Optical gain and stimulated emission in silicon nanocrystals

ABSTRACT Time-resolved luminescence measurements on silicon nanocrystal waveguides have revealeda fast recombination dynamics, related to population inversion which leads to net optical gain.The waveguide samples were obtained by thermal annealing of plasma enhanced chemicalvapour deposited thin layers of silicon rich oxide Variable stripe length measurements performedon the fast emission signal have shown an exponential growth of the amplified spontaneousemission, with net gain values of about 10 cm -1 . Both the fast component intensity and itstemporal width revealed threshold behaviour as a function of the incident pump intensity. Amodelling of the decay dynamics is suggested within an effective four level rate equationtreatment 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 onsilicon [1,2]. Following the initial observations of optical gain in highly packed siliconnanocrystals (Si-nc) prepared by ion-implantation [3], other works have recently demonstrated thepossibility of stimulated emission in Si-nc [4-6]. Nanosecond gain dynamics [4] and evidences forspeckle 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-radiativeprocesses, mainly Auger type, is present in Si-nc, which causes very fast dynamics in the opticalgain. Although a clear understanding of the microscopic gain mechanism is still under debate, ithas been realised that interface radiative states associated with oxygen atoms can play a crucialrole in determining the emission properties of the Si-nc systems [9,10]. Here we report on lightamplification dynamic studies in Si-nc and we propose a simple phenomenological model toexplain our experimental results.