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A flat microlens array whose focal length can be switched from positive to negative by electric field is demonstrated experimentally and confirmed by computer simulations. To generate the required gradient refractive index, an inhomogeneous electric field is created by a spherical indium-tin-oxide (ITO) electrode which is imbedded in the top flat substrate. The bottom substrate has a planar ITO electrode on its inner surface. A thin polymeric layer is overcoated on top of the spherical ITO to create a flat surface. The disclination lines are eliminated. Because of the employed dual-frequency liquid crystal, the microlens array has fast response times.