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Scientists combine light and matter to make particles with new behaviors

Scientists combine light and matter to make particles with new behaviors

Every type of atom in the universe has a unique fingerprint: It only absorbs or emits light at the particular energies that match the allowed orbits of its electrons. That fingerprint enables scientists to identify an atom wherever it is found. A hydrogen atom in outer space absorbs light at the same energies as one on Earth.

While physicists have learned how electric and magnetic fields can manipulate this fingerprint, the number of features that make it up usually remains constant. In work published July 3 in the journal Nature, the University of Chicago researchers challenged this paradigm by shaking electrons with lasers to create “doppelganger” features at new energies—a breakthrough that lets scientists create hybrid particles which are part-atom and part-light, with a wide variety of new behaviors.

The research is part of a greater effort in Assoc. Prof. Jonathan Simon’s lab to break down the walls between matter and light, in order to investigate their fundamental properties. In addition to learning about how materials behave at the quantum level, this work could one day help create more powerful computers or virtually “unhackable” quantum communications.

One step along the way to making matter out of light is to make individual packets of light, called photons, interact with each other like matter does. (Normally photons zip along at the speed of light and don’t react to each other at all.)

“In order to make photons collide with one another, we use atoms as a go-between,” said postdoctoral researcher Logan Clark, who led the research. “But we were running into a problem because the photons only interact with atoms whose electronic orbitals are at very particular energies. So we asked: What if we could make copies of the orbitals at whatever energies we wanted?”

Clark had already developed techniques to manipulate quantum matter by shaking it —called Floquet engineering—as part of his PhD project. The right sort of shaking naturally produces copies of quantum states at multiple energies along the way. “We had always viewed the copies as a side effect rather than the goal,” he said, “but this time, we shook our electrons with the specific intent of making the copies.”

“We asked: What if we could make copies of the orbitals at whatever energies we wanted?”

—Postdoctoral researcher Logan Clark

By varying the intensity of a laser field tuned precisely to an atomic resonance, the team was able to shift the orbitals of an electron. Shaking the orbitals by periodically varying this intensity produced the desired copies.

But these doppelgangers come with an important catch: “While the atomic orbital does appear at multiple distinct energies, it is important to note that these copies are actually bound to the original like puppets,” explained postdoctoral researcher Nathan Schine, a co-author on the study. “When any of the copies shifts, the original and all of the other copies shift with it.”

By allowing photons to interact with these shaken atoms, the team has created what they call “Floquet polaritons”—quasi-particles which are part-light and part-atom, and unlike regular photons, interact with each other quite strongly. These interactions are essential for making matter from light. Making polaritons with shaken atoms can give the polaritons much more flexibility to move around and collide with each other in new ways.

“Floquet polaritons are full of surprises; we’re still continuing to understand them better,” Clark said. “Our next order of business, though, will be to use these colliding photons to make topological ‘fluids’ of light. It is a tremendously exciting time.”

Having copies of an atomic state at multiple energies also offers exciting possibilities for optical frequency conversion—a key tool in creating secure quantum communication methods.

“It turns out shaking things is not only a lot of fun, but can lead to some really fascinating science,” Clark said.

Citation: “Interacting Floquet polaritons.” Clark et al, Nature, July 3, 2019. DOI: 10.1038/s41586-019-1354-5

Materials provided by the University of Chicago

Electron-behaving nanoparticles rock current understanding of matter

Electron-behaving nanoparticles rock current understanding of matter

It’s not an electron. But it sure does act like one.

Northwestern University researchers have made a strange and startling discovery that nanoparticles engineered with DNA in colloidal crystals — when extremely small — behave just like electrons. Not only has this finding upended the current, accepted notion of matter, it also opens the door for new possibilities in materials design.

“We have never seen anything like this before,” said Northwestern’s Monica Olvera de la Cruz, who made the initial observation through computational work. “In our simulations, the particles look just like orbiting electrons.”

With this discovery, the researchers introduced a new term called “metallicity,” which refers to the mobility of electrons in a metal. In colloidal crystals, tiny nanoparticles roam similarly to electrons and act as a glue that holds the material together.

“This is going to get people to think about matter in a new way,” said Northwestern’s Chad Mirkin, who led the experimental work. “It’s going to lead to all sorts of materials that have potentially spectacular properties that have never been observed before. Properties that could lead to a variety of new technologies in the fields of optics, electronics and even catalysis.”

