Such high-pixel-density displays will be able to provide stunning images with true-to-life detail — something that will be even more important for headset displays designed to sit just centimeters from our faces.

The advance is based on research by Stanford University materials scientist Mark Brongersma in collaboration with the Samsung Advanced Institute of Technology (SAIT). Brongersma was initially put on this research path because he wanted to create an ultra-thin solar panel design.

“We’ve taken advantage of the fact that, on the nanoscale, light can flow around objects like water,” said Brongersma, who is a professor of materials science and engineering and senior author of the Oct. 22 Science paper detailing this research. “The field of nanoscale photonics keeps bringing new surprises and now we’re starting to impact real technologies. Our designs worked really well for solar cells and now we have a chance to impact next generation displays.”

In addition to having a record-setting pixel density the new “metaphotonic” OLED displays would also be brighter and have better color accuracy than existing versions, and they’d be much easier and cost-effective to produce as well.

Hidden gems

At the heart of an OLED are organic, light-emitting materials. These are sandwiched between highly-reflective and semi-transparent electrodes that enable current injection into the device. When electricity flows through an OLED, the emitters give off red, green or blue light. Each pixel in an OLED display is composed of smaller sub-pixels that produce these primary colors. When the resolution is sufficiently high, the pixels are perceived as one color by the human eye. OLEDs are an attractive technology because they are thin, light and flexible and produce brighter and more colorful images than other kinds of displays.

This research aims to offer an alternative to the two types of OLED displays that are currently commercially available. One type — called a red-green-blue OLED — has individual sub-pixels that each contain only one color of emitter. These OLEDs are fabricated by spraying each layer of materials through a fine metal mesh to control the composition of each pixel. They can only be produced on a small scale, however, like what would be used for a smartphone.

Larger devices like TVs employ white OLED displays. Each of these sub-pixels contains a stack of all three emitters and then relies on filters to determine the final sub-pixel color, which is simpler to fabricate. Since the filters reduce the overall output of light, white OLED displays are more power-hungry and prone to having images burn into the screen.

OLED displays were on the mind of Won-Jae Joo, a SAIT scientist, when he visited Stanford from 2016 to 2018. During that time, Joo listened to a presentation by Stanford graduate student Majid Esfandyarpour about an ultrathin solar cell technology he was developing in Brongersma’s lab and realized it had applications beyond renewable energy.

“Professor Brongersma’s research themes were all very academically profound and were like hidden gems for me as an engineer and researcher at Samsung Electronics,” said Joo, who is lead author of the Science paper.

Joo approached Esfandyarpour after the presentation with his idea, which led to a collaboration between researchers at Stanford, SAI and Hanyang University in Korea.

“It was quite exciting to see that a problem that we have already thought about in a different context can have such an important impact on OLED displays,” said Esfandyarpour.

A fundamental foundation

The crucial innovation behind both the solar panel and the new OLED is a base layer of reflective metal with nanoscale (smaller than microscopic) corrugations, called an optical metasurface. The metasurface can manipulate the reflective properties of light and thereby allow the different colors to resonate in the pixels. These resonances are key to facilitating effective light extraction from the OLEDs.

“This is akin to the way musical instruments use acoustic resonances to produce beautiful and easily audible tones,” said Brongersma, who conducted this research as part of the Geballe Laboratory for Advanced Materials at Stanford.

For example, red emitters have a longer wavelength of light than blue emitters, which, in conventional RGB-OLEDs, translates to sub-pixels of different heights. In order to create a flat screen overall, the materials deposited above the emitters have to be laid down in unequal thicknesses. By contrast, in the proposed OLEDs, the base layer corrugations allow each pixel to be the same height and this facilitates a simpler process for large-scale as well as micro-scale fabrication.

In lab tests, the researchers successfully produced miniature proof-of-concept pixels. Compared with color-filtered white-OLEDs (which are used in OLED televisions) these pixels had a higher color purity and a twofold increase in luminescence efficiency — a measure of how bright the screen is compared to how much energy it uses. They also allow for an ultrahigh pixel density of 10,000 pixels-per-inch.

The next steps for integrating this work into a full-size display is being pursued by Samsung, and Brongersma eagerly awaits the results, hoping to be among the first people to see the meta-OLED display in action.

UC researchers discovered evidence of a filter system at the Corriental reservoir, an important source of drinking water for the ancient Maya in what is now northern Guatemala.

A multidisciplinary team of UC anthropologists, geographers and biologists identified crystalline quartz and zeolite imported miles from the city. The quartz found in the coarse sand along with zeolite, a crystalline compound consisting of silicon and aluminum, create a natural molecular sieve. Both minerals are used in modern water filtration.

