In optics, the era of glass lenses may be waning.

In recent years, physicists and engineers have been designing, constructing and testing different types of ultrathin materials that could replace the thick glass lenses used today in cameras and imaging systems. Critically, these engineered lenses — known as metalenses — are not made of glass. Instead, they consist of materials constructed at the nanoscale into arrays of columns or fin-like structures. These formations can interact with incoming light, directing it toward a single focal point for imaging purposes.

But even though metalenses are much thinner than glass lenses, they still rely on “high aspect ratio” structures, in which the column or fin-like structures are much taller than they are wide, making them prone to collapsing and falling over. Furthermore, these structures have always been near the wavelength of light they’re interacting with in thickness — until now.

Four ultrathin metalenses developed by University of Washington researchers and visualized under a microscope. Credit: Liu et al., Nano Letters, 2018

In a paper published Oct. 8 in the journal Nano Letters, a team from the University of Washington and the National Tsing Hua University in Taiwan announced that it has constructed functional metalenses that are one-tenth to one-half the thickness of the wavelengths of light that they focus. Their metalenses, which were constructed out of layered 2D materials, were as thin as 190 nanometers — less than 1/100,000ths of an inch thick.

“This is the first time that someone has shown that it is possible to create a metalens out of 2D materials,” said senior and co-corresponding author Arka Majumdar, a UW assistant professor of physics and of electrical and computer engineering.

Their design principles can be used for the creation of metalenses with more complex, tunable features, added Majumdar, who is also a faculty researcher with the UW’s Molecular Engineering & Sciences Institute.

Majumdar’s team has been studying the design principles of metalenses for years, and previously constructed metalenses for full-color imaging. But the challenge in this project was to overcome an inherent design limitation in metalenses: in order for a metalens material to interact with light and achieve optimal imaging quality, the material had to be roughly the same thickness as the light’s wavelength in that material. In mathematical terms, this restriction ensures that a full zero to two-pi phase shift range is achievable, which guarantees that any optical element can be designed. For example, a metalens for a 500-nanometer lightwave — which in the visual spectrum is green light — would need to be about 500 nanometers in thickness, though this thickness can decrease as the refractive index of the material increases.

Majumdar and his team were able to synthesize functional metalenses that were much thinner than this theoretical limit — one-tenth to one-half the wavelength. First, they constructed the metalens out of sheets of layered 2D materials. The team used widely studied 2D materials such as hexagonal boron nitride and molybdenum disulfide. A single atomic layer of these materials provides a very small phase shift, unsuitable for efficient lensing. So the team used multiple layers to increase the thickness, although the thickness remained too small to reach a full two-pi phase shift.

“We had to start by figuring out what type of design would yield the best performance given the incomplete phase,” said co-author Jiajiu Zheng, a doctoral student in electrical and computer engineering.

To make up for the shortfall, the team employed mathematical models that were originally formulated for liquid-crystal optics. These, in conjunction with the metalens structural elements, allowed the researchers to achieve high efficiency even if the whole phase shift is not covered. They tested the metalens’ efficacy by using it to capture different test images, including of the Mona Lisa and a block letter W. The team also demonstrated how stretching the metalens could tune the focal length of the lens.

In addition to achieving a wholly new approach to metalens design at record-thin levels, the team believes that its experiments show the promise of making new devices for imaging and optics entirely out of 2D materials.

“These results open up an entirely new platform for studying the properties of 2D materials, as well as constructing fully functional nanophotonic devices made entirely from these materials,” said Majumdar.

Additionally, these materials can be easily transferred on any substrate, including flexible materials, paving a way towards flexible photonics.

Researchers at Linköping University, Sweden, are working to develop a method to convert water and carbon dioxide to the renewable energy of the future, using the energy from the sun and graphene applied to the surface of cubic silicon carbide. They have now taken an important step towards this goal, and developed a method that makes it possible to produce graphene with several layers in a tightly controlled process.

