ABSTRACT
The mid-infrared (IR) region from about 2.5 to 25 μm wavelengths (or wavenumbers from about 400 to 4000 cm–1) (Zhang 2007) is the main platform for thermal emission from ambient objects and resonance absorption of many molecular species, thus playing a vital role in thermal management, thermal imaging, energy harvesting, chemical and biological sensing, and environmental monitoring. Besides, the atmosphere’s transparent window (8–13 μm) falls in the mid-IR region and is important for radiative cooling and free-space optical communication. Noble metals have a low emissivity in the mid-IR due to the poor impedance mismatch with air, thus precluding the potential applications in efficient thermal management and thermal signature detection. The absolute value of the permittivity being much larger than the dielectrics obscures the application prospects in sensing due to the weak confinement of light and light–matter interaction. Doped silicon and its nanostructures or related metamaterials, as alternative engineered “metals” due to its free carriers, can relieve these problems via the much smaller plasma frequency lying in the mid-IR. Besides, the resonance frequency can also be tuned by chemical doping, as in stark contrast to the fixed plasma frequency of noble metals. Therefore, more flexibility and tunability can be achieved, enabling the manipulation of both the structure design and doping control for various applications.
