Mechanics know molybdenum disulfide (MoS2) as a useful lubricant in aircraft and motorcycle engines and in the CV and universal joints of trucks and automobiles. Rice University engineering researcher Isabell Thomann knows it as a remarkably light-absorbent substance that holds promise for the development of energy-efficient optoelectronic and photocatalytic devices.

“Basically, we want to understand how much light can be confined in an atomically thin semiconductor monolayer of MoS2,” said Thomann, assistant professor of electrical and computer engineering and of materials science and nanoengineering and of chemistry. “By using simple strategies, we were able to absorb 35 to 37 percent of the incident light in the 400- to 700-nanometer wavelength range, in a layer that is only 0.7 nanometers thick.”

Thomann and Rice graduate students Shah Mohammad Bahauddin and Hossein Robatjazi have recounted their findings in a paper titled “Broadband Absorption Engineering To Enhance Light Absorption in Monolayer MoS2,” which was recently published in the American Chemical Society journal ACS Photonics. The research has many applications, including development of efficient and inexpensive photovoltaic solar panels.

“Squeezing light into these extremely thin layers and extracting the generated charge carriers is an important problem in the field of two-dimensional materials,” she said. “That’s because monolayers of 2-D materials have different electronic and catalytic properties from their bulk or multilayer counterparts.”

Thomann and her team used a combination of numerical simulations, analytical models and experimental optical characterizations. Using three-dimensional electromagnetic simulations, they found that light absorption was enhanced 5.9 times compared with using MoS2 on a sapphire substrate.

“If light absorption in these materials was perfect, we’d be able to create all sorts of energy-efficient optoelectronic and photocatalytic devices. That’s the problem we’re trying to solve,” Thomann said.

She is pleased with her lab’s progress but concedes that much work remains to be done. “The goal, of course, is 100 percent absorption, and we’re not there yet.”

Researchers from the University of Illinois at Urbana-Champaign have developed a one-step, facile method to pattern graphene by using stencil mask and oxygen plasma reactive-ion etching, and subsequent polymer-free direct transfer to flexible substrates.

Graphene, a two-dimensional carbon allotrope, has received immense scientific and technological interest. Combining exceptional mechanical properties, superior carrier mobility, high thermal conductivity, hydrophobicity, and potentially low manufacturing cost, graphene provides a superior base material for next generation bioelectrical, electromechanical, optoelectronic, and thermal management applications.

“Significant progress has been made in the direct synthesis of large-area, uniform, high quality graphene films using chemical vapor deposition (CVD) with various precursors and catalyst substrates,” explained SungWoo Nam, an assistant professor of mechanical science and engineering at Illinois. “However, to date, the infrastructure requirements on post-synthesis processing–patterning and transfer–for creating interconnects, transistor channels, or device terminals have slowed the implementation of graphene in a wider range of applications.”

“In conjunction with the recent evolution of additive and subtractive manufacturing techniques such as 3D printing and computer numerical control milling, we developed a simple and scalable graphene patterning technique using a stencil mask fabricated via a laser cutter,” stated Keong Yong, a graduate student and first author of the paper, “Rapid Stencil Mask Fabrication Enabled One-Step Polymer-Free Graphene Patterning and Direct Transfer for Flexible Graphene Devices appearing in Scientific Reports.

“Our approach to patterning graphene is based on a shadow mask technique that has been employed for contact metal deposition,” Yong added. “Not only are these stencil masks easily and rapidly manufactured for iterative rapid prototyping, they are also reusable, enabling cost-effective pattern replication. And since our approach involves neither a polymeric transfer layer nor organic solvents, we are able to obtain contamination-free graphene patterns directly on various flexible substrates.”

Nam stated that this approach demonstrates a new possibility to overcome limitations imposed by existing post-synthesis processes to achieve graphene micro-patterning. Yong envisions this facile approach to graphene patterning sets forth transformative changes in “do It yourself” (DIY) graphene-based device development for broad applications including flexible circuits/devices and wearable electronics.

