Two-photon Lithography Research Articles

Exploring the mechanical properties of additively manufactured carbon-rich zirconia 3D microarchitectures

Two-photon lithography (TPL) is a promising technique for manufacturing ceramic microstructures with nanoscale resolution. The process relies on tailor-made precursor resins rich in metal–organic and organic constituents, which can lead to carbon-based residues incorporated within the ceramic microstructures. While these are generally considered unwanted impurities, our study reveals that the presence of carbon-rich residues in the form of graphitic and disordered carbon in tetragonal (t-) ZrO2 can benefit the mechanical strength of TPL microstructures. In order to achieve a better understanding of these effects, we deconvolute the structural and materials contributions to the strength of the 3D microarchitectures by comparing them to plain micropillars. We vary the organic content by different thermal treatments, resulting in different crystal structures. The highest compression strength of 3.73 ± 0.21 GPa and ductility are reached for the t-ZrO2 micropillars, which also contain the highest carbon content. This paradoxical finding opens up new perspectives and will foster the development of “brick and mortar”-like ceramic microarchitectures.

Determining Single Photon Quantum States through Robust Waveguides on Chip

Waveguided beam splitters were microfabricated by using a commercial two-photon lithography system (Nanoscribe), Ip-Dip as the waveguides and fused silica as the substrate, and they were covered with Loctite. The gap between the waveguides in the coupler was used to determine the transmission and reflection coefficients, and our results were compared with simulation results (using OptiFDTD software). The input and output ports of the beam splitters were spliced with multimode optical fibers in a robust system that can easily be handled. Then, they were tested by leading single photons (from an SPDC) to the beam splitters to produce different quantum statistics that were rated using the Fano factor.

Protein-Based Patterning to Spatially Functionalize Biomimetic Membranes.

The bottom-up reconstitution of proteins for their modular engineering into synthetic cellular systems can reveal hidden protein functions in vitro. This is particularly evident for the bacterial Min proteins, a paradigm for self-organizing reaction-diffusion systems that displays an unexpected functionality of potential interest for bioengineering: the directional active transport of any diffusible cargo molecule on membranes. Here, the MinDE protein system is reported as a versatile surface patterning tool for the rational design of synthetically assembled 3D systems. Employing two-photon lithography, microswimmer-like structures coated with tailored lipid bilayers are fabricated and demonstrate that Min proteins can uniformly pattern bioactive molecules on their surface. Moreover, it is shown that the MinDE system can form stationary patterns inside lipid vesicles, which allow the targeting and distinctive clustering of higher-order protein structures on their inner leaflet. Given their facile use and robust function, Min proteins thus constitute a valuable molecular toolkit for spatially patterned functionalization of artificial biosystems like cell mimics and microcarriers.

Enabling high-quality transparent conductive oxide on 3D printed ZrO2 architectures through atomic layer deposition

The conformal atomic layer deposition of a transparent conductive oxide composed of Al-doped ZnO (AZO) over three-dimensional (3D) shaped ZrO2 microarchitectures produced using two-photon lithography (TPL) is reported here for the first time. The effect of ZrO2 morphology, surface roughness, and crystallographic phase (tetragonal and monoclinic) on the quality and properties of the deposited ZnO and AZO thin films is investigated. No discontinuities, domains, or areas differing from the desired chemical composition have been found in films grown over the 3D structures. Three different Al dopant concentrations (4.0 %, 4–5 %, and 5.0 % Al doping cycles) are examined and compared to undoped ZnO. AZO and ZnO optical and electrical properties are studied using cathodoluminescence (CL) and Hall effect measurements. The CL study confirms that the observed emissions from the ZnO and AZO films are associated with the near band emission of ZnO and defects, i.e., zinc and oxygen vacancies and interstitial oxygen. The AZO films exhibit n-type semiconductor behavior, and a minimum resistivity of 1.2 × 10-3 Ω cm is achieved. From a broad perspective, AZO deposition on 3D microarchitectures opens a new route towards dimensionally refined optoelectronic devices in which the ZrO2/AZO can serve as an enabling role for the production of electrodes.

Direct laser writing of 3D electrodes on flexible substrates

This report describes a 3D microelectrode array integrated on a thin-film flexible cable for neural recording in small animals. The fabrication process combines traditional silicon thin-film processing techniques and direct laser writing of 3D structures at micron resolution via two-photon lithography. Direct laser-writing of 3D-printed electrodes has been described before, but this report is the first to provide a method for producing high-aspect-ratio structures. One prototype, a 16-channel array with 300 µm pitch, demonstrates successful electrophysiological signal capture from bird and mouse brains. Additional devices include 90 µm pitch arrays, biomimetic mosquito needles that penetrate through the dura of birds, and porous electrodes with enhanced surface area. The rapid 3D printing and wafer-scale methods described here will enable efficient device fabrication and new studies examining the relationship between electrode geometry and electrode performance. Applications include small animal models, nerve interfaces, retinal implants, and other devices requiring compact, high-density 3D electrodes.

