Two-photon Polymerization Lithography Research Articles

Arbitrary engineering of spatial caustics with 3D-printed metasurfaces

Caustics occur in diverse physical systems, spanning the nano-scale in electron microscopy to astronomical-scale in gravitational lensing. As envelopes of rays, optical caustics result in sharp edges or extended networks. Caustics in structured light, characterized by complex-amplitude distributions, have innovated numerous applications including particle manipulation, high-resolution imaging techniques, and optical communication. However, these applications have encountered limitations due to a major challenge in engineering caustic fields with customizable propagation trajectories and in-plane intensity profiles. Here, we introduce the “compensation phase” via 3D-printed metasurfaces to shape caustic fields with curved trajectories in free space. The in-plane caustic patterns can be preserved or morphed from one structure to another during propagation. Large-scale fabrication of these metasurfaces is enabled by the fast-prototyping and cost-effective two-photon polymerization lithography. Our optical elements with the ultra-thin profile and sub-millimeter extension offer a compact solution to generating caustic structured light for beam shaping, high-resolution microscopy, and light-matter-interaction studies.

3D-printed multilayer structures for high-numerical aperture achromatic metalenses.

Flat optics consisting of nanostructures of high-refractive index materials produce lenses with thin form factors that tend to operate only at specific wavelengths. Recent attempts to achieve achromatic lenses uncover a trade-off between the numerical aperture (NA) and bandwidth, which limits performance. Here, we propose a new approach to design high-NA, broadband, and polarization-insensitive multilayer achromatic metalenses (MAMs). We combine topology optimization and full-wave simulations to inversely design MAMs and fabricate the structures in low-refractive index materials by two-photon polymerization lithography. MAMs measuring 20 μm in diameter operating in the visible range of 400 to 800 nm with 0.5 and 0.7 NA were achieved with efficiencies of up to 42%. We demonstrate broadband imaging performance of the fabricated MAM under white light and RGB narrowband illuminations. These results highlight the potential of the 3D-printed multilayer structures for realizing broadband and multifunctional meta-devices with inverse design.

Development of high-reflectivity polymer/air-Bragg micromirror structures for nanophotonic applications

We report the design, nanofabrication, and characterization of high-quality polymer-based micromirror structures employing the 3D two-photon polymerization lithography technique. Compared to conventional microcavity approaches, our innovative concept provides microstructures, which allow fast prototyping. Moreover, our polymer-based mirrors are cost effective and environmentally sensitive, as well as compatible with a wide range of wavelengths from near-infrared to the telecom C-band. We demonstrate polymer/air distributed Bragg reflectors and full microcavity structures with up to 14 mirror pairs with a target wavelength of 1550 nm and a reflectivity close to 99%. Additionally, our 3D printed micromirrors are reproducible and mechanically stable, and enable hybrid nanophotonic devices based on quantum dots, molecules, or 2D quantum materials as the active medium.

Pick and place process for uniform shrinking of 3D printed micro- and nano-architected materials

Two-photon polymerization lithography is promising for producing three-dimensional structures with user-defined micro- and nanoscale features. Additionally, shrinkage by thermolysis can readily shorten the lattice constant of three-dimensional photonic crystals and enhance their resolution and mechanical properties; however, this technique suffers from non-uniform shrinkage owing to substrate pinning during heating. Here, we develop a simple method using poly(vinyl alcohol)-assisted uniform shrinking of three-dimensional printed structures. Microscopic three-dimensional printed objects are picked and placed onto a receiving substrate, followed by heating to induce shrinkage. We show the successful uniform heat-shrinking of three-dimensional prints with various shapes and sizes, without sacrificial support structures, and observe that the surface properties of the receiving substrate are important factors for uniform shrinking. Moreover, we print a three-dimensional mascot model that is then uniformly shrunk, producing vivid colors from colorless woodpile photonic crystals. The proposed method has significant potential for application in mechanics, optics, and photonics.

Tunable reflection coating to reduce exposure power threshold for interference-assisted two-photon polymerization lithography

We proposed a strategy to significantly reduce the exposure power threshold for two-photon lithography on tunable reflection coating substrates. The focused light spot at the photoresist-reflective interface is modulated into a multi-layered distribution, which has an enhanced optical field compared to the incident light. The interference of incident and reflected light at the interface enhances the local light field by 1.6–3.4 times, equivalent to reducing the incident exposure power. The tailored multilayer nanowires fabricated by a single scan and the exposure power modulation mechanism proposed in this study would be of profound importance in the fields of semiconductor electronic devices.

PDMS-filled micro-spring Fabry-Perot cavity for temperature sensing.

