Following the people and events that make up the research community at Duke

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Experts Unpack the Space Debris Challenge Just Before an “Irresponsible” Russian Missile Launch

Russia sucessfully tested a direct-ascent anti-satellite missile on Monday, creating a debris field of more than 1,500 pieces of trackable orbital debris — space junk — whizzing around the planet. The crew aboard the International Space Station was ordered into their spacesuits to help them survive if one of the shards hit their home.

The Russian test, which has been strongly condemned by US officials, has created extreme hazards for satellites. US Space Command Commander General James Dickinson stated that “Russia has demonstrated a deliberate disregard for the security, safety, stability, and long-term sustainability of the space domain for all nations.”

You might be wondering, What’s the big deal?

Just last Friday on November 12th, a group of experts met with the Duke community to discuss the threats to space – an environment we often forget about – and why space junk poses a large challenge for the 21st century.

Benjamin Schmitt PhD, a postdoctoral research fellow at the Harvard-Smithsonian Center for Astrophysics, facilitated the group conversation, which featured Hugh Lewis PhD, Professor of Astronautics and Head of the Astronautics Research Group at the University of Southampton. Schmitt stated that for the last two weeks, people around the world have paused to look up at the climate with the proceedings of COP26, but they “should also tilt their heads back a bit further” and consider the problem of space junk.

The challenge of space debris requires technical and diplomatic solutions, which are often complex. This has been effectively demonstrated by the Russian launch and resultant global reactions to the “irresponsibility” of the maneuver.

Schmitt and Lewis were joined by Brit Lundgren PhD, Laura Newburgh PhD, and W. Robert Pearson JD. Lundgren is an Associate Professor of Physics and Astronomy at the University of North Carolina at Asheville, Newburgh is an Assistant Professor of Physics at Yale University, and Pearson is a retired U.S. Ambassador and current Duke University Center for International and Global Studies Fellow.

Space experts engaged in Friday’s conversation

“The space debris problem is a wicked problem,” Lewis said. And the problem is this: According to the European Space Agency, there are over 36,500 objects larger than 10cm, 1,000,000 objects over 1cm, and more than one-third of a billion objects over 1mm in size in orbit around the Earth. These numbers, though bewilderingly large, are posed to expand.

As all this junk collides with itself, there are more and more fragments and particles in space. Lewis said that unlike climate change, there is not a “tipping point.” There will not be a warning or any sudden event that pushes us into the exponential growth phase – it will just, sort of, happen.

These pieces of debris pose substantial risks to the space systems that our modern societies have come to rely on, like piloting and navigation, communication, and many forms of entertainment like television. “Without those services, all of us, the entire planet, would suffer,” Lewis said.

A visualization of the space debris currently rotating around Earth.

But this issue of space debris likely feels entirely disconnected and irrelevant for most of the world’s population. “For us down here on Earth, we are really not aware of this growing problem … and we are really not able to connect to it,” Lewis said. “Unless we make that human connection, it’s not something we would be able to address.”

The panelists all agreed that making the connections between space debris and the current functioning of our globe is a critical step to getting the public to engage with the space debris challenge.

There are also other important reasons to care about space debris. Lundgren pointed out that there has already been a global 10% increase in brightness relative to the natural, dark sky because of light-reflecting space debris. This is the kind of light pollution that you cannot escape, Lundgren stated, “You can’t just drive away like with city pollution.” For communities of people, like the Indigenous, this is also having severe impact on the cultural ways in which they use nighttime skies.

Newburgh’s scientific research uses a particular satellite frequency for data collection. This wavelength was just sold to a communication company, meaning that eventually, she will no longer be able to do her work. The frequencies used for satellites are limited, and thus an extremely valuable and expensive, monopolizable commodity. Scientists like Newburgh are gravely concerned about the protection of the future of their work and worried that we might “lose out on science.”

Because of the initiatives like Starlink, a satellite internet constellation operated by SpaceX, Newburgh said that space has begun to feel like the “Wild West” with no rules or regulation. “It feels like you could just do anything.”

This was a very important tenet of the discussion: “[Space debris] is not just a technical problem we have to solve, but a social one as well.” While technical solutions are needed to constrain the exponential growth of space debris, the even bigger challenge seems to lie in answering questions like “Who gets to use the remaining capacity in lowest orbit and how do we decide?” that Lewis asked. “Lots of companies, governments, and so on want to use space,” Lewis said.

Starlink satellites are changing to night sky. The company’s satellites can be seen traveling through space.

Ambassador Pearson said that this issue could be resolved by starting with a shared interest in the space debris issue and working outwards to points of change that are important across nations. The result would not ultimately be the full wish of any singular entity. Pearson also emphasized the pertinence of action: “It’s one thing to talk about what ought to be done and to talk about what we will do.”

