We present recent advancements in two-photon grayscale lithography (2GL®). In contrast to one-photon grayscale lithography, for 2GL®, the exposed volume pixel is strongly confined to the vicinity of the laser focus allowing for a truly 3-dimensional dose control with very high spatial resolution. Discrete and accurate steps, as well as essentially continuous topographies, can be printed with increased throughput, on any substrate, and without the need for additional lithography steps or mask fabrication. We update on throughput and quality levels of the method. As demonstrators, we fabricate and characterize optics masters for replication technologies like nanoimprint lithography.

Volumetric Additive Manufacturing (VAM) is a recently-conceived method of printing into a fixed volume of photosensitive resin in a single lithographic step. It has multiple advantages over prior printing methods. Printing into highly viscous or solid materials is possible, enabling a new range of material properties. Print times are dramatically reduced, non-contiguous parts become possible, and there are no inherent layering effects that plague other methods via inhomogeneity, anisotropy, and cosmetic defect of final parts. However, there remain significant challenges that limit the potential of VAM. 1. Poor dose contrast of tomographic reconstructions leaves no room for errors in timing, in optical uniformity, etc., without impacting print shape fidelity. 2. VAM suffers from large striations, similar in appearance to layering, and with similar penalties. Here, we present a VAM image computation algorithm that significantly improves the contrast of applied dose, along with other performance metrics. This method takes a fundamentally different approach than prior algorithms by algebraically optimizing a model of the printed object instead of directly optimizing projection images. We also present a simple and effective method of eliminating striations in VAM via the addition of a latent cure step. We show that this method, facilitated by the high contrast reconstructions from our image computation algorithm, preserves theoretically perfect print shape fidelity. We conclude by discussing extensions and future work.

Femtosecond direct laser writing is a key enabling technology for complex microoptics. Imaging and illumination applications have impressively been demonstrated in the past. Here, we take 3D-printed microoptics one step further and assess the feasibility of complex microoptical systems: From a pinhole camera to a spectrometer. The first step in successful realization of complex microoptical systems are specialized measurement techniques that match both fabrication and simulation methods. Different setups are presented to reach stray light control, isolate topographic effects, and measure the efficiency of diffractive structures with small lateral extensions. All methods are easy to implement and can be key to targeted optimization of complex systems. In a second step, the spectrum of corresponding fabrication methods to fsDLW is extended by the microfluidic addition of a functional substance. We show that the incorporation of microfluidic channels into the 3D-printed mounting structures can be used to absorb a non-transparent fluid to create aperutres. Thus, a 3D-printed micro-pinhole camera can be demonstrated. Finally, all learnings and methods from these studies are combined to create complex microoptical systems. Multiple concepts of ultra-compact 3D-printed wide-angle cameras are examined. A special focus is laid on optical and mechanical design, measurement and optimization of highly tilted refractive and catadioptric freeform surfaces. An iterative correction mechanism is developed to improve shape fidelity to realize first implementations of 180°×360° field of view multi-aperture imaging. The highest complexity of a 3D-printed microoptical system is finally reached by the realization of an entire measurement system. The feasibility of a monolithic spectrometer in a volume of only 100 × 100 × 300 μm³ is theoretically and experimentally demonstrated. The results represent the first direct spectrometer in this miniature size range and unclose a new era of complex 3D-printed microoptical (measurement) systems, enabled by novel methods for charactarization, optimization and aperture fabrication.

Additive manufacturing using regolith, the main Lunar in-situ material resource, is offering a solution for long duration stay on the Moon or Mars. Using a homemade Selective Laser Sintering (SLS) machine with various lunar soil simulants, this study compares the experimental results with those of the simulations. Various lunar dust simulants were used and characterized by XRD. Specific care was taken when manipulating the nano-size powders. The experimental process was monitored by a pyrometer. The process conditions were optimized by numerical developed model on COMSOL Muliphysics. This model is based on data from the literature to cover material properties such as latent heat of fusion and temperature dependent parameters like the thermal conductivity, the thermal capacity, and the density. The meshing was adapted to the optimize the model efficiency. The laser source was modelled as a gaussian beam. The simulation was done with both a static and a moving beam where the effect of power, beam size and scanning speed was investigated. The evolution of temperature of the melt pool, the deformation of the structures and the evolution of the built-in stress were evaluated. This study shows that the outcome from the numerical model corroborates the experimental results in terms of spot size and temperature of the melt pool. The model predicts the failure of the sample for threshold values of power and scan speed for a given spot size.

This conference presentation, “Hummink: additive manufacturing at the nanoscale” was recorded for the 3D Printed Optics and Additive Photonic Manufacturing III conference at SPIE Photonics Europe 2022.

