# Two-photon lithography
Two-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.[1](https://www.sciencedirect.com/science/article/abs/pii/S0030399221002681) 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.[2](https://www.nature.com/articles/s43586-025-00386-y)
| Key fact | Value |
|---|---|
| Typical lateral feature size (standard TPL) | ~100 nm; voxel separation ~\(\lambda/4\) (200 nm at \(\lambda = 800\ \mathrm{nm}\), NA 1.4)[2](https://www.nature.com/articles/s43586-025-00386-y) • [3](https://opn-web-afd-d3bfbkd5bcc5asbs.z02.azurefd.net/opn/media/images/pdf/2019/1019/28-35_opn_10_19.pdf?t=638452570106247176) |
| Axial resolution (standard TPL) | ~500 nm best values; voxel elongated axially by a factor of ~2.5[3](https://opn-web-afd-d3bfbkd5bcc5asbs.z02.azurefd.net/opn/media/images/pdf/2019/1019/28-35_opn_10_19.pdf?t=638452570106247176) |
| Best reported sub-diffraction features | Sub-30 nm lateral (T2A-PI, single color); 40–60 nm voxels (STED/RAPID); 60 nm (chemical inhibition)[4](https://doi.org/10.1002/lpor.202300645) • [5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a) |
| Build area (standard serial writing) | Roughly 10 μm × 10 μm to 2.2 mm × 2.2 mm[6](https://beta.iopscience.iop.org/article/10.1088/2631-7990/ab8d9a) |
| Parallel TPL methods | Most report rates below 0.1 mm³/h or \(10^{4}\) voxels/s[7](https://www.nature.com/articles/s41467-023-37163-y) |
| Parallelization records (since 2023) | \(2 \times 10^{6}\) voxels/s with 2000 holographic foci; ~\(10^{8}\) voxels/s with >120,000 metalens foci[7](https://www.nature.com/articles/s41467-023-37163-y) • [8](https://iopscience.iop.org/article/10.1088/2631-7990/ae614a) |
| Environment | No mask, mold, stamp, vacuum, or clean room required[5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a) |
## How it works
Polymerization is confined to the focus by the nonlinear intensity dependence of two-photon absorption.[9](https://ascpt.onlinelibrary.wiley.com/doi/10.1002/lpor.71530) 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.[10](https://www.cell.com/iscience/pdf/S2589-0042%2823%2900451-0.pdf) Each point exposure produces a voxel, the three-dimensional analog of a pixel.[3](https://opn-web-afd-d3bfbkd5bcc5asbs.z02.azurefd.net/opn/media/images/pdf/2019/1019/28-35_opn_10_19.pdf?t=638452570106247176)
The 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.[5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a) • [3](https://opn-web-afd-d3bfbkd5bcc5asbs.z02.azurefd.net/opn/media/images/pdf/2019/1019/28-35_opn_10_19.pdf?t=638452570106247176) 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 \(0.1 \cdot I_{0}\).[5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a) 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.[5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a)
## How it is done
The 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.[5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a) 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.[5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a) 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.[5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a)
Raster 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.[5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a) 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.[5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a) The original 1997 resin consisted of photoinitiators, urethane acrylate monomers, and urethane acrylate oligomers.[11](https://doi.org/10.1364/ol.22.000132)
## Origin
Shoji 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.[11](https://doi.org/10.1364/ol.22.000132) 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.[12](http://www.chem.ucla.edu/dept/Faculty/schwartz/schwartz_pubs/Opt_Lett_23_1745_1998.pdf)
## Variants
**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.[5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a) 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.[13](https://pmc.ncbi.nlm.nih.gov/articles/PMC10518867/) Chemical inhibition with radical inhibitors achieved 60 nm features (Farasi group).[5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a)
**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.[4](https://doi.org/10.1002/lpor.202300645) The dose dependence of the photomodification depth (degree of conversion) also enables 3D greyscale and 4D patterning.[2](https://www.nature.com/articles/s43586-025-00386-y) Projection TPL parallelizes exposure to raise printing rate by three orders of magnitude.[14](https://par.nsf.gov/biblio/10391835-minimizing-shrinkage-microstructures-printed-projection-two-photon-lithography)
## Applications
Published applications span optics, microfluidics, bioelectronics, metamaterials, and biomedical research.[9](https://ascpt.onlinelibrary.wiley.com/doi/10.1002/lpor.71530) 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.[8](https://iopscience.iop.org/article/10.1088/2631-7990/ae614a)
## Limitations and alternatives
TPL 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.[10](https://www.cell.com/iscience/pdf/S2589-0042%2823%2900451-0.pdf) Serial writing remains the bottleneck: most parallel TPL methods report rates below 0.1 mm³/hour or \(10^{4}\) voxels/s.[7](https://www.nature.com/articles/s41467-023-37163-y) 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.[7](https://www.nature.com/articles/s41467-023-37163-y) 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.[8](https://iopscience.iop.org/article/10.1088/2631-7990/ae614a) [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.[15](https://www.sciencedirect.com/science/article/pii/S2214860425002660) • [16](https://www.osti.gov/pages/biblio/3012646-suppressing-proximity-effects-during-rapid-serial-two-photon-lithography-through-tuning-reactiondiffusion-kinetics) 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.[14](https://par.nsf.gov/biblio/10391835-minimizing-shrinkage-microstructures-printed-projection-two-photon-lithography)
The 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.[6](https://beta.iopscience.iop.org/article/10.1088/2631-7990/ab8d9a) Unlike two-beam interference lithography, EUV lithography, and electron-beam lithography, TPL requires no mask, mold, stamp, vacuum, or clean-room environment.[5](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a)
## References
1. [Two-photon lithography for three-dimensional fabrication in micro/nanoscale regime: A comprehensive review](https://www.sciencedirect.com/science/article/abs/pii/S0030399221002681)
2. [Multiphoton 3D lithography (Nature Reviews Methods Primers)](https://www.nature.com/articles/s43586-025-00386-y)
3. [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)
4. [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)
5. [Two-photon polymerization-assisted 3D laser nanoprinting: from fundamentals to modern applications](https://pubs.rsc.org/en/content/articlelanding/2025/tc/d5tc02037a)
6. [Projection micro stereolithography based 3D printing and its applications](https://beta.iopscience.iop.org/article/10.1088/2631-7990/ab8d9a)
7. [Ultrafast 3D nanofabrication via digital holography](https://www.nature.com/articles/s41467-023-37163-y)
8. [Metalens-based 3D nanolithography breaks the throughput barrier](https://iopscience.iop.org/article/10.1088/2631-7990/ae614a)
9. [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)
10. [S2589 0042(23)00451 0 (cell.com)](https://www.cell.com/iscience/pdf/S2589-0042%2823%2900451-0.pdf)
11. [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)
12. [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)
13. [STED-Inspired Cationic Photoinhibition Lithography](https://pmc.ncbi.nlm.nih.gov/articles/PMC10518867/)
14. [Minimizing Shrinkage in Microstructures Printed With Projection Two-Photon Lithography](https://par.nsf.gov/biblio/10391835-minimizing-shrinkage-microstructures-printed-projection-two-photon-lithography)
15. [Control of temporal and spatial proximity effects in two-photon lithography](https://www.sciencedirect.com/science/article/pii/S2214860425002660)
16. [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)
---
*Topic: Encyclopedia › Technology and the built world › Engineering and manufacturing › Manufacturing processes and fabrication › Forming, heat treatment, and finishing › Polymer and composite additive manufacturing*
*Initially written Sep 29, 2026 · Edited: Oct 15, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license