Tunable topological interface states in a one-dimensional photonic crystal heterostructure with metallic nanoparticle inclusions.

We investigate the optical properties of a one-dimensional topological photonic crystal heterostructure in which one constituent layer is doped with silver nanoparticles. The structure comprises two photonic crystals with identical photonic band gaps but distinct topological characteristics, enabling the formation of localized interface states. Using the transfer matrix method and effective-medium theory, we analyze the effects of metallic (silver-nanoparticle) inclusions, on the photonic band structure, reflection phase, transmission spectrum, and absorption. The results show that topologically protected interface states emerge within photonic band gaps exhibiting a topological mismatch and persist despite metallic losses. Metallic inclusions provide a tunable mechanism for controlling the resonant wavelength, linewidth, and transmission of these states. Increasing the filling fraction induces a redshift and resonance broadening due to an enhanced effective refractive index and increased absorption, while variations in nanoparticle size primarily affect damping and transmission efficiency. Despite these changes, the spatial localization profile of the interface state remains robust, confirming its topological nature. The angular response exhibits strong polarization dependence, with TM states showing greater resilience to loss under oblique incidence. These findings demonstrate the interplay between effective-medium response of metallic inclusions and topological protection and provide practical guidance for designing tunable, robust photonic devices based on hybrid metallic-nanoparticle-topological structures.