GAIN-ASSISTED NEGATIVE REFRACTIVE INDEX IN A QUANTUM COHERENT MEDIUM

A new scheme for overcoming losses with incoherent optical gain in a quantum-coherent left-handed atomic vapor is suggested. In order to obtain low-loss, lossless or active left-handed media (LHM), a pump field, which aims at realizing population inversion of atomic levels, is introduced into a four-level atomic system. Both analytical and numerical results are given to illustrate that such an atomic vapor can exhibit intriguing electric and magnetic responses required for achieving simultaneously negative permittivity and permeability (and hence a gain-assisted quantum-coherent negative refractive index would emerge). The quantum-coherent left-handed atomic vapor presented here could have four fascinating characteristics: i) three-dimensionally isotropic negative refractive index, ii) doublenegative atomic medium at visible and infrared wavelengths, iii) high-gain optical amplification, and iv) tunable negative refractive index based on quantum coherent control. Such a three-dimensionally isotropic gain medium with negative refractive index at visible and infrared frequencies would have a potential application in design of new quantum optical and photonic devices, including superlenses for perfect imaging and subwavelength focusing.