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Two-photon polymerization, optionally combined with stimulated emission depletion (STED) lithography, allows two and three dimensional polymer fabrication with structure sizes and resolution below the diffraction limit. Structuring of polymers with photons, whose wavelength is within the visible range of the electromagnetic spectrum, gives new opportunities to a large field of applications e.g. in the field of biotechnology and tissue engineering [1]. Radical photoinitiator molecules (fluorophores) in an acrylic negative tone photoresist are excited with a near infrared laser via two photon absorption; this allows writing of features as small as ~100 nm. To achieve spatial polymerization restriction similar to STED-microscopy [2, 3], the excited photoinitiators are depleted in the outer rim of the excitation volume via stimulated emission by a second laser beam. An appropriate beam shaping shrinks the volume of excited photoinitiators. Thereby, polymerization initiation is furtherly confined. The feature size can be decreased to several tens of nanometers in any desired geometry using stimulated emission depletion (STED) [2-5]. Currently, feature sizes as small as 55 nm and a lateral resolution of 120 nm of adjacent lines can be achieved [5, 6]. Feature size as well as structure resolution are mainly limited by the used photoresists.
Future applications of sub-diffraction optical lithography include optical data storage and nanophotonic devices. Recently, STED lithography allows us to produce well characterized, biocompatible nanoanchors as platforms for single, biochemically active proteins, applicable to many biological assays [6, 7].
1. Maruo, S., O. Nakamura, and S. Kawata, Three-dimensional microfabrication with two-photon-absorbed photopolymerization. Opt Lett, 1997. 22(2): p. 132-4.
2. Klar, T.A. and S.W. Hell, Subdiffraction resolution in far-field fluorescence microscopy. Opt Lett, 1999. 24(14): p. 954-6.
3. Klar, T.A., et al., Fluorescence microscopy with diffraction resolution barrier broken by stimulated emission. Proc Natl Acad Sci U S A, 2000. 97(15): p. 8206-10.
4. Fischer, J., G. von Freymann, and M. Wegener, The Materials Challenge in Diffraction-Unlimited Direct-Laser-Writing Optical Lithography. Advanced Materials, 2010. 22(32): p. 3578-+.
5. Wollhofen, R., et al., 120 nm resolution and 55 nm structure size in STED-lithography. Opt Express, 2013. 21(9): p. 10831-40.
6. Klar, T.A., R. Wollhofen, and J. Jacak, Sub-Abbe resolution: from STED microscopy to STED lithography. Physica Scripta, 2014. 162: p.14049
7. Wolfesberger, C., et al., Streptavidin functionalized polymer nanodots fabricated by visible light lithography. J Nanobiotechnology, 2015. 13(1): p. 27.
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