Two-photon Photopolymerization Research Articles

Fabrication of predesigned 3D carbon based microstructures via two-photon vat photopolymerization and susceptor-assisted microwave post-processing

This study presents a fabrication strategy for 3D carbon microstructures using via two-photon vat photopolymerization (2p-VPP) and susceptor-assisted microwave pyrolysis (SMWP). Fabricating carbon-based electrode materials with miniaturized functional structures is pivotal for developing high-performance electrochemical microdevices. However, efficiently producing these structures in the submicron regime with desired materials is still challenging. To address this, a hybrid microfabrication strategy was developed, and 3D carbon-based microstructures with submicron resolution were successfully produced. A carbon nanotube nanocomposite photoresist (SCNT-PR1) with improved dispersibility was first prepared. After a comprehensive printability assessment, 3D microstructures with a resolution of 833 ± 54 nm were produced using 2p-VPP. Through SMWP, the microstructures were transformed into pyrolytic carbon (PyC) nanocomposite microstructures with retained geometrical features. SMWP was shown to produce PyC with a less disordered carbon atomic structure, thanks to the accelerated pyrolysis reaction under MW irradiation, when compared to pyrolysis using a conventional furnace. The resistivity was reduced by over 75% from 0.69 ± 0.13 Ω cm to 0.16 ± 0.01 Ω cm, and enhanced electrochemical performance was confirmed. The fabricated PyC nanocomposite showed a further 12% reduction in electrical resistivity and a 20% lower charge transfer resistance for the redox reaction when using SCNT-PR1 as the precursor. Overall, this hybrid fabrication strategy demonstrates the advantages of producing 3D carbon microstructures with precise control over the geometric features and enhancing electrical and electrochemical properties compared to the conventional pyrolysis method. Its potential could be extended to the fabrication of miniaturized electrochemical devices, including microelectronics and point-of-care devices.

Femtosecond Two-Photon Photopolymerization—Creating 3D Microstructures for Optical Applications

Femtosecond Two-Photon Photopolymerization—Creating 3D Microstructures for Optical Applications

Three-dimensional printing of a beam expander to enable the combination of hundred-micron optical elements and a single-mode fiber.

We use a flexible two-photon photopolymerization direct laser writing to fabricate an integrated diffractive lens system on a fiber tip to expand the output beam of the fiber. The results show that the micro-integrated beam expander based on double lenses (axial size of about 100 μm) has a magnification of 5.9 and a loss of 0.062 dB. Subsequently, we demonstrate the fabrication of a spiral phase plate (diffractive optical elements) and micro-lens arrays (refractive optical elements) on an integrated beam expander, and their optical properties are measured and analyzed, respectively. This Letter is an exploration of the future integrated micro-optical systems on an optical fiber tip.

X-photon laser direct write 3D nanolithography

ABSTRACT Multiphoton photopolymerisation (MPP), also known as 3D nanoprinting, was studied using a wavelength-tunable femtosecond laser. The possibility of using any colour of the spectrum from 500 to 1200 nm with a fixed pulse width of 100 fs revealed an interplay of photophysical mechanisms more delicate than just two-photon photopolymerisation. An effective order of absorption, i.e. the X-photon absorption, as well as optimal exposure conditions were assessed for photosensitised and pure SZ2080 pre-polymer. The tunability of wavelength greatly influenced the dynamic fabrication window (DFW), optimised conditions resulting in a 10-fold increase. Furthermore, a non-trivial energy deposition by X-photon absorption was noted with an onset of a strong lateral size increase at longer wavelengths and can be understood as due to reaching epsilon-near-zero conditions. Such a control over the voxel aspect ratio and, consequently, the photopolymerised volume, may boost 3D nanoprinting efficiency. Overall, the results reveal wavelength being an important degree of freedom to tailor the MPP process and, if optimised, benefiting broad applications in areas of micro-optics, nanophotonic devices, metamaterials and tissue engineering.

Two-photon photopolymerization directly initiated by spiropyran photochromic molecules

Two-photon photopolymerization directly initiated by spiropyran photochromic molecules

Benzylidene Cyclopentanone Derivative Photoinitiator for Two-Photon Photopolymerization-Photochemistry and 3D Structures Fabrication for X-ray Application.

