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.

中文翻译：

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基于纳米腔的拉曼硅激光器中光增益的强烈非对称波长依赖性

尽管使用带间跃迁实现硅激光器在技术上仍然存在问题，但利用拉曼散射工艺似乎是最可行的选择。基于光子晶体纳米腔的拉曼硅激光器可提供亚微瓦级阈值和CMOS兼容性。因此，这种类型的激光器适用于Si光子电路中的密集集成。然而，由于缺乏合适的表征技术，对于提高激光性能至关重要的增益机制的细节很少被讨论。在这里，我们报告高质量的基于纳米腔的拉曼硅激光器中光学增益的激发波长依赖性。为此，我们采用了所谓的受激拉曼散射激发（SRE）光谱，这使我们能够揭示使激光工作的激发波长范围，最大输出的激发条件，增益峰值的偏移以及包括非线性光学损耗在内的拉曼增益的增强。特别地，我们发现随着激发功率的增加，激光输出在腔共振的长波长区域显着降低。数值模拟表明，由双光子吸收引起的自由载流子吸收引起的光损耗基本上增长到某个阈值以上。