Polarization and angle dependent subwavelength coupling and dramatic reflection enhancement/reduction from an ITO–LiNbO3 interface

Abstract

Two dimensional electron gases (2DEGs) were evidenced in the indium-tin-oxide (ITO) coated lithium niobate (LN) slabs in monitoring the very first reflection (VFR). Being dictated by light-matter interaction within half-a-wavelength range, two-fold enhancement of the VFR was observed at one incidence angle, while two-and-half-fold reduction was obtained at another angle, when illuminating a Z-cut ITO/LN slab with two laser beams at 532 nm from the same side. In terms of exponential gain coefficient (EGC), such VFR enhancement/reduction correspond to a range from -7.416×10⁴ cm^-1 to +4.68×10⁴ cm^-1, three orders of magnitude higher than that expected in the conventional photorefractive (PR) theory. The nanometer thick 2DEG and the coupling role played by the evanescent field of SPPs impose much loose coherency requirement for the interacting light beams, which was verified by the solid energy coupling observed while rotating the polarization state of one laser beam. As high as 18.4 mW of the side diffraction mode associated with the light track formed on the slab surface shed some light about the dramatic coupling dynamics. All these findings above were far beyond the reach of conventional PR theory, yet consistent well with a physical picture of 2DEG supported surface plasmon polaritons (SPPs) and their interactions at the ITO/LN interface, offering good opportunity in conceiving and designing future photonic/electronic devices, readily for making optical modulators.