  A perfect lens with unlimited resolution has always posed a challenge to both
theoretical and experimental physicists. Recent developments in optical
meta-materials promise an attractive approach towards perfect lenses using
negative refraction to overcome the diffraction limit, improving resolution.
However, those artificially engineered meta-materials usually company by high
losses from metals and are extremely difficult to fabricate. An alternative
proposal using negative refraction by four-wave mixing has attracted much
interests recently, though most of existing experiments still require metals
and none of them has been implemented for an optical lens. Here we
experimentally demonstrate a metal-free flat lens for the first time using
negative refraction by degenerate four-wave mixing with a thin glass slide. We
realize optical lensing effect utilizing a nonlinear refraction law, which may
have potential applications in microscopy.