Volumetric Additive Manufacturing (VAM) is a novel light-based 3D printing technique which radially exposes a volume of material from 0 to 360º with different grayscale images to fabricate arbitrary 3D geometries within the material. Here, we leverage the ability to print grayscale structures into a solid volume of material with VAM to 3D print gradient index (GRIN) lens refractive index profiles into a flexible photopolymer. To realize 3D printed flexible GRIN lenses, the patterning optics and the gradient index material were co-designed and experimentally characterized. We also discuss further work needed to 3D print high quality, flexible GRIN lenses. VAM offers new approaches to additive manufacturing (AM) that are not possible with traditional AM techniques. One advantage over other AM techniques is the lack of layering effects due to the simultaneous printing of an entire 3D part as opposed to layer-by-layer deposition printing. Like other light-based AM techniques, control over local material properties is realized by modulating local polymer conversion. In a region of material, the product of exposure intensity and time determines the degree of conversion of that region, giving a method to spatially control conversion and thus the refractive index locally within a print. Additionally, due to the ability to print into a volume of material with VAM, we can print into solid volumes of materials. A solid, flexible photopolymer consisting of a low refractive index polyurethane host matrix, a high refractive index acrylate writing monomer, and a low absorption photoinitiator was developed for solid VAM printing. Upon exposure, polymerization and subsequent monomer transport occurs, changing the composition and index of refraction of that region dependent on exposure conditions. The grayscale images required to fabricate a flexible GRIN lens are computed by filtered back projection and optimized through an iterative algorithm for a more accurate GRIN profile.

Tailoring light beams by altering the initial properties is a key feature in many aspects of optical systems and applications. Hence, incoming light could be manipulated to fit various necessities such as focusing, rerouting, filtering, and general phase or amplitude beam shaping. The conventional way to manipulate light beams is usually carried out using various bulky optic elements, either spatial light modulators or mode converters. However, these approaches have several disadvantages, mainly due to their physical sizes, demanding fabrication process, and economical considerations. Furthermore, the ability to integrate such light functionalities directly onto optical elements is advantageous. The most common methods for such integrations use focused ion beam milling, electron beam lithography, and 3D-direct laser writing (3D-DLW). The latter approach enables 2-photon polymerization n in a three-dimensional printed photo-polymer resin, which allows the construction of readily assembled structures with sub-100 nm lateral resolution. The main advantages of the 3D-DLW technology are mask-less and single step processes, precise fabrication, and cost effective procedures. Furthermore, complicated micron-scale 3D structures can be fabricated and integrated directly onto other systems in a relatively short and rather simple manner. The Conversion of light beams might be achieved by changing the amplitude, phase or both of the incoming beams. Phase modulation is advantageous with respect to amplitude modulation, since there is no energy loss. In this work, we demonstrate an approach for tailoring light beams using micron-scale 3D printed phase elements directly on fiber tips. Hence, making an on-fiber integrated optical system. Here we show demonstrations for shaping light exiting the fibers into flattened intensity distribution beams (top-hat beams) and vortex beams. This demonstration may serve as a benchmark for various on-fiber photonics applications.

Bessel beams (BBs) were first introduced by Durnin in 1987 and have a transverse intensity distribution dictated by the Bessel function. BBs are characterized by their diffraction-free propagation and self-healing nature. The family of BBs is categorized into two sets - zeroth-order Bessel beams (ZOBBs) with narrow high-intensity center and high-order Bessel beams (HOBBs) with phase singularity and dark center. HOBBs are vortex beams as they carry orbital angular momentum (OAM). Several approaches to generate BBs have been devised; some of them include transforming a narrow annular beam with a lens, using an axicon, or using spatial light modulators. Nevertheless, these techniques involve space-consuming and expensive table-top diffractive optical elements. In recent years, the on-fiber generation of BBs has gained prominence as it offers miniaturized optical probes that can find exciting applications in different fields, ranging from bio-imaging to communications. Here, we present on-fiber 3D printed complex photonic structures that convert the Gaussian-like mode from single-mode fibers into BBs of various orders. Remarkably, we report for the first time the generation of HOBBs from optical fibers. Our technique is inspired by Durnin's approach of generating BBs due to the transformation of an annular beam through a lens. Our novel design has three sections; the first and second sections contain photonic crystal waveguides that convert the input Gaussian-like mode into an annular beam of arbitrary radius and width, which is then transformed into BBs with the help of a micro-lens. To generate HOBBs, we also integrated a spiral phase plate in the stacked structure. We compared the experimentally generated BB parameters with what predicted from theory and found an excellent match. For HOBBs, we performed modal decomposition to confirm the existence of OAM. Overall, we showcase the results of various BBs with orders up to 20.