First-principles variational formulation of polarization effects in geometrical optics

The propagation of electromagnetic waves in isotropic dielectric media with local dispersion is studied under the assumption of small but nonvanishing λ/ℓ, where λ is the wavelength and ℓ is the characteristic inhomogeneity scale. It is commonly known that, due to nonzero λ/ℓ, such waves can experience polarization-driven bending of ray trajectories and polarization dynamics that can be interpreted as the precession of the wave "spin". The present work reports how Lagrangians describing these effects can be deduced, rather than guessed, within a strictly classical theory. In addition to the commonly known ray Lagrangian that features the Berry connection, a simple alternative Lagrangian is proposed that naturally has a canonical form. The presented theory captures not only the eigenray dynamics but also the dynamics of continuous-wave fields and rays with mixed polarization, or "entangled" waves. The calculation assumes stationary lossless media with isotropic local dispersion, but generalizations to other media are straightforward.