Holographic images have real depth because they are three dimensional, whereas monitors merely simulate depth on a 2D screen. Because we see in three dimensions, holographic images could be integrated seamlessly into our normal view of the everyday world.

The result is a virtual and augmented reality display that has the potential to be truly immersive, the kind where you can move your head normally and never lose the holographic images from view. “To get a similar experience using a monitor, you would need to sit right in front of a cinema screen,” said Felix Heide, assistant professor of computer science and senior author on a paper published April 22 in Nature Communications.

And you wouldn’t need to wear a screen in front of your eyes to get this immersive experience. Optical elements required to create these images are tiny and could potentially fit on a regular pair of glasses. Virtual reality displays that use a monitor, as current displays do, require a full headset. And they tend to be bulky because they need to accommodate a screen and the hardware necessary to operate it.

“Holography could make virtual and augmented reality displays easily usable, wearable and ultrathin,” said Heide. They could transform how we interact with our environments, everything from getting directions while driving, to monitoring a patient during surgery, to accessing plumbing instructions while doing a home repair.

One of the most important challenges is quality. Holographic images are created by a small chip-like device called a spatial light modulator. Until now, these modulators could only create images that are either small and clear or large and fuzzy. This tradeoff between image size and clarity results in a narrow field of view, too narrow to give the user an immersive experience. “If you look towards the corners of the display, the whole image may disappear,” said Nathan Matsuda, research scientist at Meta and co-author on the paper.

Heide, Matsuda and Ethan Tseng, doctoral student in computer science, have created a device to improve image quality and potentially solve this problem. Along with their collaborators, they built a second optical element to work in tandem with the spatial light modulator. Their device filters the light from the spatial light modulator to expand the field of view while preserving the stability and fidelity of the image. It creates a larger image with only a minimal drop in quality.

Image quality has been a core challenge preventing the practical applications of holographic displays, said Matsuda. “The research brings us one step closer to resolving this challenge,” he said.

The new optical element is like a very small custom-built piece of frosted glass, said Heide. The pattern etched into the frosted glass is the key. Designed using AI and optical techniques, the etched surface scatters light created by the spatial light modulator in a very precise way, pushing some elements of an image into frequency bands that are not easily perceived by the human eye. This improves the quality of the holographic image and expands the field of view.

Still, hurdles to making a working holographic display remain. The image quality isn’t yet perfect, said Heide, and the fabrication process for the optical elements needs to be improved. “A lot of technology has to come together to make this feasible,” said Heide. “But this research shows a path forward.”

Osmotic energy can be generated anywhere salt gradients are found, but the available technologies to capture this renewable energy have room for improvement. One method uses an array of reverse electrodialysis (RED) membranes that act as a sort of “salt battery,” generating electricity from pressure differences caused by the salt gradient. To even out that gradient, positively charged ions from seawater, such as sodium, flow through the system to the freshwater, increasing the pressure on the membrane. To further increase its harvesting power, the membrane also needs to keep a low internal electrical resistance by allowing electrons to easily flow in the opposite direction of the ions. Previous research suggests that improving both the flow of ions across the RED membrane and the efficiency of electron transport would likely increase the amount of electricity captured from osmotic energy. So, Dongdong Ye, Xingzhen Qin and colleagues designed a semipermeable membrane from environmentally friendly materials that would theoretically minimize internal resistance and maximize output power.

The researchers’ RED membrane prototype contained separate (i.e., decoupled) channels for ion transport and electron transport. They created this by sandwiching a negatively charged cellulose hydrogel (for ion transport) between layers of an organic, electrically conductive polymer called polyaniline (for electron transport). Initial tests confirmed their theory that decoupled transport channels resulted in higher ion conductivity and lower resistivity compared to homogenous membranes made from the same materials. In a water tank that simulated an estuary environment, their prototype achieved an output power density 2.34 times higher than a commercial RED membrane and maintained performance during 16 days of non-stop operation, demonstrating its long-term, stable performance underwater. In a final test, the team created a salt battery array from 20 of their RED membranes and generated enough electricity to individually power a calculator, LED light and stopwatch.

