Two-photon grayscale lithography for free-form micro-optical arrays

Additive manufacturing of double-sided centimeter-scale optics

In this article we demonstrate rapid manufacturing of an illumination quality double-sided centimeter-scale optics without any post-processing. Two methods are presented: additive manufacturing (AM) using an optically transparent material with a water-soluble support material (support method), and alternatively, using only the optically transparent material but flipping the optics mid-process (flipping method). The main advantage of the flipping method is that only one material is needed. However, the use of support material is more straightforward, allows better alignment between the two sides of the lens and enables more complex lenses manufactured in the future. Both methods result in sufficient surface quality, i.e. surface roughness Rq = 11.48 ±3.32 nm and form accuracy of ±10 μm, for the purposes of illumination optics.

Generic fabrication solution of freeform Fresnel optics using ultra-precision turning

Freeform Fresnel optics represent an emerging category of modern optics that reproduces powerful optical functionalities while maintaining an ultra-compact volume. The existing ultra-precision machining (UPM) technique faces technical challenges in meeting the fabrication requirements for freeform Fresnel optics because of the absence of appropriate geometry definition and corresponding tool path planning strategy to overcome the extreme asymmetry and discontinuity. This study proposes a new scheme for ultra-precision machining using four axes (X, Y, Z, C) to fabricate freeform Fresnel optics, including a general geometry description for freeform Fresnel optics, the quasi-spiral tool path generation strategy to overcome the lack of rotary symmetry, and the adaptive tool pose manipulation method for avoiding tool interference. In addition, the tool edge compensation and the adaptive timestep determination are also introduced to enhance the performance and efficiency of the proposed scheme. The machining of two exemplary freeform Fresnel lenses is successfully demonstrated. Overall, this study introduces a comprehensive routine for the fabrication of freeform Fresnel optics and proposes the adaptive tool pose manipulation scheme, which has the potential for broader applications in the ultra-precision machining of complex or discontinuous surfaces.

Industrial perspectives for personalized microneedles

Microneedles and, subsequently, microneedle arrays are emerging miniaturized medical devices for painless transdermal drug delivery. New and improved additive manufacturing methods enable novel microneedle designs to be realized for preclinical and clinical trial assessments. However, current literature reviews suggest that industrial manufacturers and researchers have focused their efforts on one-size-fits-all designs for transdermal drug delivery, regardless of patient demographic and injection site. In this perspective article, we briefly review current microneedle designs, microfabrication methods, and industrialization strategies. We also provide an outlook where microneedles may become personalized according to a patient's demographic in order to increase drug delivery efficiency and reduce healing times for patient-centric care.

Fabrication of a multifaceted mapping mirror using two-photon polymerization for a snapshot image mapping spectrometer

A design and fabrication technique for making high-precision and large-format multifaceted mapping mirrors is presented. The method is based on two-photon polymerization, which allows more flexibility in the mapping mirror design. The mirror fabricated in this paper consists of 36 2D tilted square pixels, instead of the continuous facet design used in diamond cutting. The paper presents a detailed discussion of the fabrication parameters and optimization process, with particular emphasis on the optimization of stitching defects by compensating for the overall tilt angle and reducing the printing field of view. The fabricated mirrors were coated with a thin layer of aluminum (93 nm) using sputter coating to enhance the reflection rate over the target wave range. The mapping mirror was characterized using a white light interferometer and a scanning electron microscope, which demonstrates its optical quality surface (with a surface roughness of 12 nm) and high-precision tilt angles (with an average of 2.03% deviation). Finally, the incorporation of one of the 3D printed mapping mirrors into an image mapping spectrometer prototype allowed for the acquisition of high-quality images of the USAF resolution target and bovine pulmonary artery endothelial cells stained with three fluorescent dyes, demonstrating the potential of this technology for practical applications.

Two Functions from a Single Photoresist: Tuning Microstructure Degradability from Light-Stabilized Dynamic Materials

A photoresist-based on a light-stabilized dynamic material driven by an out-of-equilibrium photo-Diels-Alder reaction of triazolinediones with naphthalenes-whose ability to intrinsically degrade postprinting can be tuned by a simple adjustment of laser intensity during 3D laser lithography is introduced. The resist's ability to form stable networks under green light irradiation that degrade in the dark is transformed into a tunable degradable 3D printing material platform. In-depth characterization of the printed microstructures via atomic force microscopy before and during degradation reveals the high dependency of the final structures' properties on the writing parameters. Upon identifying the ideal writing parameters and their effect on the network structure, it is possible to selectively toggle between stable and fully degradable structures. This simplifies the direct laser writing manufacturing process of multifunctional materials significantly, which typically requires the use of separate resists and consecutive writing efforts to achieve degradable and nondegradable material sections.

