Abstract Quantum cascade lasers are highly desirable for chemical, physical, and biological research scenarios. Among the applications, mid-infrared frequency combs based on quantum cascade lasers have sparked increasing interest due to their unique advantages in small footprint, high power, and flexible designability. Despite significant performance improvement over a decade, the lasing spectrum bandwidths of the quantum cascade laser frequency combs are still limited to ~100 cm −1 , limiting their applications in multi-gas spectroscopy and posing severe challenges in tracking their carrier-envelope offset frequency. To achieve a broad-gain spectrum, heterogeneous active regions consisting of multiple stacks of different wavelengths have been implemented. For example, stacking active regions of four different wavelengths results in a full width at half maximum of approximately 0.92 μm (110 cm −1 ) at 290 K, and of ~2.7 μm (360 cm −1 ) at 80 K. However, as more stages are stacked, ensuring a homogeneous and flat gain profile from both design and growth perspectives becomes very challenging. In this work, we present a demonstration of ultra-broadband quantum cascade lasers with a diagonal multi-state-to-continuum active region design. The proposed active region design exhibits a surprisingly wide electroluminescence with a full width at half maximum of ~600 cm −1 at 298 K. Devices, with a total peak output power of 2.72 W and a slope efficiency of 1.3 W/A, have shown a lasing spectrum of ~1 μm over 43% of the current dynamic range, with a maximum bandwidth of 1.2 μm around the rollover current. Moreover, a much broader lasing bandwidth of 1.93 μm is obtained from the same device at 80 K, accounting for 22% of the center wavelength. This work represents substantial progress on the single-stack ultra-broadband mid-infrared semiconductor lasers and may provide a novel platform for mid-infrared frequency combs, which are of paramount importance to broadband high-precision spectroscopy, imaging, and free-space communication systems.
Liu et al. (Fri,) studied this question.