Electromagnetic–Thermal Coupling Competition in Ag@TiO2 Core–Shell Nanorods Under Distance-Dependent Interaction

The photothermal response of plasmonic nanomaterials is strongly affected by the electromagnetic and thermal interactions between neighboring particles. In this work, the finite element method was employed to investigate the distance-dependent photothermal behavior of Ag@TiO2 core–shell nanorods under 808 nm laser irradiation. A single-nanorod model was first used to analyze the optical absorption and temperature distribution of an isolated nanorod, and an idealized two-nanorod model was then established to examine the coupled electromagnetic and thermal fields at different inter-particle spacings. The results show that reducing the inter-particle distance induces two competing effects: thermal-field superposition enhances local heat accumulation, whereas strong near-field plasmonic coupling at small separations modifies the electromagnetic field distribution, induces resonance shifts, and reduces the effective absorption cross-section, thereby weakening heat generation. Consequently, the temperature response exhibits a non-monotonic dependence on inter-particle distance, reflecting the competition between thermal-field overlap and plasmonic coupling. This work helps clarify the electromagnetic–thermal coupling mechanism of Ag@TiO2 core–shell nanorods under near-infrared irradiation and provides a theoretical reference for understanding their distance-dependent photothermal response.