Porous silicon grains in SiO 2 matrix: Ultrafast photoluminescence and optical gain q K. Luterova ´ a, * , K. Dohnalova ´ a , F. Troja ´nek b , K. Neudert b , P. Gilliot c , B. Honerlage c , P. Maly ´ b , I. Pelant a a Department of Thin Films, Institute of Physics, Academy of Sciences, Cukrovarnicka ´ 10, CZ-162 53 Prague 6, Czech Republic b Department of Chemical Physics and Optics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 121 16 Prague 2, Czech Republic c Groupe d’Optique Non Line ´aire et d’Optoe ´lectronique, IPCMS, Unite ´ mixte CNRS – ULP (UMR 7504), 23, rue du Loess – BP 43, 67034 Strasbourg cedex 2, France Received 19 March 2006 Available online 27 June 2006 Abstract We search for the presence of stimulated emission in samples of porous silicon embedded in the sol–gel derived SiO 2 matrix. By mod- ifying the etching conditions of the porous silicon using hydrogen peroxide, we decrease substantially the nanocrystal size and produce a significant blue shift of the PL emission. Femtosecond variable-stripe length experiments combined with the shifting-excited spot tech- nique demonstrates positive optical gain (modal gain 25 cm 1 ) in the range 550–730 nm. Ultrafast photoluminescence dynamics indi- cates the origin of the stimulated emission as possibly due to recombination of excitonic states inside silicon nanocrystals. Ó 2006 Elsevier B.V. All rights reserved. PACS: 78.45.+h; 78.55.Mb; 81.05.Ys Keywords: Nanocrystals; Optical properties; Lasers; Luminescence; Non-linear optics; Ultrafast processes and measurements; Upconversion; Time resolved measurements 1. Introduction A successful demonstration of the silicon-based laser is nowadays one of the main challenges in the silicon micro- and optoelectronics technology. Due to the indirect band gap of bulk crystalline silicon and therefore very low light emission efficiency, a prospective way to produce an effi- cient silicon light source lies in the silicon nanotechnology. Each laser comprises three main components: an active material where population inversion and light amplifica- tion by stimulated emission can be achieved, a suitable pumping, most simply optical yet for applications prefera- bly electrical injection in a p–n junction, and an optical resonator which provides the optical feed-back to sustain the laser action. Stimulated emission (optical gain) from nanocrystalline silicon in the visible has become a hot topic during past years. The interest in silicon lasing was triggered in 2000, when the group of Professor Pavesi from the University of Trento announced observation of the optical gain in an ensemble of silicon nanocrystals [1]. In spite of many experimental problems in detecting low values of the gain coefficient (i.e. negative absorption), with high confidence the positive optical gain in silicon nanomaterials was independently demonstrated in several other laboratories since then [2–9]. Quite recently, moreover, a successful 0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.04.021 q This paper was presented at the 21st International Conference on Amorphous and Nanocrystalline Semiconductors, Lisbon, Portugal, 4–9 September 2005. * Corresponding author. Tel.: +420 220 318 569; fax: +420 220 318 468. E-mail address: luterova@fzu.cz (K. Luterova ´). www.elsevier.com/locate/jnoncrysol Journal of Non-Crystalline Solids 352 (2006) 3041–3046