Mid-Infrared Transparent Materials: from Mechanisms to Cutting-Edge Applications

The mid-infrared (MIR) spectral window, typically spanning wavelengths from 2.5 to 20 μm (or wave numbers 500-4000 cm^-1), constitutes a pivotal domain of the electromagnetic spectrum, where molecular vibrational and rotational transitions enable precise spectroscopic identification and tunable thermal radiation modulation. Mastery over this spectral range underpins a broad and growing suite of technologies, encompassing high-resolution MIR imaging and spectroscopic gas sensing, advanced thermal management via radiative cooling/heating and dynamic emissivity control, integrated photonic platforms featuring low-loss optical windows and waveguides, as well as MIR laser systems that leverage broadband transparency for efficient frequency conversion and beam delivery. High MIR transmittance (T_MIR) is therefore essential for driving MIR photonic innovations, enabling efficient photon transmission, modulation, and targeted heat control. Yet, the fundamental interplay among material structure, photonic/electronic behavior, and MIR optical performance remains underexplored. This review comprehensively evaluates high T_MIR materials, with an emphasis on their optical mechanisms, structural attributes, synthesis routes, and performance benchmarks. By elucidating structure-property relationships and offering design strategies for MIR transparency, this review provides a roadmap for developing high-performance MIR transparent materials for advanced thermal management, infrared optics, and next-generation photonic systems.