While Pearson says that he does not believe there is a way to avoid national competition in space, it is essential to develop rules to mitigate and govern international interactions in space. This is likely to be a long process and has been on the minds of experts for decades already. But as Pearson reminded the audience it took almost 40 years to “get the ball rolling on climate change” and 10 years for the first nuclear disarmament.

The conversation ultimately kept returning to the need to engage the public and the impact that unconstrained space debris would have on their lives. Pearson said it is important to let the public know that the access to health, technology, communications, and many facets of society people had come to expect in their lives, would be severely impacted by damage to our space infrastructure.

“Whenever you think about the environment down here that we all occupy, that we are all connected to, we have to also think about the environment in space,” Lewis said.

He ended the conversation with a quote from the science fiction movie, Terminator 2: There is no fate but what we make for ourselves. This fate is dependent on cooperation between scientists, diplomats, regulatory and technical experts, and the public around the world.

Post by Cydney Livingston, Class of 2022

Duke has 38 of the World’s Most Highly-Cited Scientists

Peak achievement in the sciences isn’t measured by stopwatches or goals scored, it goes by citations – the number of times other scientists have referenced your findings in their own academic papers. A high number of citations is an indication that a particular work was influential in moving the field forward.

Nobel laureate Bob Lefkowitz made the list in two categories this year.

And the peak of this peak is the annual “Highly Cited Researchers” list produced each year by the folks at Clarivate, who run the Institute for Scientific Information. The names on this list are drawn from publications that rank in the top 1% by citations for field and publication year in the Web of Science™ citation index – the most-cited of the cited.

Duke has 38 names on the highly cited list this year — including Bob Lefkowitz twice because he’s just that good — and two colleagues at the Duke NUS Medical School in Singapore. In all, the 2021 list includes 6,602 researchers from more than 70 countries.

The ISI says that US scientists are a little less than 40 percent of the highly cited list this year – and dropping. Chinese researchers are gaining, having nearly doubled their presence on the roster in the last four years.

“The headline story is one of sizeable gains for Mainland China and a decline for the United States, particularly when you look at the trends over the last four years,” said a statement from David Pendlebury, Senior Citation Analyst at the Institute for Scientific Information. “(This reflects) a transformational rebalancing of scientific and scholarly contributions at the top level through the globalization of the research enterprise.”

Without further ado, let’s see who our champions are!

Biology and Biochemistry

Charles A. Gersbach

Robert J. Lefkowitz

Clinical Medicine

Pamela S. Douglas

Christopher Bull Granger

Adrian F. Hernandez

Manesh R.Patel

Eric D. Peterson


Richard Becker

Antonio Bertoletti (NUS)

Yiran Chen

Stefano Curtarolo

Derek J. Hausenloy (NUS)

Ru-Rong Ji

Jie Liu

Jason W. Locasale

David B. Mitzi

Christopher B. Newgard

Ram Oren

David R. Smith

Heather M. Stapleton

Avner Vengosh

Mark R. Wiesner

Environment and Ecology

Emily S. Bernhardt


Drew T. Shindell


Edward A. Miao


Barton F. Haynes

Neuroscience and Behavior

Quinn T. Ostrom

Pharmacology and Toxicology

Robert J. Lefkowitz

Plant and Animal Science

Xinnian Dong

Sheng Yang He

Philip N. Benfey

Psychiatry and Psychology

Avshalom Caspi

E. Jane Costello

Honalee Harrington

Renate M. Houts

Terrie E. Moffitt

Social Sciences

Michael J. Pencina

Bryce B. Reeve

John W. Williams

Post by Karl Bates

Introducing: The Duke Space Initiative


Engineers, medical students, ecologists, political scientists, ethicists, policymakers — come one, come all to the Duke Space Initiative (DSI), “the interdisciplinary home for all things space at Duke.”

At Duke Polis’ “Perspectives on Space: Introducing the Duke Space Initiative” on Sept. 9, DSI co-founder and undergraduate student Ritika Saligram introduced the initiative and moderated a discussion on the current landscape of space studies both at Duke and beyond.

William R. & Thomas L. Perkins Professor of Law Jonathan Wiener began by expressing his excitement in the amount of interest he’s observed in space at Duke. 

One of these interested students was Spencer Kaplan. Kaplan, an undergraduate student studying public policy, couldn’t attend Wiener’s Science & Society Dinner Dialogue about policy and risk in the settlement of Mars. Unwilling to miss the learning opportunity, Kaplan set up a one-on-one conversation with Wiener. One thing led to another: the two created a readings course on space law — Wiener hired Kaplan as a research assistant and they worked together to compile materials for the syllabus — then thought, “Why stop there?” 

Wiener and Kaplan, together with Chase Hamilton, Jory Weintraub, Tyler Felgenhauer, Dan Buckland, and Somia Youssef, created the Bass Connections project “Going to Mars: Science, Society, and Sustainability,” through which a highly interdisciplinary team of faculty and students discussed problems ranging from the science and technology of getting to Mars, to the social and political reality of living on another planet. 