Micron- and submicron-scale 3D structure realization nowadays is possible due to the two-photon photopolymerization (TPP) direct laser writing photolithography (DLW photolithography) method. However, the achievement of lithographic features with dimensions less than 100 nm is in demand for the fabrication of micro-optical elements with high curvature values, including X-ray microlenses. Spectroscopic and photochemical study of a photoinitiator (PI) based on a methyl methacrylate derivative of 2,5-bis(4-(dimethylamino)benzylidene) cyclopentanone was performed. Enhanced intersystem crossing in the methyl methacrylate derivative results in increased radical generation for the subsequent initiation of polymerization. A comprehensive study of the new photocompositions was performed, with particular emphasis on photochemical constants, the degree of photopolymerization, and topology. The optimal parameters for the fabrication of mechanically stable structures were determined in this research. The threshold dose parameters for lithography (radiation power of 5 mW at a speed of 180 µm/s) when trying to reach saturation values with a conversion degree of (35 ± 1) % were defined, as well as parameters for sub-100 nm feature fabrication. Moreover, the 45 nm feature size for elements was reached. Fabrication of X-ray lens microstructures was also demonstrated.

Characterization of two-photon photopolymerization fabrication using high-speed optical diffraction tomography

Two-photon photopolymerization (TPP) has recently become a popular method for the fabrication of three-dimensional (3D) micro- and nanostructures. The reproduction fidelity of the designed micro- and nanostructures is influenced by experimental writing conditions, including laser power, exposure time, etc. To determine the appropriate writing parameters, characterization of morphological features and surface roughness during the experiment is needed. Traditional characterization methods for TPP, e.g., scanning electron microscopy and atomic force microscopy, have limited speed and cannot study internal structures without invasive approaches. Optical diffraction tomography (ODT) is an emerging label-free 3D imaging technique based on reconstructing the object’s 3D refractive index (RI) distribution with diffraction-limited resolution. Here, we propose a non-invasive solution to fully characterize the TPP-fabricated structures using a high-speed ODT technique, which can eliminate the need for complex sample preparation, such as fluorescence labelling or metal-coating, and achieve a full 3D measurement time of 6 ms. By visualizing and studying different TPP-fabricated structures, including embedded spirals and cubes, via the ODT system, the fabrication quality, including 3D morphological features, exposure levels, and surface roughness, can be examined quantitatively. The results suggest our method can effectively improve the fabrication quality and reproducibility of TPP, generating impacts on the nanofabrication community.

Improved two-photon photopolymerisation and optical trapping with aberration-corrected structured light

We demonstrate the effectiveness of phase only aberration corrections of structured light and their application to versatile optical trapping setups. We calculate phase corrections before (ex-situ) and after (in-situ) a high numerical aperture microscope objective using a spatial light modulator (SLM), and investigate how these corrections can be used to improve the efficiency and resolution of micro-structures fabricated through two-photon-photopolymerisation (2PP). We apply a phase retrieval algorithm to correct for distortions in a femtosecond laser that enables the fabrication of 3D structures using as many as 50 simultaneous foci. The inclusion of aberration correction in the fabrication process shows improved confinement of optically trapped particles and more efficient polymerisation while minimising intensity variations at individual foci, which potentially damage the structure during fabrication. We find that phase corrections allow for consistent voxel sizes, increased sharpness, and an expanded effective printing range when using an SLM, while also allowing for closer proximity of individual trap foci, minimising interference effects that hinder fabrication resolution.

Unsymmetrical Methacrylate-Containing Benzylidene Cyclopentanone Dyes in One- and Two-Photon Photopolymerization

Unsymmetrical Methacrylate-Containing Benzylidene Cyclopentanone Dyes in One- and Two-Photon Photopolymerization

Luminescence confocal microscopy of 3D components of photonic integrated circuits fabricated by two-photon photopolymerization

Direct laser writing (DLW) lithography has been growing rapidly and is widely used for photonic integrated circuit (PIC) fabrication manufacturing in micro optics and photonics to fabricate 2D, 2.5D, and 3D components due to low cost, high precision and no need for mask exposure. There is a huge demand for PIC for instruments, systems and complexes of optoelectronic navigation instrumentation, as well as for use in a wide range of areas: biophotonics, signal detection and processing, robotics, telecommunications. The technique of DLW is based on nonlinear interaction between laser radiation and the photoresist medium (two photon absorption and consequent photopolymerization). The traditional method for studying the morphology of obtained PIC elements is the method of scanning electron microscopy. An alternative method of morphological analysis of PICs obtained by two photon absorption-direct laser writing (TPA-DLW) is proposed here - scanning luminescence confocal microscopy method. It is an effective method that enables obtaining conclusive information on the geometrical parameters as the morphology properties as well as the luminescence information.

Femtosecond Green Light Generation Using a MgO-Doped Periodically Poled Lithium Niobate Crystal Pumped by a Yb-Doped Fiber Laser

We demonstrate, in this paper, the creation of a femtosecond green laser achieved from a MgO-doped periodically poled lithium niobate crystal pumped by a home-made Yb-doped MOPA laser system. With an incident fundamental average power of 2 W, 820 mW (41 nJ) is produced at 515 nm with pulse duration of ~540 fs, corresponding to conversion efficiency of 40%. The standard deviation of the 515 nm pulse-to-pulse intensity is calculated as 0.007 (normalized), and the average power keeps stable with a root-mean-square value of 0.18% in an eight hour measurement. The spectral characters of the green laser and laser-induced damage thresholds of the crystal are also investigated. This stable laser system provides a compact and portable laser source for two-photon photopolymerization application.

