Weakly Tapered Silicon Nanopillar Resonators with Spatially Well-Separated Whispering Gallery Modes for Si-Based Lasers

A silicon-based laser is a critical component in the realization of full silicon (Si) photonic integrated circuits due to advantages in cost-competitive integration with the mature digital technology. The poor optical gain stemming from the indirect energy band structure of Si, however, has been a hurdle to realize this goal. An efficient Si nanocavity can become a key enabler to overcome the hurdle by earning the interaction time for ephemeral photons. Here, we propose a weakly tapered Si nanopillar whispering-gallery-mode resonator as a wavelength-scale Si cavity platform, which is suitable for lasing. We observe spatially well-separated and nondegenerate modes in the wide near-infrared spectral regime while maintaining a high quality factor. Mode analysis via experiments and simulations confirms spectrally and spatially well-separated resonant modes, suggesting that the diameter and slope of the Si nanopillar are crucial determinants for the formation and control of whispering gallery modes. Considering the versatile spectral tunability and selectivity with the high reproducibility, the proposed nanopillars can be a pathway that leads to compact Si-based lasers.