High‐Contrast and Ultra‐Wideband Microwave Modulation Enabled by Electrochemically Mediated Semiconducting‐Enriched Carbon Nanotube Assemblies

Adaptive surfaces that can modulate the propagation of gigahertz (GHz) microwaves are emerging as a technology to address increasingly complex electromagnetic environments. However, achieving high‐contrast and broadband modulation remains challenging due to difficulties in dynamically mediating the bulk properties of GHz interference materials. Here, it is demonstrated that electrochemically mediated, micrometer‐thick assemblies of semiconducting‐enriched carbon nanotubes can achieve transmission modulation of 35 dB across 8–40 GHz, enabling 3‐mode functionality switching among transmission, reflection, and absorption. Using in situ optical and electrical characterizations, it is confirmed that this tunability arises from electrolyte‐mediated charge accumulation throughout the entire volume of carbon nanotube assemblies, which, in turn, significantly alters their complex permittivity. Based on the Drude model and transfer‐matrix method, it is further revealed that non‐localized charge conduction, which governs the imaginary part of the permittivity, is responsible for the modulation depth of the GHz surfaces. In contrast, the dielectric relation of bound charges has minimal impact on the modulation depth. Film conductivity and thickness are interchangeable parameters for tuning the power coefficients. Finally, switching speed and modulation efficacy are improved through device configuration optimization. These electro‐active materials and theoretical insights will support adaptive radar and 5G/6G technologies.