The metalens meets the stars

Metalenses have been used to image microscopic features of tissue and resolve details smaller than a wavelength of light. Now they are going bigger.

Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a 10-centimeter-diameter glass metalens that can image the sun, the moon and distant nebulae with high resolution.

It is the first all-glass, large-scale metalens in the visible wavelength that can be mass produced using conventional CMOS fabrication technology.

The research is published in ACS Nano.

"The ability to accurately control the size of tens of billions of nanopillars over an unprecedentedly large flat lens using state-of-the-art semiconductor foundry processes is a nanofabrication feat that opens exciting new opportunities for space science and technology," said Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS and senior author of the paper.

Most flat metalenses, which use millions of pillar-like nanostructures to focus light, are about the size of a piece of glitter.

In 2019, Capasso and his team developed a centimeter-scale metalens using a technique called deep-ultraviolet (DUV) projection lithography,which projects and forms a nanostructure pattern that can be directly etched into the glass wafer, eliminating the time-consuming writing and deposition processes that were required for previous metalenses.

DUV projection lithography is commonly used to pattern fine lines and shapes in silicon chips for smartphones and computers.

Joon-Suh Park, a former graduate student at SEAS and current postdoctoral fellow in Capasso's team, demonstrated that the technique could not only be used to mass produce metalenses but also increase their size for applications in virtual and augmented reality.

But making the metalens even larger for applications in astronomy and free-space optical communications posed an engineering problem.

"There is a major limitation with the lithography tool because these tools are used to make computer chips, so chip size is restricted to no more than 20 to 30 millimeters," said Park, co-first author of the paper.

"In order to make a 100-millimeter diameter lens, we needed to find a way around this limitation."

Park and the team developed a technique to stitch together several patterns of nanopillars using the DUV projection lithography tool.

By dividing the lens into 25 sections but using only the 7 sections of a quadrant considering the rotational symmetry, the researchers showed that DUV projection lithography could pattern 18.7 billion designed nanostructures onto a 10-centimeter circular area in a matter of minutes.

The team also developed a vertical glass etching technique that allows the creation of high-aspect ratio, smooth-sidewall nanopillars etched into glass.

"Using the same DUV projection lithography, one could produce large-diameter, aberration-correcting meta-optics or even larger lenses on larger glass diameter wafers as the corresponding CMOS foundry tools become increasingly available in the industry," said Soon Wei Daniel Lim, a postdoctoral fellow at SEAS and co-first author of the paper.

Lim played a lead role in the full simulation and characterization of all the possible fabrication errors that could arise during mass-manufacturing processes and how they could impact the optical performance of metalenses.

After addressing possible manufacturing challenges, the researchers demonstrated the power of the metalens in imaging celestial objects.

Mounting the metalens on a tripod with a color filter and camera sensor, Park and the team took to the roof of Harvard's Science Center.

There, they imaged the Sun, the moon and the North America nebula, a dim nebula in the constellation Cygnus about 2,590 light years away.

"We were able to get very detailed images of the Sun, the moon and the nebula that are comparable to images taken by conventional lenses" said Arman Amirzhan, a graduate student in the Capasso Lab and co-author of the paper.

Using only the metalens, the researchers were able to image the same cluster of sunspots as a NASA image taken that same day.

The team also demonstrated that the lens could survive exposure to extreme heat, extreme cold and the intense vibrations that would occur during a space launch without any damage or loss in optical performance.

Because of its size and monolithic glass composition, the lens could also be used for long-range telecommunications and directed energy transport applications.

Read more at Science Daily

Surface melting of glass

In 1842, the famous British researcher Michael Faraday made an amazing observation by chance: A thin layer of water forms on the surface of ice, even though it is well below zero degrees. So the temperature is below the melting point of ice, yet the surface of the ice has melted. This liquid layer on ice crystals is also why snowballs stick together.