A highly sensitive fiber-tipped temperature sensor based on polydimethylsiloxane (PDMS)-filled spring Fabry-Perot (FP) cavity has been proposed and experimentally demonstrated. The spring FP cavity is first fabricated on the fiber endface by the two-photon polymerization lithography. After that, PDMS is filled into the cavity to drive the elongation of the flexible spring and thus to functionalize high-performance temperature sensing. Benefiting from the large thermal expansion coefficient of PDMS, the proposed sensor exhibits a maximal temperature sensitivity of 704.3 pm/°C with excellent operating repeatability and stability. Besides, by selecting a proper spring constant k, the FP sensitivity can be precisely adjusted in the range of 100-700 pm/°C. Thanks to the advantages of high fabrication accuracy and designable property, the proposed sensor could promote numerous usages for customizable temperature sensing.

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.

Two‐Photon Polymerization Lithography for Optics and Photonics: Fundamentals, Materials, Technologies, and Applications

AbstractThe rapid development of additive manufacturing has fueled a revolution in various research fields and industrial applications. Among the myriad of advanced 3D printing techniques, two‐photon polymerization lithography (TPL) uniquely offers a significant advantage in nanoscale print resolution, and has been widely employed in diverse fields, for example, life sciences, materials sciences, mechanics, and microfluidics. More recently, by virtue of the optical transparency of most of the resins used, TPL is finding new applications in optics and photonics, with nanometer to millimeter feature dimensions. It enables the minimization of optical elements and systems, and exploration of light‐matter interactions with new degrees of freedom, never possible before. To review the recent progress in the TPL related optical research, it starts with the fundamentals of TPL and material formulation, then discusses novel fabrication methods, and a wide range of optical applications. These applications notably include diffractive, topological, quantum, and color optics. With a panoramic view of the development, it is concluded with insights and perspectives of the future development of TPL and related potential optical applications.

Anatomic, physiologic, and proteomic consequences of repeated microneedle-mediated perforations of the round window membrane

BackgroundWe have developed 3D-printed microneedle technology for diagnostic aspiration of perilymph and intracochlear delivery of therapeutic agents. Single microneedle-mediated round window membrane (RWM) perforation does not cause hearing loss, heals within 48–72 h, and yields sufficient perilymph for proteomic analysis. In this study, we investigate the anatomic, physiologic, and proteomic consequences of repeated microneedle-mediated perforations of the same RWM at different timepoints. Methods100-μm-diameter hollow microneedles were fabricated using two-photon polymerization (2PP) lithography. The tympanic bullae of Hartley guinea pigs (n = 8) were opened with adequate exposure of the RWM. Distortion product otoacoustic emissions (DPOAE) and compound action potential (CAP) were recorded to assess hearing. The hollow microneedle was introduced into the bulla and the RWM was perforated; 1 μL of perilymph was aspirated from the cochlea over the course of 45 s. 72 h later, the above procedure was repeated with aspiration of an additional 1 μL of perilymph. 72 h after the second perforation, RWMs were harvested for confocal imaging. Perilymph proteomic analysis was completed using liquid chromatography-tandem mass spectrometry (LC-MS/MS). ResultsTwo perforations and aspirations were performed in 8 guinea pigs. In six, CAP, DPOAE, and proteomic analysis were obtained; in one, only CAP and DPOAE results were obtained; and in one, only proteomics results were obtained. Hearing tests demonstrated mild hearing loss at 1–4 kHz and 28 kHz, most consistent with conductive hearing loss. Confocal microscopy demonstrated complete healing of all perforations with full reconstitution of the RWM. Perilymph proteomic analysis identified 1855 proteins across 14 samples. The inner ear protein cochlin was observed in all samples, indicating successful aspiration of perilymph. Non-adjusted paired t-tests with p < 0.01 revealed significant changes in 13 of 1855 identified proteins (0.7%) between the first and second aspirations. ConclusionsWe demonstrate that repeated microneedle perforation of the RWM is feasible, allows for complete healing of the RWM, and minimally changes the proteomic expression profile. Thus, microneedle-mediated repeated aspirations in a single animal can be used to monitor the response to inner ear treatments over time.

Pyrolyzed Ultrasharp Glassy Carbon Microneedles.

Polymeric microneedles fabricated via two-photon polymerization (2PP) lithography enable safe medical access to the inner ear. Herein, the material class for 2PP-lithography-based microneedles is expanded by pyrolyzing 2PP-fabricated polymeric microneedles, resulting in glassy carbon microneedles. During pyrolysis the microneedles shrink up to 81% while maintaining their complex shape when the exposed surface-area-to-volume ratio (SVR) is 0.025 < SVR < 0.04, for the temperature history protocol used herein. The derived glassy carbon is confirmed with energy-dispersive X-ray spectroscopy and Raman spectroscopy. The pyrolyzed glassy carbon has Young's modulus 9.0 GPa. As a brittle material, the strength is stochastic. Using the two-parameter Weibull distribution, the glassy carbon has Weibull modulus of 3.1 and characteristic strength of 710 MPa. The viscoelastic response has characteristic time scale of about 10000 s. In vitro experiments demonstrate that the glassy carbon microneedles introduce controlled perforations across the guinea pig round window membrane (RWM) from the middle ear space into the inner ear, without damaging the microneedle. The resultant controlled perforation of RWM is known to enhance diffusion of therapeutics across the RWM in a predictable fashion. Hence, the glassy carbon microneedles can be deployed for mediating inner ear delivery.

