A novel stimulated trapped electron-acoustic wave scattering instability by a linearly polarized laser interacting with a subcritical density plasma layer is observed by particle simulation. Its spectrum in the early stage is well-explained by a resonant three-wave parametric decay process and it takes place whether the ion dynamics are taken into account or not. When ion dynamics are considered, the excitation of the ion electrostatic wave, the generation of the electromagnetic (EM) soliton and the formation of ion vortices due to the large EM soliton, etc., are studied.