Symmetry-resolved separation of topological surface and bulk conductivity in Bi2Se3 using a hybrid plasmonic racetrack resonator

Characterizing the intrinsic electrodynamics of topological insulator (TI) surface states remains a persistent challenge for conventional optical techniques, owing to the overwhelming isotropic background from the bulk. Here, we report a high-sensitivity, angle-resolved on-chip methodology for resolving and quantifying the surface-state sheet conductivity of Bi2Se3 thin films at visible wavelengths. By integrating a Bi2Se3 overlayer onto an Au–SiO2 hybrid plasmonic racetrack resonator (PRR), we realize a mode-selective probing mechanism in which surface plasmon polariton modes (Q = 4.494 × 103) provide intense interfacial field enhancement, while optically dominated modes serve as quasi-isotropic references. Loading the PRR with Bi2Se3 films of varying thickness (30, 70, and 120 nm) produces a pronounced, thickness-dependent redshift and extinction enhancement that appears exclusively in the TM-polarized plasmonic resonances. By systematically rotating the resonator orientation relative to the fixed TI crystallographic axes, we isolate a reproducible threefold (120°) harmonic modulation in the resonance shift, a direct optical fingerprint of the hexagonally warped Dirac cone. Applying a variational Kramers–Kronig constrained inversion, we decompose the total effective sheet conductivity perturbation Δσtot(ω, θ) into an angle-independent bulk background ΔσBulk(ω) and a symmetry-locked surface contribution ΔσSurface(ω, θ). The anisotropic surface response is primarily encoded in the absorptive (imaginary) component of the complex refractive index, with the plasmonic mode exhibiting an anisotropy ratio of 6.40% at 30 nm thickness, approximately five times that of non-plasmonic modes (1.28%). These results establish the hybrid PRR platform as a robust, contact-free route for symmetry-resolved extraction of topological surface admittance.