Aurelia Butler

Femtosecond pulsed lasers — which emit light in ultrafast bursts lasting a millionth of a billionth of a second — are powerful tools used in a range of applications from medicine and manufacturing, to sensing and precision measurements of space and time. Today, these lasers are typically expensive table-top systems, which limits their use in applications that have size and power consumption restrictions.
An on-chip femtosecond pulse source would unlock new applications in quantum and optical computing, astronomy, optical communications and beyond. However, it’s been a challenge to integrate tunable and highly efficient pulsed lasers onto chips.
Now, researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a high-performance, on-chip femtosecond pulse source using a tool that seems straight out of science fiction: a time lens.
The research is published in Nature.
“Pulsed lasers that produce high-intensity, short pulses consisting of many colors of light have remained large,” said Marko Lon?ar, the Tiantsai Lin Professor of Electrical Engineering at SEAS and senior author of the study. “To make these sources more practical, we decided to shrink a well-known approach, used to realize conventional — and large — femtosecond sources, leveraging a state of the art integrated photonics platform that we have developed. Importantly, our chips are made using microfabrication techniques like those used to make computer chips, which ensures not only reduced cost and size, but also improved performance and reliability of our femtosecond sources.”
Traditional lenses, like contact lenses or those found in magnifying glasses and microscopes, bend rays of light coming from different directions by altering their phase so that they hit the same location in space — the focal point. More

Researchers at Purdue University have discovered new waves with picometer-scale spatial variations of electromagnetic fields which can propagate in semiconductors like silicon. The research team, led by Dr. Zubin Jacob, Elmore Associate Professor of Electrical and Computer Engineering and Department of Physics and Astronomy (courtesy), published their findings in APS Physics Review Applied in a paper titled, “Picophotonics: Anomalous Atomistic Waves in Silicon.”
“The word microscopic has its origins in the length scale of a micron which is a million times smaller than a meter. Our work is for light matter interaction within the picoscopic regime which is far smaller, where the discrete arrangement of atomic lattices changes light’s properties in surprising ways,” says Jacob.
These intriguing findings demonstrate that natural media host a variety of rich light-matter interaction phenomena at the atomistic level. The use of picophotonic waves in semiconducting materials may lead researchers to design new, functional optical devices, allowing for applications in quantum technologies.
Light-matter interaction in materials is central to several photonic devices from lasers to detectors. Over the past decade, nanophotonics, the study of how light flows on the nanometer scale in engineered structures such as photonic crystals and metamaterials have led to important advances. This existing research can be captured within the realm of classical theory of atomic matter. The current finding leading to picophotonics was made possible by a major leap forward using a quantum theory of atomistic response in matter. The team consists of Jacob as well as Dr. Sathwik Bharadwaj, research scientist at Purdue University, and Dr. Todd Van Mechelen, former post-doc at Purdue University.
The long-standing puzzle in the field was the missing link between atomic lattices, their symmetries and the role it plays on deeply picoscopic light fields. To answer this puzzle, the theory team developed a Maxwell Hamiltonian framework of matter combined with a quantum theory of light induced response in materials.
“This is a pivotal shift from the classical treatment of light flow applied in nanophotonics,” says Jacob. “The quantum nature of light’s behavior in materials is the key for the emergence of picophotonics phenomena.”
Bharadwaj and colleagues showed that hidden amidst traditional well-known electromagnetic waves, new anomalous waves emerge in the atomic lattice. These light waves are highly oscillatory even within one fundamental building block of the silicon crystal (sub-nanometer length scale).
“Natural materials itself have rich intrinsic crystal lattice symmetries and light is strongly influenced by these symmetries,” says Bharadwaj. “The immediate next goal is to apply our theory to the plethora of quantum and topological materials and also verify the existence of these new waves experimentally.”
“Our group has been leading the frontier of research on pico-scale electrodynamic fields inside matter at the atomistic level,” says Jacob. “We recently initiated the picoelectrodynamics theory network where we are bringing together diverse researchers to explore macroscopic phenomena stemming from microscopic pico-electrodynamic fields inside matter.”
This research was funded by the DARPA QUEST program.
Writer: Cheryl Pierce, communications specialist, Earth, Atmospheric, Planetary Sciences | Physics/Astronomy, Purdue University
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Materials provided by Purdue University. Original written by Cheryl Pierce. Note: Content may be edited for style and length. More