The team produced a website, research papers, policy memos and recommendations, and a policy report for stakeholders including NASA and some prestigious actors in the private sector. According to Saligram, through their work, the team realized the need for a concerted “space for space” at Duke, and the DSI was born. The Initiative seeks to serve more immediately as a resource center for higher education on space, and eventually as the home of a space studies certificate program for undergraduates at Duke. 

Wiener sees space as an “opportunity to reflect on what we’ve learned from being on Earth” — to consider how we could avoid mistakes made here and “try to do better if we settle another planet.” He listed a few of the many problems that the Bass Connections examined. 

The economics of space exploration have changed: once, national governments funded space exploration; now, private companies like SpaceX, Blue Origin, and Virgin Galactic seek to run the show. Space debris, satellite and launch junk that could impair future launches, is the tragedy of the commons at work — in space. How would we resolve international disputes on other planets and avoid conflict, especially when settlements have different missions? Can we develop technology to ward off asteroids? What if we unintentionally brought microorganisms from one planet to another? How will we make the rules for the settlement of other planets?

These questions are vast — thereby reflecting the vastness of space, commented Saligram — and weren’t answerable within the hour. However, cutting edge research and thinking around them can be found on the Bass Connections’ website.

Earth and Climate Sciences Senior Lecturer Alexander Glass added to Wiener’s list of problems: “terraforming” — or creating a human habitat — on Mars. According to Glass, oxygen “isn’t a huge issue”: MOXIE can buzz Co2 with electricity to produce it. A greater concern is radiation. Without Earth’s magnetosphere, shielding of some sort will be necessary; it takes sixteen feet of rock to produce the same protection. Humans on Mars might have to live underground. 

Glass noted that although “we have the science to solve a lot of these problems, the science we’re lagging in is the human aspects of it: the psychological, of humanity living in conditions like isolation.” The engineering could be rock solid. But the mission “will fail because there will be a sociopath we couldn’t predict beforehand.”

Bass Connections project leader and PhD candidate in political science Somia Youssef discussed the need to examine deeply our laws, systems, and culture. Youssef emphasized that we humans have been on Earth for six million years. Like Wiener, she asked how we will “apply what we’ve learned to space” and what changes we should make. How, she mused, do prevailing ideas about humanity “transform in the confines, the harsh environment of space?” Youssef urged the balancing of unity with protection of the things that make us different, as well as consideration for voices that aren’t being represented.

Material Science Professor, Assistant Professor of Surgery, and NASA Human System Risk Manager Dr. Dan Buckland explained that automation has exciting potential in improving medical care in space. If robots can do the “most dangerous aspects” of mission medical care, humans won’t have to. Offloading onto “repeatable devices” will reduce the amount of accidents and medical capabilities needed in space. 

Multiple panelists also discussed the “false dichotomy” between spending resources on space and back home on Earth. Youssef pointed out that many innovations which have benefited (or will benefit) earthly humanity have come from the excitement and passion that comes from investing in space. Saligram stated that space is an “extension of the same social and policy issues as the ones we face on Earth, just in a different context.” This means that solutions we find in our attempt to settle Mars and explore the universe can be “reverse engineered” to help Earth-dwelling humans everywhere.

Saligram opened up the panel for discussion, and one guest asked Buckland how he ended up working for NASA. Buckland said his advice was to “be in rooms you’re not really supposed to be in, and eventually people will start thinking you’re supposed to be there.” 

Youssef echoed this view, expressing the need for diverse perspectives in space exploration. She’s most excited by all the people “who are interested in space, but don’t know if there’s enough space for them.”

If this sounds like you, check out the Duke Space Initiative. They’ve got space.

Post by Zella Hanson

A New Algorithm for “In-Betweening” images applied to Covid, Aging and Continental Drift

Collaborating with a colleague in Shanghai, we recently published an article that explains the mathematical concept of ‘in-betweening,’in images – calculating intermediate stages of changes in appearance from one image to the next.

Our equilibrium-driven deformation algorithm (EDDA) was used to demonstrate three difficult tasks of ‘in-betweening’ images: Facial aging, coronavirus spread in the lungs, and continental drift.

Part I. Understanding Pneumonia Invasion and Retreat in COVID-19

The pandemic has influenced the entire world and taken away nearly 3 million lives to date. If a person were unlucky enough to contract the virus and COVID-19, one way to diagnose them is to carry out CT scans of their lungs to visualize the damage caused by pneumonia.

However, it is impossible to monitor the patient all the time using CT scans. Thus, the invading process is usually invisible for doctors and researchers.

To solve this difficulty, we developed a mathematical algorithm which relies on only two CT scans to simulate the pneumonia invasion process caused by COVID-19.