Full Geometric Control of Hidden Color Information in Diffraction Gratings under Angled White Light Illumination.

Under white light illumination, gratings produce an angular distribution of wavelengths dependent on the diffraction order and geometric parameters. However, previous studies of gratings are limited to at least one geometric parameter (height, periodicity, orientation, angle of incidence) kept constant. Here, we vary all geometric parameters in the gratings using a versatile nanofabrication technique, two-photon polymerization lithography, to encode hidden color information through two design approaches. The first approach hides color information by decoupling the effects of grating height and periodicity under normal and oblique incidence. The second approach hides multiple sets of color information by arranging gratings in sectors around semicircular pixels. Different images are revealed with negligible crosstalk under oblique incidence and varying sample rotation angles. Our analysis shows that an angular separation of ≥10° between adjacent sectors is required to suppress crosstalk. This work has potential applications in information storage and security watermarks.

Inner Ear Diagnostics and Drug Delivery via Microneedles.

Objectives: Precision medicine for inner ear disorders has seen significant advances in recent years. However, unreliable access to the inner ear has impeded diagnostics and therapeutic delivery. The purpose of this review is to describe the development, production, and utility of novel microneedles for intracochlear access. Methods: We summarize the current work on microneedles developed using two-photon polymerization (2PP) lithography for perforation of the round window membrane (RWM). We contextualize our findings with the existing literature in intracochlear diagnostics and delivery. Results: Two-photon polymerization lithography produces microneedles capable of perforating human and guinea pig RWMs without structural or functional damage. Solid microneedles may be used to perforate guinea pig RWMs in vivo with full reconstitution of the membrane in 48–72 h, and hollow microneedles may be used to aspirate perilymph or inject therapeutics into the inner ear. Microneedles produced with two-photon templated electrodeposition (2PTE) have greater strength and biocompatibility and may be used to perforate human RWMs. Conclusions: Microneedles produced with 2PP lithography and 2PTE can safely and reliably perforate the RWM for intracochlear access. This technology is groundbreaking and enabling in the field of inner ear precision medicine.

Characterization of two-photon-polymerization lithography structures via Raman spectroscopy and nanoindentation

Additive manufacturing using two-photon polymerization (TPP) lithography is increasingly used in industry and research. Parameter sweeps of cuboid structures fabricated using TPP lithography were investigated across the parameters of the laser power and scan speed to find dependent mechanical material properties. The employed photoresists were examined using Raman spectroscopy to find the degree of conversion (DC) of monomer to polymer, and subsequently, micro- or nanoindentation was used to find Young’s modulus (E). For the photoresist IP-Dip, the attained DC and E ranged from 20% to 45% and 1 to 2.1 GPa, respectively. The results were compared with reports found in the literature. For IP-Q, the attained DC and E ranged from 53% to 80% and 0.5 to 1.3 GPa, respectively. The characterized properties of IP-Q manifest as the current state of knowledge of the material.

Two-photon direct laser writing of micro Fabry-Perot cavity on single-mode fiber for refractive index sensing.

Using the two-photon polymerization (TPP) lithography, here we propose and experimentally demonstrate a fiber-tipped Fabry-Perot interferometer (FPI) for liquid refractive index (RI) measurement. To fit the aqueous environment, the FPI is designed as an open-cell microstructure consisting of well-crafted surfaces together with supporting rods, where the major spectral interference occurs between the waveguide's facet and the printed surface. Subsequently, the sensing performances of the fiber FPI are comprehensively studied under various RI as well as temperature configurations. The RI sensitivity is obtained to be ∼1058 nm/RIU with a low detection limit of 4.5× 10-6 RIU, which is comparable to that of previous reported FPIs. And the temperature cross-sensitivity reaches a value of 8.2 × 10-5 RIU/°C, indicating the good reliability for RI monitoring. Compared to other fiber FPIs, our sensor exhibits substantial advantages such as ease of fabrication, highly smooth cavity surfaces, and sufficient mechanical strength, providing a practical and competitive solution for chemical and biological sensing.