This little robot can go almost anywhere.
Researchers at Carnegie Mellon University’s School of Computer Science and the University of California, Berkeley, have designed a robotic system that enables a low-cost and relatively small legged robot to climb and descend stairs nearly its height; traverse rocky, slippery, uneven, steep and varied terrain; walk across gaps; scale rocks and curbs; and even operate in the dark.
“Empowering small robots to climb stairs and handle a variety of environments is crucial to developing robots that will be useful in people’s homes as well as search-and-rescue operations,” said Deepak Pathak, an assistant professor in the Robotics Institute. “This system creates a robust and adaptable robot that could perform many everyday tasks.”
The team put the robot through its paces, testing it on uneven stairs and hillsides at public parks, challenging it to walk across stepping stones and over slippery surfaces, and asking it to climb stairs that for its height would be akin to a human leaping over a hurdle. The robot adapts quickly and masters challenging terrain by relying on its vision and a small onboard computer.
The researchers trained the robot with 4,000 clones of it in a simulator, where they practiced walking and climbing on challenging terrain. The simulator’s speed allowed the robot to gain six years of experience in a single day. The simulator also stored the motor skills it learned during training in a neural network that the researchers copied to the real robot. This approach did not require any hand-engineering of the robot’s movements — a departure from traditional methods.
Most robotic systems use cameras to create a map of the surrounding environment and use that map to plan movements before executing them. The process is slow and can often falter due to inherent fuzziness, inaccuracies, or misperceptions in the mapping stage that affect the subsequent planning and movements. Mapping and planning are useful in systems focused on high-level control but are not always suited for the dynamic requirements of low-level skills like walking or running over challenging terrains. More

When fighting the spread of misinformation, social media platforms typically place most users in the passenger seat. Platforms often use machine-learning algorithms or human fact-checkers to flag false or misinforming content for users.
“Just because this is the status quo doesn’t mean it is the correct way or the only way to do it,” says Farnaz Jahanbakhsh, a graduate student in MIT’s Computer Science and Artificial Intelligence Laboratory (CSAIL).
She and her collaborators conducted a study in which they put that power into the hands of social media users instead.
They first surveyed people to learn how they avoid or filter misinformation on social media. Using their findings, the researchers developed a prototype platform that enables users to assess the accuracy of content, indicate which users they trust to assess accuracy, and filter posts that appear in their feed based on those assessments.
Through a field study, they found that users were able to effectively assess misinforming posts without receiving any prior training. Moreover, users valued the ability to assess posts and view assessments in a structured way. The researchers also saw that participants used content filters differently — for instance, some blocked all misinforming content while others used filters to seek out such articles.
This work shows that a decentralized approach to moderation can lead to higher content reliability on social media, says Jahanbakhsh. This approach is also more efficient and scalable than centralized moderation schemes, and may appeal to users who mistrust platforms, she adds. More

New wearable electronics paired with artificial intelligence could transform screening for health problems.
Flexible, wearable electronics are making their way into everyday use, and their full potential is still to be realized. Soon, this technology could be used for precision medical sensors attached to the skin, designed to perform health monitoring and diagnosis. It would be like having a high-tech medical center at your instant beck and call.
Such a skin-like device is being developed in a project between the U.S. Department of Energy’s (DOE) Argonne National Laboratory and the University of Chicago’s Pritzker School of Molecular Engineering (PME). Leading the project is Sihong Wang, assistant professor in UChicago PME with a joint appointment in Argonne’s Nanoscience and Technology division.
Worn routinely, future wearable electronics could potentially detect possible emerging health problems — such as heart disease, cancer or multiple sclerosis — even before obvious symptoms appear. The device could also do a personalized analysis of the tracked health data while minimizing the need for its wireless transmission. “The diagnosis for the same health measurements could differ depending on the person’s age, medical history and other factors,” Wang said. “Such a diagnosis, with health information being continuously gathered over an extended period, is very data intensive.”
Such a device would need to collect and process a vast amount of data, well above what even the best smartwatches can do today. And it would have to do this data crunching with very low power consumption in a very tiny space.
To address that need, the team called upon neuromorphic computing. This AI technology mimics operation of the brain by training on past data sets and learning from experience. Its advantages include compatibility with stretchable material, lower energy consumption and faster speed than other types of AI. More

