FIGURE 1. Microfabrication and nanofabrication at subdiffraction-limit resolution.

A titanium sapphire laser operating in mode-lock at 76 MHz and 780 nm with a 150-femtosecond pulse width was used as an exposure source. The laser was focused by an objective lens of high numerical aperture (~1.4). a–c, Bull sculpture produced by raster scanning; the process took 180 min. d–f, The surface of the bull was defined by two-photon absorption (TPA; that is, surface-profile scanning) and was then solidified internally by illumination under a mercury lamp, reducing the TPA-scanning time to 13 min. g, Achievement of subdiffraction-limit resolution, where A, B and C respectively denote the laser-pulse energy below, at and above the TPA-polymerization threshold (dashed line). The yellow line represents the range of single-photon absorption. TPA-P, TPA probability. h, Scanning electron micrograph of voxels formed at different exposure times and laser-pulse energies. i, Dependence of lateral spatial resolution on exposure time. The laser-pulse energy was 137 pJ. The same data are presented using both logarithmic (triangles; bottom axis) and linear (circles; top axis) coordinates, to show the logarithmic dependence and threshold behaviour of TPA photopolymerization. Scale bars, 2 m.