This study systematically investigates how trunk length and branch connectivity govern surface plasmon resonances in silver nanodendrites in the infrared (IR) region using a computational modeling strategy. We show that a continuous conductive trunk is essential for exciting long-wavelength collective plasmon modes. In a simulated bottom-up construction scheme, the trunk length is gradually increased to conductively connect additional branches to the backbone. Our results reveal that the fundamental \(\:\delta\:\) mode resonance can be deterministically tuned across the mid-infrared spectrum (from 3840 nm to 4360 nm) primarily by controlling the trunk connectivity. As the number of connected branches grows, the lowest-order collective resonance peak exhibits a systematic redshift, and its resonance wavelength scales linearly with the effective dipole length \(\:{\mathrm L}_{\text{e}\text{f}\text{f}}\) of the electron oscillation path. Concurrently, new higher-order modes emerge as local resonances of the connected substructures. These observations indicate that interrupting the conductive pathway causes a global collective mode to decompose into multiple resonances associated with more weakly coupled subsystems. The established linear scaling relationship provides a highly predictable design rule for this “programmable” connectivity, offering a robust platform for advanced applications such as multi-spectral infrared imaging, selective chemical sensing, and surface-enhanced infrared absorption (SEIRA) spectroscopy, where precise, a priori control over narrow-band infrared resonances is essential.
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