It was not until about 140 years later, in 1985, that this "surface melting" could be scientifically confirmed under controlled laboratory conditions. By now, surface melting has been demonstrated in a variety of crystalline materials and is scientifically well understood: Several degrees below the actual melting point, a liquid layer only a few nanometres thick forms on the surface of the otherwise solid material. Because the surface properties of materials play a crucial role in their use as, e.g. catalysts, sensors, battery electrodes and more, surface melting is not only of fundamental importance but also in view of technical applications.

It must be emphasized that this process has absolutely nothing to do with the effect of, say, taking an ice cube out of the freezer and exposing it to ambient temperature. The reason why an ice cube melts on its surface first under such conditions is that the surface is significantly warmer than the ice cube's interior.

Surface melting detected in glass

In crystals with periodically arranged atoms, the thin liquid layer on the surface is typically detected by scattering experiments, which are very sensitive to the presence of atomic order. Since liquids are not arranged in a regular pattern, such techniques can therefore clearly resolve the appearance of a thin liquid film on top of the solid. This approach, however, does not work for glasses (i.e. disordered, amorphous materials) because there is no difference in the atomic order between the solid and the liquid. Therefore, surface melting of glasses has remained rather unexplored with experiments.

To overcome the above-mentioned difficulties, Clemens Bechinger, physics professor at the University of Konstanz, and his colleague Li Tian used a trick: instead of studying an atomic glass, they produced a disordered material made of microscopic glass spheres known as colloids. In contrast to atoms, these particles are about 10,000 times larger and can be observed directly under a microscope.

The researchers were able to demonstrate the process of surface melting in such a colloidal glass because the particles near the surface move much faster compared to the solid below. At first glance, such behaviour is not entirely unexpected, since the particle density at the surface is lower than in the underlying bulk material. Therefore, particles close to the surface have more space to move past each other, which makes them faster.

A surprising discovery

What surprised Clemens Bechinger and Li Tian, however, was the fact that even far below the surface, where the particle density has reached the bulk value, the particle mobility is still significantly higher compared to the bulk material. The microscope images show that this previously unknown layer is up to 30 particle diameters thick and continues from the surface into the deeper regions of the solid in a streak-like pattern. "This layer which reaches far into the material has interesting material properties since it combines liquid and solid features," Bechinger explains.

Read more at Science Daily

Beads of glass in meteorites help scientists piece together how solar system formed

Ever since scientists started looking at meteorites with microscopes, they've been puzzled -- and fascinated -- by what's inside. Most meteorites are made of tiny beads of glass that date back to the earliest days of the solar system, before the planets were even formed.

Scientists with the University of Chicago have published an analysis laying out how these beads, which are found in many meteorites, came to be -- and what they can tell us about what happened in the early solar system.

"These are big questions," said UChicago alum Nicole Xike Nie, PhD'19, a postdoctoral fellow at the Carnegie Institution for Science and first author of the study. "Meteorites are snapshots that can reveal the conditions this early dust experienced -- which has implications for the evolution of both Earth and other planets."

'This question goes back 50 years'

The beads of glass inside these meteorites are called chondrules. Scientists think they are bits of rock left over from the debris that was floating around billions of years ago, which eventually coalesced into the planets we now know and love. These are immensely useful to scientists, who can get their hands on pieces of the original stuff that comprised the solar system -- before the constant churn of volcanoes and tectonic plates of Earth changed all the rock we can find on the planet itself.

But what exactly caused the formation of these chondrules remains unclear.

"We have the same theories we had 50 years ago," said study co-author and UChicago postdoctoral researcher Timo Hopp. "Even though there have been advances in many other areas, this one has been stubborn."

Scientists can find clues about the early days of the solar system by looking at the types of a given element in a rock. Elements can come in several different forms, called isotopes, and the proportion in each rock varies according to what happened when that rock was born -- how hot it was, whether it cooled slowly or was flash-frozen, what other elements were around to interact with it. From there, scientists can piece together a history of likely events.

