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 "title": "Two-photon lithography",
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 "excerpt": "Two-photon lithography is a mask-free additive microfabrication method that uses a pulsed laser to polymerize resin, building 3D microstructures with features down to tens of nanometers.",
 "snippet": "Two-photon lithography is a mask-free additive microfabrication method that uses a pulsed laser to polymerize resin, building 3D microstructures with features down to tens of nanometers.",
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 "markdown": "# Two-photon lithography\n\nTwo-photon lithography (TPL) is a mask-free additive microfabrication method in which the simultaneous absorption of two photons from a pulsed laser initiates polymerization of a photosensitive resin only at the beam focus, building free-form three-dimensional microstructures and microoptics with features down to tens of nanometers. Because the process is nonlinear, it writes true three-dimensional geometries directly, in contrast to conventional one-photon lithography, which produces two-dimensional patterns.<sup>[1](https://www.sciencedirect.com/science/article/abs/pii/S0030399221002681)</sup> Multiphoton 3D lithography (MP3DL), as the method is also called, reaches 100 nm feature sizes and millimeter-scale object dimensions, printing materials from organic resins to inorganic ceramics.<sup>[2](https://www.nature.com/articles/s43586-025-00386-y)</sup>\n\n| Key fact | Value |\n|---|---|\n| Typical lateral feature size (standard TPL) | ~100 nm; voxel separation ~\\(\\lambda/4\\) (200 nm at \\(\\lambda = 800\\ \\mathrm{nm}\\), NA 1.4)<sup>[2](https://www.nature.com/articles/s43586-025-00386-y)</sup><sup> • </sup><sup>[3](https://opn-web-afd-d3bfbkd5bcc5asbs.z02.azurefd.net/opn/media/images/pdf/2019/1019/28-35_opn_10_19.pdf?t=638452570106247176)</sup> |\n| Axial resolution (standard TPL) | ~500 nm best values; voxel elongated axially by a factor of ~2.5<sup>[3](https://opn-web-afd-d3bfbkd5bcc5asbs.z02.azurefd.net/opn/media/images/pdf/2019/1019/28-35_opn_10_19.pdf?t=638452570106247176)</sup> |\n| Best reported sub-diffraction features | Sub-30 nm lateral (T2A-PI, single color); 40–60 nm voxels (STED/RAPID); 60 nm (chemical inhibition)<sup>[4](https://doi.org/10.1002/lpor.202300645)</sup><sup> • </sup><sup>[5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a)</sup> |\n| Build area (standard serial writing) | Roughly 10 μm × 10 μm to 2.2 mm × 2.2 mm<sup>[6](https://beta.iopscience.iop.org/article/10.1088/2631-7990/ab8d9a)</sup> |\n| Parallel TPL methods | Most report rates below 0.1 mm³/h or \\(10^{4}\\) voxels/s<sup>[7](https://www.nature.com/articles/s41467-023-37163-y)</sup> |\n| Parallelization records (since 2023) | \\(2 \\times 10^{6}\\) voxels/s with 2000 holographic foci; ~\\(10^{8}\\) voxels/s with >120,000 metalens foci<sup>[7](https://www.nature.com/articles/s41467-023-37163-y)</sup><sup> • </sup><sup>[8](https://iopscience.iop.org/article/10.1088/2631-7990/ae614a)</sup> |\n| Environment | No mask, mold, stamp, vacuum, or clean room required<sup>[5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a)</sup> |\n\n## How it works\n\nPolymerization is confined to the focus by the nonlinear intensity dependence of two-photon absorption.<sup>[9](https://ascpt.onlinelibrary.wiley.com/doi/10.1002/lpor.71530)</sup> A photoinitiator in the resin is sensitized by two-photon excitation, and the monomer or oligomer is cross-linked by an intense pulsed laser beam; because the process is nonlinear, excitation is appreciable only where the beam is most concentrated, at the focal point.<sup>[10](https://www.cell.com/iscience/pdf/S2589-0042%2823%2900451-0.pdf)</sup> Each point exposure produces a voxel, the three-dimensional analog of a pixel.<sup>[3](https://opn-web-afd-d3bfbkd5bcc5asbs.z02.azurefd.net/opn/media/images/pdf/2019/1019/28-35_opn_10_19.pdf?t=638452570106247176)</sup>\n\nThe polymerization threshold is what makes sub-diffraction features possible. The threshold is defined as the minimum peak intensity required to generate a sufficient concentration of radicals to initiate polymerization and form a stable solid structure; below the corresponding dose the developer washes insufficiently cross-linked material away, above it the material remains.