The paper will be published on Friday, June 21 in the journal Science.

Olvera de la Cruz is the Lawyer Taylor Professor of Materials Science and Engineering in Northwestern’s McCormick School of Engineering. Mirkin is the George B. Rathmann Professor of Chemistry in Northwestern’s Weinberg College of Arts and Sciences.

Mirkin’s group previously invented the chemistry for engineering colloidal crystals with DNA, which has forged new possibilities for materials design. In these structures, DNA strands act as a sort of smart glue to link together nanoparticles in a lattice pattern

“Over the past two decades, we have figured out how to make all sorts of crystalline structures where the DNA effectively takes the particles and places them exactly where they are supposed to go in a lattice,” said Mirkin, founding director of the International Institute for Nanotechnology.

In these previous studies, the particles’ diameters are on the tens of nanometers length scale. Particles in these structures are static, fixed in place by DNA. In the current study, however, Mirkin and Olvera de la Cruz shrunk the particles down to 1.4 nanometers in diameter in computational simulations. This is where the magic happened.

“The bigger particles have hundreds of DNA strands linking them together,” said Olvera de la Cruz. “The small ones only have four to eight linkers. When those links break, the particles roll and migrate through the lattice holding together the crystal of bigger particles.”

When Mirkin’s team performed the experiments to image the small particles, they found that Olvera de la Cruz’s team’s computational observations proved true. Because this behaviour is reminiscent of how electrons behave in metals, the researchers call it “metallicity.”

“A sea of electrons migrates throughout metals, acting as a glue, holding everything together,” Mirkin explained. “That’s what these nanoparticles become. The tiny particles become the mobile glue that holds everything together.”

Olvera de la Cruz and Mirkin next plan to explore how to exploit these electron-like particles in order to design new materials with useful properties. Although their research used gold nanoparticles, Olvera de la Cruz said “metallicity” applies to other classes of particles in colloidal crystals.

“In science, it’s really rare to discover a new property, but that’s what happened here,” Mirkin said. “It challenges the whole way we think about building matter. It’s a foundational piece of work that will have a lasting impact.”

Materials provided by Northwestern University

Irregular Galaxy NGC 4449

For the second time, astronomers discover a galaxy with no dark matter

In 2018, astronomers discovered a galaxy with almost zero dark matter. This was a groundbreaking discovery as scientists always considered dark matter to be a crucial component of galaxies. This galaxy named NGC1052-DF2, is 65 million light-years away from us in the constellation Cetus. It is of the same size as Milky Way but has one star per 200 found in Milky Way. Because of lack of stars and absence of dark matter, it has a very low mass.

The main evidence of dark matter is the calculations which show that galaxies would have flown away instead of rotating if they did not contain matter which is not visible otherwise. As dark matter has not been observed directly, it must have a bare minimum interaction with ordinary baryonic matter, except only through gravity. In the standard model of cosmology, dark matter consists of about 85% of total mass while dark matter and dark energy combined accounts for 95% of total mass energy content.

Now again, researchers have found one more galaxy without any dark matter. This one is named NGC1052-DF4. The research team at Yale University which discovered DF2 was skeptical about its findings, but now the discovery of DF4 clears the confusion and it proves that galaxies do exist devoid of any dark matter. This galaxy like the previous one is also an ultra diffuse galaxy, large, spread out and can be faintly observed. These galaxies have very less stars thus making them very difficult to observe.

The astronomers tracked orbital motion of multiple dense clusters of stars which are called globular clusters using a device called Low Resolution Imaging Spectrometer. This velocity was found to be consistent with the one calculated using the mass of the normal matter of the galaxy. Scientists are hoping such galaxies can be found in more numbers and more research work can be carried on how they were formed.

Scientists also conducted more research work to confirm the past results for DF2. They used the Keck Cosmic Web Imager and tracked the orbital motion of 10 globular clusters. The results were consistent with a lack of dark matter.

But it has not been found yet that why these galaxies do not have dark matter. This is still unsolved. It might be possible that they had dark matter present but then it was lost. There can be multiple hypotheses to this question and this can only be solved by finding more such galaxies. Researchers want more evidence before drawing any conclusion, so that they can finally solve the mystery of dark matter.