Abstract: The Volkov state, as an exact description of the quantum dynamics of a free charged particle in a plane electromagnetic field, is a central ingredient of theoretical frameworks for lasermatter interaction. In this work, within a unified planewave model, the Dirac equation is solved to obtain the Dirac–Volkov solution, which is then reduced, under nonrelativistic and fixedspin approximations, to a Schrdinger–Volkov state that explicitly incorporates nondipole modification. The Hamilton–Jacobi equation is further employed to construct the corresponding classical action picture. The analysis clarifies that Volkov descriptions formulated in different theoretical frameworks share an intrinsically unified phase structure, which reveals mechanisms of the particle’s transverse oscillations, longitudinal drift, and effectivemass effects. It is also shown that nondipole modification can be interpreted as the effective coupling between the particle momentum and the laser field. 

Key words: charged particle, strong laser field, volkov state