Two-photon direct laser writing of ultracompact multi-lens objectives Timo Gissibl 1 * , Simon Thiele 2 , Alois Herkommer 2 and Harald Giessen 1 Current lens systems are restricted in size, shape and dimensions by limitations of manufacturing. Multi-lens elements with non-spherical shapes are required for high optical performance and to correct for aberrations when imaging at wide angles and large fields. Here we present a novel concept in optics that overcomes all of the aforementioned difficulties and opens the new field of 3D printed micro- and nano-optics with complex lens designs. We demonstrate the complete process chain, from optical design, manufacturing by femtosecond two-photon direct laser writing and testing to the application of multi-lens objectives with sizes around 100 μm, and validate their high performance and functionality by quantitative measurements of the modulation transfer function and aberrations. The unprecedented flexibility of our method paves the way towards printed optical miniature instruments such as endoscopes, fibre-imaging systems for cell biology, new illumination systems, miniature optical fibre traps, integrated quantum emitters and detectors, and miniature drones and robots with autonomous vision. A dditive manufacturing enables new and unprecedented engineering and production possibilities that are predicted to have an enormous impact in the twenty-first century. The technology allows for the simple three-dimensional (3D) print- ing of volumetric objects directly from a computer-aided design 1 . So far, additively manufactured objects are mostly fabricated from metals, ceramics and opaque plastics. There are a number of differ- ent fabrication methods to manufacture small and high-performance micro-optical systems 2–10 ; however, these technologies suffer from drawbacks such as limited miniaturization, inability to combine multiple elements, restrictions in designing the surfaces 2–4,11 and problems with the alignment 12 . Multiphoton lithography is one of various 3D printing technologies that realize the fabrication of 3D objects 13–15 . Using femtosecond laser pulses and two-photon absorption, this manufacturing method takes 3D printing down to submicrometre feature sizes and therefore pushes the ongoing trend of miniaturiza- tion forwards. Direct laser writing with highly transparent photo- resists enables 3D printing to enter the realm of manufacturing optical elements at the micro- and nanometre scale 16–22 . Thus the precise fabrication of complex optical elements on demand becomes possible. We demonstrate that 3D direct laser writing is a suitable tool for fabricating complex multi-lens optical systems that show high optical performances and tremendous compactness. Until now multi-lens optics that have comparable performances are consider- ably larger 12,23 and at the same time do not show the manifold compound structures and possibilities presented here. Our optical devices consist of several different free-form lens elements with air in between. This work is right at the interface between micro- and nano-optics and represents a paradigm shift for micro-optics. It takes only a few hours from lens design through production and testing to the final working optical device. Endoscopic applications will allow for non-invasive and non- destructive examination of small objects in the medical as well as the industrial sector and serve as a hallmark application of this new technology. Figure 1 depicts an optical fibre equipped with a 3D printed multi-lens system for imaging the interior of a hollow organ or a cavity inside the body. Using a very small injection cannula with an outer diameter of only 412 μm (27 gauge) together with the fibre-coupled printed compound microscope objective lens, the insertion can be easily accomplished. In this Article we demonstrate the capabilities of 3D dip-in laser lithography to manufacture high-quality optical compound lenses with outstanding performances 24 . We realize a variety of optical elements with different features for numerous applications. The optical performance is quantified and analysed by measuring the optical modulation transfer function (MTF) and the longitudinal (axial) chromatic aberration. We also characterize the roughness of the surfaces by atomic force microscope measurements. As 500 μm 100 μm Figure 1 | Coloured SEM image of a triplet lens objective attached to an optical fibre inserted into the hollow needle of a syringe. The compound objective lens (blue) consists of five refractive surfaces for imaging applications and is directly fabricated on the optical fibre (red). The fibre is emerging from a hollow needle (27 gauge, outer diameter 412 μm, inner diameter 210 μm) to demonstrate the possibility of endoscopic applications. The objective lens is fabricated in a cutout fashion for better visibility. The inset shows a magnified image of the fibre tip with the objective. 1 4th Physics Institute and Research Center SCoPE, University of Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany. 2 Institute for Applied Optics (ITO) and Research Center SCoPE, University of Stuttgart, Pfaffenwaldring 9, 70569 Stuttgart, Germany. *e-mail: t.gissibl@pi4.uni-stuttgart.de ARTICLES PUBLISHED ONLINE: 27 JUNE 2016 | DOI: 10.1038/NPHOTON.2016.121 NATURE PHOTONICS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturephotonics 1 © 2016 Macmillan Publishers Limited. All rights reserved