Finite voxel size compensation for microprinting of parabolic X-ray lenses by two-photon lithography

3D-microprinting by two-photon lithography can be applied to fabricate X-ray lenses. However, when the lens’ curvature radius approaches voxel size, the lens’ refractive surface essentially differs from the defined model. This shape distortion leads to defocusing of X-ray irradiation and astigmatism. In current work we suggest the compensation of the parabolic lens model taking into account finite voxel size.

Fabrication of GRIN microstructures by two-photon lithography

The method of two-photon lithography is used to fabricate GRIN microstructures. Test rectangular structures with sizes 25 × 25 × 3 micrometers were used with varying laser intensity by linear or gaussian distribution in one dimension. The resulting refractive index has been tuned in the range of 0.03. The suggested method can be applied to produce arbitrarily shaped 3D GRIN micro-optical elements.

3D-patterned inverse-designed mid-infrared metaoptics

Modern imaging systems can be enhanced in efficiency, compactness, and application through the introduction of multilayer nanopatterned structures for manipulation of light based on its fundamental properties. High transmission multispectral imaging is elusive due to the commonplace use of filter arrays which discard most of the incident light. Further, given the challenges of miniaturizing optical systems, most cameras do not leverage the wealth of information in polarization and spatial degrees of freedom. Optical metamaterials can respond to these electromagnetic properties but have been explored primarily in single-layer geometries, limiting their performance and multifunctional capacity. Here we use advanced two-photon lithography to realize multilayer scattering structures that achieve highly nontrivial optical transformations intended to process light just before it reaches a focal plane array. Computationally optimized multispectral and polarimetric sorting devices are fabricated with submicron feature sizes and experimentally validated in the mid-infrared. A final structure shown in simulation redirects light based on its angular momentum. These devices demonstrate that with precise 3-dimensional nanopatterning, one can directly modify the scattering properties of a sensor array to create advanced imaging systems.

Aberration-compensated supercritical lens for sub-diffractive focusing within 20° field of view.

The supercritical lens has shown a remarkable capability of achieving far-field sub-diffraction limited focusing through elaborating a modulated interference effect. Benefiting from the relative high energy utilization efficiency and weak sidelobe properties, the supercritical lens holds significant advantage in a series of application scenarios. However, all of the demonstrated supercritical lenses mainly work in the on-axis illumination condition, so the off-axis aberration effect will severely deteriorate its sub-diffraction limit focusing capability for the illuminating beam with an oblique angle. In this work, an aberration-compensated supercritical lens with single-layer configuration is proposed and experimentally demonstrated. Such a single-layer supercritical lens consists of multilevel phase configurations patterned with the two-photon polymerization lithography technique. The simulation and experimental recorded results show that the aberration-compensated supercritical lens with a numerical aperture value of 0.63 could achieve a far-field sub-diffraction limited focusing property within 20° field of view at a wavelength of λ = 633 nm. This monochromatic aberration-compensated supercritical lens with single-layer configuration indicates excellent potential in the development of laser scanning ultrahigh optical storage and label free super-resolution imaging.

High‐Resolution Patterning of Organic–Inorganic Photoresins for Tungsten and Tungsten Carbide Microstructures

Tungsten is an important material for high‐temperature applications due to its high chemical and thermal stability. Its carbide, that is, tungsten carbide, is used in tool manufacturing because of its outstanding hardness and as a catalyst scaffold due to its morphology and large surface area. However, microstructuring, especially high‐resolution 3D microstructuring of both materials, is a complex and challenging process which suffers from slow speeds and requires expensive specialized equipment. Traditional subtractive machining methods, for example, milling, are often not feasible because of the hardness and brittleness of the materials. Commonly, tungsten and tungsten carbide are manufactured by powder metallurgy. However, these methods are very limited in the complexity and resolution of the produced components. Herein, tungsten ion‐containing organic–inorganic photoresins, which are patterned by two‐photon lithography (TPL) at micrometer resolution, are introduced. The printed structures are converted to tungsten or tungsten carbide by thermal debinding and reduction of the precursor or carbothermal reduction reaction, respectively. Using TPL, complex 3D tungsten and tungsten carbide structures are prepared with a resolution down to 2 and 7 μm, respectively. This new pathway of structuring tungsten and its carbide facilitates a broad range of applications from micromachining to metamaterials and catalysis.

