The authors investigate how point-wise laser dose control in two-photon polymerization can encode spatial material properties rather than geometry alone. They provide a MATLAB slicer converting voxelized refractive-index volumes to position-and-power print instructions, and experimentally calibrate IP-Dip2 refractive index using two-immersion digital holographic microscopy. Printed cell and C. elegans anterior-region phantoms demonstrate internal optical contrast, while grayscale lithography reduces measured surface roughness from 49 to 17 nm at 300 nm slicing. The accessible refractive-index

A microscopic object can have more than a shape. Its interior can be programmed with light.

One resin. One print. A different refractive index at every point.

Usually, 3D printing tells a material where to exist. This work also tells it what to be.

Michał Ziemczonok and Koen Vanmol combine a voxel-based slicer with a precise optical calibration to change how much a resin polymerizes, point by point. The local exposure sets its refractive index: how strongly it slows light.

Their striking demonstration is a printed phantom of the front of C. elegans. Its mouth and pharynx appear through differences in refractive index inside a single material.

Same outline. A different inside.

Start with uniform exposure. Then switch on the index map and choose how much contrast to encode. Watch the light write the volume.

Choose the exposure

The material shares one index throughout.

Explore a rescaled version of the supplied design.

The calibration comes from the supplementary IP-Dip2 dataset.

Colors indicate refractive index, not fluorescence. The scan illustrates fabrication; it does not represent print time.

What changes? More exposure changes the degree of resin conversion. The calibrated material response converts a desired index into local laser power.

What stays the same? The outer shape. The slicer adds a fourth instruction to each position: (x, y, z, power).

Localize the reaction

Two-photon absorption confines polymerization near the focus of an intense laser beam. Moving the focus writes a microscopic volume.

Choose the conversion

Adjusting exposure changes how much the resin reacts. The resulting conversion changes material density and refractive index.

Assign every voxel

A 3D array of target indices is converted into calibrated laser powers and scanning coordinates, all in one print file.

Anatomy, written as optical contrast.

Explore the actual supplied design and the optical tomography measurement of the printed phantom. Sweep through the volume to find the internal structure.

A virtual section through a printed specimen

The two volumes have independent depth axes. Equal normalized depths are not registered physical planes. Violet → gold represents lower → higher refractive index on the same fixed color scale (n = 1.507–1.564). The background is immersion medium.

A phantom is a known target. A reproducible internal index map can help benchmark microscopes and their reconstruction or analysis pipelines.

From design to measurement. Biological index contrast was reduced by about a factor of two to fit the resin. Printing and limited-angle tomography elongate features along z and reduce recovered contrast.

Volume data: the authors’ Zenodo supplement, CC BY 4.0. Resized and recolored for this view; index values and depth order retained. Underlying biological dataset courtesy of Shwetadwip Chowdhury, cited in the paper.

Give the dose a physical meaning.

A useful power map needs a measured relationship between laser power and refractive index. The authors measure phase delay through protected resin chambers in two different immersion liquids.

Using two liquids lets them solve for the resin index without a separate height measurement. A highly converted outer shell protects the less converted interior during measurement.

The phase shift depends on index contrast and path length. Two known surrounding indices separate those contributions.

Dose controls the surface, too.

Laser exposure also changes voxel size. Instead of adding finer layers, grayscale lithography adjusts voxel heights to better match a desired surface.

How would you print the slope?

Constant power · 300 nm slicing

Coarse layers leave a staircase on the slope.

Roughness is the measured standard deviation after removing shape and waviness, not a value calculated by this schematic.

Grayscale printing reached 17 nm roughness at 300 nm slicing—close to the 14 nm of six times finer layers—with no additional print time relative to the coarse version.

Design beyond the boundary.

A volume can carry a designed optical function, not just a silhouette.

The demonstrated phantoms turn biological imaging data into repeatable physical targets. The same workflow could support gradient-index optics, beam-shaping elements, and other structures whose function depends on a spatially varying material response.

Designs operate within the resin’s available index modulation, about 0.03 here. Low-conversion material needs protection and careful handling; calibration belongs to the particular resin and printing setup.

In the aging experiment, samples protected from light in immersion medium were followed for 303 days. Material state and storage are part of the design.

The experiments used a Nanoscribe Photonic Professional GT2 and IP-Dip2 resin with a 63×, 1.4 NA objective. Typical fabrication used 0.15 μm hatching, 0.3 μm slicing, and a scan speed of 10,000 μm s−1.

Laser power (LP) is reported as a percentage of 50 mW at the objective input. Calibration, desired refractive index, sampling, and scanning orientation feed the slicer; its output lists position and local power.

The volume applet uses the supplied design and calibration. Its contrast slider scales index deviations around 1.5457 within the calibrated dataset range. The 3D view samples the design and omits the surrounding medium; its fixed shape contains voxels above index 1.53298. It illustrates dose mapping rather than predicting a new experimental result.

Turning Light Into Matter: Programmable Microstructures Via Dose Control in Two-Photon Polymerization

Michał Ziemczonok & Koen Vanmol Advanced Optical Materials (2026), e71770

Open data, slicer & supplementary material ↗

Article: Creative Commons Attribution. Supplement: CC BY 4.0. Figures and volume data credited where shown. AI-generated conceptual artwork is an interpretive visual, not experimental data.

AI-generated conceptual illustration, informed by Figures 2 and 5. Visual adaptation, not experimental data. On a small screen, scroll to explore.