Luminescence properties of two-photon excited silicon nanocrystals

Visible photoluminescence (PL) from silicon nanocrystals (NCs) in porous silicon (PSi) at two-photon excitation (two-photon absorption) has been studied. For resonant excitation at low temperatures the PL response near to the excitation energy differs significantly from that observed at one-photon excitation. Contrary to one-photon excitation no spectral gap between the excitation energy and the onset of the two-photos excited PL is observed. This is explained in the framework of selection rules for dipole allowed and forbidden optical transitions in silicon NCs. At room temperature one- and two-photon excitation results in a similar PL spectra. However; the degree of linear polarization (p) is significantly larger for the later one. This enhancement of p is a consequence of the dielectric nanostructure of PSi and the excitation of ellipsoidal NCs with linearly polarized light by a higher-order, nonlinear, process. (C) 2001 Elsevier Science B.V. All rights reserved.