Composition Control of Plasmon-Phonon Interaction Using Topological Quantum-Phase Transition in Photoexcited (Bi_1-xIn_x)₂Se₃

Plasmonics is a technology aiming at light modulation via collective charge oscillations. Topological insulators, where Dirac-like metallic surfaces coexist with normal insulating bulk, have recently attracted great attention in plasmonics due to their topology-originated outstanding properties. Here, we introduce a new methodology for controlling the interaction of a plasmon with a phonon in topological insulators, which is a key for utilizing the unique spectral profiles for photonic applications. By using both static and ultrafast terahertz spectroscopy, we show that the interaction can be tuned by controlling the chemical composition of (Bi_1-xIn_x)₂Se₃ microribbon arrays. The topological quantum-phase transition induced by varying the composition drives a dramatic change in the strength of the plasmon-phonon interaction. This was possible due to the availability of manipulating the spatial overlap between topological surface plasmonic states and underlying bulk phonons. Especially, we control the laser-induced ultrafast evolution of the transient spectral peaks arising from the plasmon-phonon interaction by varying the spatial overlap across the topological phase transition. This study may provide a new platform for realizing topological insulator-based ultrafast plasmonic devices.