Deep Mid‐Infrared Coherent Lasers: From Engineered Materials to Integrated Systems

Advancements in coherent mid‐infrared light sources are a critical driver of progress across science and technology. While well‐established platforms exist for shorter wavelengths, the region beyond 5 µm is challenging as fundamental material limitations become severe here. This vital spectral window enables transformative applications, including environmental monitoring, non‐invasive medical diagnostics, standoff threat detection, and high‐capacity optical communications. Progress in this domain is inherently limited by the ‘phonon wall’ from rapid multi‐phonon relaxation in dielectrics and by increasing Auger recombination in semiconductors. This review examines the shift toward engineered materials and novel device architectures created to overcome these limitations. It covers direct emission strategies from optically pumped chalcogenide materials to quantum‐engineered cascade lasers for electrical operation and monolithic integration. Indirect generation through nonlinear frequency conversion is also detailed, including tunable parametric oscillators and amplification schemes alongside broadband supercontinuum generation across bulk, fiber, and integrated platforms. The review then demonstrates the pivotal role these sources play in real‐world applications, achieving ppb‐level chemical sensing, high‐speed communication, and metrology‐grade precision spectroscopy. This research shows a clear trend moving from fundamental material discovery to systems engineering, replacing discrete components with fully integrated on‐chip systems to unlock the full potential of mid‐infrared light.