A number of applications require true 3D fabrication, such as micro/nanophotonics, microelectromechanical systems (MEMS), microfluidics, biomedical implants, and microdevices. Two-photon polymerization (2PP) is the only technology that can provide this with nanoscale resolution. Classic 3D prototyping techniques such as UV laser stereolithography and 3D inkjet printing can also be employed, but the resolution these approaches provide is only a few microns. Classic lithographic techniques such as electron-beam lithography have superior resolution, but they can only produce high-aspect-ratio, 2D structures.
Although 2PP is an established research tool, it is not widely used. This is because of the relatively high costs of femtosecond lasers, positioning systems, and optics, as well as its slow processing speed and small volume. There is also a lack of specially designed materials with good optical, mechanical, and biomedical properties. Nonlinear, optical-microstereolithography laser technology based on 2PP allows submicron fabricaton of 3D structures.1 When a femtosecond IR laser beam is tightly focused into a photosensitive material, polymerization can be initiated by nonlinear absorption within the focal volume. By moving the laser focus through the material, 3D structures can be fabricated (see Figure 1). The technique has been used with a variety of photostructurable materials and several components and devices have been fabricated, including photonic-crystal templates, mechanical devices, and microscopic models.
The majority of teams involved in 2PP research use commercially available negative (but also positive) photoresists designed for planar lithographic applications. The main disadvantage of those materials is they cannot easily be modified or functionalized with active components. We have been collaborating with the Laser Zentrum Hannover e.V. (LZH) to develop new, organic/inorganic hybrid sol-gel materials for 2PP applications. The composite materials are a powerful tool for making photostructurable compounds and are easy to prepare, modify, and process. In addition, after processing they have high optical quality and are chemically and electrochemically inert. Our newly designed compounds include highly transparent, biocompatible photopolymers that do not distort during polymerization and active materials with nonlinear optical functionalities and metal-binding affinity.2
Figure 1. Two-photon polymerization. By moving a laser focus through a photosensitive material, 3D structures can be fabricated.
A current application of 2PP enables fabrication of complicated photonic nanostructures such as 3D photonic crystals. These periodic structures can be made with other techniques such as holographic interferometry and colloidal self-assembly. However, for their practical application in photonic circuits, it is essential to remove symmetry and introduce defects, which is not possible using other methods. An example of a photonic crystal made using 2PP (see Figure 2) uses the zirconium/silicon hybrid sol gel SZ2080 we developed with LZH.3,4 This material does not shrink or distort during structuring, thus making design and fabrication of photonic-crystal structures simple and repeatable.
Figure 2. A woodpile structure fabricated using a zirconium/silicon hybrid composite.
2PP direct laser writing is a potentially useful technology in other areas of micro/nanophotonics fabrication, e.g., for diffractive optical elements, and microprism and microlens arrays. Another application involves artificial metamaterials5 that can exhibit magnetism at optical frequencies, thus making it essential that the structures are conducting. Fabrication of good-quality, high-resolution 3D metallic structures has not been achieved to date. A popular approach is to first produce dielectric structures and then metallize them. Two methods of post-fabrication metallization have been demonstrated: silver chemical-vapor deposition and electroless-plating metallization. We used the latter to develop new, metal-binding, organic/inorganic hybrid materials that show significant promise in both resolution and metallization capability (see Figure 3). Using these materials, which contain a metal-binding amine group, structures with features of 100nm have been fabricated (see Figure 4).
Figure 3. Metal-coated 3D structure fabricated using a metal-binding sol gel and subsequently coated with silver using electroless plating.
Figure 4. The resolution achieved with a metal-binding material is of order 100nm.
Until recently, wider implementation of 2PP in nanophotonic-device fabrication was hindered by the lack of specialized materials and ready-to-use equipment. This is changing because LZH has also developed an autonomous, compact 2PP machine.6 As there is no other realistic alternative to full 3D nanomanufacturing, expansion of 2PP as a nanophotonic-fabrication technology looks very promising. Our future research plans include developing new active and passive photopolymers and their implementation into photonic devices.
This work resulted from a collaboration between the Institute of Electronic Structure and Laser/Foundation for Research and Technology Hellas (Maria Farsari, Maria Vamvakaki, David Gray, Ioanna Sakellari, Konstantina Terzaki, Arune Gaidukeviciute, Anastasia Giakoumaki, and Costas Fotakis) and LZH (Boris N. Chichkov, Carsten Reinhardt, and Aleksandr Ovsianikov).
Institute of Electronic Structure and Laser Foundation for Research and Technology Hellas
Maria Farsari received her undergraduate degree from the Physics Department of the University of Crete (Greece) and her PhD from the Physics Department of the University of Durham (UK). She subsequently worked as a postdoctoral researcher at the Universities of Durham and Sussex (UK) and as a senior scientist for DeLaRue Holographics and Xsil Ltd. Her main research interests are nonlinear lithography, nanophotonics, and materials processing using ultrafast lasers.