Jianwu Sun at Linköping University inspecting the growth reactor for growth of cubic silicon carbide. Credit: Thor Balkhed/LiU

The research group has also shown that graphene acts as a superconductor in certain conditions. Their results have been published in the scientific journals Carbon and Nano Letters.

Carbon, oxygen and hydrogen. These are the three elements you would get if you took apart molecules of carbon dioxide and water. The same elements are the building blocks of chemical substances that we use for fuel, such as ethanol and methane. The conversion of carbon dioxide and water to renewable fuel, if possible, would provide an alternative to fossil fuels, and contribute to reducing our emission of carbon dioxide to the atmosphere. Jianwu Sun, senior lecturer at Linköping University, is trying to find a way to do just that.

The first step is to develop the material they plan to use. Researchers at Linköping University have previously developed a world-leading method to produce cubic silicon carbide, which consists of silicon and carbon. The cubic form has the ability to capture energy from the sun and create charge carriers. This is, however, not sufficient. Graphene, one of the thinnest materials ever produced, plays a key role in the project. The material comprises a single layer of carbon atoms bound to each other in a hexagonal lattice. Graphene has a high ability to conduct an electric current, a property that would be useful for solar energy conversion. It also has several unique properties, and possible uses of graphene are being extensively studied all over the world.

In recent years, the researchers have attempted to improve the process by which graphene grows on a surface in order to control the properties of the graphene. Their recent progress is described in an article in the scientific journal Carbon.

“It is relatively easy to grow one layer of graphene on silicon carbide. But it’s a greater challenge to grow large-area uniform graphene that consists of several layers on top of each other. We have now shown that it is possible to grow uniform graphene that consists of up to four layers in a controlled manner”, says Jianwu Sun, of the Department of Physics, Chemistry and Biology at Linköping University.

One of the difficulties posed by multilayer graphene is that the surface becomes uneven when different numbers of layers grow at different locations. The edge when one layer ends has the form of a tiny, nanoscale, staircase. For the researchers, who want large flat areas, these steps are a problem. It is particularly problematic when steps collect in one location, like a wrongly built staircase in which several steps have been united to form one large step. The researchers have now found a way to remove these united large steps by growing the graphene at a carefully controlled temperature. Furthermore, the researchers have shown that their method makes it possible to control how many layers the graphene will contain. This is the first key step in an ongoing research project whose goal is to make fuel from water and carbon dioxide.

In a closely related article in the journal Nano Letters, the researchers describe investigations into the electronic properties of multilayer graphene grown on cubic silicon carbide.

“We discovered that multilayer graphene has extremely promising electrical properties that enable the material to be used as a superconductor, a material that conducts electrical current with zero electrical resistance. This special property arises solely when the graphene layers are arranged in a special way relative to each other”, says Jianwu Sun.

Theoretical calculations had predicted that multilayer graphene would have superconductive properties, provided that the layers are arranged in a particular way. In the new study, the researchers demonstrate experimentally for the first time that this is the case. Superconducting materials are used in, among other things, superconducting magnets – extremely powerful magnets found in magnet resonance cameras for medical investigations, and in particle accelerators for research. There are many potential areas of application for superconductors, such as electrical supply lines with zero energy loss, and high-speed trains that float on a magnetic field. Their use is currently limited by the inability to produce superconductors that function at room temperature: currently available superconductors function only at extremely low temperatures.

Two-dimensional magnetism has long intrigued and motivated researchers for its potential to unleash new states of matter and utility in nano-devices.

In part the excitement is driven by predictions that the magnetic moments of electrons – known as “spins” – would no longer be able to align in perfectly clean systems. This enhancement in the strengths of the excitations could unleash numerous new states of mater, and enable novel forms of quantum computing.

A key challenge has been the successful fabrication of perfectly clean systems and their incorporation with other materials. However, for more than a decade, materials known as “van der Waals” crystals, held together by friction, have been used to isolate single-atom-thick layers leading to numerous new physical effects and applications.

