We exploit micro-nano structuration to achieve multifunctional windows offering outstanding optical and fluidic properties to enhance the operation of surveillance or detection devices under rainy conditions. These windows are based on synthesis of an artificial index gradient for antireflection properties and improvement of their water repellency property thanks to their structuration at a subwavelength scale with controlled conical geometries. We demonstrate the realization of multifunctional germanium windows for LWIR camera, using two approaches: nanoimprint lithography, well-known for its very high resolution enabling applications from visible to thermal infrared domain, followed by etching techniques, and 3D direct laser writing based on Two-Photon Polymerization (TPP), which is of interest thanks to its ability to manufacture complex 3D structuration directly. Optical characterization shows the ability of such windows to improve optical transmission within 8-14μm spectral range, as compared to non-structured window. In terms of water repellency, the structured windows enable an increase of the contact angle up to 160° with a very low hysteresis. To evaluate the advantage of the multifunctional windows for imaging devices, the windows are integrated in front of a thermal infrared camera and images analysis shows that the camera sensitivity is increased for the nanoimprint window thanks to the multifunctional window and high water repellency in presence of water.
KEYWORDS: 3D printing, Lithography, Two photon polymerization, Manufacturing, Ultraviolet radiation, Surface roughness, Printing, Laser systems engineering, Cartilage, Bone
Based on the underlying printing resolution Two-Photon Polymerization (TPP) can be distinguished into 3D Lithography and Micro 3D Printing applications. Both of these fields will be discussed in terms of the requirements on the fabrication process such as exposure strategy, overall resolution and accessible print height among others. Enabling both 3D Lithography and Micro 3D Printing in one TPP laser system imposes certain challenges which will be addressed with solutions being presented.
The use of two-photon absorption (TPA) for polymerization, also known as 3D Lithography, Direct Laser Writing, or High-Precision 3D Printing is gaining increasing attraction in industrial fabrication of micro- and nanostructures. Mainly due to its vast freedom in design and high-resolution capabilities, TPA enables the fabrication of designs which are not feasible or far too complicated to be achieved with conventional fabrication methods. TPA is a scanning technology and fabrication in 3D requires axial overwritings. High industrial throughput fabrication can be achieved by intelligent fabrication strategies combined with an excellent material basis. Further boosting the throughput can be achieved by multispot exposure strategies. In this paper, massive parallelization is demonstrated which was realized by using a beam splitting diffractive optical element (DOE). Simultaneous fabrication using commercially available acrylate-based hybrid resin with 121 parallel focal spots arranged as 11 x 11 array is reported. Structures fabricated by a single laser beam and by 121 parallel beams are compared to each other with regard to shape and polymerization threshold. It was found that polymerization is strongly increased when parallel beams are used, especially for the central beams. As a result, polymerization threshold is lower in the center of the 11 x 11 array compared to the edges of the array. Furthermore, structures at the center of the 11 x 11 array are bigger compared to structures at the edges of the array when assigning equal intensity to all diffracted beams. These results are attributed to diffusion of photo initiators, quenchers, and radicals.
Industrial High-precision 3D Printing via two-photon absorption (TPA) as a potential disruptive tool for microfabrication enables novel products for diverse applications in the field of optics, photonics, biomedicine, and life sciences. Especially the freedom in design provides one-step fabrication of structures that are not feasible with conventional fabrication techniques or need combined technologies with a required changeover of the workpiece.
Up to now, 2PP-fabrication has only been used in the community for structures on the micro and mesoscale due to limited travelling ranges of the translation stages and the field-of-view (FoV) of microscope objectives in combination with galvoscanners to deflect the laser instead of moving the sample relative to the focus. Macroscale elements can be realized via stitching strategies but, however, often induce obvious joints that hinder aimed applications. For this purpose, different fabrication strategies for large scale elements are revealed in this contribution without relying on stitching. Modular machine configurations like inverted focusing through a bath of photoresist (LithoBath3D) enable objects several millimeters in size with micrometer resolution. Additionally, 3D scanning by translation stages only can be efficiently used for the fabrication of large scale DOE structures. For optical elements with high surface quality, precise fabrication is required. As galvoscanners enable high throughput at several 100 mm/s scan speed, TPA-fabricated microlenses are limited to the FoV of the corresponding microscope objective, typically less than 0.5 mm. This limit can be overcome by sophisticated exposure strategies like a synchronized movement of translation stages and galvoscanner (infinite FoV) in combination with advanced beam steering.
Driven by IoT, Industry 4.0, and social media the amount of data to be transferred is tremendously increasing, pushing the need for energy-efficient device concepts for a vast variety of products such as photonic integrated circuits or sensors. Low energy data transfer can be achieved, for example, by replacing part of the electronic circuitry by optical data lines in chip-level packaging, or by introducing optical elements such as specially designed microlenses into semiconductor laser packaging. This also allows to drastically reduce footprints of systems, and – at the same time – to increase functionality. On the other hand, a significant demand is seen in providing lower cost and scalable manufacturing processes with technologies which provide highest flexibility.
High Precision 3D Printing as novel emerging fabrication technology is a promising tool for optical packaging. It enables to reduce the necessary process steps for packaging to only three to five, independently of the packaging task. This is enabled by a novel and versatile packaging concept where the chips and the dies are already mounted prior to the fabrication of optically functional elements such as optical interconnects or microoptics to couple, for example chip-to-chip or dies to fiber, with passive alignment only. Flexible exposure strategies using High Precision 3D Printing provide both, scalability and high throughput with fabrication times from seconds for optical waveguides and single microlenses to only a few minutes for more complex lens systems. The impact of the fabrication strategy will be discussed with respect to the performance of the optical devices.
Diffractive optical elements (DOEs) are widely used in various applications such as material processing, illumination, medical, and sensor applications by providing a shape on demand of laser beams. In contrast to refractive optical elements, the effect of DOEs is based on modulating the phase of the beam locally. This creates an interference pattern of the beam. The more height levels are implemented in a DOE, the higher its diffraction efficiency.
Rapid fabrication and testing in practice of new designs is desirable to shorten the prototyping development cycle of DOEs. High Precision 3D Printing via a two-photon absorption (TPA) process initiating a polymerization reaction allows manufacturing of virtually any 3D-shaped object, thus being the technique of choice to fabricate DOEs at high precision with an arbitrary number of levels within short time periods.
We demonstrate the use of High Precision 3D Printing to fabricate DOEs to be used as beam-shaping and beam-splitting elements, respectively. Different exposure strategies in polymer-like materials are used to fabricate DOEs which have significant impact on the fabrication time. The quality of the fabricated DOEs will be assessed by a variety of characterization methods such as metrology investigations for determination of the surface quality (for example, shape deviations, roughness), AFM, and optical characterization. The impact of different exposure strategies on the final DOEs will be presented and discussed.
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