Parallel two-photon lithography achieving uniform sub-200 nm features with thousands of individually controlled foci.

The limited throughput of nano-scale laser lithography has been the bottleneck for its industrial applications. Although using multiple laser foci to parallelize the lithography process is an effective and straightforward strategy to improve rate, most conventional multi-focus methods are plagued by non-uniform laser intensity distribution due to the lack of individual control for each focus, which greatly hinders the nano-scale precision. In this paper, we present a highly uniform parallel two-photon lithography method based on a digital mirror device (DMD) and microlens array (MLA), which allows the generation of thousands of femtosecond (fs) laser foci with individual on-off switching and intensity-tuning capability. In the experiments, we generated a 1,600-laser focus array for parallel fabrication. Notably, the intensity uniformity of the focus array reached 97.7%, where the intensity-tuning precision for each focus reached 0.83%. A uniform dot array structure was fabricated to demonstrate parallel fabrication of sub-diffraction limit features, i.e., below 1/4 λ or 200 nm. The multi-focus lithography method has the potential of realizing rapid fabrication of sub-diffraction, arbitrarily complex, and large-scale 3D structures with three orders of magnitude higher fabrication rate.

Finite Voxel Size Compensation for Microprinting of Parabolic X-ray Lenses by Two-Photon Lithography

Finite Voxel Size Compensation for Microprinting of Parabolic X-ray Lenses by Two-Photon Lithography

Ultrafast 3D nanofabrication via digital holography

There has been a compelling demand of fabricating high-resolution complex three-dimensional (3D) structures in nanotechnology. While two-photon lithography (TPL) largely satisfies the need since its introduction, its low writing speed and high cost make it impractical for many large-scale applications. We report a digital holography-based TPL platform that realizes parallel printing with up to 2000 individually programmable laser foci to fabricate complex 3D structures with 90 nm resolution. This effectively improves the fabrication rate to 2,000,000 voxels/sec. The promising result is enabled by the polymerization kinetics under a low-repetition-rate regenerative laser amplifier, where the smallest features are defined via a single laser pulse at 1 kHz. We have fabricated large-scale metastructures and optical devices of up to centimeter-scale to validate the predicted writing speed, resolution, and cost. The results confirm our method provides an effective solution for scaling up TPL for applications beyond laboratory prototyping.

An Autofocus Method Based on Improved Differential Confocal Microscopy in Two-Photon Lithography

Two-photon lithography (TPL) plays a vital role in microstructure fabrication due to its high processing accuracy and maskless characteristics. To optimize the manufacturing quality deteriorated by the defocus of the substrate, an autofocus approach based on improved differential confocal microscopy (IDCM) is proposed in this paper. Through analyzing the intensity response signals from two detectors with symmetrical axial offset, the defocus amount is measured and compensated for with high precision and noise immunity to stabilize the substrate. The verification experiments on the coverslip reported a detection sensitivity of 5 nm, a repetitive measurement accuracy of less than 15 nm, and a focusing accuracy reaching around 5 nm. The consistency between simulation and characterization demonstrated the effectiveness and superior performance of the autofocus system for the high production quality of the metalens array. The proposed autofocus method shows promise for further application to the fabrication of complex structures on various substrates.

Ketocoumarin-Based Photoinitiators for High-Sensitivity Two-Photon Lithography

Ketocoumarins are attractive and distinct photosensitizers due to their high molar extinction coefficients, high intersystem crossing coefficients, and high photochemical stability. As a classic commercial ketocoumarin-based two-photon initiator, 7-diethylamino-3-thenoylcoumarin (DETC) was widely used in two-photon lithography. However, the large fluorescence quantum yield and low two-photon absorption cross section value greatly limit its application in high-throughput nanofabrication. In this work, a series of DETC derivatives were developed by extending the length of the alkyl chain and integrating different donor and acceptor groups. These ketocoumarin-based initiators, namely, compounds 1–7, were designed, synthesized, and unambiguously characterized. Compared with DETC, these compounds exhibit higher molar extinction coefficient, lower fluorescence quantum yield, higher two-photon absorption cross section, and improved sensitivity in two-photon lithography. Among these molecules, compound 7 with expanded π-electron systems and structures with enhanced intramolecular charge transfer exhibits the best sensitivity in two-photon lithography. With compound 7-based photoresist, many kinds of complex three-dimensional patterns can be fabricated using two-photon lithography at a writing speed of up to 60 mm s–1. The high two-photon initiation sensitivity makes these compounds promising candidates for commercialization and provides a new design concept for the development of new initiators.

