Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging, optical data storage, and lithographic microfabrication. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures.

Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication

Processing of materials by ultrashort laser pulses has evolved significantly over the last decade and is starting to reveal its scientific, technological and industrial potential. In ultrafast laser manufacturing, optical energy of tightly focused femtosecond or picosecond laser pulses can be delivered to precisely defined positions in the bulk of materials via two-/multi-photon excitation on a timescale much faster than thermal energy exchange between photoexcited electrons and lattice ions. Control of photo-ionization and thermal processes with the highest precision, inducing local photomodification in sub-100-nm-sized regions has been achieved. State-of-the-art ultrashort laser processing techniques exploit high 0.1–1 μm spatial resolution and almost unrestricted three-dimensional structuring capability. Adjustable pulse duration, spatiotemporal chirp, phase front tilt and polarization allow control of photomodification via uniquely wide parameter space. Mature opto-electrical/mechanical technologies have enabled laser processing speeds approaching meters-per-second, leading to a fast lab-to-fab transfer. The key aspects and latest achievements are reviewed with an emphasis on the fundamental relation between spatial resolution and total fabrication throughput. Emerging biomedical applications implementing micrometer feature precision over centimeter-scale scaffolds and photonic wire bonding in telecommunications are highlighted.

/pdf/ultrafast-laser-processing-of-materials-from-science-to-rducz2yjvs.pdf

Ultrafast laser processing of materials: from science to industry

Ultrafast laser nanostructuring of photopolymers: a decade of advances

Fabrication technology for three-dimensional microstructures with submicrometer accuracy has been needed in the fields of modern optics, such as micro mechanical system driven with photon pressure[1, 2] and laser-trapping near-filed optical microscopy[3]. However, the present accuracy with stereolithography[4] is not yet satisfactory to this purpose. Moreover, it is not so flexible to make a three- dimensional structure with the present technique. In this paper, we propose a new microfabrication method in which a point in three-dimensional volume of UV photopolymerizing resin is photopolymerized through two-photon absorption process. The microfabrication with two-photon absorption drastically improves the depth resolution due to a nonlinearlity between the power of the irradiation and that of the absorption[5].

Three-dimensional microfabrication with two-photon absorbed photopolymerization

With the advent of femtosecond lasers in the early 1990s, ultrafast laser processing has proven to be an imperative tool for micro/nano machining. Two-photon lithography (TPL) is one such unique microfabrication technique exploiting the nonlinear dependency of the polymerization rate on the irradiating light intensity to produce true three-dimensional structures with feature sizes beyond the diffraction limit. This characteristic has revolutionized laser material processing for the fabrication of micro and nanostructures. In this paper, an overview of TPL including its working principle, experimental setup, and materials is presented. Then, the effect of resolution with a focus on techniques adopted to improve the final resolution of the structures is covered. Insights to improve throughput and speed of fabrication to pave a way for industrialization of this technique are provided. Finally, TPL for microfabrication of structures with the emphasis on metamaterials is thoroughly reviewed and presented.

Two-photon lithography for three-dimensional fabrication in micro/nanoscale regime: A comprehensive review

We report a method to replicate not only complex three-dimensional (3D) microstructures with closed loops but also a rotary microstructure restricted to a shaft by using a membrane-assisted transfer molding with a poly(dimethylsiloxane) (PDMS) mold. In this method, the use of supercritical CO2 drying allows the fabrication of a 3D master structure with partition membranes of submicron thickness by two-photon microfabrication. Because the membranes attached to the master structure separate the PDMS mold under closed loops, the PDMS mold can be released from the master structure that has closed loops or movable microparts. In particular, rotary micromechanisms such as microrotors and micromanipulators were replicated using a cylindrical partition membrane. It was also demonstrated that the replicated movable microparts can be driven in water. The movable microparts will be useful for microfluidic devices and as manipulation tools for biological samples.

https://iopscience.iop.org/article/10.1143/JJAP.48.06FH05/pdf

Replication of Three-Dimensional Rotary Micromechanism by Membrane-Assisted Transfer Molding

Optoelectronic micromachines driven by laser-induced dielectrophoretic force were developed by two-photon microfabrication. The optoelectronic driving system consists of the lower photoconductive layer of amorphous silicon on an indium tin oxide (ITO)-coated glass substrate and the upper transparent, conductive ITO-coated glass substrate. Micromachines are fabricated on the upper ITO-coated substrate by two-photon microfabrication. To estimate the driving condition, we examined the displacement of a microparticle by focusing a laser beam. Micromachines such as manipulators and microgears were also driven at proper driving condition. Furthermore a prototype of microfluidic components was developed. The optoelectronic micromachines will be powerful tools applicable to high-performance, low-cost lab-on-a-chip devices.

Polymeric micromachines driven by laser-induced negative dielectrophoresis

PROBLEM TO BE SOLVED: To provide a technique capable of driving a movable member, a minute article using an objective lens of a low numerical aperture with low output laser lights having an output of about several mW, rather than several W for a conventional technique. SOLUTION: This micromachine has a structure such that a movable member is movably supported by a supporting member, and comprises a face constituted to hold fluid surrounding the movable member to be operated, means for applying an electric field to the movable member surrounded by the fluid, at least one photoconductive area on the face provided for converting received lights into a local electric field near the received lights, and a light source collecting and emitting lights near the movable member in the fluid via the photoconductive area, and generating a driving force by an uneven dielectric polarization force generated to the movable member according to the induced local electric field. COPYRIGHT: (C)2011,JPO&INPIT

Photoelectric driving micromachine and driving method of minute movable member

In high-precision two-photon microfabrication of three-dimensional (3-D) polymeric microstructures, supercritical CO2 drying was employed to reduce surface tension, which tends to cause the collapse of micro/nano structures. Use of supercritical drying allowed high-aspect ratio microstructures, such as micropillars and cantilevers, to be fabricated. We also propose a single-anchor supporting method to eliminate non-uniform shrinkage of polymeric structures otherwise caused by attachment to the substrate. Use of this method permitted frame models such as lattices to be produced without harmful distortion. The combination of supercritical CO2 drying and the single-anchor supporting method offers reliable high-precision microfabrication of sophisticated, fragile 3-D micro/nano structures.

Single-anchor support and supercritical CO_2 drying enable high-precision microfabrication of three-dimensional structures

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Replication of Three-Dimensional Rotary Micromechanism by Membrane-Assisted Transfer Molding

Polymeric micromachines driven by laser-induced negative dielectrophoresis

Photoelectric driving micromachine and driving method of minute movable member

Single-anchor support and supercritical CO_2 drying enable high-precision microfabrication of three-dimensional structures