Pumped quantum systems: immersion fluids of the future?
  Quantum optical techniques may yield immersion fluids with high indices of refraction without absorption. We describe one such technique in which a probe field experiences a large index of refraction with amplification rather than absorption, and examine its practicality for an immersion lithography application. Enhanced index can be observed in a three-level system with a tunable, near-resonant, coherent probe and incoherent pump field that inverts population of the probe transition. This observation contradicts the common belief that large indices of refraction are impossible without absorption, however it is well in accord with existing electromagnetic theory and practice. Calculations show that a refractive index >> 2 is possible with practical experimental parameters. A scheme with an incoherent mixture of pumped and unpumped atoms is also examined, and is seen to have a lower refractive index (~2) accompanied by neither gain nor loss. 
Vikas Anant, Magnus R{\aa}dmark, Ayman F. Abouraddy, Thomas C.   Killian, Karl K. Berggren