New nondestructive optical technique reveals the structure of mother-of-pearl
Most people know mother-of-pearl, an iridescent biomineral also called nacre, from buttons, jewelry, instrument inlays and other decorative flourishes. Scientists, too, have admired and marveled at nacre for decades, not only for its beauty and optical properties but because of its exceptional toughness.
“It’s one of the most-studied natural biominerals,” says Pupa Gilbert, a University of Wisconsin–Madison physics professor who has studied nacre for more than a decade. “It may not look like much — just a shiny, decorative material. But it can be 3,000 times more resistant to fracture than aragonite, the mineral from which it’s made. It has piqued the interest of materials scientists because making materials better than the sum of their parts is extremely attractive.”
Now, a new, nondestructive optical technique will unlock even more knowledge about nacre, and in the process could lead to a new understanding of climate history. Gilbert, UW–Madison electrical engineering professor Mikhail Kats — who is also an affiliate professor of physics — their students, and collaborators described the technique, called hyperspectral interference tomography, today in the journal Proceedings of the National Academy of Sciences.
Surprising communication between atoms could improve quantum computing
A group of University of Wisconsin–Madison physicists has identified conditions under which relatively distant atoms communicate with each other in ways that had previously only been seen in atoms closer together — a development that could have applications to quantum computing.
The physicists’ findings, published Oct. 14 in the journal Physical Review A, open up new prospects for generating entangled atoms, the term given to atoms that share information at large distances, which are important for quantum communications and the development of quantum computers.
“Building a quantum computer is very tough, so one approach is that you build smaller modules that can talk to each other,” says Deniz Yavuz, a UW–Madison physics professor and senior author of the study. “This effect we’re seeing could be used to increase the communication between these modules.”
The scenario at hand depends on the interplay between light and the electrons that orbit atoms. An electron that has been hit with a photon of light can be excited to a higher energy state. But electrons loathe excess energy, so they quickly shed it by emitting a photon in a process known as decay. The photons atoms release have less energy than the ones that boosted the electron up — the same phenomenon that causes some chemicals to fluoresce, or some jellyfish to have a green-glowing ring.
“Now, the problem gets very interesting if you have more than one atom,” says Yavuz. “The presence of other atoms modifies the decay of each atom; they talk to each other.”
Physics grad students share hands-on physics, art lessons with local fifth graders
UW–Madison physics grad student Aedan Gardill has been illustrating physics concepts with art for years, such as through his Instagram account, where he shares ink drawings. Earlier this year, he applied for a grant from the Madison Arts Commission to create hidden portraits of women in the physical sciences that could only be revealed by using polarized lenses. He also planned to visit local schools to explain the concept behind his art and help students make their own images based on his technique.
By the time Gardill learned he had been awarded the grant, the pandemic was in full force, and his plans had to change. While he could still present his portraits at the Wisconsin Science Festival, school visits were no longer in the cards.
“With the realization this summer that school was going to most likely be online in the fall, I had to rethink how I was going to use the funding from the grant,” Gardill explains. “And that has morphed into providing at-home, hands-on learning experiences that we’ll lead virtually.”
Funding for Gardill’s work is provided by a grant from the Madison Arts Commission, with additional funds from the Wisconsin Arts Board, the Optical Society of America, the International Society for Optics and Photonics, and the UW–Madison Department of Physics, with special thanks to Arts + Literature Laboratory. UW–Madison physics graduate student volunteers include Abby Bishop, Praful Gagrani, Jimena Gonzalez, Ben Harpt, Preston Huft, Brent Mode, Bryan Rubio Perez, Susan Sorensen, and Jessie Thwaites.
Manipulating the magnetic response to light in natural materials
When light moves from one material into another, it bends — like how a partially submerged object appears distorted under water when viewed from above. What if, instead of bending, a material could change the light so much that the material was no longer visible at all?
In a study published in Physical Review A, University of Wisconsin–Madison researchers have shown for the first time that a similar response can be obtained and manipulated in naturally-occurring materials. The findings have implications from the development of “perfect” lenses for improved microscopy to Harry Potter-esque invisibility cloaks.
Visible light is made of both magnetic and electric fields, and the refractive index of a material — how much it bends the light — is determined by how the material interacts with those two fields. Nearly all materials we encounter in everyday life, though, interact entirely with light’s electric field.
Researchers have spent the past two decades developing artificial materials that more strongly interact with light’s magnetic field by manipulating the refractive index. With a strong enough response, the material could eventually have a negative refractive index, leading to unique optical properties. However, the response in synthetic materials is limited by the size of their repeating units. A naturally-occurring crystal that has much smaller unit cells is likely a better choice.
“Part of producing a negative refractive index is that the material needs to have a strong response to both electric and magnetic fields, so the big challenge is getting that magnetic response in natural materials,” explains Zach Buckholtz, a graduate student in UW–Madison physics professor Deniz Yavuz’s group and lead author of the study. “A few years ago, we published a paper showing that the crystal we’re working with has a magnetic response, and in this study, we were able to manipulate the response.”
The natural material Buckholtz is working with is a silicon-based crystal, which in general is optically ordinary, except that it has been “doped” with the rare earth metal Europium. Rare earth metals are unique in that they contain an abundance of electrons in the atoms’ outer energy shells. Those electrons can then work together to create a bigger magnetic response, but only if they are all in tune with each other.
“If you have some magnetic response and a much larger electric response to light, you can connect those two responses,” Buckholtz says. “To get to a negative refractive index from there, you have to set up coherences between the energy levels, meaning you have to make sure all those energy levels are oscillating together.”
To show they can manipulate the magnetic response, Buckholtz and Yavuz did two things. First, because the crystal is a mix of ions with slightly different electron responses, they needed to set up their experimental system to select for one class of ion. This uniformity allows for a cleaner interpretation of the results.
“We send a laser into the crystal, and then measure how much of the light is transmitted. But because the crystal isn’t perfect, instead of seeing a narrow peak for the transitions, you’ll see a really broad transition,” Buckholtz explains. “So, we do this procedure known as spectral hole burning to clear out the ions we don’t want and then we’ll be left with just one transition, which is necessary to move on to experiments that involve coherence.”
Next, they wanted to show if they could increase the magnetic response. To do so, they needed to take those selected ions, put them in coherence, and then measure the response compared to ions not in coherence. In these experiments, they shined one (a probe beam) or two (probe and coupling beam) wavelengths of laser at the ions. Both lasers excite electrons in the ions to a higher-energy state, and the scientists can again measure how might light is transmitted through the crystal as a readout of the electron transitions.
“With just the probe beam, we see just the normal transition, and that’s what we did in our previous study. But with the coupling beam added in, it connects and adds another transition state in there,” Buckholtz says. “If those states are in coherence, they cancel each other out, and we see that effect as a peak in transmitted light, which means the index of refraction is going toward zero.”
Buckholtz notes that the magnetic response they see is not yet large enough to produce the materials with interesting new optical properties they are hoping for. Still, he says, this work provides a path forward to continue manipulations to improve the response, such as investigating different rare earth metals.
“We have a magnetic response, we can set up coherence, and we can manipulate the response,” Buckholtz says. “Now, we want to increase the scale of the response to with a goal of eventually making the refractive index below zero.”