We compared a series of CT scans of a Chinese patient taken at different times. This patient had severe pneumonia caused by COVID-19 but recovered after a successful treatment. Our simulation clearly revealed the pneumonia invasion process in the patient’s lungs and the fading away process after the treatment.

Our simulation results also identify several significant areas in which the patient’s lungs are more vulnerable to the virus and other areas in which the lungs have better response to the treatment. Those areas were perfectly consistent with the medical analysis based on this patient’s actual, real-time CT scan images. The consistency of our results indicates the value of the method.

The COVID-19 pneumonia invading (upper panel) and fading away (lower panel) process from the data-driven simulations. Red circles indicate four significant areas in which the patient’s lungs were more vulnerable to the pneumonia and blue circles indicate two significant areas in which the patient’s lungs had better response to the treatment. (Image credit: Gao et al., 2021)
We also applied this algorithm to simulate human facial changes over time, in which the aging processes for different parts of a woman’s face were automatically created by the algorithm with high resolution. (Image credit: Gao et al., 2021. Video)

Part II. Solving the Puzzle of Continental Drift

It has always been mysterious how the continents we know evolved and formed from the ancient single supercontinent, Pangaea. But then German polar researcher Alfred Wegener proposed the continental drift hypothesis in the early 20th century. Although many geologists argued about his hypothesis initially, more sound evidence such as continental structures, fossils and the magnetic polarity of rocks has supported Wegener’s proposition.

Our data-driven algorithm has been applied to simulate the possible evolution process of continents from Pangaea period.

The underlying forces driving continental drift were determined by the equilibrium status of the continents on the current planet. In order to describe the edges that divide the land to create oceans, we proposed a delicate thresholding scheme.

The formation and deformation for different continents is clearly revealed in our simulation. For example, the ‘drift’ of the Antarctic continent from Africa can be seen happening. This exciting simulation presents a quick and obvious way for geologists to establish more possible lines of inquiry about how continents can drift from one status to another, just based on the initial and equilibrium continental status. Combined with other technological advances, this data-driven method may provide a path to solve Wegener’s puzzle of continental drift.

The theory of continental drift reconciled similar fossil plants and animals now found on widely separated continents. The southern part after Pangaea breaks (Gondwana) is shown here evidence of Wegener’s theory. (Image credit: United States Geological Survey)
The continental drift process of the data-driven simulations. Black arrow indicates the formation of the Antarctic. (Image credit: Gao et al., 2021)

The study was supported by the Department of Mathematics and Physics, Duke University.

CITATION: “Inbetweening auto-animation via Fokker-Planck dynamics and thresholding,” Yuan Gao, Guangzhen Jin & Jian-Guo Liu. Inverse Problems and Imaging, February, 2021, DOI: 10.3934/ipi.2021016. Online:

Yuan Gao

Yuan Gao is the William W. Elliot Assistant Research Professor in the department of mathematics, Trinity College of Arts & Sciences.

Jian-Guo Liu is a Professor in the departments of mathematics and physics, Trinity College of Arts & Sciences.

Jian-Guo Liu

Hard-Won Answer Was Worth the Wait

Most of Physics Professor Haiyan Gao’s students see their doctoral dissertations posted on her lab’s web site very soon after they have been awarded their Ph.Ds.

But Yang Zhang, Ph.D. 2018, had to wait two years, because his thesis work had a very good chance of being accepted by a major journal. And this week, it has been published in the journal Science.

What Zhang did was to create the world’s most precise value for a subatomic nuclear particle called a neutral pion. It’s a quark and an antiquark comprising a meson. The neutral pion (also known as p0) is the lightest of the mesons, but a player in the strong attractive force that holds the atom’s nucleus together.

Haiyan Gao (left) with newly-minted physics Ph.D. Yang Zhang in 2018. (Photo courtesy of Min Huang, Ph.D. ’16)

And that, in turn, makes it a part of the puzzle Gao and her students have been trying to solve for many years. The prevailing theory about the strong force is called quantum chromodynamics (QCD), and it’s been probed for years by high-energy physics. But Gao, Zhang and their collaborators are trying to study QCD under more normal energy states, a notoriously difficult problem.

Yang Zhang spent six years analyzing and writing up the data from a Primakoff  (PrimEx-II) experiment in Hall B at Thomas Jefferson National Accelerator Facility (Jefferson Lab) in Newport News, VA. His work was done on equipment supported by both the National Science Foundation and the Department of Energy.  

This is the quark structure of the positive pion – an up quark and an anti-down quark. The strong force is from gluons, represented as the wavy lines (Arpad Horvath via Wikimedia Commons)

In a Primakoff experiment, a photon beam is directed on a nuclear target, producing neutral pions. In both the PrimEx-I and PrimEx-II experiments at Jefferson Lab, the two photons from the decay of a neutral pionwere subsequently detected in an electromagnetic calorimeter. From that, Zhang extracted the pion’s ‘radiative decay width.’ That decay width is a handy thing to have, because it is directly related to the pion’s life expectancy, and QCD has a direct prediction for it.

