Researchers at Lawrence Livermore National Laboratory (LLNL) have optimized and 3D-printed helix structures as optical materials for Terahertz (THz) frequencies, a potential way to address a technology gap for next-generation telecommunications, non-destructive evaluation, chemical/biological sensing and more.
The printed microscale helixes reliably create circularly polarized beams in the THz range and, when arranged in patterned arrays, can function as a new type of Quick Response (QR) for advanced encryption/decryption. Their results, published in Advanced Science, represent the first full parametric analysis of helical structures for THz frequencies and show the potential of 3D printing for fabricating THz devices.
Lending a handedness
The THz frequency on the electromagnetic spectrum is the backbone of 5G/6G telecommunications and a potential non-ionizing alterative to X-rays and gamma rays, in addition to being able to detect chemical and biological signatures that are inaccessible at other wavelengths. However, common optical components like waveplates and cameras are difficult to realize because the THz frequency is too high for electronics and its wavelength is too long for photonics. In the study, the team focused on one of these missing pieces: quarter waveplates to generate circularly polarized beams.
"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.
Circularly polarized beams twist like a spiral staircase in a right-handed or left-handed direction, giving them a "handedness" property known as chirality. Chirality is a fundamental property of biomolecules like amino acids, DNA and proteins, and using a chiral beam to investigate their molecular vibrations can reveal critical information about structure, composition and biological activity. The THz regime supercharges this capability, making it possible to study much larger groups of atoms and detect vibrations resulting from long-range ordering and secondary-bonding networks. This can help quickly identify or characterize diseases and potentially hazardous materials like drugs and explosives.
Creating circularly polarized beams involves using chiral structures as quarter waveplates, which introduce a 90-degree phase shift between two orthogonal components of a wave's electric field. Previous attempts at making THz chiral structures have resulted in limited transmission and frequency range, but the team saw an opportunity to make something much more optimal with two-photon polymerization (2PP), an ultrahigh resolution light-based 3D printing technique.
"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.
Optimization to innovation
With the flexibility to print nearly any shape, the team focused on optimizing helixes. Helixes are difficult to design because they have extra variables to consider like number of turns, radius, height and handedness, but their inherent circular geometry makes them ideal for creating strong circular polarization.
"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 printed helixes demonstrated strong broadband activity in the THz range and reliably created circularly polarized beams at nearly any azimuthal angle. The team also discovered that single helixes have distinct left-handed or right-handed signals, and that arranging them in an array had a coupling effect that enhanced the response of both types. This inspired the world's first "chiral QR code."
"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."
Made from a printed helical array, the chiral QR code's information is inaccessible unless viewed through a special "handedness" filter that is properly polarized and within the correct electromagnetic frequency. This makes it possible either hide or add an important layer of security in sensitive environments.
"For hospitals or banks or military purposes, sometimes we might need to add encryption while maintaining the convenience of the rapid scan," said Choi.
New applications
For Xia's team, the study was an opportunity to test their innovative parallel printing technique, which uses novel optical materials called metalenses to tightly focus the laser for direct 2PP printing. A large metalens array can generate more than 100,000 focal spots at the same time, creating a 3D printing assembly line. The array can also be turned on and off to control printing path and create complex, layered and aperiodic structures significantly faster.
"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."
Potential applications for the helixes include chiral molecular sensing, band-pass filtering for 5G/6G telecommunications, as well detection and sensing for fields like medicine, biology, astronomy and more. The study also shows the potential of combining high-throughput 3D printing with materials science and optimization to create new THz technologies that take advantage of the frequency's full potential.
Other co-authors include LLNL's Widi Moestopo, Songyun Gu, Jae-Hyuck Yoo and Michael Armstrong, and Professor Taeil Lee from Gachon University in South Korea.
-Noah Pflueger-Peters
