Two-field Optical Vernier to Probe Energy-Dependent Feshbach resonances*

Optical control of two-body interactions in ultracold gases enables new high resolution probes of energy-dependent narrow Feshbach resonances, which are strongly dependent on the relative momentum of colliding atom pairs. We demonstrate a two-field optical vernier using electromagnetically induced transparency, which expands kHz (mG) magnetic field detunings into MHz optical field detunings, providing new insights into the momentum-dependent two body interactions. Two-photon atom loss spectra are measured near the narrow Feshbach resonance in $^6$Li atoms, revealing rich structure in good agreement with our theoretical model.