Nonlinear plasmonics is a growing field since the power threshold for observable nonlinear light emission of new frequencies can be lowered greatly due to dramatic electromagnetic field confinement. Along another research line, 2D electron gases (2DEGs) formed at interfaces of oxides have been drawing broader attention globally because the metallic constituents can be eliminated, and hence the inherent huge loss associated with uses of metals in plasmonic applications can be circumvented. Once the nonlinear materials are proximal to 2DEGs and surface plasmon polaritons (SPPs) are excited, the electromagnetic field can be strengthened several orders in magnitude. Consequently, the nonlinear processes can take place at a quite low incident light power. Considering much greater dispersion of SPPs, the second order nonlinear processes can be easily realized in terms of meeting phase matching conditions. In this paper, nonuniform 2DEG formed at the interface of a Z-cut Fe doped LiNbO<sub>3</sub> (LN) slab and an indium-tin-oxide (ITO) thin film was analyzed with semi-classical Thomas Fermi screening model, and dispersion of index of refraction was given accordingly. A laser beam at 532 nm and a white light source illuminated the slab from the opposite directions collinearly and a remarkable light emission redistribution was observed with a continuous spectrum of short visible light peaked around 437 nm. Several confirming experimental results with ITO coated Y-cut slabs are presented and phase grating mediated SPP excitation is proposed to explain the related findings, suggesting that second order nonlinear processes strengthened by SPPs are behind the light emission redistribution.
Two dimensional electron gases (2DEGs) formed at interfaces of two oxides have been drawing growing attentions for their intriguing magnetic, 2D superconducting and optical properties. To investigate optically 2DEG formed at LiNbO<sub>3</sub>/indium-tin-oxide interface, the power of the very first reflection beam was monitored under illumination of one (two) laser beam(s). It was found the very first reflection can be reduced to as low as 1.13% from the original 12.9%, pointing unambiguously to a subwavelength coupling and corresponding to conservatively estimated exponential gain coefficient of -78525 cm<sup>-1</sup> by taking half a wavelength as the coupling range, since the 1<sup>st</sup> reflection is dictated by what occurs in that range. Such high exponential gain coefficient, far beyond the reach of conventional photorefractive theoretical framework, is consistent with a physical picture of 2DEG supported interface plasmon polaritons. Such dramatic reflection reduction and corresponding high exponential gain coefficient are highly valuable in designing nanometric photonic devices, such as waveguides, attenuators (amplifiers), modulators and sensors, which are compatible to photonic circuits nowadays. In addition, such a material system is promising for nonlinear plasmonic applications.