Dressed-state mixed-parity transitions for realizing negative refractive index

Abstract

The dressed states that are linear combinations of two bare levels of an atom (e.g., an alkali-metal atom) can be realized by a strong-coupling laser beam. As the dressed states have mixed parities, both electric- and magnetic-dipole-allowed transitions can occur between the dressed states and a third level with a definite (pure) parity. It is shown that such dressed-state mixed-parity transitions in an atomic vapor (the concept also applies in the solid state) can give rise to a negative refractive index. The produced negative refractive index is isotropic with atomic-scale microscopic structure units, and the negative real part can emerge in the optical frequency band. Also examined is the case of a fully quantized probe photonic field which interrogates the bottom dressed state and the third-level state. Similarities between the semiclassical approach for the weaker probe field and its fully quantum mechanical second-quantization treatment are discussed in regard to the off-diagonal density matrix element for the reduced $2\ifmmode\times\else\texttimes\fi{}2$ manifold, and its implications for the refractive index.