Zhang’s hard-won answer: The neutral pion has a radiative decay width of 7.8 electron-volts, give or take. That makes it an important piece of the dauntingly huge puzzle about QCD. Gao and her colleagues will continue to ask the fundamental questions about nature, at the finest but perhaps most profound scale imaginable.

The PrimEx-I and PrimEx-II collaborations were led by Prof. Ashot Gasparian from North Carolina A&T State University. Gao and Zhang joined the collaboration in 2011.

“Precision Measurement of the Neutral Pion Lifetime,” appears in Science May 1. Dr. Yang Zhang is now a quantitative researcher at JPMorgan Chase & Co.

Researchers created a tiny circuit through a single water molecule, and here’s what they found

Graphic by Limin Xiang, Arizona State University

Many university labs may have gone quiet amid coronavirus shutdowns, but faculty continue to analyze data, publish papers and write grants. In this guest post from Duke chemistry professor David Beratan and colleagues, the researchers describe a new study showing how water’s ability to shepherd electrons can change with subtle shifts in a water molecule’s 3-D structure:

Water, the humble combination of hydrogen and oxygen, is essential for life. Despite its central place in nature, relatively little is known about the role that single water molecules play in biology.

Researchers at Duke University, in collaboration with Arizona State University, Pennsylvania State University and University of California-Davis have studied how electrons flow though water molecules, a process crucial for the energy-generating machinery of living systems. The team discovered that the way that water molecules cluster on solid surfaces enables the molecules to be either strong or weak mediators of electron transfer, depending on their orientation. The team’s experiments show that water is able to adopt a higher- or a lower-conducting form, much like the electrical switch on your wall. They were able to shift between the two structures using large electric fields.

In a previous paper published fifteen years ago in the journal Science, Duke chemistry professor David Beratan predicted that water’s mediation properties in living systems would depend on how the water molecules are oriented.

Water assemblies and chains occur throughout biological systems. “If you know the conducting properties of the two forms for a single water molecule, then you can predict the conducting properties of a water chain,” said Limin Xiang, a postdoctoral scholar at University of California, Berkeley, and the first author of the paper.

“Just like the piling up of Lego bricks, you could also pile up a water chain with the two forms of water as the building blocks,” Xiang said.

In addition to discovering the two forms of water, the authors also found that water can change its structure at high voltages. Indeed, when the voltage is large, water switches from a high- to a low-conductive form. In fact, it is may be possible that this switching could gate the flow of electron charge in living systems.

This study marks an important first step in establishing water synthetic structures that could assist in making electrical contact between biomolecules and electrodes. In addition, the research may help reveal nature’s strategies for maintaining appropriate electron transport through water molecules and could shed light on diseases linked to oxidative damage processes.

The researchers dedicate this study to the memory of Prof. Nongjian (NJ) Tao.

CITATION: “Conductance and Configuration of Molecular Gold-Water-Gold Junctions Under Electric Fields,” Limin Xiang, Peng Zhang, Chaoren Liu, Xin He, Haipeng B. Li, Yueqi Li, Zixiao Wang, Joshua Hihath, Seong H. Kim, David N. Beratan and Nongjian Tao. Matter, April 20, 2020. DOI: 10.1016/j.matt.2020.03.023

Guest post by David Beratan and Limin Xiang

Origami-inspired robots that could fit in a cell?

Imagine robots that can move, sense and respond to stimuli, but that are smaller than a hair’s width. This is the project that Cornell professor and biophysicist Itai Cohen, who gave a talk on Wednesday, January 29 as a part of Duke’s Physics Colloquium, has been working on with and his team. His project is inspired by the microscopic robots in Paul McEuen’s book Spiral. Building robots at such a small scale involves a lot more innovation than simply shrinking all of the parts of a normal robot. At low Reynolds number, fluids are viscous instead of inertial, Van der Waals forces come into play, as well as other factors that affect how the robot can move and function. 

Cohen’s team designs robots that fold similar to origami creatures. Image from

To resolve this issue, Cohen and his team decided to build and pattern their micro robots in 2D. Then, inspired by origami, a computer would print the 2D pattern of a robot that can fold itself into a 3D structure. Because paper origami is scale invariant, mechanisms built at one scale will work at another, so the idea is to build robot patterns than can be printed and then walk off of the page or out of a petri dish. However, as Cohen said in his talk last Wednesday, “an origami artist is only as good as their origami paper.” And to build robots at a microscopic scale, one would need some pretty thin paper. Cohen’s team uses graphene, a single sheet of which is only one atom thick. Atomic layer deposition films also behave very similarly to paper, and can be cut up, stretch locally and adopt a 3D shape. Some key steps to making sure the robot self-folds include making elements that bend, and putting additional stiff pads that localize bends in the pattern of the robot. This is what allows them to produce what they call “graphene bimorphs.” 

