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.