Two dimensional electron gases (2DEGs) formed at interfaces between two oxides have been drawing growing attention for their intriguing magnetic, superconducting, and optical properties. Remarkable anisotropic transmission was observed from an indium-tin-oxide (ITO) coated lithium niobate (LN) slab, implying that a 2DEG is formed at one of the ITO/LN interfaces, and this is seemingly behind the anisotropic transmission. To optically probe 2DEG formation at one of the ITO/LN interfaces, the first reflected beam was monitored with one (two) laser beam(s). Reflective dynamics as large as 5%-15% were observed, pointing unambiguously to a subwavelength coupling and corresponding to, conservatively, exponential gain coefficients of −26 800 to +2700 cm−1 with half a wavelength as the coupling range. All observations are far beyond the reach of conventional bulk photorefractive effects, but align well with a picture of surface plasmon polariton excitation based on 2DEG formation. The 2DEG proximal to the LN substrate is a promising candidate for designing nonlinear plasmonics based nanometric waveguides, rectifiers, modulators, and sensors, which are compatible with current photonic circuits.