Researchers of Delft University of Technology, Vrije Universiteit Amsterdam and VSL have developed an alternative positioning system that is more robust and accurate than GPS, especially in urban settings. The working prototype that demonstrated this new mobile network infrastructure achieved an accuracy of 10 centimeter. This new technology is important for the implementation of a range of location-based applications, including automated vehicles, quantum communication and next-generation mobile communication systems. The results were published in Nature today.
A lot of our vital infrastructure relies on global navigation satellite systems such as the US GPS and EU Galileo. Yet these systems that rely on satellites have their limitations and vulnerabilities. Their radio signals are weak when received on Earth, and accurate positioning is no longer possible if the radio signals are reflected or blocked by buildings. “This can make GPS unreliable in urban settings, for instance” says Christiaan Tiberius of Delft University of Technology and coordinator of the project, “which is a problem if we ever want to use automated vehicles. Also, citizens and our authorities actually depend on GPS for many location-based applications and navigation devices. Furthermore, so far we had no back-up system.”
The aim of the project entitled SuperGPS was to develop an alternative positioning system that makes use of the mobile telecommunication network instead of satellites and that could be more robust and accurate than GPS. ‘We realized that with a few cutting-edge innovations, the telecommunication network could be transformed into a very accurate alternative positioning system that is independent of GPS,’ says Jeroen Koelemeij of Vrije Universiteit Amsterdam. “We have succeeded and have successfully developed a system that can provide connectivity just like existing mobile and Wi-Fi networks do, as well as accurate positioning and time distribution like GPS.”
An atomic clock
One of these innovations is to connect the mobile network to a very accurate atomic clock, so that it can broadcast perfectly timed messages for positioning, just like GPS satellites do with the help of the atomic clocks they carry on board. These connections are made through the existing fiber-optic network. “We had already been investigating techniques to distribute the national time produced by our atomic clocks to users elsewhere through the telecommunication network,” says Erik Dierikx of VSL. “With these techniques we can turn the network into a nationwide distributed atomic clock — with many new applications such as very accurate positioning through mobile networks. With the hybrid optical-wireless system that we have demonstrated now, in principle anyone can have wireless access to the national time produced at VSL. It basically forms an extremely accurate radio clock that is good to one billionth of a second.”
Furthermore, the system employs radio signals with a bandwidth much larger than commonly used. “Buildings reflect radio signals, which can confuse navigation devices. The large bandwidth of our system helps sorting out these confusing signal reflections, and enables higher positioning accuracy,” Gerard Janssen of Delft University of Technology explains. “At the same time, bandwidth within the radio spectrum is scarce and therefore expensive. We circumvent this by using a number of related small bandwidth radio signals spread over a large virtual bandwidth. This has the advantage that only a small fraction of the virtual bandwidth is actually used and the signals can be very similar to those of mobile phones.”
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Researchers at North Carolina State University have created a ring-shaped soft robot capable of crawling across surfaces when exposed to elevated temperatures or infrared light. The researchers have demonstrated that these “ringbots” are capable of pulling a small payload across the surface — in ambient air or under water, as well as passing through a gap that is narrower than its ring size.
The ringbots are made of liquid crystal elastomers in the shape of looped ribbon, resembling a bracelet. When you place the ringbot on a surface that is at least 55 degrees Celsius (131 degrees Fahrenheit), which is hotter than the ambient air, the portion of the ribbon touching the surface contracts, while the portion of the ribbon exposed to the air does not. This induces a rolling motion in the ribbon. Video of the ringbots can be found here: https://youtu.be/yL5gVAjh1mQ.
Similarly, when researchers shine infrared light on the ringbot, the portion of the ribbon exposed to the light contracts, while the portion shielded from the light does not. This also induces a rolling motion in the ribbon.
In practical terms, this means that the crawling ringbot moves from the bottom up when placed on a hot surface. But when exposed to infrared light, the movement begins from the top down.
One of the things that drives this continuous motion is the fact that the ringbots are bistable, meaning that there are two shapes when it is at rest. If the ribbon begins to twist, it will either snap back to its original shape, or snap forward into the other bistable state.
Picture a rubber bracelet shaped like a ribbon. If you fold two ends of the bracelet forward a little bit, then let go, it will snap back to its original shape. But if you fold the ends over far enough, it will snap over — essentially folding the bracelet inside out. More