Cilia on the surface of a cell. Image from MedicalXpress.

Cohen and his team are looking to use microscopic robots in making artificial cilia, which are small leg-like protrusions in cells. Cilia can be sensory or used for locomotion. In the brain, there are cavities where neurotransmitters are redirected based on cilial beatings, so if one can control the individual beating of cilia, they can control where neurotransmitters are directed. This could potentially have biomedical implications for detecting and resolving neurological disorders. 

Right now, Cohen and his lab have microscopic robots made of graphene, which have photovoltaics attached to their legs. When a light shines on the photovoltaic receptor, it activates the robot’s arm movement, and it can wave hello. The advantage of using photovoltaics is that to control the robot, scientists can shine light instead of supplying voltage through a probe—the robot doesn’t need any tethers. During his presentation, Cohen showed the audience a video of his “Brobot,” a robot that flexes its arms when a light shines on it. His team has also successfully made microscopic robots with front and back legs that can walk off a petri dish. Their dimensions are 70 microns long, 40 microns wide and two microns thick. 

Cohen wants to think critically about what problems are important to use technology to solve; he wants make projects that can predict the behavior of people in crowds, predict the direction people will go in response to political issues, and help resolve water crises. Cohen’s research has the potential to find solutions for a wide variety of current issues. Using science fiction and origami as the inspiration for his projects reminds us that the ideas we dream of can become tangible realities. 

By Victoria Priester

Scientists Made a ‘T-Ray’ Laser That Runs on Laughing Gas

‘T-Ray’ laser finally arrives in practical, tunable form. Duke physicist Henry Everitt worked on it over two decades. Courtesy of Chad Scales, US Army Futures Command

It was a Frankenstein moment for Duke alumnus and adjunct physics professor Henry Everitt.

After years of working out the basic principles behind his new laser, last Halloween he was finally ready to put it to the test. He turned some knobs and toggled some switches, and presto, the first bright beam came shooting out.

“It was like, ‘It’s alive!’” Everitt said.

This was no laser for presenting Powerpoint slides or entertaining cats. Everitt and colleagues have invented a new type of laser that emits beams of light in the ‘terahertz gap,’ the no-man’s-land of the electromagnetic spectrum between microwaves and infrared light.

Terahertz radiation, or ‘T-rays,’ can see through clothing and packaging, but without the health hazards of harmful radiation, so they could be used in security scanners to spot concealed weapons without subjecting people to the dangers of X-rays.

It’s also possible to identify substances by the characteristic frequencies they absorb when T-rays hit them, which makes terahertz waves ideal for detecting toxins in the air or gases between the stars. And because such frequencies are higher than those of radio waves and microwaves, they can carry more bandwidth, so terahertz signals could transmit data many times faster than today’s cellular or Wi-Fi networks.

“Imagine a wireless hotspot where you could download a movie to your phone in a fraction of a second,” Everitt said.

Yet despite the potential payoffs, T-rays aren’t widely used because there isn’t a portable, cheap or easy way to make them.

Now Everitt and colleagues at Harvard University and MIT have invented a small, tunable T-ray laser that might help scientists tap into the terahertz band’s potential.

While most terahertz molecular lasers take up an area the size of a ping pong table, the new device could fit in a shoebox. And while previous sources emit light at just one or a few select frequencies, their laser could be tuned to emit over the entire terahertz spectrum, from 0.1 to 10 THz.

The laser’s tunability gives it another practical advantage, researchers say: the ability to adjust how far the T-ray beam travels. Terahertz signals don’t go very far because water vapor in the air absorbs them. But because some terahertz frequencies are more strongly absorbed by the atmosphere than others, the tuning capability of the new laser makes it possible to control how far the waves travel simply by changing the frequency. This might be ideal for applications like keeping car radar sensors from interfering with each other, or restricting wireless signals to short distances so potential eavesdroppers can’t intercept them and listen in.

Everitt and a team co-led by Federico Capasso of Harvard and Steven Johnson of MIT describe their approach this week in the journal Science. The device works by harnessing discrete shifts in the energy levels of spinning gas molecules when they’re hit by another laser emitting infrared light.

Their T-ray laser consists of a pencil-sized copper tube filled with gas, and a 1-millimeter pinhole at one end. A zap from the infrared laser excites the gas molecules within, and when the molecules in this higher energy state outnumber the ones in a lower one, they emit T-rays.

The team dubbed their gizmo the “laughing gas laser” because it uses nitrous oxide, though almost any gas could work, they say.

Duke professor Henry Everitt and MIT graduate student Fan Wang and colleagues have invented a new laser that emits beams of light in the ‘terahertz gap,’ the no-man’s-land of the electromagnetic spectrum.

