Strongly asymmetric wavelength dependence of optical gain in nanocavity-based Raman silicon lasers

While the realization of silicon lasers using interband transitions is still technically problematic, utilization of Raman scattering processes seems to be the most feasible alternative. Raman silicon lasers based on photonic crystal nanocavities provide sub-microwatt thresholds and CMOS compatibility. Therefore, this type of laser is suitable for dense integration in Si photonic circuits. However, details of the gain mechanism, which are important for improving laser performance, have rarely been discussed due to the lack of a suitable characterization technique. Here, we report on the excitation-wavelength dependence of optical gain in a high-quality nanocavity-based Raman silicon laser. For this, we employ a so-called stimulated-Raman-scattering excitation (SRE) spectroscopy, which allows us to reveal the range of excitation wavelengths enabling laser operation, the excitation condition for maximum output, shift of the gain peak, and enhancement of Raman gain including nonlinear optical losses. In particular, we find that laser output remarkably decreases in the long-wavelength region of cavity resonance as excitation power increases. Numerical simulations suggest that optical loss due to free-carrier absorption induced by two-photon absorption grows substantially above a certain threshold.