Coherent resonant interactions and slow light with molecules confined in   photonic band-gap fibers
  We investigate resonant nonlinear optical interactions and demonstrate induced transparency in acetylene molecules in a hollow-core photonic band-gap fiber at 1.5$\mu$m. The induced spectral transmission window is used to demonstrate slow-light effects, and we show that the observed broadening of the spectral features is due to collisions of the molecules with the inner walls of the fiber core. Our results illustrate that such fibers can be used to facilitate strong coherent light-matter interactions even when the optical response of the individual molecules is weak. 
Saikat Ghosh, Jay E. Sharping, Dimitre G. Ouzounov and Alexander L.   Gaeta