The filters would have removed harmful microbes, nitrogen-rich compounds, heavy metals such as mercury and other toxins from the water, said Kenneth Barnett Tankersley, associate professor of anthropology and lead author of the study.

“What’s interesting is this system would still be effective today and the Maya discovered it more than 2,000 years ago,” Tankersley said.

UC’s discovery was published in the journal Scientific Reports.

The Maya created this water filtration system nearly 2,000 years before similar systems were used in Europe, making it one of the oldest water treatment systems of its kind in the world, Tankersley said.

Researchers from UC’s College of Arts and Sciences traced the zeolite and quartz to steep ridges around the Bajo de Azúcar about 18 miles northeast of Tikal. They used X-ray diffraction analysis to identify zeolite and crystalline quartz in the reservoir sediments.

At Tikal, zeolite was found exclusively in the Corriental reservoir.

For the ancient Maya, finding ways to collect and store clean water was of critical importance. Tikal and other Maya cities were built atop porous limestone that made ready access to drinking water difficult to obtain for much of the year during seasonal droughts.

UC geography professor and co-author Nicholas Dunning, who has studied ancient civilizations most of his career, found a likely source of the quartz and zeolite about 10 years ago while conducting fieldwork in Guatemala.

“It was an exposed, weathered volcanic tuff of quartz grains and zeolite. It was bleeding water at a good rate,” he said. “Workers refilled their water bottles with it. It was locally famous for how clean and sweet the water was.”

Dunning took samples of the material. UC researchers later determined the quartz and zeolite closely matched the minerals found at Tikal.

UC assistant research professor Christopher Carr, an expert in geographic information system mapping, also conducted work on the UC projects at Bajo de Azúcar and Corriental.

“It was probably through very clever empirical observation that the ancient Maya saw this particular material was associated with clean water and made some effort to carry it back,” Dunning said.

UC anthropology professor emeritus Vernon Scarborough, another co-author, said most research on ancient water management has tried to explain how civilizations conserved, collected or diverted water.

“The quality of water put to potable ends has remained difficult to address,” Scarborough said. “This study by our UC team has opened the research agenda by way of identifying the quality of a water source and how that might have been established and maintained.”

Of course, reconstructing the lives, habits and motivations of a civilization 1,000 years ago is tricky.

“We don’t have absolute proof, but we have strong circumstantial evidence,” Dunning said. “Our explanation makes logical sense.”

“This is what you have to do as an archaeologist,” UC biologist and co-author David Lentz said. “You have to put together a puzzle with some of the pieces missing.”

Lentz said the filtration system would have protected the ancient Maya from harmful cyanobacteria and other toxins that might otherwise have made people who drank from the reservoir sick.

“The ancient Maya figured out that this material produced pools of clear water,” he said.

Complex water filtration systems have been observed in other ancient civilizations from Greece to Egypt to South Asia, but this is the first observed in the ancient New World, Tankersley said.

“The ancient Maya lived in a tropical environment and had to be innovators. This is a remarkable innovation,” Tankersley said. “A lot of people look at Native Americans in the Western Hemisphere as not having the same engineering or technological muscle of places like Greece, Rome, India or China. But when it comes to water management, the Maya were millennia ahead.”

They’re hypothesized to have formed about 100 million years after the Big Bang out of universal darkness from the primordial gases of hydrogen, helium, and trace light metals. These gases cooled, collapsed, and ignited into stars up to 1,000 times more massive than our sun. The bigger the star, the faster they burn out. The first stars probably only lived a few million years, a drop in the bucket of the age of the universe, at about 13.8 billion years. They’re unlikely to ever be observed, lost to the mists of time.

As the metal-free first stars collapsed and exploded into supernovae, they forged heavier elements such as carbon that seeded the next generation of stars. One type of these second stars is called a carbon-enhanced metal-poor star. They’re like fossils to astrophysicists. Their composition reflects the nucleosynthesis, or fusion, of heavier elements from the first stars.

“We can get results from indirect measurements to get the mass distribution of metal-free stars from the elemental abundances of metal-poor stars,” said Gen Chiaki, a post-doctoral researcher in the Center for Relativistic Astrophysics, School of Physics, Georgia Tech.

Chiaki is the lead author of a study published in the September 2020 issue of the Monthly Notices of the Royal Astronomical Society. The study modeled for the first time faint supernovae of metal-free first stars, which yielded carbon-enhanced abundance patterns through the mixing and fallback of the ejected bits.

Their simulations also showed the carbonaceous grains seeding the fragmentation of the gas cloud produced, leading to formation of low-mass ‘giga-metal-poor’ stars that can survive to the present day and possibly be found in future observations.