Hybrid achromatic microlenses with high numerical apertures and focusing efficiencies across the visible

Compact visible wavelength achromats are essential for miniaturized and lightweight optics. However, fabrication of such achromats has proved to be exceptionally challenging. Here, using subsurface 3D printing inside mesoporous hosts we densely integrate aligned refractive and diffractive elements, forming thin high performance hybrid achromatic imaging micro-optics. Focusing efficiencies of 51-70% are achieved for 15μm thick, 90μm diameter, 0.3 numerical aperture microlenses. Chromatic focal length errors of less than 3% allow these microlenses to form high-quality images under broadband illumination (400-700 nm). Numerical apertures upwards of 0.47 are also achieved at the cost of some focusing efficiency, demonstrating the flexibility of this approach. Furthermore, larger area images are reconstructed from an array of hybrid achromatic microlenses, laying the groundwork for achromatic light-field imagers and displays. The presented approach precisely combines optical components within 3D space to achieve thin lens systems with high focusing efficiencies, high numerical apertures, and low chromatic focusing errors, providing a pathway towards achromatic micro-optical systems.

Complex aspherical singlet and doublet microoptics by grayscale 3D printing

We demonstrate 3D printed aspherical singlet and doublet microoptical components by grayscale lithography and characterize and evaluate their excellent shape accuracy and optical performance. The typical two-photon polymerization (2PP) 3D printing process creates steps in the structure which is undesired for optical surfaces. We utilize two-photon grayscale lithography (2GL) to create step-free lenses. To showcase the 2GL process, the focusing ability of a spherical and aspherical singlet lens are compared. The surface deviations of the aspherical lens are minimized by an iterative design process and no distinct steps can be measured via confocal microscopy. We design, print, and optimize an air-spaced doublet lens with a diameter of 300 µm. After optimization, the residual shape deviation is less than 100 nm for the top lens and 20 nm for the bottom lens of the doublet. We examine the optical performance with an USAF 1951 resolution test chart to find a resolution of 645 lp/mm.

Towards efficient structure prediction and pre-compensation in multi-photon lithography

Microscale 3D printing technologies have been of increasing interest in industry and research for several years. Unfortunately, the fabricated structures always deviate from the respective expectations, often caused by the physico-chemical properties during and after the printing process. Here, we show first steps towards a simple, fast and easy to implement algorithm to predict the final structure topography for multi-photon lithography - also known as Direct Laser Writing (DLW). The three main steps of DLW, (i) exposure of a photo resin, (ii) cross-linking of the resin, and (iii) subsequent shrinkage are approximated by mathematical operations, showing promising results in coincidence with experimental observations. For example, the root-mean-square error (rmse) between the unmodified 3D print of a radial-symmetrically chirped topography and our predicted topography is only 0.46 µm, whereas the rmse between this 3D print and its target is 1.49 µm. Thus, our robust predictions can be used prior to the printing process to minimize undesired deviations between the target structure and the final 3D printed structure. Using a Downhill-Simplex algorithm for identifying the optimal prediction parameters, we were able to reduce the rmse from 4.04 µm to 0.33 µm by only two correction loops in our best-case scenario (rmse = 0.72 µm after one loop). Consequently, this approach can eliminate the need for many structural optimization loops to produce highly conformal and high quality micro structures in the future.

Model for polymerization and self-deactivation in two-photon nanolithography

A mathematical model is developed to describe the photochemical processes in two-photon nanolithography, including two-step absorption leading to initiation and self-deactivation of the photoinitiator by laser irradiance, polymer chain propagation, termination, inhibition, and inhibitor and photoinitiator diffusion. This model is solved numerically to obtain the concentrations of the reaction species as a function of time and space as a laser beam is scanned through a volume of photoresist, from which a voxel size or linewidth is determined. The most impactful process parameters are determined by fitting the model to experimentally measured linewidths for a range of laser powers and scanning speeds, while also obtaining effective nonlinearities that are similar to previously measured values. The effects and sensitivities of the different process parameters are examined. It is shown that the photopolymerization process is dominated by diffusion of photoinitiators and oxygen inhibitors, and that self-deactivation can lead to higher effective nonlinearities in two-photon nanolithography.

Freeform optics: introduction

This feature issue of Optics Express highlights 28 state-of-the-art articles that capture a snapshot of the recent developments in the field of freeform optics. As an introduction, the editors provide an overview of all published articles, which cover a broad range of topics in freeform optics. The wide variety of applications presented here demonstrates that freeform optics is a growing and vibrant field with many more innovations to come.