Abstract
This review highlights the development and diverse applications of surface-emitting ring quantum cascade lasers. These lasers, featuring a ring-shaped cavity and a second-order distributed feedback grating, enable controlled surface emission, stable single-mode operation, and collimated, circularly symmetric beams. These advancements have expanded their use in scientific and industrial applications, with significant progress in extending their operation from the mid-infrared to the terahertz regime. Innovations such as microring designs, phase-shifted gratings and buried heterostructures have enhanced their scalability, beam quality, and suitability for continuous-wave operation at room temperature, unlocking opportunities in spectroscopy, imaging, and communication. On-chip optical solutions and integrated architectures combining emission and detection within a single chip further pave the way for compact and efficient systems for remote sensing and gas monitoring. Progress in miniaturization, thermal stability, and wavelength consistency strengthens the role of ring-based lasers in industrial, environmental, and medical diagnostics. These advancements, alongside scalable fabrication techniques and enhanced modulation schemes, position ring quantum cascade lasers as key enablers for the next generation of robust, energy-efficient sensors tailored to diverse scientific and industrial needs.