Third-order nonlinear optical susceptibility and photoluminescence in porous silicon

Third-order nonlinear-optical-susceptibility spectra ‖${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$(-3ω;ω,ω,ω)‖ have been obtained for free-standing porous silicon films by third-harmonic-generation (THG) measurements in a third-harmonic photon energy region from 540 to 720 nm, which covers the visible photoluminescence (PL) wavelength region. In the THG wavelength region between 540 and 720 nm, the ‖${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$‖ spectrum is not sensitive to the excitation laser wavelength and shows about 0.5×${10}^{\mathrm{-}12}$ esu. This suggests that there is no excitonic resonance enhancement of the ‖${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$‖ spectrum in the PL wavelength region. The optical absorption and luminescence properties of porous silicon are discussed.