<sup>[5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a)</sup><sup> • </sup><sup>[3](https://opn-web-afd-d3bfbkd5bcc5asbs.z02.azurefd.net/opn/media/images/pdf/2019/1019/28-35_opn_10_19.pdf?t=638452570106247176)</sup> Operating just above threshold solidifies only the center of the Gaussian focus: setting the threshold at \\(0.9 \\cdot I_{0}\\) yields a voxel approximately 5 times smaller than a threshold of \\(0.1 \\cdot I_{0}\\).<sup>[5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a)</sup> Dissolved oxygen assists by acting as a quenching agent, inhibiting polymerization from propagating beyond the focal-spot center where radical generation cannot overcome the quenching.<sup>[5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a)</sup>\n\n## How it is done\n\nThe workflow starts from a CAD model of the desired microstructure, converted to an [STL file](https://www.edgepedia.org/stl-file-format), which is sliced into layers and transmitted as instructions to the laser writing system, which polymerizes resin layer by layer.<sup>[5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a)</sup> A typical writing system consists of a laser source, a focusing objective lens, and a 3D scanning stage; oil-immersion objectives of NA ~1.4–1.45 focus the beam into the resin.<sup>[5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a)</sup> Two scanning modes are used: galvo-mirror beam scanning, faster (several m/s) but over a small patterning area, and PZT sample scanning, slower but with millimeter-to-centimeter travel and more precise motion resolution; hybrid galvo/PZT synchronization combines the two.<sup>[5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a)</sup>\n\nRaster scanning polymerizes the structure voxel by voxel and layer by layer for high precision, while contour scanning fabricates only a shell that is later UV-cured, significantly reducing processing time.<sup>[5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a)</sup> After printing, unexposed negative-tone resist is washed out with a developer such as ethanol, leaving a freestanding 3D microstructure; real-time process monitoring is possible via refractive-index contrast under a white-light microscope.<sup>[5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a)</sup> The original 1997 resin consisted of photoinitiators, urethane acrylate monomers, and urethane acrylate oligomers.<sup>[11](https://doi.org/10.1364/ol.22.000132)</sup>\n\n## Origin\n\nShoji Maruo, Osamu Nakamura, and [Satoshi Kawata](https://www.edgepedia.org/satoshi-kawata) reported three-dimensional microfabrication with two-photon-absorbed photopolymerization in Optics Letters in 1997, using a [Ti:sapphire laser](https://www.edgepedia.org/ti-sapphire-laser) with a 790 nm wavelength and 200 fs pulse width, and verified the method by fabricating several kinds of microstructure in the urethane acrylate resin.<sup>[11](https://doi.org/10.1364/ol.22.000132)</sup> Earlier two-photon-absorption work had required multiple laser shots at each voxel; a 1998 Optics Letters paper showed that single-shot two-photon exposure of the commercial photoresist SU-8 allowed structures to be produced as fast as the laser repetition rate, 80–100 million voxels per second, limited only by voxel-to-voxel scanning.<sup>[12](http://www.chem.ucla.edu/dept/Faculty/schwartz/schwartz_pubs/Opt_Lett_23_1745_1998.pdf)</sup>\n\n## Variants\n\n**Photoinhibition lithography** adds a second beam that depletes excited photoinitiator molecules at the voxel periphery. STED-inspired inhibition depopulates the S1 state via stimulated emission before intersystem crossing, while RAPID depletes the triplet state \\(T_{1}\\) through non-radiative relaxation using a wavelength similar to the ~800 nm writing beam. Both typically enhance resolution nearly 2-fold, halving voxel size to around 40–60 nm, but nearly double system cost, reduce printing speed, and require specially engineered photoinitiators and higher-powered lasers, so they are not compatible with all photoresists.