The use of fundamental modelocking to generate short terahertz (THz) pulses and THz frequency combs from semiconductor lasers has become a routine affair, using quantum cascade lasers (QCLs) as a gain medium. However, unlike classic laser diodes, no demonstrations of harmonic modelocking, active or passive, have been shown in THz QCLs, where multiple pulses per round trip are generated when the laser is modulated at the harmonics of the cavity’s fundamental round-trip frequency. Here, using time-resolved THz techniques, we show for the first time harmonic injection and mode-locking in which THz QCLs are modulated at the harmonics of the round-trip frequency. We demonstrate the generation of the harmonic electrical beatnote within a QCL, its injection locking to an active modulation and its direct translation to harmonic pulse generation using the unique ultrafast nature of our approach. Finally, we show indications of self-starting harmonic emission, i.e., without external modulation, where the QCL operates exclusively on a harmonic (up to its 15th harmonic) of the round-trip frequency. This behaviour is supported by time-resolved simulations of induced gain and loss in the system and shows the importance of the electronic, as well as photonic, nature of QCLs. These results open up the prospect of passive harmonic modelocking and THz pulse generation, as well as the generation of low-noise microwave generation in the hundreds of GHz region.

使用量子级联激光器（QCL）作为增益介质，使用基波模型ockock来从半导体激光器产生短太赫兹（THz）脉冲和THz频率梳已经成为日常事务。但是，与传统的激光二极管不同，在THz QCL中没有显示有源或无源谐波锁模的演示，当在腔体基本往返频率的谐波上调制激光器时，每次往返都会产生多个脉冲。在这里，我们使用时间分辨的THz技术，首次展示了谐波注入和锁模，其中THz QCL在往返频率的谐波处进行调制。我们演示了QCL中谐波电子节拍的产生，使用我们的方法独特的超快特性，它可以将注入锁定为有效调制，并直接转换为谐波脉冲生成。最后，我们显示了自启动谐波发射的指示，即，没有外部调制，其中QCL仅在往返频率的谐波（最高15次谐波）上运行。这种行为得到了系统中感应增益和损耗的时间分辨仿真的支持，并显示了QCL的电子性质和光子性质的重要性。这些结果开辟了无源谐波震荡和太赫兹脉冲产生以及在数百GHz区域产生低噪声微波产生的前景。QCL仅在往返频率的谐波（最高15次谐波）上运行。这种行为得到了系统中感应增益和损耗的时间分辨仿真的支持，并显示了QCL的电子性质和光子性质的重要性。这些结果开辟了无源谐波震荡和太赫兹脉冲产生以及在数百GHz区域产生低噪声微波产生的前景。QCL仅在往返频率的谐波（最高15次谐波）上运行。这种行为得到了系统中感应增益和损耗的时间分辨仿真的支持，并显示了QCL的电子性质和光子性质的重要性。这些结果开辟了无源谐波对接和太赫兹脉冲产生以及在数百GHz区域产生低噪声微波产生的前景。