Freestanding and scalable VO₂/CMC composite film for thermal and optical THz modulation

Despite striking advances in terahertz (THz) sources and detectors, the exploitation of active materials that can alter their properties in response to external excitations for the manipulation of THz waves is another major issue. Here, freestanding and easily scalable composite film based on vanadium dioxide/carboxymethyl cellulose (VO₂/CMC) is proposed to realize thermally and optically tunable THz waves. This VO₂/CMC composite film exhibits broadband modulation with a modulation depth of up to 76%, a response time of about 1.2 s and high stability both in mechanical deformation and in air. Based on the above VO₂/CMC composite film, a THz metamaterial with absorption-reflection switchability is demonstrated. By controlling the temperature-dependent phase transition of VO₂ particles, the reflection dip of the metamaterial at 0.423 THz with an amplitude of -25 dB first shifts to lower frequencies accompanied by an increase in amplitude, and finally disappears with the THz metamaterial reflecting THz waves almost completely. The experimental results agree well with numerical simulations and the modulation mechanism of the metamaterial is also investigated. This work provides a simple and low-cost method for developing VO₂-based THz multifunctional smart devices with promising applications in areas such as but not limited to THz modulators and sensors.