<sup>[5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a)</sup> A STED-inspired cationic variant with the resist EPOX failed to produce lower-dimensional structures such as individual voxels, and postbaking did not improve the outcome.<sup>[13](https://pmc.ncbi.nlm.nih.gov/articles/PMC10518867/)</sup> Chemical inhibition with radical inhibitors achieved 60 nm features (Farasi group).<sup>[5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a)</sup>\n\n**T2A-PI** (two-photon two-step absorption and photoinhibition), reported by Chenliang Ding and colleagues in Laser & Photonics Review in 2023, uses a single color instead of three; with benzil as photoinitiator it achieved sub-30 nm (<\\(\\lambda/17.5\\)) lateral feature sizes and minimum 80 nm lateral and 160 nm axial dimensions.<sup>[4](https://doi.org/10.1002/lpor.202300645)</sup> The dose dependence of the photomodification depth (degree of conversion) also enables 3D greyscale and 4D patterning.<sup>[2](https://www.nature.com/articles/s43586-025-00386-y)</sup> Projection TPL parallelizes exposure to raise printing rate by three orders of magnitude.<sup>[14](https://par.nsf.gov/biblio/10391835-minimizing-shrinkage-microstructures-printed-projection-two-photon-lithography)</sup>\n\n## Applications\n\nPublished applications span optics, microfluidics, bioelectronics, metamaterials, and biomedical research.<sup>[9](https://ascpt.onlinelibrary.wiley.com/doi/10.1002/lpor.71530)</sup> A massively parallel metalens-based system fabricated mechanical and electromagnetic [terahertz metamaterials](https://www.edgepedia.org/terahertz-metamaterial) with three distinct lattice types in 10 mm × 5 mm × 0.6 mm slabs containing approximately 240,000 unit cells.<sup>[8](https://iopscience.iop.org/article/10.1088/2631-7990/ae614a)</sup>\n\n## Limitations and alternatives\n\nTPL is not yet a scalable commercial technology; the drawbacks enumerated in a materials-science review are low processing speed, lack of commercial TPP photoresists, low processing volume, low dynamic range because of the higher writing threshold, and the low two-photon absorption cross-section of the photoresist.<sup>[10](https://www.cell.com/iscience/pdf/S2589-0042%2823%2900451-0.pdf)</sup> Serial writing remains the bottleneck: most parallel TPL methods report rates below 0.1 mm³/hour or \\(10^{4}\\) voxels/s.<sup>[7](https://www.nature.com/articles/s41467-023-37163-y)</sup> A digital holography platform with up to 2000 individually programmable laser foci, driven by a 1 kHz femtosecond [regenerative amplifier](https://www.edgepedia.org/regenerative-amplifier) with the smallest features defined by a single laser pulse, reached 90 nm lateral and 141 nm axial resolution, a fabrication rate of 2,000,000 voxels/s, and volume printing speeds of 4.5–54.0 mm³/h, which its authors report is better than state-of-the-art commercial solutions by three orders of magnitude.<sup>[7](https://www.nature.com/articles/s41467-023-37163-y)</sup> More recently, a 12 cm² metalens array with spatially adaptive illumination generated more than 120,000 coordinated focal spots, enabling a peak printing rate of ~\\(10^{8}\\) voxels·s⁻¹ with features as small as 113 nm.<sup>[8](https://iopscience.iop.org/article/10.1088/2631-7990/ae614a)</sup> [Performance](https://www.edgepedia.org/performance) is often limited by the inability to meet desired tolerances due to proximity effects; tuning reaction–diffusion kinetics reduced proximity-effect feature sizes from >400 to 260 nm during printing of nanoporous 3D woodpiles at 50 mm/s scanning speed.<sup>[15](https://www.sciencedirect.com/science/article/pii/S2214860425002660)</sup><sup> • </sup><sup>[16](https://www.osti.gov/pages/biblio/3012646-suppressing-proximity-effects-during-rapid-serial-two-photon-lithography-through-tuning-reactiondiffusion-kinetics)</sup> In projection TPL, most polymerization occurs through dark reactions while the light is off, giving a lower degree of polymerization and high shrinkage; UV post-curing while submerged in a photoinitiator solution reduced areal shrinkage from more than 45% to 1% without limiting the geometric design space.