Ye, Qin and their team members say their findings expand the range of ecological materials that could be used to make RED membranes and improve osmotic energy-harvesting performance, making these systems more feasible for real-world use.

The authors acknowledge funding from the National Natural Science Foundation of China.

A team of researchers led by Director Rod RUOFF at the Center for Multidimensional Carbon Materials (CMCM) within the Institute for Basic Science (IBS), including graduate students at the Ulsan National Institute of Science and Technology (UNIST), have grown diamonds under conditions of 1 atmosphere pressure and at 1025 °C using a liquid metal alloy composed of gallium, iron, nickel, and silicon, thus breaking the existing paradigm. The discovery of this new growth method opens many possibilities for further basic science studies and for scaling up the growth of diamonds in new ways.

Director Ruoff, who is also a UNIST Distinguished Professor notes, “This pioneering breakthrough was the result of human ingenuity, unremitting efforts, and the concerted cooperation of many collaborators.” Researchers led by Ruoff conducted a series of experiments, involving several hundred parameter adjustments and a variety of experimental approaches before they finally succeeded in growing diamonds using a ‘home-built’ cold-wall vacuum system.

Ruoff notes “We had been running our parametric studies in a large chamber (named RSR-A with an interior volume of 100 liters) and our search for parameters that would yield growth of diamond was slowed due to the time needed to pump out air (about 3 minutes), purge with inert gas (90 minutes), followed by pumping down again to vacuum level (3 minutes) so that the chamber could then be filled with 1 atmosphere pressure of quite pure hydrogen/methane mixture (again 90 minutes); that is over 3 hours before the experiment could be started! I asked Dr. Won Kyung SEONG to design & build a much smaller chamber to greatly reduce the time needed to start (and finish!) the experiment with the liquid metal exposed to the mixture of methane and hydrogen.” Seong adds, “Our new homebuilt system (named RSR-S, with an interior volume of only 9 liters) can be pumped out, purged, pumped out, and filled with methane/hydrogen mixture, in a total time of 15 minutes. Parametric studies were greatly accelerated, and this helped us discover the parameters for which diamond grows in the liquid metal!”

The team discovered that diamond grows in the sub-surface of a liquid metal alloy consisting of a 77.75/11.00/11.00/0.25 mix (atomic percentages) of gallium/nickel/iron/silicon when exposed to methane and hydrogen under 1 atm pressure at ~1025 °C.

Yan GONG, UNIST graduate student and first author, explains “One day with the RSR-S system when I ran the experiment and then cooled down the graphite crucible to solidify the liquid metal, and removed the solidified liquid metal piece, I noticed a ‘rainbow pattern’ spread over a few millimeters on the bottom surface of this piece. We found out that the rainbow colors were due to diamonds! This allowed us to to identify parameters that favored the reproducible growth of diamond.”

The initial formation occurs without the need for diamond or other seed particles commonly used in conventional HPHT and chemical vapor deposition synthesis methods. Once formed, the diamond particles merge to form a film, which can be easily detached and transferred to other substrates, for further studies and potential applications.

The synchrotron two-dimensional X-ray diffraction measurements confirmed that the synthesized diamond film has a very high purity of the diamond phase. Another intriguing aspect is the presence of silicon-vacancy color centers in the diamond structure, as an intense zero-phonon line at 738.5 nm in the photoluminescence spectrum excited by using a 532 nm laser was found.

Coauthor Dr. Meihui WANG notes, “This synthesized diamond with silicon-vacancy color centers may find applications in magnetic sensing and quantum computing.”

The research team delved deeply into possible mechanisms for diamonds to nucleate and grow under these new conditions. High-resolution transmission electron microscope (TEM) imaging on cross-sections of the samples showed about 30-40 nm thick amorphous subsurface region in the solidified liquid metal that was directly in contact with the diamonds. Coauthor Dr. Myeonggi CHOE notes, “Approximately 27 percent of atoms that were present at the top surface of this amorphous region were carbon atoms, with the carbon concentration decreasing with depth.”

