Multiphoton lithography | EPFL Graph Search

Multiphoton lithography (also known as direct laser lithography or direct laser writing) of polymer templates has been known for years by the photonic crystal community. Similar to standard photolithography techniques, structuring is accomplished by illuminating negative-tone or positive-tone photoresists via light of a well-defined wavelength. The fundamental difference is, however, the avoidance of reticles. Instead, two-photon absorption is utilized to induce a dramatic change in the solubility of the resist for appropriate developers. Hence, multiphoton lithography is a technique for creating small features in a photosensitive material, without the use of complex optical systems or photomasks. This method relies on a multi-photon absorption process in a material that is transparent at the wavelength of the laser used for creating the pattern. By scanning and properly modulating the laser, a chemical change (usually polymerization) occurs at the focal spot of the laser and can be

Micro-mechanics of 3D architectured metals synthesized by electrodeposition

Maxime Alain Baptiste Mieszala

Cellular metallic materials have emerged as a new promising class of materials due to their lightweight porous structures and advanced multi-functional properties. Originally limited to random metallic foams, modern lithography techniques have enabled the design of hierarchical cellular structures with tailored mechanical properties. In addition, the mechanical properties of micro-cellular metals can be further improved by taking advantage of the properties of metals, which are size-sensitive at this length scale. It is therefore expected that the combination of architectured cellular designs with size effects in metals would result in materials with unprecedented mechanical properties. The present thesis aims at investigating and understanding the influence of size effects on the mechanical properties of three-dimensional micro-architectured metal structures synthesized using advanced lithography techniques and electrochemical deposition of metals. Electrodeposited copper films with engineered microstructures and nickel-based hierarchical micro-cellular structures were manufactured and tested under uniaxial loading conditions. The results of the tests were used to evaluate the contribution of the size effects and/or architecture to the change in mechanical properties and deformation behavior. The influence of the intrinsic size effect was first demonstrated in electrodeposited pure copper films. Using nanoscale twins to engineer the grain boundaries, it was possible to produce copper films with high strength and significantly tune the mechanical properties by adjusting the twin orientation. The combination of size effects was then investigated for two types of micro-architecture. The first type of metal micro-architecture was obtained by plating nanocrystalline Ni inside a self-assembled colloidal crystal of microspheres in order to produce a regular inverse opal. The second type was fabricated by metallizing a 3D polymer template produced by 3D laser lithography with an amorphous NiB coating. Results show that there is a complex interaction between the characteristic external dimension and cellular structure. For regular Ni inverse opal, there is a complex interaction between the ligament size and the grain size which defines the overall strength of cellular metals. In the case of hybrid NiB/polymer structures, the brittle-to-ductile transition in the amorphous NiB coating and the architecture effect are coupled. By varying the NiB thickness and the architecture geometry, it is possible to control the deformation mechanism from global buckling to brittle failure and to tune the energy absorption characteristics of the micro-architectures. Through these case studies, it was demonstrated that the combination of grain boundary engineering, sub-micron geometrical features and overall architecture can yield 3D metallic micro-architectures, which are both light and strong and have tailored mechanical properties. The mechanical properties of some of these materials exceed the properties of either of the parent materials. Therefore, the constitutive mechanical laws for porous metals should be modified to account for these three effects all together. The results confirmed the potential of our experimental approach and provide a practical way to extend the material property-space.

EPFL2018

Three-dimensional fabrication of sub-micron optical waveguides in PDMS and other polymer materials

Giulia Panusa

Optical waveguides are one of the most important photonic components. They are indispensable tools in many of todayâs technologies because of their capacity of guiding light. In particular, compact, low loss, flexible polymer optical waveguides are crucial in optofluidic and microfluidic devices for a dense integration of optical functionalities. The sought after suitable materials and innovative fabrication techniques to achieve low loss long polymer optical waveguides and interconnects has proven to be challenging. In a fiber optic endoscope, thousands of single-mode (SM) optical fibers acting as single pixels are closely packed together, delivering the local information to a camera chip. Biocompatible polymer-based waveguides increasingly became valid alternatives to silica fibers to deliver and collect light for optical diagnosis, therapy and surgery. In particular, polydimethylsiloxane (PDMS) is well known to be a suitable polymer for biomedical implantation devices, thanks to its excellent physical and chemical properties. In this thesis, I demonstrate the fabrication of compact optical waveguides in PDMS through multiphoton laser direct writing (MP-LDW). The core of this research consists of the investigation of suitable combinations of monomer and PI capable of efficient photopolymerization in a cured PDMS matrix. We achieved, for the first time, the photoinitiator-free fabrication of optical waveguides employing phenylacetylene as the photosensitive monomer via multi-photon absorption. Because of the dense Ï-electrons in phenylacetylene, we achieved a high refractive index contrast (În â¥ 0.06) between the waveguide core and the PDMS cladding. This allowed for efficient waveguiding at a core size of 1.3-Âµm with a measured loss of 0.03 dB/cm in the spectral band of 650-700 nm.Motivated by the need of minimizing self-focusing, we investigated alternative chemical schemes and demonstrated the fabrication of submicron optical waveguides in PDMS using divinylbenzene (DVB) as the monomer through two-photon polymerization (2PP). We show that the commercial oxime ester photoinitiator Irgacure OXE02 is suitable for triggering the DVB polymerization, resulting in a stable and controllable fabrication process for the fabrication of defect-free, 5-cm long waveguides. Moreover, I present the methodologies we have developed for the fabrication of polymer rectangular step-index (STIN) optical waveguides using a commercial 3D printing system (Photonic Professional GT, Nanoscribe GmbH), using the proprietary IP-dip resist. We performed a full calibration and implemented a printing strategy for the fabrication of a 720 ÎŒm long SM-fiber bundle. We characterized it in terms of refractive index, transmission loss and imaging capabilities. We further demonstrate how a convolutional neural network (CNN) can reconstruct the original images from a scrambled output from the waveguide bundle due to crosstalk. To achieve this, we have constructed a CNN of the U-net type and trained the network with the input images and their corresponding output from the bundle. Overall, the presented work provides innovative materials and new insights into the fabrication of polymer optical microstructures, opening new scenarios for future technological development in the field of microfabrication using MP-LDW. We expect such waveguides will receive a wide range of applications in biosensors, microfluidic flow cytometry and wearable photonic devices.

EPFL2021