Gedruckte Helix metamaterials: LLNL generates THz circular polarization via 2PP 3D printing

Researchers at the Lawrence Livermore National Laboratory (LLNL) have designed microstructured helix geometries as optical components for terahertz (THz) frequencies and fabricated them using high-resolution 3D printing. The results, published in Advanced Science, address a well-known gap: Many classical optical components such as waveplates are difficult to implement in the THz range because the frequencies are too high for conventional electronics and the wavelengths are too long for many photonic approaches.

At the center of the work are printed quarter-wave plates that create a 90-degree phase shift between two orthogonal field components, thereby providing circularly polarized THz radiation.

“Metamaterials are the most effective way to generate circularly polarized beams in the THz frequency range using optimized geometries, as there are currently no optical crystals available for such long electromagnetic wavelengths,” said Materials Science Division scientist and Lawrence fellow Wonjin Choi, who led the project.

“At around 300 µm, the wavelength of the THz frequency is a sweet spot [for 2PP], so we can create any geometries in that length scale comfortably and control it very nicely,” said Materials Engineering Division (MED) staff engineer Xiaoxing Xia, who led the printing efforts for the project.

“One of the most intuitive and powerful approaches to inducing chirality is to create a helix,” said Choi. “We nearly perfectly optimized these parameters through simulations and then precisely 3D-printed the structures to achieve the desired functionality.”

The helix shape provides an intuitive source of chirality, but requires careful parameterization, for example of the number of turns, radius, height, and handedness.

The structures were fabricated using two-photon polymerization (2PP), a light-based micro-3D-printing technique.

“I realized we can make pixelation if we make black pixels right-handed and white pixels left-handed,” said Choi. “A typical QR code encodes information in binary amplitude or brightness, but this one does so in phase with left- and right-handed polarization rotation.”

“For hospitals or banks or military purposes, sometimes we might need to add encryption while maintaining the convenience of the rapid scan,” said Choi.

“Because we have active control of the focal spots, we can selectively print helixes of different handedness in different locations,” said Xia. “It would take a very long time to print on a commercial printer, but our parallel printer really improves the throughput to make the application tangible.”

The system-level aspect is also exciting: Combined in arrays, left- and right-handed helices can be used as “pixels” to encode information not via brightness, but via polarization rotation. The pattern becomes readable only with the appropriate polarization and frequency window—an approach that could be interesting for access control or additional security layers. Parallelization via metalens arrays with many focal spots is also intended to significantly increase throughput compared to conventional 2PP.