Ring resonators are an interesting alternative cavity solution to the commonly used ridge-type waveguide for terahertz (THz) quantum cascade lasers. They either support a standing-wave pattern showing spatial hole burning if there are defects implemented or a traveling mode in a defect-free cavity. Here, we report on ring-shaped THz quantum cascade lasers emitting between 3.2 and 4.1 THz operating in four different emission regimes. The presence of defects in the cavities force the THz quantum cascade laser into a standing-wave pattern. The measurements show a complex behavior highlighting the effect of strong confinement and the optical nonlinearities leading to the generation of a harmonic state, as well as to a fundamental comb, exhibiting over 30 equidistant modes and covering a bandwidth of 622 GHz. The results are explained by numerical calculations based on the Maxwell–Bloch formalism, including the linewidth enhancement factor and reflection points. The compact geometry and high output power (4 mW detected) make these devices extremely appealing for on-chip frequency comb applications in the terahertz region.

中文翻译：

---

太赫兹量子级联环形激光器中的梳状操作

环形谐振器是用于太赫兹 (THz) 量子级联激光器的常用脊型波导的一种有趣的替代腔解决方案。如果存在缺陷，它们要么支持显示空间烧孔的驻波模式，要么支持无缺陷腔中的行进模式。在这里，我们报告了在 3.2 到 4.1 THz 之间发射的环形太赫兹量子级联激光器在四种不同的发射方式下运行。空腔中缺陷的存在迫使太赫兹量子级联激光器进入驻波模式。测量结果显示了一种复杂的行为，突出了强约束和光学非线性的影响，导致谐波状态的产生，以及基本梳，表现出超过 30 种等距模式并覆盖 622 GHz 的带宽。结果由基于 Maxwell-Bloch 形式主义的数值计算解释，包括线宽增强因子和反射点。紧凑的几何形状和高输出功率（检测到 4 mW）使这些器件非常适合太赫兹区域的片上频率梳应用。