volume filling factor f=2×10-4), and the coefficient βSi for pure silicon with a hypothetical volume filling factor f≈1. For the laser radiation wavelength λ=497 nm and the pulse duration τ=0.5 ns, these coefficients are βSi-gl=1.2×10-8 cm W-1 and βSi =7.36×10-5 cm W-1, while the corresponding values for λ=532 nm and τ=10 ns are βSi-gl=5.36×10-5 cm W-1 and βSi =0.25 cm W-1. Experiments with 540-nm, 20-ps laser pulses performed for two different filling factors equal to 2×10-4 and 3×10-3 gave nonlinear absorption coefficients βSi-gl=2×10-7 and 3.6×10-6 cm W-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.