Everitt started working on terahertz laser designs 35 years ago as a Duke undergraduate in the mid-1980s, when a physics professor named Frank De Lucia offered him a summer job.

De Lucia was interested in improving special lasers called “OPFIR lasers,” which were the most powerful sources of T-rays at the time. They were too bulky for widespread use, and they relied on an equally unwieldy infrared laser called a CO2 laser to excite the gas inside.

Everitt was tasked with trying to generate T-rays with smaller gas laser designs. A summer gig soon grew into an undergraduate honors thesis, and eventually a Ph.D. from Duke, during which he and De Lucia managed to shrink the footprint of their OPFIR lasers from the size of an axe handle to the size of a toothpick.

But the CO2 lasers they were partnered with were still quite cumbersome and dangerous, and each time researchers wanted to produce a different frequency they needed to use a different gas. When more compact and tunable sources of T-rays came to be, OPFIR lasers were largely abandoned.

Everitt would shelf the idea for another decade before a better alternative to the CO2 laser came along, a compact infrared laser invented by Harvard’s Capasso that could be tuned to any frequency over a swath of the infrared spectrum.

By replacing the CO2 laser with Capasso’s laser, Everitt realized they wouldn’t need to change the laser gas anymore to change the frequency. He thought the OPFIR laser approach could make a comeback. So he partnered with Johnson’s team at MIT to work out the theory, then with Capasso’s group to give it a shot.

The team has moved to patent their design, but there is still a long way before it finds its way onto store shelves or into consumers’ hands. Nonetheless, the researchers — who couldn’t resist a laser joke — say the outlook for the technique is “very bright.”

This research was supported by the U.S. Army Research Office (W911NF-19-2-0168, W911NF-13-D-0001) and by the National Science Foundation (ECCS-1614631) and its Materials Research Science and Engineering Center Program (DMR-1419807).

CITATION: “Widely Tunable Compact Terahertz Gas Lasers,” Paul Chevalier, Arman Armizhan, Fan Wang, Marco Piccardo, Steven G. Johnson, Federico Capasso, Henry Everitt. Science, Nov. 15, 2019. DOI: 10.1126/science.aay8683.

How Small is a Proton? Smaller Than Anyone Thought

The proton, that little positively-charged nugget inside an atom, is fractions of a quadrillionth of a meter smaller than anyone thought, according to new research appearing Nov. 7 in Nature.

Haiyan Gao of Duke Physics

In work they hope solves the contentious “proton radius puzzle” that has been roiling some corners of physics in the last decade, a team of scientists including Duke physicist Haiyan Gao have addressed the question of the proton’s radius in a new way and discovered that it is 0.831 femtometers across, which is about 4 percent smaller than the best previous measurement using electrons from accelerators. (Read the paper!)

A single femtometer is 0.000000000000039370 inches imperial, if that helps, or think of it as a millionth part of a billionth part of a meter. And the new radius is just 80 percent of that.

But this is a big — and very small — deal for physicists, because any precise calculation of energy levels in an atom will be affected by this measure of the proton’s size, said Gao, who is the Henry Newson professor of physics in Trinity College of Arts & Sciences.

Bohr model of Hydrogen. One proton, one electron, as simple as they come.

What the physicists actually measured is the radius of the proton’s charge distribution, but that’s never a smooth, spherical point, Gao explained. The proton is made of still smaller bits, called quarks, that have their own charges and those aren’t evenly distributed. Nor does anything sit still. So it’s kind of a moving target.

One way to measure a proton’s charge radius is to scatter an electron beam from the nucleus of an atom of hydrogen, which is made of just one proton and one electron. But the electron must only perturb the proton very gently to enable researchers to infer the size of the charge involved in the interaction. Another approach measures the difference between two atomic hydrogen energy levels. Past results from these two methods have generally agreed.

Artist’s conception of a very happy muon by Particle Zoo

But in 2010, an experiment at the Paul Scherrer Institute replaced the electron in a hydrogen atom with a muon, a much heavier and shorter-lived member of the electron’s particle family. The muon is still negatively charged like an electron, but it’s about 200 times heavier, so it can orbit much closer to the proton. Measuring the difference between muonic hydrogen energy levels, these physicists obtained a proton charge radius that is highly precise, but much smaller than the previously accepted value. And this started the dispute they’ve dubbed the “proton charge radius puzzle.”

To resolve the puzzle, Gao and her collaborators set out to do a completely new type of electron scattering experiment with a number of innovations. And they looked at electron scattering from both the proton and the electron of the hydrogen atom at the same time. They also managed to get the beam of electrons scattered at near zero degrees, meaning it came almost straight forward, which enabled the electron beam to “feel” the proton’s charge response more precisely.

Voila, a 4-percent-smaller proton. “But actually, it’s much more complicated,” Gao said, in a major understatement.

