High-aspect-ratio microtubes with variable diameter and uniform wall thickness by compressing Bessel hologram phase depth

Giant Helical Dichroism of Single Chiral Nanostructures with Photonic Orbital Angular Momentum

Optical activity, demonstrating the chiral light-matter interaction, has attracted tremendous attention in both fundamental theoretical research and advanced applications of high-efficiency enantioselective sensing and next-generation chiroptical spectroscopic techniques. However, conventional chiroptical responses are normally limited in large assemblies of chiral materials by circularly polarized light, exhibiting extremely weak chiroptical signals in a single chiral nanostructure. Here, we demonstrate that an alternative chiral freedom of light-orbital angular momentum-can be utilized for generating strong helical dichroism in single chiral nanostructures. The helical dichroism by monochromatic vortex beams can unambiguously distinguish the intrinsic chirality of nanostructures, in an excellent agreement with theoretical predictions. The single planar-chiral nanostructure can exhibit giant helical dichroism of ∼20% at the visible wavelength. The vortex-dependent helical dichroism, expanding to single nanostructures and two-dimensional space, has implications for high-efficiency chiroptical detection of planar-chiral nanostructures in chiral optics and nanophotonic systems.

Stimuli-Responsive Actuator Fabricated by Dynamic Asymmetric Femtosecond Bessel Beam for In Situ Particle and Cell Manipulation

Microscale intelligent actuators capable of sensitive and accurate manipulation under external stimuli hold great promise in various fields including precision sensors and biomedical devices. Current microactuators, however, are often limited to a multiple-step fabrication process and multimaterials. Here, a pH-triggered soft microactuator (<100 μm) with simple structure, one-step fabrication process, and single material is proposed, which is composed of deformable hydrogel microstructures fabricated by an asymmetric femtosecond Bessel beam. To further explore the swelling-shrinking mechanism, the hydrogel porosity difference between expansion and contraction states is investigated. In addition, by introducing the dynamic holographic processing and splicing processing method, more complex responsive microstructures (S-shaped, C-shaped, and tortile chiral structures) are rapidly fabricated, which exhibit tremendous expected deformation characteristics. Finally, as a proof of concept, a pH-responsive microgripper is fabricated for in situ capturing polystyrene (PS) particles and neural stem cells rapidly. This flexible, designable, and one-step approach manufacturing of intelligent actuator provides a versatile platform for micro-objects manipulation and drug delivery.

Amplitude-phase optimized long depth of focus femtosecond axilens beam for single-exposure fabrication of high-aspect-ratio microstructures

Fabrication of high-aspect-ratio (HAR) micro/nanostructures by two-photon polymerization (TPP) has become a hot topic because of the advantages of ultra-high resolution and true 3D printing ability. However, the low efficiency caused by point-by-point scanning strategy limits its application. In this Letter, we propose a strategy for the rapid fabrication of HAR microstructures by combining TPP with an amplitude-phase optimized long depth of focus laser beam (LDFB). The optimization of the LDFB is implemented by modulating the amplitude and phase on a phase-only spatial light modulator, which can suppress the side lobe and smooth energy oscillations effectively. The LDFB is used for rapid fabrication of HAR micropillars and various microstructures, which greatly increases the fabrication efficiency. As a demonstration, several typical HAR microstructures such as assemblies, microchannels, microtubes, and cell scaffolds are prepared. Moreover, the microcapture arrays are rapidly fabricated for the capture of microspheres and the formation of microlens arrays, which show focusing and imaging ability.

UV-NIR femtosecond laser hybrid lithography for efficient printing of complex on-chip waveguides

We propose UV-IR femtosecond laser hybrid lithography for the efficient printing of complex on-chip waveguides, which offers good performance in terms of processing efficiency and accuracy. With this three-dimensional printing technology, waveguides with complex cross-section shapes, such as owls and kittens, can be easily fabricated with an efficiency increased by 1500% (for ${6}\;\unicode{x00B5} {\rm m}\; \times \;{6}\;\unicode{x00B5} {\rm m}$6µm×6µm). In addition, a circular cross-section waveguide with an extremely low birefringence and complex ${8} \times {8}$8×8 random walk networks were quickly customized, which implies that in the design and preparation of the large-scale optical chips, the proposed maskless method allows for the preparation of highly customized devices.

