Hollow-core photonic-crystal fibers are shown to offer the unique possibility of coherent excitation and probing of Raman-active vibrations in molecules by isolated air-guided modes of electromagnetic radiation. A $3\text{−}\mathrm{cm}$ section of a hollow photonic-crystal fiber is used to prepare isolated air-guided modes of pump and probe fields for a coherent excitation of $2331{\text{−}\mathrm{cm}}^{-1}$ $Q$-branch vibrations of molecular nitrogen in the gas filling the fiber core, enhancing coherent anti-Stokes Raman scattering through these vibrations by a factor of 15 relative to the regime of tight focusing.

• Received 21 April 2004

DOI:https://doi.org/10.1103/PhysRevA.70.045802