  Strong enough electric field is predicted to spontaneously create
electron-positron pairs in vacuum via Schwinger effect. A somewhat similar
effect is predicted to occur in sufficiently strong inhomogeneous gravitational
fields. However, due to necessity of very large field strength, these effects
were never observed in the experiment. Here we demonstrate that optical analogs
of very strong gravitational fields (up to ~10^24 g) and very strong
gravitational field gradients (up to ~10^31 s-2) may be created in
electromagnetic metamaterial waveguides, leading to an optical analog of
particle production in gravitational fields. Such waveguide geometries may also
potentially be used to search for axion-like particles weakly interacting with
electromagnetic field.