Recently this class has been expanded to include magnetic materials, and it may offer one of the most ambitious platforms yet in scientific efforts to investigate and manipulate phases of matter at the nanoscale, researchers from Boston College, the University of Tennessee, and Seoul National University, write in the latest edition of the journal Nature.

Two-dimensional magnetism, the subject of theoretical explorations and experimentation for the past 80 years, is enjoying a resurgence thanks to a group of materials and compounds that are relatively plentiful and easy to manipulate, according to Boston College Associate Professor of Physics Kenneth Burch, a first author of the article “‘Magnetism in two-dimensional van der Waals materials.”

The most oft-cited example of these materials is graphene, a crystal constructed in uniform, atom-thick layers. A procedure as simple as applying a piece of scotch tape to the crystal can remove a single layer, providing a thin, uniform section to serve as a platform to create novel materials with a range of physical properties open to manipulation.

“What’s amazing about these 2-D materials is they’re so flexible,” said Burch. “Because they are so flexible, they give you this huge array of possibilities. You can make combinations you could not dream of before. You can just try them. You don’t have to spend this huge amount of time and money and machinery trying to grow them. A student working with tape puts them together. That adds up to this exciting opportunity people dreamed of for a long time, to be able to engineer these new phases of matter.”

At that single layer, researchers have focused on spin, what Burch refers to as the “magnetic moment” of an electron. While the charge of an electron can be used to send two signals – either “off” or “on”, results represented as either zero or one – spin excitations offer multiple points of control and measurement, an exponential expansion of the potential to signal, store or transmit information in the tiniest of spaces.

“One of the big efforts now is to try to switch the way we do computations,” said Burch. “Now we record whether the charge of the electron is there or it isn’t. Since every electron has a magnetic moment, you can potentially store information using the relative directions of those moments, which is more like a compass with multiple points. You don’t just get a one and a zero, you get all the values in between.”

Potential applications lie in the areas of new “quantum” computers, sensing technologies, semiconductors, or high-temperature superconductors.

“The point of our perspective is that there has been a huge emphasis on devices and trying to pursue these 2-D materials to make these new devices, which is extremely promising,” said Burch. “But what we point out is magnetic 2D atomic crystals can also realize the dream of engineering these new phases – superconducting, or magnetic or topological phases of matter, that is really the most exciting part. It is not just fundamentally interesting to realize these theorems that have been around for 40 years. These new phases would have applications in various forms of computing, whether in spintronics, producing high temperature superconductors, magnetic and optical sensors and in topological quantum computing.”

Burch and his colleagues – the University of Tennessee’s David Mandrus and Seoul National University’s Je-Geun Park – outline four major directions for research into magnetic van der Waals materials:

• Discovering new materials with specific functionality. New materials with isotropic or complex magnetic interactions, could play significant roles in the development of new supercondcutors.
• These new materials can also lead to a deeper understanding of fundamental issues in condensed matter physics, serving as unique platforms for experimentation.
• The materials will be tested for the potential to become unique devices, capable of delivering novel applications. The two-dimensional structure of these materials makes them more receptive to external signals.
• These materials possess quantum and topological phases that could potentially lead to exotic states, such as quantum spin liquids, “skyrmions,” or new iterations of superconductivity.

Germano Iannacchione, a National Science Foundation (NSF) program officer who oversees grants to Burch and other materials scientists, said the co-authors offer the broader community of scientists ideas that can serve to guide a dynamic field pushing beyond boundaries in materials research.

“Magnetism in 2D van Der Waals materials has grown into a vibrant field of study,” said Iannacchione. “Its investigators have matured from highly focused researchers to statesmen shepherding a field, broadening applications into as many channels as possible. The review captures the multiplicative aspect of steady, focused, and sometimes risky research that opens vast new frontiers, with tremendous potential for applications in quantum computing and spintronics.”