Tetramethacrylic benzylidene cyclopentanone dye for one- and two-photon photopolymerization

The crystalline one-component methacrylate containing photoinitiator 4MetBAC was synthesized and characterized by 1H NMR, 13C NMR, IR-, UV–Vis spectrum, and X-Ray analysis. The one- and two-photon absorption, photoluminescence, and electrochemical properties of 4MetBAC in solution were investigated. The polymerization of PETA was induced directly by 4MetBAC (2.5–11.5 mM) under irradiation with LED@380 (1PP) or focused beam of femtosecond laser with wavelength 780 nm (2PP). The polymer nanostructures with intricate architecture («sleepers-rails»-, «cell scaffold»- and cylindrical spiral structures) were fabricated by the DLW method. The polymerization threshold of the composition based on 4Met-BAC (11.5 mM) was 0.9 mW at the recording speed of 170 μm/s. The smallest stable structures with the linear fragment sizes of 72 ± 5 nm (less than λ/10) have been obtained by nanolithography. Due to the polymerizable groups, 4Met-BAC has the ability to link into the polymer networks. Experiments with human dermal fibroblasts and cells of the HEK 293 line have been demonstrated that the final polymers prepared by both one- and two-photon photopolymerization (cell scaffold) are biosafety. Hence we believe that the PETA composition based on one-component methacrylate-containing photoinitiator 4Met-BAC opens up wide capabilities for creating three-dimensional topological structures with high submicron resolution by the DLW method for optoelectronic and biological applications.

Near-Infrared Self-Written Optical Waveguides for Fiber-to-Chip Self-Coupling

The light-induced self-written (LISW) waveguide technique is a promising candidate for the practical realization of a passive alignment between telecommunication and silicon photonic devices. While silicon is transparent to wavelengths above 1100 nm, the one-photon polymerization wavelength range is limited below 1100 nm. In this study, LISW waveguide fabrication at telecommunication wavelengths was demonstrated using a single-photon photopolymerizing resin. The LISW waveguide fabrication threshold values are lower than that of the two-photon photopolymerization system by six to seven orders of magnitude. LISW waveguides are fabricated from a single-mode fiber, a vertical-cavity surface-emitting laser, and a silicon waveguide. Using this technique, fiber-to-chip self-coupling between silicon photonic devices at telecommunication wavelengths is demonstrated. The results demonstrate the potential of the LISW technique for optical self-coupling of telecommunication and silicon photonic devices.

Elastomeric nematic colloids, colloidal crystals and microstructures with complex topology.

Control of physical behaviors of nematic colloids and colloidal crystals has been demonstrated by tuning particle shape, topology, chirality and surface charging. However, the capability of altering physical behaviors of such soft matter systems by changing particle shape and the ensuing responses to external stimuli has remained elusive. We fabricated genus-one nematic elastomeric colloidal ring-shaped particles and various microstructures using two-photon photopolymerization. Nematic ordering within both the nano-printed particle and the surrounding medium leads to anisotropic responses and actuation when heated. With the thermal control, elastomeric microstructures are capable of changing from genus-one to genus-zero surface topology. Using these particles as building blocks, we investigated elastomeric colloidal crystals immersed within a liquid crystal fluid, which exhibit crystallographic symmetry transformations. Our findings may lead to colloidal crystals responsive to a large variety of external stimuli, including electric fields and light. Pre-designed response of elastomeric nematic colloids, including changes of colloidal surface topology and lattice symmetry, are of interest for both fundamental research and applications.

Xolography for linear volumetric 3D printing.

The range of applications for additive manufacturing is expanding quickly, including mass production of athletic footwear parts1, dental ceramics2 and aerospace components3 as well as fabrication of microfluidics4, medical devices5, and artificial organs6. The light-induced additive manufacturing techniques7 used are particularly successful owing to their high spatial and temporal control, but such techniques still share the common motifs of pointwise or layered generation, as do stereolithography8, laser powder bed fusion9, and continuous liquid interface production10 and its successors11,12. Volumetric 3D printing13-20 is the next step onward from sequential additive manufacturing methods. Here we introduce xolography, a dual colour technique using photoswitchable photoinitiators to induce local polymerization inside a confined monomer volume upon linear excitation by intersecting light beams of different wavelengths. We demonstrate this concept with a volumetric printer designed to generate three-dimensional objects with complex structural features as well as mechanical and optical functions. Compared to state-of-the-art volumetric printing methods, our technique has a resolution about ten times higher than computed axial lithography without feedback optimization, and a volume generation rate four to five orders of magnitude higher than two-photon photopolymerization. We expect this technology to transform rapid volumetric production for objects at the nanoscopic to macroscopic length scales.

