Enhanced stimulated Raman scattering in silicon nanocrystals embedded in silicon-rich nitride/silicon superlattice structure

In the last few years several strategies have been developed to engineer efficient light sources and amplifiers in Si-based materials, with the aim to demonstrate a convenient path to monolithic integration of optical and electronic devices within the mainstream Si technology In particular, light amplification by Stimulated Raman Scattering (SRS) in silicon waveguides has been recently demonstrated despite intrinsic limitations related to the nature of the bulk Si materials have been pointed out. The narrow-band (105 GHz) of stimulated Raman gain in Si limits its applicability in the context of Si photonics, and makes it unsuitable for its use in broad band division multiplexing (WDM) applications, unless expensive multi-pump schemes are implemented. Additionally, Raman amplification in Si is a small effect. Therefore, in order to build a laser based on stimulated Raman effects in Si, very high power intensity and very low absorption losses are required. Finally, Raman gain in Si is further reduced by the competing nonlinear effect of two-photon absorption. This effect generates electron-hole pairs, which remain excited in the sample for a long time (micro to milliseconds) and lead to strong absorption at both the pump and signal frequencies In our previous papers [1,2], we pointed out some of the intrinsic advantages of the use of Si nanostructures for Raman gain with respect to bulk Si. In particular, we pointed out the broadening of spontaneous Raman scattering and the tuning of the Stokes shift. Furthermore, we speculated that the general trade-off between gain and bandwidth, which limits the amplification in bulk solids, could be overcome in low dimensional material structures [2]. In this paper, we experimentally demonstrate a significant enhancement of the gain coefficient in Raman amplifier using nanostructured silicon clusters embedded in Si-rich nitride/Silicon superlattice structures (SRN/Si-SLs). The measured gain enhancement is approximately a factor four larger than the value (0.076 cm/MW) reported for bulk silicon. Our previous results on the broadening of the Raman gain spectra [1,2] combined with the present observation of enhanced Raman gain lead us to conclude that the traditional trade-off between gain and bandwidth is overcome in low dimensional materials[3].