The work was done at the Department of Energy’s Thomas Jefferson National Accelerator Facility in Newport News, Virginia, using new equipment supported by both the National Science Foundation and the Department of Energy, and some parts that were purpose-built for this experiment. “To solve the argument, we needed a new approach,” Gao said.

Gao said she has been interested in this question for nearly 20 years, ever since she became aware of two different values for the proton’s charge radius, both from electron scattering experiments.  “Each one claimed about 1 percent uncertainty, but they disagreed by several percent,” she said.

And as always in modern physics, had the answer not worked out so neatly, it might have called into question parts of the Standard Model of particle physics. But alas, not this time.

“This is particularly important for a number of reasons,” Gao said. The proton is a fundamental building block of visible matter, and the energy level of hydrogen is a basic unit of measure that all physicists rely on.

The new measure may also help advance new insights into quantum chromodynamics (QCD), the theory of strong interaction in quarks and gluons, Gao said. “We really don’t understand how QCD works.”

“This is a very, very big deal,” she said. “The field is very excited about it. And I should add that this experiment would not have been so successful without the heroic contributions from our highly talented and hardworking graduate students and postdocs from Duke.”

This work was funded in part by the U. S. National Science Foundation (NSF MRI PHY-1229153) and by the U.S. Department of Energy (Contract No. DE-FG02-03ER41231), including contract No. DE-AC05-06OR23177 under which Jefferson Science Associates, LLC operates Thomas Jefferson National Accelerator Facility.

CITATION: “A Small Proton Charge Radius from An Electron-Proton Scattering Experiment,”  W. Xiong, A. Gasparian, H. Gao, et al. Nature, Nov. 7, 2019. DOI: 10.1038/s41586-019-1721-2 (ONLINE)

Nature Shows a U-Turn Path to Better Solar Cells

The technical-sounding category of “light-driven charge-transfer reactions,” becomes more familiar to non-physicists when you just call it photosynthesis or solar electricity.

When a molecule (in a leaf or solar cell) is hit by an energetic photon of light, it first absorbs the little meteor’s energy, generating what chemists call an excited state. This excited state then almost immediately (like trillionths of a second) shuttles an electron away to a charge acceptor to lower its energy. That transference of charge is what drives plant life and photovoltaic current.

A 20 Megawatt solar farm ( Aerial Innovations via wikimedia commons)

The energy of the excited state plays an important role in determining solar energy conversion efficiency. That is, the more of that photon’s energy that can be retained in the charge-separated state, the better. For most solar-electric devices, the excited state rapidly loses energy, resulting in less efficient devices.

But what if there were a way to create even more energetic excited states from that incoming photon?

Using a very efficient photosynthesizing bacterium as their inspiration, a team of Duke chemists that included graduate students Nick Polizzi and Ting Jiang, and faculty members David Beratan and Michael Therien, synthesized a “supermolecule” to help address this question.

“Nick and Ting discovered a really cool trick about electron transfer that we might be able to adapt to improving solar cells,” said Michael Therien, the William R. Kenan, Jr. Professor of Chemistry. “Biology figured this out eons ago,” he said.

“When molecules absorb light, they have more energy,” Therien said. “One of the things that these molecular excited states do is that they move charge. Generally speaking, most solar energy conversion structures that chemists design feature molecules that push electron density in the direction they want charge to move when a photon is absorbed. The solar-fueled microbe, Rhodobacter sphaeroides, however, does the opposite. What Nick and Ting demonstrated is that this could also be a winning strategy for solar cells.”

Ting Jiang
Nick Polizzi

The chemists devised a clever synthetic molecule that shows the advantages of an excited state that pushes electron density in the direction opposite to where charge flows. In effect, this allows more of the energy harvested from a photon to be used in a solar cell. 

“Nick and Ting’s work shows that there are huge advantages to pushing electron density in the exact opposite direction where you want charge to flow,” Therien said in his top-floor office of the French Family Science Center. “The biggest advantage of an excited state that pushes charge the wrong way is it stops a really critical pathway for excited state relaxation.”

“So, in many ways it’s a Rube Goldberg Like conception,” Therien said. “It is a design strategy that’s been maybe staring us in the face for several years, but no one’s connected the dots like Nick and Ting have here.”

In a July 2 commentary for the Proceedings of the National Academy of Sciences, Bowling Green State University chemist and photoscientist Malcom D.E. Forbes calls this work “a great leap forward,” and says it “should be regarded as one of the most beautiful experiments in physical chemistry in the 21st century.”

Here’s a schematic from the paper.
(Image by Nick Polizzi)

CITATION: “Engineering Opposite Electronic Polarization of Singlet and Triplet States Increases the Yield of High-Energy Photoproducts,” Nicholas Polizzi, Ting Jiang, David Beratan, Michael Therien. Proceedings of the National Academy of Sciences, June 10, 2019. DOI: 10.1073/pnas.1901752116 Online:

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