Toughness Amplification via Controlled Nanostructure in Lightweight Nano-Bouligand Materials.

The enhanced properties of nanomaterials make them attractive for advanced high-performance materials, but their role in promoting toughness has been unclear. Fabrication challenges often prevent the proper organization of nanomaterial constituents, and inadequate testing methods have led to a poor knowledge of toughness at small scales. In this work, the individual roles of nanomaterials and nanoarchitecture on toughness are quantified by creating lightweight materials made from helicoidal polymeric nanofibers (nano-Bouligand). Unidirectional ( = 0°) and nano-Bouligand beams ( = 2°-90°) are fabricated using two-photon lithography and are designed in a micro-single edge notch bend (µ-SENB) configuration with relative densities between 48% and 81%. Experiments demonstrate two unique toughening mechanisms. First, size-enhanced ductility of nanoconfined polymer fibers increases specific fracture energy by 70% in the 0° unidirectional beams. Second, nanoscale stiffness heterogeneity created via inter-layer fiber twisting impedes crack growth and improves absolute fracture energy dissipation by 48% in high-density nano-Bouligand materials. This demonstration of size-enhanced ductility and nanoscale heterogeneity as coexisting toughening mechanisms reveals the capacity for nanoengineered materials to greatly improve mechanical resilience in a new generation of advancedmaterials.

Selective Positioning of Different Cell Types on 3D Scaffolds via DNA Hybridization.

Three-dimensional (3D) microscaffolds for cell biology have shown their potential in mimicking physiological environments and simulating complex multicellular constructs. However, controlling the localization of cells precisely on microfabricated structures is still complex and usually limited to two-dimensional assays. Indeed, the implementation of an efficient method to selectively target different cell types to specific regions of a 3D microscaffold would represent a decisive step toward cell-by-cell assembly of complex cellular arrangements. Here, we use two-photon lithography (2PL) to fabricate 3D microarchitectures with functional photoresists. UV-mediated click reactions are used to functionalize their surfaces with single-stranded DNA oligonucleotides, using sequential repetition to decorate different scaffold regions with individual DNA addresses. Various immortalized cell lines and stem cells modified by grafting complementary oligonucleotides onto the phospholipid membranes can then be immobilized onto complementary regions of the 3D structures by selective hybridization. This allows controlled cocultures to be established with spatially separated arrays of eukaryotic cells in 3D.

Two-photon shape-modulated maskless lithography of positive photoresist of S1813

The authors systematically investigated influence factors for two-photon maskless lithography of S1813 photoresist. A trench with a width of 155 nm, about 1/5 of the laser wavelength, was fabricated by accurate tuning the scanning speed. Two-photon shape-modulated maskless lithography, a method which employs different combined scanning and exposing approaches was proposed and studied. The patterns of rectangle, parallelogram, rice grain, arc, and “x” letter, and the quatrefoil hole patterns with different shape hearts, were all fabricated by using this method. This work is a protocol for lithography of S1813 photoresist with a high spatial resolution and flexibility. The fabricated trenches and diverse micro/nano patterns can be applied in the fabrication of integrated circuits, metamaterials structures, and SERS and biosensor substrates. This work will be conducive to the development and application of the two-photon maskless lithography of S1813.

Efficient fabrication method for non-periodic microstructures using one-step two-photon lithography and a metal lift-off process.

This paper presents a mask-less, flexible, efficient, and high-resolution fabrication method for non-periodic microstructures. Sub-wavelength micro-polarizer arrays, (MPAs) which are the most essential part of the focal plane polarimeters, are typical non-periodic structures. The grating ridges of each polarizer were oriented in four different directions offset by 45°, corresponding to different polarization directions. The finite element method was introduced to optimize the structural parameters of the MPA in the far-infrared region. The numerical results demonstrated that the designed MPA had a TM transmittance of more than 55% and an extinction ratio no less than 7dB. An aluminum MPA that operates in the 8-14µm infrared region was prepared by one-step two-photon lithography (TPL) and the metal lift-off process. The femtosecond laser exposed the photoresist with only a single scan, making TPL very efficient. The fabricated single-layer sub-wavelength MPAs with a period of 3µm, a duty cycle of 0.35-0.5, and a height of 150nm, were analyzed by an optical microscope and an atomic force microscope. The successful fabrication of the MPA indicated that one-step TPL could be a viable and efficient method for pattern preparation in the fabrication of non-periodic microstructures.

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.