Control of diameter and numerical aperture of microlens by a single ultra-short laser pulse

We demonstrate a versatile method for fast and flexible fabrication of either one or an array of microlenses. Multi-foci axial intensity distribution generated by a phase pattern displayed on a spatial light modulator irradiates silica, causing ablation and its internal modification. The following wet etching step defines the diameter r, while the radius of curvature R (hence, the focal length f) is maintained the same. As a result, the numerical aperture NA=r/f changes from 0.2 to 0.4 for the same pulse energy (but different number of multi-foci) during ablation. An isotropic wet etching of silica becomes highly anisotropic for the initial stages of etching following the irradiated pattern. Subsequent evolution of the shape is governed by an isotropic silica etch and forms a spherical lens. This method can be extended to other materials and geometries of micro-optical elements.

Holographic femtosecond laser integration of microtube arrays inside a hollow needle as a lab-in-a-needle device

In this Letter, the femtosecond laser holographic two-photon polymerization (HTPP) method is adopted to rapidly realize a unique lab-in-a-needle (LIN) device by manufacturing microtube arrays inside a needle. The HTPP method is to modulate a Gaussian beam into a ring Bessel beam by a spatial light modulator (SLM) loaded with a Bessel hologram, and can fabricate microtube arrays with controllable inside diameter (1-10 μm) and designable patterns on such complex three-dimensional (3D) substrates by optimizing experimental parameters. A single LIN device can be processed by this method in about 4 min, which is not possible with traditional micronano technology and is much faster than the traditional two-photon polymerization method (at least several hours). To further demonstrate the functionality of this LIN device, a particle separation experiment is carried out. For the purpose of achieving greater functionality and integration of the on-chip system, this HTPP method provides a powerful processing method for integrating 3D functional microstructures on 3D nonplanar substrates.

Two-Photon Vertical-Flow Lithography for Microtube Synthesis

Two-photon vertical-flow lithography is demonstrated for synthesis of complex-shaped polymeric microtubes with a high aspect ratio (>100:1). This unique microfluidic approach provides rigorous control over the morphology and surface topology to generate thin-walled (<1 µm) microtubes with a tunable diameter (1-400 µm) and pore size (1-20 µm). The interplay between fluid-flow control and two-photon lithography presents a generic high-resolution method that will substantially contribute toward the future development of biocompatible scaffolds, stents, needles, nerve guides, membranes, and beyond.

Fabrication of high-aspect-ratio structures using Bessel-beam-activated photopolymerization

Microfabrication based on photopolymerization is typically achieved by scanning a focal spot within the material point by point, which significantly limits fabrication speed. In this paper, we explore a method for rapid fabrication of high-aspect-ratio microstructures based on photopolymerization using a femtosecond laser beam that is converted into a Bessel beam by an axicon. With stationary exposure, a polymer fiber measured at 200 μm in length and 400 nm in width (500∶1 aspect ratio) was fabricated within 50 ms of exposure time. The exposure conditions can be adjusted to produce fibers with variable widths. A phenomenological polymerization-threshold model is adapted for Bessel-beam exposure. The revised model is applied to analyze the structure width and estimate the order of multi-photon absorption. Examination of the cross section of the fibers shows that they are nearly monolithic, suggesting that active species diffuse during photopolymerization. By scanning the Bessel beam in the plane transverse to the direction of beam propagation, mesh structures are fabricated with a single-pass scan, showing the potential of this method for rapid fabrication of large-scale high-aspect-ratio microstructures for applications in photonics, micro-machines, and tissue engineering.