Permanent Electronic Excitation of a Molecular Layer on a Surface through Phase Conjugation

Abstract

The dynamics of an atom (or molecule) located near the surface of a four-wave mixing phase conjugator is studied. This device consists of a slab of nonlinear material, which is illuminated by two strong counterpropagating laser beams. The atomic dipole field induces a polarization in the nonlinear material, and as a result, a phase-conjugated replica is emitted toward the atom. This effectively leads to a four-wave mixing process at the site of the atom, during which two pump photons are annihilated in favor of an electronic excitation of the atom. The mismatch in energy is emitted as a single fluorescent photon. In the steady state the atom acquires a finite population of the excited level, although there is no direct optical pumping of the atomic transition. It is shown that this population is determined entirely by the Fresnel reflection coefficients of the medium. In case of magnetic degeneracy of the levels, the distribution over the sublevels is furthermore determined by geometrical Clebsch−Gordan coefficients. Three cases of low angular momentum are worked out in detail, as examples.