The structure of matter shapes the passage of light. An opal bends and curves it, producing iridescence. A prism separates it into its constituent parts, producing a rainbow. A mirror reflects it, producing a 2D you.
Scientist Feng Pan creates materials with sculptural features that manipulate light not for their visual effects, but to encode information.
Unlike an opal or prism, his materials are practically invisible. Only with a powerful microscope can one view the 2D etchings that are his handiwork. These metamaterials — materials exhibiting effects not found in nature — are miniature bas reliefs that reliably store and deliver quantum information.
“… we can engineer the metamaterials with the desired chirality and then couple to other materials to potentially create chiral polaritons. … Using polaritons will be powerful and important for information storage.” — Feng Pan, Stanford University
“I think the best part is to play with the optics and to build the setup that can characterize these materials,” said Pan, a Stanford University postdoctoral researcher working under Professor Jennifer Dionne.
Pan is a member of Q-NEXT, a U.S. Department of Energy (DOE) National Quantum Information Science Research Center led by DOE’s Argonne National Laboratory.
Precision design for quantum information storage
Pan’s metamaterials feature notches, carvings and forms with fun names such as “nanobars” and “nanodiscs,” each as wide as 1/1,000th of the diameter of a human hair. The result often looks like a nanoscopic apple pie with bites taken from the edge.
Whimsical descriptions notwithstanding, these features are precisely designed. They steer or bend light in unusual ways, and they can store light energy for a millionth of a second — a long time in the quantum realm.
“We control a lot of the metamaterial’s geometric parameters or intrinsic properties to design unique nanostructures that perform distinct but desired functions,” Pan said.
Reliable information delivery and storage is crucial for the development of quantum technologies, whose impact is expected to be revolutionary. In the future, quantum computers could tackle today’s most intractable problems in mere hours, compared to the thousands of years today’s traditional computers would need to solve them — if they can solve them at all.
But quantum information storage is a tricky business. Quantum information is packaged into bits called qubits, which are exceedingly delicate. One small disturbance in the environment, and poof — the qubit disintegrates.
As part of his Q-NEXT research, Pan is designing his metamaterials to be able to exercise tight control over how they emit photons — particles of light and carriers of quantum information — and so protect the fragile qubits.
One of the goals is for the material to produce particles with a well-defined chirality — a fancy word for the particle’s innate right- or left-handedness.
In particular, Pan pursues the production of half-light, half-matter particles called chiral polaritons. These particles can flow and interact with one another in ways that photons can’t, ways that are critical for quantum information storage and simulation.
Pan’s metamaterials bring chirality to polaritons, which must be distinctly left- or right-handed. Wishy-washy, imperfect chirality will not do. That property gives scientists an important, additional knob to turn to control quantum information storage.
“Using polaritons will be powerful and important for information storage,” Pan said. “We can use them to store even more information.”
The science and craft of making metamaterials
How does Pan create his metamaterials? It’s a three-step process.
First, he and his team use computer-aided numerical simulations to design the metamaterials.
Second, he fabricates them in the cleanroom. To start, he uses an electron beam to define the 2D pattern and print it onto a special compound. The pattern is transferred onto silicon layer mere hundreds of nanometers thick, 1/1,000th as thick as a sheet of paper, to produce the metamaterial. The metamaterial is integrated with a second layer, an atomically thin semiconductor material.
Third, he and his team measure how the integrated whole behaves. What are the characteristics of its emitted photons? Can its design be improved? How? The team iterates on the design and repeats the process from step one. The entire procedure can take weeks or months to optimize.
“You have to trial and error this process to tweak the parameters for the goals,” Pan said. “There’s often some discrepancy between the design and the real structure. You can do beautiful simulations using computers, but it sometimes turns out that it isn’t the design you want because you didn’t account for fabrication errors. It’s a challenging task.”
The connection between the silicon metamaterial layer and semiconductor layer is key. The longer the photons and the semiconductor layer can interact, the higher the polaritons’ quality. And that’s one reason Pan and his team like using 2D materials: The materials’ flatness increases the ease of integrating these two ultrathin layers, making it easier to control the interaction between them.
“I think the most important aspect that differentiates our work from others is that we can engineer the metamaterials with the desired chirality and then couple to other materials to potentially create chiral polaritons,” Pan said.
Learning to manipulate light
Pan remembers the first time he conquered the task of creating a metamaterial. He’d just begun his stint as a Stanford postdoc. As a chemistry graduate student at the University of Wisconsin–Madison, he’d never done any materials fabrication.
After two months, he managed to make a thin silicon film the size of a compact disc. The type of silicon he needed wasn’t commercially available, so he had to make it himself. He even developed a process to bond the silicon to glass.
“One day I had a four-inch wafer of this silicon thin film on a glass substrate, which was very exciting,” Pan said. “The recipe I came up with could be very useful for making crystalline silicon on glass metamaterials.”
He cut the wafer into about 50 chips, and the team can use them to mold their metamaterials.
Right now, Pan’s integrated materials work only at ultracold temperatures, which means having to operate them in a cryogenic station. The moonshot: Create materials that operate at room temperature, which would make fashioning them cost-effective and massively deployable.
Pan loves the versatility of these compact metamaterials, which are already used in holograms and in the creation of virtual or augmented reality environments.
“There are vast opportunities for these metamaterials. They’re a powerful candidate for manipulating any properties of light,” he said. “There will be more and more people diving into this field to bring these devices to many quantum applications.”
For those who do want to dive in, Pan’s advice is simple:
“Always be hungry for new science,” he said. “There always an uphill and downhill in this pursuit of science.”
As to his own research for quantum storage metamaterials, he’s optimistic.
“We’re ready for any surprises,” he said. “And we’re not at the finish line yet, but we’re on track.”
This work was supported by the DOE’s Office of Science National Quantum Information Science Research Centers as part of the Q-NEXT center.
Q-NEXT is a U.S. Department of Energy National Quantum Information Science Research Center led by Argonne National Laboratory. Q-NEXT brings together world-class researchers from national laboratories, universities and U.S. technology companies with the goal of developing the science and technology to control and distribute quantum information. Q-NEXT collaborators and institutions will create two national foundries for quantum materials and devices, develop networks of sensors and secure communications systems, establish simulation and network test beds, and train the next-generation quantum-ready workforce to ensure continued U.S. scientific and economic leadership in this rapidly advancing field. For more information, visit https://q-next.org/.
Argonne National Laboratory seeks solutions to pressing national problems in science and technology. The nation’s first national laboratory, Argonne conducts leading-edge basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance America’s scientific leadership and prepare the nation for a better future. With employees from more than 60 nations, Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy’s Office of Science.
The U.S. Department of Energy’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit https://energy.gov/science.