NEW YORK, Oct. 7, 2020 — Researchers in David Pine’s NYU laboratory have developed and introduced a method for the reliable creation of colloidal diamonds. Using a steric interlock mechanism to spontaneously produce the staggered bonds that make a physical diamond formation possible, the researchers created pyramidal colloids that approached and subsequently linked to one another in an orientation necessary to the self-assembly of a diamond formation.
Colloids, if assembled and arranged in a diamond formation, produce a bandgap for visible light, filtering out distinct wavelengths. The photonic technique and the diamond lattice it produced could cut manufacturing costs and processes necessary to the manufacture of highly efficient optical circuits and switches, advancing optical computers, lasers, and light filters.
Further, the stable, self-assembled 3D photonic crystals the discovery enables support increasingly lightweight and highly efficient devices, as well as precision light control and the management of certain thermal signatures.
Spheres that are hundreds of times smaller than the diameter of a human hair make up colloidal structures, which can be physically structured and assembled into different crystalline shapes depending on the way(s) in which the spheres link to one another. DNA strands glued to a colloid’s surface allow the multiple colloids to attach to one another. When colloids collide in a liquid bath, the DNA snags and the colloids link. How and where the DNA is attached to each colloid allows the colloids to spontaneously assemble into complex structures.
Until now, though, those complex structures did not include or produce a bandgap for visible light in a process that could be reasonably performed at a commercial scale. Previous assemblies demonstrated colloidal strings and tightly packed cubic crystals, but not a diamond.
The idea of making a material with photonic bandgaps, Pine said, dates to the 1980s. It wasn’t until 1990 that scientists showed a definitive calculation leading to a structure that contained one. Pine and his group have been attempting to self-assemble photonic crystals since 1997, focused for much of that period on doing so via a diamond lattice.
“There were two challenges, at least,” Pine said. “One was to make colloidal particles that would bind to each other so that they were tetrahedrally coordinated.” It is a challenging arrangement to complete, as it involves an open and generally unstable structure.
Colloids arranged in crystalline shapes. Courtesy of David Pine.
The more immense obstacle, Pine said, is that when two patches on different particles bind to each other, the other three patches on both particles must “anti-align” in what is known as the staggered confirmation.
“We had no way of doing that. It was last year that we hit on a way of doing it,” Pine said. “Once we ironed out and honed that idea, we were optimistic this would work.”
The new method emerged when Mingxin He, lead author of the study describing the discovery, synthesized colloids in a strictly pyramidal structure. He and teammates drew out all the many ways they could link the structures with which they were working, ultimately identifying a distinct interlinked structure capable of reliably forming diamonds.
The method forgoes the use of nanomachines, instead relying on colloids to structure themselves without external interferences. In addition to self-assembled, the diamond formation is highly stable, even when removed from the liquid in which they assemble.
Once assembled, the next step, as it relates to implementation in photonic devices, is to invert the crystals, Pine said. This allows the physicists to fill the spaces between the particles that make up the lattice with a material or materials with a high refractive index, bending the light traveling through the formation.
The team is currently at work on that stage. Members will next attempt to make waveguides, resonators, and filters by writing patterns into samples. The team finally aims to expand the range of optical frequencies their method covers, a pursuit that may require advances to colloidal materials.
“What we conceptually start to think about now is the perfect diamond crystal, where every colloidal particle is a perfect lattice,” Pine said. “It turns out that isn’t very useful. What becomes useful is when you start to introduce defects, breaking the symmetry of the perfect lattice. This gives you the practical applications.”
These applications use waveguides and optical circuits, including those with splitters, filters, switches, and transistors, the creation of which are supported by the new method.
“One of the things that is nice about photonic crystals is that you ought to be able to use them to make the smallest optical circuits that are theoretically possible, namely where the width of the circuit wire is the same wavelength, or maybe even half the wavelength of light. You cannot really get smaller than that and still have light pass through,” Pine said.
A small photonic circuit can increase efficiency, requiring less power to achieve optimal performance, while still allowing scientists to intentionally write in defects and attributes, like cavities, useful in lasing and other processes.
Another advantage of the colloidal diamond construction is that it intrinsically makes 3D structures. Photonic integrated circuits exist in 2D, though adding a dimension has proved to be a difficult lift. The technology and approach for constructing 2D circuits closely resembles the methods scientists and engineers have historically used to build electronic circuits; the processes are highly developed and refined.
Conversely, shifting into the third dimension, especially with self-assembly, is a new undertaking. Developmentally, it parallels the new method itself. Whereas silicon, for example, had existing application in electronics prior to the widespread emergence of the field of silicon photonics at the start of the century, work with colloidal diamonds, and even photonic crystals, is in a more primitive stage, Pine said.
Energy and Defense
According to the U.S. Army Research Office, which funded the work, by advancing the way researchers are able to develop optical technologies such as photonic circuits and filters, the discovery also charts a course for laser and sensor development — devices boasting a range of applications, from industry to defense, and aerospace to communications.
“This long-sought demonstration of the first self-assembled colloidal diamond lattices will unlock new research and development opportunities for important Department of Defense technologies which could benefit from 3D photonic crystals,” said Evan Runnerstrom, Army Research Office program manager.
Lasers operating at high efficiency that support precision sensing and directed energy systems are applications key to Army modernization priorities, including air and missile defense, a U.S. Army Research Lab press release said.
For quantum computing, atomic clocks, gyroscopes for precision navigation and timing, and optimally sized, weighted, and powered optical systems, photonic circuits will play a similarly important function.
The National Science Foundation provided additional funding for the work. The research was published in Nature (www.doi.org/10.1038/s41586-020-2718-6).