Dynamic tuning of ENZ wavelength in conductive polymer films via polaron excitation

Traditional metal and n-type doped semiconductor materials serve as emerging epsilon-near-zero (ENZ) materials, showcasing great potential for nonlinear photonic applications. However, a significant limitation for such materials is the lack of versatile ENZ wavelength tuning, and thus dynamic tuning of the ENZ wavelength remains a technical challenge, thereby restricting their potential applications, such as multi-band communications. Here, dynamic tuning of the ENZ wavelength in p-type organic PEDOT: PSS films is achieved through a reversible change in hole concentrations originating from the polaron formation/decoupling following optical excitation, and a tunable ENZ wavelength shift up to 150 nm is observed. Experimental investigations about ultrafast dynamics of polaron excitation reveal a ∼80 fs time constant for polaron buildup and a ∼280 fs time constant for polaron decoupling, indicating the reversible ultrafast switching for the ENZ wavelength within subpicosecond time scale. These findings suggest that p-type organic semiconductors can serve as what is believed to be a novel platform for dynamically tuning the ENZ wavelength through polaron excitation, opening what we feel are new possibilities for ENZ-based nonlinear optical applications in flexible optoelectronics.