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Two-step absorption instead of two-photon absorption in 3D nanoprinting


The quadratic optical nonlinearity arising from two-photon absorption provides the crucial spatial concentration of optical excitation in three-dimensional (3D) laser nanoprinting, with widespread applications in technical and life sciences. Femtosecond lasers allow for obtaining efficient two-photon absorption but are accompanied by a number of issues, including higher-order processes, cost, reliability and size. Here we introduce two-step absorption replacing two-photon absorption as the primary optical excitation process. Under suitable conditions, two-step absorption shows the same quadratic optical nonlinearity as two-photon absorption. We present a photoresist system based on a photoinitiator supporting two-step absorption, a scavenger and a well-established triacrylate. We show that this system allows for printing state-of-the-art 3D nanostructures and beyond. In these experiments, we use ~100 μW optical power from an inexpensive, compact continuous-wave semiconductor laser diode emitting at 405 nm wavelength. Our work opens the door to drastic miniaturization and cost reduction of 3D laser nanoprinters.

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Fig. 1: 3D laser nanoprinting using two-photon absorption or two-step absorption.
Fig. 2: Simplified energy-level model and rate-equation calculations for a two-step-absorption photoinitiator.
Fig. 3: Benzil as a two-step-absorption photoinitiator.
Fig. 4: Laser, laser focus and effective nonlinearity of the photoresist.
Fig. 5: Two-step-absorption printing resolution in two and three dimensions.
Fig. 6: Gallery of oblique-view electron micrographs of further 3D-printed nanostructures.

Data availability

The data underlying the plots within this paper, Supplementary Information and related 3D printing files are published on the open-access data repository of the Karlsruhe Institute of Technology (

Code availability

The code for the computations shown in Fig. 2 is published on the open-access data repository of the Karlsruhe Institute of Technology (


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We acknowledge fruitful discussions with C. Barner-Kowollik (Queensland Institute of Technology), F. Mayer (Karlsruhe Institute of Technology (KIT)), P. Müller (previously at KIT) and T. Schlöder (KIT). V.H. is presently funded by the Max Planck School of Photonics. This research has additionally been funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy via the Excellence Cluster ‘3D Matter Made to Order’ (EXC-2082/1-390761711), which has also been supported by the Carl Zeiss Foundation through the ‘Carl Zeiss Foundation-Focus@HEiKA’, by the State of Baden-Württemberg, and by KIT. We further acknowledge support by the Helmholtz program ‘Materials Systems Engineering (MSE)’, and by the Karlsruhe School of Optics & Photonics (KSOP) and by the Ministry of Science, Research and the Arts of Baden-Württemberg as part of the sustainability financing of the projects of the Excellence Initiative II. R.R.S. acknowledges funding from the Federal Ministry for Education and Research (BMBF) under grant no. 13N14476.

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Authors and Affiliations



V.H. and M.W. had the idea to use two-step absorption instead of two-photon absorption. V.H. screened possible photoinitiator candidates suitable for two-step absorption. V.H., T.M. and N.M.B. performed all the experiments and the corresponding analysis. E.R.C. and I.W. performed the sample preparation, ultramicrotomy and electron microscopy of the cross sections. R.R.S. prepared the 3D reconstruction of the woodpile structures. M.W. and E.B. supervised the project. M.W. drafted the first version of the paper. All the authors contributed to the interpretation of the results and writing of the manuscript.

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Correspondence to Vincent Hahn.

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Competing interests

V.H., T.M. and M.W. are inventors on a patent application on two-step absorption. The authors declare no other competing interests.

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Peer review information Nature Photonics thanks Paul Braun and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

Supplementary Figs. 1–9, Table 1, methods and references.

Supplementary Video 1

Three-dimensional volume rendering of a woodpile structure with a = 300 nm.

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Hahn, V., Messer, T., Bojanowski, N.M. et al. Two-step absorption instead of two-photon absorption in 3D nanoprinting. Nat. Photon. 15, 932–938 (2021).

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