Surface metallization from ab initio theory and subwavelength coupling via surface plasmon polaritons in Cu-doped lithium niobate/tantalate owing to charge accumulation

Abstract Surface metallization in Z-cut Cu-doped lithium niobate and its isomorphous lithium tantalate slabs was achieved by exhibiting metal optical properties in the visible regime using ab initio calculation. The giant opposite surface potentials were measured with sKPFM, indicating large amount of surface charges accumulated. The metallization was firstly evidenced from reflectivity enhancement under single laser beam irradiation on a Cu-doped LT slab, which is compared with the calculated reflectivity from a metallic film, seemingly amplified by energy coupling from visible surface plasmon polaritons supported by the metallic film. Since the very first reflection (VFR) is dictated by what happens in the subwavelength range, it leads us a direct way to monitoring SPPs excitation in the metallic layer. The VFR varies from 17% to 86% under two counterpropagating beams illumination, pointing clearly to a subwavelength energy coupling via SPPs. In addition, several intriguing phenomena were observed and aligned well with the SPPs picture. This work should be universal to all n-type ferroelectric oxides with photorefractive and photovoltaic effects, unveiling a missing important aspect in the past and opening an avenue towards designing novel photonic circuits, devices and active plasmonics applications in a truly nanometric scale.