Far-infrared transparent conductors.

The long-standing challenge in designing far-infrared transparent conductors (FIRTC) is the combination of high plasma absorption edge (λ p ) and high conductivity (σ). These competing requirements are commonly met by tuning carrier concentration or/and effective carrier mass in a metal oxide/oxonate with low optical dielectric constant (ε opt  = 2-7). However, despite the high σ, the transparent band is limited to mid-infrared (λ p  < 5 μm). In this paper, we break the trade-off between high σ and λ p by increasing the "so-called constant" ε opt that has been neglected, and successfully develop the material family of FIRTC with ε opt  > 15 and λ p  > 15 μm. These FIRTC crystals are mainly octahedrally-coordinated heavy-metal chalcogenides and their solid solutions with shallow-level defects. Their high ε opt relies on the formation of electron-deficiency multicenter bonds resulting in the great electron-polarization effect. The new FIRTC enables us to develop the first "continuous film" type far-infrared electromagnetic shielder that is unattainable using traditional materials. Therefore, this study may inaugurate a new era in far-infrared optoelectronics.