Index of refraction engineering in five level DIGS atoms
  We present a five level atomic system in which the index of refraction of a probe laser can be enhanced or reduced below unity with vanishing absorption in the region between pairs of absorption and gain lines formed by dressing of the atoms with a control laser and RF/microwave fields. By weak incoherent pumping of population into a single metastable state, one can create several narrow amplifying resonances. At frequencies between these gain lines and additional absorption lines, there exist regions of vanishing absorption but resonantly enhanced index of refraction. In $Rb$ vapors with density $N$ in units of $cm^{-3}$, we predict an index of refraction up to $n \approx \sqrt{1+1.2\times10^{-14}N}$ for the D1 line, which is more than an order of magnitude larger than other proposals for index of refraction enhancement. Furthermore, the index can be readily reduced below 1 by simply changing the sign of the probe or RF field detunings. This enhancement is robust with respect to homogeneous and inhomogeneous broadening. 
Steven A. Sagona-Stophel, James Owen Weatherall, and Christopher P.   Search
quant-ph physics.atom-ph physics.optics