A method to directly explore the effect of electron density on the surface plasmon wave and the corresponding laser-induced structures is demonstrated by controllable doping of silver nitrate (AgNO3) into a water-soluble polymer polyvinyl alcohol (PVA). It is shown that periodic nanostructures on PVA appeared only when the electron density was more than 6×1020 cm-3 and became uniform when doping density corresponded to 1.2×1021 cm-3. Photo-excitation of electrons in the Ag-doped PVA defined by laser fluence determined the conditions of surface plasmon wave and formation of nanostructures on the surface. The Drude-Lorentz model was used to describe the formation conditions of surface wave and to estimate the period of the structures. The demonstrated photo-electron doping brings required means to control the formation of laser-induced patterns with an optical sub-wavelength resolution.