Microscale white light emitters fabricated by two-photon polymerization lithography on functional resist

Micrometric white light emitters were fabricated by applying two-photon polymerization (TPP) lithography to a chemically modified resist. Such a functional resist (FR) was doped with fluorescent molecules to turn it emitting into the whole visible region. The TPP has shown to be a versatile, low cost maskless technique capable to fabricate highly resolved structures in our FR's. The spectroscopic and chromaticity properties of the FR and fabricated microscale structures were investigated. Structures emitting high quality of white light were obtained and classified into two categories: physical and physiological white light emitters. Finally, a lifetime study was performed with the aim to apply such structures as light sources for on chip integrated photonic devices.

2.5D, 3D and 4D printing in nanophotonics - a progress report

Additive manufacturing produces 3D geometries by depositing material in a layer-by-layer fashion. Commonly, the resolution of additive manufacturing is in the millimeter and micrometer scales, due to limitations in mechanical precision and material properties. In the past two decades, nanoscale additive manufacturing has been achieved using femtosecond lasers that crosslink resin through a two-photon polymerization lithography (TPL) process. In this progress report, we present some of the recent efforts on nanoscale printing using TPL. We provide examples of nanoscale 2.5D, 3D and 4D printing by TPL and their corresponding applications in nanophotonics. We discuss the benefits of fabricating these geometries by the printing approach. Finally, we summarize the challenges and provide our perspective to improve the performance of nanoscale printing.

Structural color prints combined with microlens arrays for sustainable autostereoscopic displays

Structural colour printing is a sustainable printing technology that avoids the use of environmentally harmful dyes or pigments. Structural colours are produced from the interaction of light with nanostructures through physical mechanisms such as scattering, instead of selective absorption caused by chemical properties. Because of these physical mechanisms, structural colours are durable and non-fading, which find them potential applications in artworks and security prints. However, structural colour prints have a fixed two-dimensional image appearance that limits the perception of three-dimensional information. An effective solution is to combine structural colour prints with microlens arrays. This solution enables three-dimensional information to be perceived by the naked eyes under white light illumination without the use of specialized optics. As microlens arrays are compact and lightweight, they can be easily integrated with structural colour prints. Here, we demonstrate that the combination of structural colour prints with microlens arrays produces dynamic moiré and autostereoscopic effects that can be used to display-three-dimensional information. To achieve these effects, we fabricate polymer structures on glass substrate using straightforward fabrication methods (primarily, two-photon polymerization lithography). Furthermore, these structures can be dissolved using organic solvents for recycling the substrate. We believe that this work contributes towards displaying a colourful and three-dimensional world with a sustainable future.

Two-photon polymerization lithography enabling the fabrication of PEDOT:PSS 3D structures for bioelectronic applications.

Conductive 3D platforms have gained increasing attention in bioelectronics thanks to the improvement in the cell-chip coupling. PEDOT:PSS is nowadays widely employed in bioelectronic applications thanks to its electrical and mechanical properties. In this work, an innovative fabrication method for the realization of PEDOT:PSS-based conductive micropillars and 3D cage-like structures is presented, combining two-photon lithography and electrodeposition techniques.

High-resolution light field prints by nanoscale 3D printing

A light field print (LFP) displays three-dimensional (3D) information to the naked-eye observer under ambient white light illumination. Changing perspectives of a 3D image are seen by the observer from varying angles. However, LFPs appear pixelated due to limited resolution and misalignment between their lenses and colour pixels. A promising solution to create high-resolution LFPs is through the use of advanced nanofabrication techniques. Here, we use two-photon polymerization lithography as a one-step nanoscale 3D printer to directly fabricate LFPs out of transparent resin. This approach produces simultaneously high spatial resolution (29–45 µm) and high angular resolution (~1.6°) images with smooth motion parallax across 15 × 15 views. Notably, the smallest colour pixel consists of only a single nanopillar (~300 nm diameter). Our LFP signifies a step towards hyper-realistic 3D images that can be applied in print media and security tags for high-value goods.

Full Color and Grayscale Painting with 3D Printed Low-Index Nanopillars.

Sculpting nanostructures into different geometries in either one or two dimensions produces a wide range of colorful elements in microscopic prints. However, achieving different shades of gray and control of color saturation remain challenging. Here, we report a complete approach to color and grayscale generation based on the tuning of a single nanostructure geometry. Through two-photon polymerization lithography, we systematically investigated color generation from the basic single nanopillar geometry in low-refractive-index (n < 1.6) material. Grayscale and full color palettes were achieved that allow decomposition onto hue, saturation, and brightness values. This approach enabled the "painting" of arbitrary colorful and grayscale images by mapping desired prints to precisely controllable parameters during 3D printing. We further extend our understanding of the scattering properties of the low-refractive-index nanopillar to demonstrate grayscale inversion and color desaturation and steganography at the level of single nanopillars.