The electron density profile is solved self-consistently in circularly polarized femtosecond laser vertically interacting with a linear plasma. The result show s that a set of electron islands and cavities are produced in the relativistic p lasma. The laser energy is partially trapped in these cavities due to strong sta tic electric fields caused by the separation of electrons and ions. The dependen ce of the trapped static electric energy on the laser intensity is also discusse d.