Active Control of Terahertz Transmission via Humidity-Responsive Swelling of Submicron Poly(vinyl alcohol)-Coated Nanoresonators

This study demonstrates active control of terahertz (THz) transmission by exploiting the humidity-driven swelling of a submicron poly(vinyl alcohol) (PVA) film integrated with metallic nanoresonators. Conventional THz modulators typically require bulk-like membranes tens of micrometers thick to compensate for weak light-matter interaction, which inevitably results in slow, diffusion-limited response times. In contrast, our approach utilizes field confinement near nanometer gaps to achieve high modulation depth with an ultrathin layer, offering a potential route to high-speed operation. Under 90% relative humidity, the 193 nm PVA film swells to 380 nm, exhibiting a rapid response time of 373 ms─nearly three times faster than 1.25 μm films. The hybrid structure achieves a 13.6% transmission modulation, driven by a localized dielectric transition at electric-field hot-spots. These findings provide a robust strategy for high-performance THz metasurfaces that bypass the traditional trade-off between modulation depth and response speed.