Tuning of Surface Plasmon Resonance in Sputtered Al/In Co-Doped ZnO for Telecommunication Wavelength: An Investigation Using Kretschmann Configuration

Doped ZnO thin films with high carrier density of the order of ~ 10²⁰/cm³ and negative real permittivity in near infrared, possessing specific optical property such as surface plasmon resonance (SPR), have been of interest for applications in sensing, optical communication, etc. This work reports the development of plasmonic thin film, prepared by radio frequency sputtering technique using co-doped mixture of Al and In in ZnO. We studied the structural, electrical and optical properties of co-doped ZnO films by varying the co-dopant ratio of Al:In. The chances for Raman active inter-conduction band transitions occurring for heavily doped n type materials were examined using the Fano resonance fitting. The bandgap variation of the films with co-doping was in accordance with the band renormalisation effect. The Drude-Lorentz model was used for the theoretical estimation of wavelength range of real negative permittivity, the same was observed for wavelengths beyond 1350 nm for all co-doped films. The experimental demonstration of surface plasmon resonance via the Kretschmann setup was consistent with theoretical expectations. Moreover, we showed that adjusting the co-dopant ratios of Al:In tune the SPR in co-doped ZnO from 1360 to 1830 nm. Additionally, for Al:In = 100:0 and 75:25, the resonance wavelengths appeared in the crucial telecommunication range, with SPR propagation lengths of 0.357 μm and 0.263 μm, respectively making it a promising plasmonic material for communication. The tunable SPR wavelength of these films provided a novel method for metal-free SPR applications.