Biomaterials obtained by photopolymerization: from UV to two photon

Photopolymerizable materials are commonly used in the industry for a long time, being extended their use in adhesives, coatings, paint and printing ink industries, composite materials, or even in the food industry. Their use in biomedical industry was until recently quite limited to acrylic derivatives used for dental restauration. However, the increase of the biocompatible monomers and initiators added to the widespread use of the 3D printing or the evolution of the two-photon photopolymerization has renew the interest on this methodology for the fabrication of biomaterials. In this review, the main photopolymerization processes from the conventional methods to more demanding methods such as two-photon polymerization are summarized, and the main applications of biomaterials obtained from photopolymerization have been reviewed.

Refractive twisted microaxicons.

Complex-shaped light fields with specially designed intensity, phase, and polarization distributions are highly demanded for various applications including optical tweezers, laser material processing, and lithography. Here, we propose a novel (to the best of our knowledge) optical element formed by the twisting of a conic surface, a twisted microaxicon, allowing us to controllably generate high-quality spiral-shaped intensity patterns. Performance of the proposed element was analyzed both analytically and numerically using ray approximation and the rigorous finite difference time domain (FDTD) solution of Maxwell's equation. The main geometric parameters, an apex cone angle and a degree of twisting, were considered to control and optimize the generated spiral-shaped intensity patterns. The three-dimensional structure of such a microaxicon cannot be described by an unambiguous height function; therefore, it has no diffraction analogue in the form of a thin optical element. Such an element can be produced via direct laser ablation of transparent targets with structured laser beams or direct laser writing via two-photon photopolymerization and can be used in various micro- and nano-optical applications.

Tuning the performance of polymeric microring resonator with femtosecond laser

Microring resonator realized by femtosecond laser-induced two-photon photopolymerization in photoresist SU-8 and the factors influencing the performance of the microring resonator are demonstrated in this study with unique advantages of flexibility, high precision, and possible three-dimensional fabrication. By varying the laser parameters, it is possible to tune the performance of the polymeric microring resonator through the adjustment of different specifications, including the magnitudes of the radius and cross section. It is revealed that the free spectral range of the microring resonator is closely related with the parameters such as ring radius, resonance wavelength, cross section of the microring, and the polarization of the light. The ring resonator with a radius of 60μm shows a free spectral range of 3.9 nm, which linearly increases as the curvature of the ring increases. Though the value of the free spectral range does not vary for a multiple microring, the full-width half-maximum of the resonance is larger for the microring resonator consisting of multiple rings. The polarization modes of the microring resonator exhibit different resonance wavelengths due to different effective refractive indices, which opens new venues for the applications of microring resonators.

Automated Indirect Transportation of Biological Cells with Optical Tweezers and a 3D Printed Microtool

Optical tweezers are widely used for noninvasive and precise micromanipulation of living cells to understand biological processes. By focusing laser beams on cells, direct cell manipulation with optical tweezers can achieve high precision and flexibility. However, direct exposure to the laser beam can lead to negative effects on the cells. These phenomena are also known as photobleaching and photodamage. In this study, we proposed a new indirect cell micromanipulation approach combined with a robot-aided holographic optical tweezer system and 3D nano-printed microtool. The microtool was designed with a V-shaped head and an optical handle part. The V-shaped head can push and trap different sizes of cells as the microtool moves forward by optical trapping of the handle part. In this way, cell exposure to the laser beam can be effectively reduced. The microtool was fabricated with a laser direct writing system by two-photon photopolymerization. A control strategy combined with an imaging processing algorithm was introduced for automated manipulation of the microtool and cells. Experiments were performed to verify the effectiveness of our approach. First, automated microtool transportation and rotation were demonstrated with high precision. Second, indirect optical transportations of cells, with and without an obstacle, were performed to demonstrate the effectiveness of the proposed approach. Third, experiments of fluorescent cell manipulation were performed to confirm that, indicated by the photobleaching effect, indirect manipulation with the microtool could induce less laser exposure compared with direct optical manipulation. The proposed method could be useful in complex biomedical applications where precise cell manipulation and less laser exposure are required.

Hybrid plasmonic nanosystem with controlled position of nanoemitters

Quantum dots optically excited in close proximity to a silver nanowire can launch surface plasmons. The challenge related to this promising hybrid system is to control the position of nanoemitters on the nanowire. We report on the use of a two-photon photopolymerization process to strategically position quantum dots on nanowires at controlled sites. A parametric study of the distance between the quantum dots and the nanowire extremity shows that precise control of the position of the launching sites enables command of light intensity at the wire end through surface plasmon propagation.