Detrimental Fluctuation of Frequency Spacing Between the Two High-Quality Resonant Modes in a Raman Silicon Nanocavity Laser

To understand how we can maximize the fabrication yield, we study the fluctuations of the cavity parameters of an ultralow-threshold Raman Si laser that employs two high-quality resonant modes of a photonic crystal heterostructure nanocavity. We measure the quality (Q) factors, the resonant wavelengths (λ), and the frequency spacing between the resonant modes (Δf) for 40 nanocavity-based Raman Si lasers with the same design. The Q, λ, and Δf fluctuate due to random variations of the cavity geometry and we find that 12 out of 40 devices provide laser emission. While the fluctuations of Q and λ in these 12 cavities are basically the same as those in the other 28 cavities, the detuning between Δf and the Raman shift of Si for the 12 cavities is less than 0.024 THz, which is strictly smaller than the detuning for the other 28 cavities. A large fluctuation of Δf is, thus, detrimental for the yield of Raman Si lasers. Numerical simulations reveal that a decrease in the random variations of the air hole positions and radii is important for high yields.