Three orders of magnitude two-beam-coupling gain increase from Cu-doped (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6 crystal owing to huge depolarization field-induced surface metallization

Metallization at the nanoscale on ferroelectric-oxide surface was reported, stemming from depolarization of the huge self-spontaneous polarization. The striking metallization layer is found responsible for various exotic effects never seen in the bulk. Very encouragingly, by using density functional theory, along with the Thomas–Fermi semi-classic model, we have demonstrated the surface metallization in Cu-doped (K_0.5Na_0.5)_0.2(Sr_0.75Ba_0.25)_0.9Nb₂O₆ (Cu:KNSBN) crystal. The electron density in such a metallization layer is up to that in noble metals, making it possible in supporting visible surface plasmon polaritons. Since the very first specular reflection (VFSR) is dictated by the optical properties within the range of half a wavelength, we used it to probe experimentally the existence of the metallization layer. Indeed, as high as 5 × 10⁴ cm⁻¹ exponential gain coefficients were obtained by monitoring the marked increase and decrease of the VFSR on the ± C-face of a Cu:KNSBN crystal cuboid, which were three orders of magnitude higher than those of the previously reported results. Such high gains are beyond the limits of current theory, including the well-established photorefractive theory, yet can be explained well with phase grating mediated excitation of surface plasmon polaritons. This work expands, both experimentally and theoretically, the general picture regarding surface metallization in ferroelectric oxides. [ABSTRACT FROM AUTHOR]