Optical gain and stimulated emission in silicon nanocrystals
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Optical gain and stimulated emission in silicon nanocrystals L. Dal Negroa, M. Cazzanellia, Z.Gaburroa, P.Bettottia, L. Pavesia D.Pacificib, G. Franzòb, F. Priolob and F. Iaconac a INFM-Dipartimento di Fisica, via Sommarie 14, Università I-38050 Povo (Trento), Italy b INFM-Dipartimento di Fisica, Università di Catania, I-95129 Catania c CNR-IMM, Sezione di Catania, I-95121 Catania, Italy
di
Trento,
ABSTRACT Time-resolved luminescence measurements on silicon nanocrystal waveguides have revealed a fast recombination dynamics, related to population inversion which leads to net optical gain. The waveguide samples were obtained by thermal annealing of plasma enhanced chemical vapour deposited thin layers of silicon rich oxide Variable stripe length measurements performed on the fast emission signal have shown an exponential growth of the amplified spontaneous emission, with net gain values of about 10 cm-1. Both the fast component intensity and its temporal width revealed threshold behaviour as a function of the incident pump intensity. A modelling of the decay dynamics is suggested within an effective four level rate equation treatment including the delicate interplay among stimulated emission and Auger recombinations.
INTRODUCTION One goal for silicon microphotonics consists in the demonstration of a laser device based on silicon [1,2]. Following the initial observations of optical gain in highly packed silicon nanocrystals (Si-nc) prepared by ion-implantation [3], other works have recently demonstrated the possibility of stimulated emission in Si-nc [4-6]. Nanosecond gain dynamics [4] and evidences for speckle patterns [5] in the spatially coherent emission have been recently reported. As in other quantum dot based systems [7,8], a severe competition with efficient non-radiative processes, mainly Auger type, is present in Si-nc, which causes very fast dynamics in the optical gain. Although a clear understanding of the microscopic gain mechanism is still under debate, it has been realised that interface radiative states associated with oxygen atoms can play a crucial role in determining the emission properties of the Si-nc systems [9,10]. Here we report on light amplification dynamic studies in Si-nc and we propose a simple phenomenological model to explain our experimental results. TIME RESOLVED VARIABLE STRIPE LENGTH RESULTS We studied Si-nc samples produced by high temperature annealing of substoichiometric silicon oxide (SiOx) thin films grown by plasma enhanced chemical vapour deposition (PE-CVD) on a quartz substrate. The structural and luminescence properties of these systems have been fully discussed elsewhere [11]. Here we focus on a representative sample produced with a total Si content of 42 at.% and with an annealing temperature of 1250 °C for one hour which yields closely packed Si-nc with a mean diameter of 1.7 nm. The Si-nc rich layer (250 nm thick) was embedded between two 100 nm thick SiO2 layers to form an optical waveguide. Considering the
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measured refractive index of 1.82, the
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