Optical projection lithography at half the Rayleigh resolution limit by two-photon exposure

Photographic media can be exposed by two-photon absorption, rather than the more usual one-photon absorption. This leads to the question of whether the simultaneous absorption of a pair of photons could be accompanied by a twofold spatial-resolution enhancement. We find that ordinary two-photon absorption merely enhances the photographic contrast, or gamma. While this improves the spatial resolution somewhat, it does so at the expense of requiring tighter control over the incident light intensity. Instead, we introduce a new type of exposure arrangement employing a multiplicity of two-photon excitation frequencies, which interfere with one another to produce a stationary image that exhibits a true doubling of the spatial resolution. (C) 1999 Society of Photo-Optical instrumentation Engineers. [S0091-3286(99)00402-X].