We report an optical investigation of cubic Silicon Carbide (3C-SiC) layers grown on 6H-SiC substrates by Chemical Vapour Deposition and Vapour-Liquid-Solid mechanism. Micro- Infrared reflectance ('-IR), micro-Raman ('-Raman) and low temperature photoluminescence spectroscopies were used for the characterisation of such layers. '-IR measurements showed unusual optical behaviour of 3C-SiC layers. The difference of refraction index between the 3C-SiC film and the 6H-SiC substrate cannot explain this result. The experimental '-IR reflectance spectrum was modelled by introducing a thin (thickness ≤ 0.5 'm) metallic-like (doping ≥ 1020 at.cm-3) interfacial film between the layer and the substrate. The photoluminescence spectra revealed the presence of a peak which may be attributed to recombination at the 3C/6H interface. All these results suggest the presence of a two dimensional electron gas at the interface.