Ion‐Water Interaction Strength Governs Terahertz Dielectric Response in Aqueous Systems

ABSTRACT Developing transparent terahertz (THz) absorbers that simultaneously achieve strong electromagnetic attenuation and high optical transmittance remains challenging. Despite the pronounced THz dielectric responses in aqueous ionic systems, the structural origins of energy dissipation and their quantitative link to ion‐water interactions remain insufficiently resolved. This study systematically quantifies ion‐water interaction strength in various ionic solutions and correlates it with water‐state distribution and THz dielectric dissipation. Our findings indicate that weaker ion‐water interactions are associated with enhanced conductive and dissipation pathways. Guided by this correlation, we designed a polydimethylsiloxane (PDMS)‐encapsulated CsCl hydrogel with a thickness of only 0.14 mm, which achieves an average reflection loss of 52.24 dB and an effective absorption bandwidth (EAB) covering 0.3–1.2 THz, while maintaining over 90% optical transmittance and high mechanical deformability. This work provides a structure‐informed strategy for the rational design of transparent THz absorbing materials.