Ruoff elaborates, “The presence of such a high concentration of carbon ‘dissolved’ in a gallium-rich alloy could be unexpected, as carbon is reported to be not soluble in gallium. This may explain why this region is amorphous — while all other regions of the solidified liquid metal are crystalline. This sub-surface region is where our diamonds nucleate and grow and we thus focused on it.”

Researchers exposed the Ga-Fe-Ni-Si liquid metal to the methane/hydrogen for short periods of time to try to understand the early growth stage — well prior to the formation of a continuous diamond film. They then analyzed the concentrations of carbon in the subsurface regions using time-of-flight secondary ion mass spectrometry depth profiling. After a 10-minute run, no diamond particles were evident but there were ~65 at% carbon atoms present in the region where the diamond typically grows. Diamond particles began to be found after a 15-minute run, and there was a lower subsurface C atom concentration of ~27 at%.

Ruoff explains, “The concentration of subsurface carbon atoms is so high at around 10 minutes that this time exposure is close to or at supersaturation, leading to the nucleation of diamonds either at 10 minutes or sometime between 10 and 15 minutes. The growth of diamond particles is expected to occur very rapidly after nucleation, at some time between about 10 minutes and 15 minutes.”

The temperature in 27 different locations in the liquid metal was measured with an attachment to the growth chamber having an array of nine thermocouples that was designed and built by Seong. The central region of the liquid metal was found to be at a lower temperature compared to the corners and sides of the chamber. It is thought that this temperature gradient is what drives carbon diffusion towards the central region, facilitating diamond growth.

The team also discovered that silicon plays a critical role in this new growth of diamond. The size of the grown diamonds becomes smaller and their density higher as the concentration of silicon in the alloy was increased from the optimal value. Diamonds could not be grown at all without the addition of silicon, which suggests that silicon may be involved in the initial nucleation of diamond.

This was supported by the various theoretical calculations conducted to uncover the factors that may be responsible for the growth of diamonds in this new liquid metal environment. Researchers found that silicon promotes the formation and stabilization of certain carbon clusters by predominantly forming sp³ bonds like carbon. It is thought that small carbon clusters containing Si atoms might serve as the ‘pre-nuclei’, which can then grow further to nucleate a diamond. It is predicted that the likely size range for an initial nucleus is around 20 to 50 C atoms.

Ruoff states, “Our discovery of nucleation and growth of diamond in this liquid metal is fascinating and offers many exciting opportunities for more basic science. We are now exploring when nucleation, and thus the rapid subsequent growth of diamond, happens. Also ‘temperature drop’ experiments where we first achieve supersaturation of carbon and other needed elements, followed by rapidly lowering the temperature to trigger nucleation — are some studies that seem promising to us.”

The team discovered their growth method offers significant flexibility in the composition of liquid metals. Researcher Dr. Da LUO remarks, “Our optimized growth was achieved using the gallium/nickel/iron/silicon liquid alloy. However, we also found that high-quality diamond can be grown by substituting nickel with cobalt or by replacing gallium with a gallium-indium mixture.”

Ruoff concludes, “Diamond might be grown in a wide variety of relatively low melting point liquid metal alloys such as containing one or more of indium, tin, lead, bismuth, gallium, and potentially antimony and tellurium — and including in the molten alloy other elements such as manganese, iron, nickel, cobalt and so on as catalysts, and others as dopants that yield color centers. And there is a wide range of carbon precursors available besides methane (various gases, and also solid carbons). New designs and methods for introducing carbon atoms and/or small carbon clusters into liquid metals for diamond growth will surely be important, and the creativity and technical ingenuity of the worldwide research community seem likely to me, based on our discovery, to rapidly lead to other related approaches and experimental configurations. There are numerous intriguing avenues to explore!”

This research was supported by the Institute for Basic Science and has been published in the journal Nature.