Multiphoton-Excited Deep-Ultraviolet Photolithography for 3D Nanofabrication

Light-matter interactions in the deep ultraviolet (DUV) wavelength region exhibits a variety of optical effects such as luminescence, photoisomerization, and polymerization in many materials. Despite the rich photochemistry and high spatial resolution due to the short wavelength, the notorious lack of DUV-compatible optical components and devices precludes use of DUV light in microscopy and lithography as a routine laboratory tool. Here, we present the use of two-photon excitation with visible laser light to realizes photo-polymerization of molecules with an excitation energy equivalent to DUV light. Using standard optics for visible light, methacrylate oligomers were polymerized with 400 nm femtosecond pulses without any addition of photo-initiators and sensitizers. By scanning the laser focus in 3D, a series of fine 3D structures were created with the smallest resolved line-space features of 80 nm. We found DUV polymerizations induced by two-photon absorption is surprisingly efficient and requires laser intensity only on the order of 100 kW/cm^2. With the variety of successful demonstrations including organic- and inorganic-material-made-structures presented, our direct nano-3D-printing method would be a valuable tool for nanofabrication in 3D.