“This method allows rapid design iterations and pattern replications, and the polymer-free patterning technique promotes graphene of cleaner quality than other fabrication techniques,” Nam said. “We have shown that graphene can be patterned into varying geometrical shapes and sizes, and we have explored various substrates for the direct transfer of the patterned graphene.”

Tiny units of matter and chemistry that they are, atoms constitute the entire universe. Some rare atoms can store quantum information, an important phenomenon for scientists in their ongoing quest for a quantum Internet.

New research from UC Santa Barbara scientists and their Dutch colleagues exploits a system that has the potential to transfer optical quantum information to a locally stored solid-state quantum format, a requirement of quantum communication. The team’s findings appear in the journal Nature Photonics.

“Our research aims at creating a quantum analog of current fiber optic technology in which light is used to transfer classical information — bits with values zero or one — between computers,” said author Dirk Bouwmeester, a professor in UCSB’s Department of Physics. “The rare earth atoms we’re studying can store the superpositions of zero and one used in quantum computation. In addition, the light by which we communicate with these atoms can also store quantum information.”

Atoms are each composed of a nucleus typically surrounded by inner shells full of electrons and often have a partially filled outer electron shell. The optical and chemical properties of the atoms are mainly determined by the electrons in the outer shell.

Rare earth atoms such as erbium and ytterbium have the opposite composition: a partially filled inner shell surrounded by filled outer shells. This special configuration is what enables these atoms to store quantum information.

However, the unique composition of rare earth atoms leads to electronic transitions so well shielded from the surrounding atoms that optical interactions are extremely weak. Even when implanted in a host material, these atoms maintain those shielded transitions, which in principle can be addressed optically in order to store and retrieve quantum information.

Bouwmeester collaborated with John Bowers, a professor in UCSB’s Department of Electrical and Computer Engineering, and investigators at Leiden University in the Netherlands to strengthen these weak interactions by implanting ytterbium into ultra-high-quality optical storage rings on a silicon chip.

“The presence of the high-quality optical ring resonator — even if no light is injected — changes the fundamental optical properties of the embedded atoms, which leads to an order of magnitude increase in optical interaction strength with the ytterbium,” Bouwmeester said. “This increase, known as the Purcell effect, has an intricate dependence on the geometry of the optical light confinement.”

The team’s findings indicate that new samples currently under development at UCSB can enable optical communication to a single ytterbium atom inside optical circuits on a silicon chip, a phenomenon of significant interest for quantum information storage. The experiments also explore the way in which the Purcell effect enhances optical interaction with an ensemble of a few hundred rare earth atoms. The grouping itself has interesting collective properties that can also be explored for the storage of quantum information.

Key is an effect called a photon echo, the result of two distinct light pulses, the first of which causes atoms in ytterbium to become partially excited.

“The first light pulse creates a set of atoms we ‘talk’ to in a specific state and we call that state ‘in phase’ because all the atoms are created at the same time by this optical pulse,” Bouwmeester explained. “However, the individual atoms have slightly different frequencies because of residual coupling to neighboring atoms, which affects their time evolution and causes decoherence in the system.” Decoherence is the inability to keep track of how the system evolves in all its details.

“The trick is that the second light pulse changes the state of the system so that it evolves backwards, causing the atoms to return to the initial phase,” he continued. “This makes everything coherent and causes the atoms to collectively emit the light they absorbed from the first pulse.”

The strength of the photon echo contains important information about the fundamental properties of the ytterbium in the host material. “By analyzing the strength of these photon echoes, we are learning about the fundamental interactions of ytterbium with its surroundings,” Bouwmeester said. “Now we’re working on strengthening the Purcell effect by making the storage rings we use smaller and smaller.”

According to Bouwmeester, quantum computation needs to be compatible with optical communication for information to be shared and transmitted. “Our ultimate goal is to be able to communicate to a single ytterbium atom; then we can start transferring the quantum state of a single photon to a single ytterbium atom,” he added. “Coupling the quantum state of a photon to a quantum solid state is essential for the existence of a quantum Internet.”