“We find that these stars have very low iron content compared to the observed carbon-enhanced stars with billionths of the solar abundance of iron. However, we can see the fragmentation of the clouds of gas. This indicates that the low mass stars form in a low iron abundance regime. Such stars have never been observed yet. Our study gives us theoretical insight of the formation of first stars,” Chiaki said.

The investigations of Wise and Chiaki are a part of a field called ‘galactic archaeology.’ They liken it to searching for artifacts underground that tell about the character of societies long gone. To astrophysicists, the character of long-gone stars can be revealed from their fossilized remains.

“We can’t see the very first generations of stars,” said study co-author John Wise, an associate professor also at the Center for Relativistic Astrophysics, School of Physics, Georgia Tech. “Therefore, it’s important to actually look at these living fossils from the early universe, because they have the fingerprints of the first stars all over them through the chemicals that were produced in the supernova from the first stars.”

“These old stars have some fingerprints of the nucleosynthesis of metal-free stars. It’s a hint for us to seek the nucleosynthesis mechanism happening in the early universe,” Chiaki said.

“That’s where our simulations come into play to see this happening. After you run the simulation, you can watch a short movie of it to see where the metals come from and how the first stars and their supernovae actually affect these fossils that live until the present day,” Wise said.

The scientists first modeled the formation of their first star, called a Population III or Pop III star, and ran three different simulations that corresponded to its mass at 13.5, 50, and 80 solar masses. The simulations solved for the radiative transfer during its main sequence and then after it dies and goes supernova. The last step was to evolve the collapse of the cloud of molecules spewed out by the supernova that involved a chemical network of 100 reactions and 50 species such as carbon monoxide and water.

The majority of the simulations ran on the Georgia Tech PACE cluster. They were also awarded computer allocations by the National Science Foundation (NSF)-funded Extreme Science and Engineering Discovery Environment (XSEDE). Stampede2 at the Texas Advanced Computing Center (TACC) and Comet at the San Diego Supercomputer Center (SDSC) ran some of the main sequence radiative transfer simulations through XSEDE allocations.

“The XSEDE systems Comet at SDSC and Stampede2 at TACC are very fast and have a large storage system. They were very suitable to conduct our huge numerical simulations,” Chiaki said.

“Because Stampede2 is just so large, even though it has to accommodate thousands of researchers, it’s still an invaluable resource for us,” Wise said. “We can’t just run our simulations on local machines at Georgia Tech.”

Chiaki said he was also happy with the fast queues on Comet at SDSC. “On Comet, I could immediately run the simulations just after I submitted the job,” he said.

Wise has been using XSEDE system allocations for over a decade, starting when he was a postdoc. “I couldn’t have done my research without XSEDE.”

XSEDE also provided expertise for the researchers to take full advantage of their supercomputer allocations through the Extended Collaborative Support Services (ECSS) program. Wise recalled using ECSS several years ago to improve the performance of the Enzo adaptive mesh refinement simulation code he still uses to solve the radiative transfer of stellar radiation and supernovae.

“Through ECSS, I worked with Lars Koesterke at TACC, and I found out that he used to work in astrophysics. He worked with me to improve the performance by about 50 percent of the radiation transport solver. He helped me profile the code to pinpoint which loops were taking the most time, and how to speed it up by reordering some loops. I don’t think I would have identified that change without his help,” Wise said.

Wise has also been awarded time on TACC’s NSF-funded Frontera system, the fastest academic supercomputer in the world. “We haven’t gotten to full steam yet on Frontera. But we’re looking forward to using it, because that’s even a larger, more capable resource.”

Wise added: “We’re all working on the next generation of Enzo. We call it Enzo-E, E for exascale. This is a total re-write of Enzo by James Bordner, a computer scientist at the San Diego Supercomputer Center. And it scales almost perfectly to 256,000 cores so far. That was run on NSF’s Blue Waters. I think he scaled it to the same amount on Frontera, but Frontera is bigger, so I want to see how far it can go.”

The downside, he said, is that since the code is new, it doesn’t have all the physics they need yet. “We’re about two-thirds of the way there,” Wise said.

He said that he’s also hoping to get access to the new Expanse system at SDSC, which will supersede Comet after it retires in the next year or so. “Expanse has over double the compute cores per node than any other XSEDE resource, which will hopefully speed up our simulations by reducing the communication time between cores,” Wise said.

According to Chiaki, the next steps in the research are to branch out beyond the carbon features of ancient stars. “We want to enlarge our interest to the other types of stars and the general elements with larger simulations,” he said.

Said Chiaki: “The aim of this study is to know the origin of elements, such as carbon, oxygen, and calcium. These elements are concentrated through the repetitive matter cycles between the interstellar medium and stars. Our bodies and our planet are made of carbon and oxygen, nitrogen, and calcium. Our study is very important to help understand the origin of these elements that we human beings are made of.”