To try and understand what had happened to the chondrules, Nie, Hopp and other scientists at the Dauphas Origins Lab at UChicago tried applying a unique angle to the isotopes.

First, Nie took extremely rigorous, precise measurements of the concentrations and isotopes of two elements that are depleted in meteorites, potassium and rubidium, which helped narrow down the possibilities of what could have happened in the early solar system.

From this information, the team pieced together what must have been happening as the chondrules formed. The elements would have been part of a clump of dust that got hot enough to melt, and then to vaporize. Then, as the material cooled, some of that vapor coalesced back into chondrules.

"We can also tell you how fast it cooled, because it was fast enough that not everything condensed," said Nicolas Dauphas, Professor of Geophysical Sciences at UChicago. "That must mean the temperature was dropping at a rate of around 500 degrees Celsius per hour, which is really fast."

Based on these constraints, scientists can theorize what kind of event would have been sudden and violent enough to cause this extreme heating and cooling. One scenario that fits would be massive shockwaves passing through the early nebula. "Large planetary bodies nearby can create shocks, which would have heated and then cooled the dust as it passed through," Dauphas said.

Over the past half-century, people have proposed different scenarios to explain the formation of the chondrules -- lightning, or collisions between rocks -- but this new evidence tips the balance toward shockwaves as an explanation.

This explanation may be the key to understanding a persistent finding that has bedeviled scientists for decades, involving a category of elements that are "moderately volatile," including potassium and rubidium. The Earth has less of these elements than scientists would expect, based on their general understanding of how the solar system formed. They knew the explanation could be traced to some complex chain of heating and cooling, but no one know the exact sequence. "It's a huge question in the field of cosmochemistry." said Dauphas.

Now, finally, the team is happy to have put a significant dent in the mystery.

"We know other processes happened -- this is just one part of the story -- but this really solves one step in the formation of planets," said Hopp.

Nie agreed: "It's really cool to be able to say quantitatively, this is what happened."

Read more at Science Daily

New ultrahard diamond glass synthesized

Carnegie's Yingwei Fei and Lin Wang were part of an international research team that synthesized a new ultrahard form of carbon glass with a wealth of potential practical applications for devices and electronics. It is the hardest known glass with the highest thermal conductivity among all glass materials. Their findings are published in Nature.

Function follows form when it comes to understanding the properties of a material. How its atoms are chemically bonded to each other, and their resulting structural arrangement, determines a material's physical qualities -- both those that are observable by the naked eye and those that are only revealed by scientific probing.

Carbon is unrivaled in its ability to form stable structures -- alone and in combination with other elements. Some forms of carbon are highly organized, with repeating crystalline lattices. Others are more disordered, a quality termed amorphous.

The type of bond holding a carbon-based material together determine its hardness. For example, soft graphite has two-dimensional bonds and hard diamond has three-dimensional bonds.

"The synthesis of an amorphous carbon material with three-dimensional bonds has been a long-standing goal," explained Fei. "The trick is to find the right starting material to transform with the application of pressure."

"For decades Carnegie researchers have been at the forefront of the field, using laboratory techniques to generate extreme pressures to produce novel materials or mimic the conditions found deep inside planets," added Carnegie Earth and Planets Laboratory Director Richard Carlson.

Because of its extremely high melting point, it's impossible to use diamond as the starting point to synthesize diamond-like glass. However, the research team, led by Jilin University's Bingbing Liu and Mingguang Yao -- a former Carnegie visiting scholar -- made their breakthrough by using a form of carbon composed of 60 molecules arranged to form a hollow ball. Informally called a buckyball, this Nobel Prize-winning material was heated just enough to collapse its soccer-ball-like structure to induce disorder before turning the carbon to crystalline diamond under pressure.

The team used a large-volume multi-anvil press to synthesize the diamond-like glass. The glass is sufficient large for characterization. Its properties were confirmed using a variety of advanced, high-resolution techniques for probing atomic structure.

Read more at Science Daily