  It has been predicted that application of a strong electric field
perpendicular to the plane of bilayer graphene can induce a significant band
gap. We have measured the optical conductivity of bilayer graphene with an
efficient electrolyte top gate for a photon energy range of 0.2 - 0.7 eV. We
see the emergence of new transitions as a band gap opens. A band gap
approaching 200 meV is observed when an electric field ~ 1 V/nm is applied by
an electrolyte gate to the bilayer system, inducing a carrier density of about
10^13 cm-2. The magnitude of the band gap and the features observed in the
infrared conductivity spectra are broadly compatible with calculations within a
tight-binding model.
