High-resolution two-photon spectroscopy of rubidium within a confined geometry

We present two-photon spectroscopy of a thermal rubidium vapor confined to the hollow core of a photonic-crystal fiber. Linewidths as narrow as 10 MHz were observed on the 5S(1/2) -> 5D(5/2) transition enabling the hyperfine splitting of the excited state to be resolved. Very strong nonlinear absorption (>90%) was observed, with substantial absorption maintained over large detunings (9 GHz) from an intermediate state. These attributes make this system ideal for many frequency metrology and quantum optics applications. DOI: 10.1103/PhysRevA.87.013818