<sup>[14](https://par.nsf.gov/biblio/10391835-minimizing-shrinkage-microstructures-printed-projection-two-photon-lithography)</sup>\n\nThe main alternative, projection micro-[stereolithography](https://www.edgepedia.org/stereolithography) (PμSL), prints complex 3D structures with 0.6–30 μm resolution over areas up to ~90 mm × 50 mm, whereas TPP offers ~100 nm resolution but covers only small areas from 10 μm × 10 μm to 2.2 mm × 2.2 mm even with DMD-based multiscale approaches; a contradictory relation between printing area and printing resolution runs across 3D printing technologies.<sup>[6](https://beta.iopscience.iop.org/article/10.1088/2631-7990/ab8d9a)</sup> Unlike two-beam interference lithography, EUV lithography, and electron-beam lithography, TPL requires no mask, mold, stamp, vacuum, or clean-room environment.<sup>[5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a)</sup>\n\n## References\n\n1. [Two-photon lithography for three-dimensional fabrication in micro/nanoscale regime: A comprehensive review](https://www.sciencedirect.com/science/article/abs/pii/S0030399221002681)\n2. [Multiphoton 3D lithography (Nature Reviews Methods Primers)](https://www.nature.com/articles/s43586-025-00386-y)\n3. [3-D Laser Nanoprinting (Optics & Photonics News, 2019)](https://opn-web-afd-d3bfbkd5bcc5asbs.z02.azurefd.net/opn/media/images/pdf/2019/1019/28-35_opn_10_19.pdf?t=638452570106247176)\n4. [Chenliang Ding and colleagues (2023). Subdiffraction 3D Nanolithography by Two‐Photon Two‐Step Absorption and Photoinhibition. Laser & Photonics Review.](https://doi.org/10.1002/lpor.202300645)\n5. [Two-photon polymerization-assisted 3D laser nanoprinting: from fundamentals to modern applications](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a)\n6. [Projection micro stereolithography based 3D printing and its applications](https://beta.iopscience.iop.org/article/10.1088/2631-7990/ab8d9a)\n7. [Ultrafast 3D nanofabrication via digital holography](https://www.nature.com/articles/s41467-023-37163-y)\n8. [Metalens-based 3D nanolithography breaks the throughput barrier](https://iopscience.iop.org/article/10.1088/2631-7990/ae614a)\n9. [Mechanical Properties and Testing Strategies for Two-Photon Lithography: A Review of Current Practices and Emerging Challenges](https://ascpt.onlinelibrary.wiley.com/doi/10.1002/lpor.71530)\n10. [S2589 0042(23)00451 0 (cell.com)](https://www.cell.com/iscience/pdf/S2589-0042%2823%2900451-0.pdf)\n11. [Shoji Maruo, Osamu Nakamura, Satoshi Kawata (1997). Three-dimensional microfabrication with two-photon-absorbed photopolymerization. Optics Letters.](https://doi.org/10.1364/ol.22.000132)\n12. [Single-shot two-photon exposure of commercial photoresist for the production of three-dimensional structures](http://www.chem.ucla.edu/dept/Faculty/schwartz/schwartz_pubs/Opt_Lett_23_1745_1998.pdf)\n13. [STED-Inspired Cationic Photoinhibition Lithography](https://pmc.ncbi.nlm.nih.gov/articles/PMC10518867/)\n14. [Minimizing Shrinkage in Microstructures Printed With Projection Two-Photon Lithography](https://par.nsf.gov/biblio/10391835-minimizing-shrinkage-microstructures-printed-projection-two-photon-lithography)\n15. [Control of temporal and spatial proximity effects in two-photon lithography](https://www.sciencedirect.com/science/article/pii/S2214860425002660)\n16. [Suppressing proximity effects during rapid serial two-photon lithography through tuning of reaction–diffusion kinetics](https://www.osti.gov/pages/biblio/3012646-suppressing-proximity-effects-during-rapid-serial-two-photon-lithography-through-tuning-reactiondiffusion-kinetics)\n\n---\n*Topic: Encyclopedia › Technology and the built world › Engineering and manufacturing › Manufacturing processes and fabrication › Forming, heat treatment, and finishing › Polymer and composite additive manufacturing*\n\n*Initially written Sep 29, 2026 · Edited: Oct 15, 2026*\n\n*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*\n\nLicense: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license\n",
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 "speakable": "Two-photon lithography is a mask-free additive microfabrication method that uses a pulsed laser to polymerize resin, building 3D microstructures with features down to tens of nanometers."
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