  We describe two-field optical techniques to control interactions in Feshbach
resonances for two-body scattering in ultra-cold gases. These techniques create
a molecular dark state in the closed channel of a magnetically tunable Feshbach
resonance, greatly suppressing optical scattering compared to single optical
field methods. The dark-state method enables control of the effective range, by
creating narrow features that modify the energy dependence of the scattering
phase shift, as well as control of the elastic and inelastic parts of the
zero-energy s-wave scattering amplitude. We determine the scattering length and
the effective range from an effective range expansion, by calculating the
momentum-dependent scattering phase shift from the two-body scattering state.
