Researchers at the University of California San Diego have successfully demonstrated a laser based on an unconventional physics phenomenon called bound states in the continuum (BIC). The new BIC laser technology potentially could transform development of surface lasers by making them more compact and energy-efficient for communications and computing applications, and could also be developed as high-power lasers for industrial and defense uses.
“Lasers are ubiquitous in the present day world, from simple everyday laser pointers to complex laser interferometers used to detect gravitational waves. Our current research will impact many areas of laser applications,” said Ashok Kodigala, an electrical engineering doctoral student at UC San Diego and first author of the study.
“Because they are unconventional, BIC lasers offer unique and unprecedented properties that haven’t yet been realized with existing laser technologies,” said Boubacar Kanté, electrical engineering professor at the UC San Diego Jacobs School of Engineering, leader of the research team.
BIC lasers can be tuned to emit beams of different wavelengths, which is useful for medical lasers precisely targeting cancer cells without damaging normal tissue. BIC lasers can also emit beams with specially engineered shapes (spiral, donut or bell curve)—called vector beams—which could enable increasingly powerful computers and optical communication systems that can carry up to 10 times more information than existing ones.
“Light sources are key components of optical data communications technology in cell phones, computers and astronomy, for example. In this work, we present a new kind of light source that is more efficient than what’s available today in terms of power consumption and speed,” said Babak Bahari, an electrical engineering PhD student in Kanté’s lab and a co-author of the study.
The research was published Jan. 12 in the journal Nature. Bound states in the continuum are phenomena that have been predicted to exist since 1929, consisting of waves that remain perfectly confined, or bound, in an open system. Conventional waves in an open system escape, but BICs defy this norm—they stay localized and do not escape despite having open pathways to do so. In a previous study, Kanté and his team demonstrated, at microwave frequencies, that BICs could be used to efficiently trap and store light to enable strong light-matter interaction. Now, they’re harnessing BICs to demonstrate new types of lasers.
The BIC laser in this work is constructed from a thin semiconductor membrane made of indium, gallium, arsenic and phosphorus, the scientists said, and the membrane is structured as an array of nano-sized cylinders suspended in air. The cylinders are interconnected by a network of supporting bridges, which provide mechanical stability to the device. By powering the membrane with a high frequency laser beam, researchers induced the BIC system to emit its own lower frequency laser beam (at telecommunication frequency).
“Right now, this is a proof of concept demonstration that we can indeed achieve lasing action with BICs,” Kanté said. “And what’s remarkable is that we can get surface lasing to occur with arrays as small as 8 × 8 particles,” he said. In comparison, the surface lasers that are widely used in data communications and high-precision sensing, called VCSELs (vertical-cavity surface-emitting lasers), need much larger (100 times) arrays—and thus more power—to achieve lasing.
“The popular VCSEL may one day be replaced by what we’re calling the ‘BICSEL’—bound state in the continuum surface-emitting laser, which could lead to smaller devices that consume less power,” Kanté said.
The team has filed a patent for the new type of light source. The team’s next step is to make BIC lasers that are electrically powered, rather than optically powered by another laser. “An electrically pumped laser is easily portable outside the lab and can run off a conventional battery source,” Kanté said.
The research was supported by a National Science Foundation Career Award (ECCS-1554021), the Office of Naval Research Multi-University Research Initiative (N000014-13-1-0678) and UC San Diego.
Treated Carbon Material Pulls Radioactive Elements from Water
Ateam of researchers from Rice University (Houston) and Kazan Federal University (Kazan, Russia) have developed a method to extract radioactivity from water, a discovery that the researchers say could help purify the hundreds of millions of gallons of contaminated water stored after the Fukushima nuclear plant accident.
The team reported their oxidatively modified carbon (OMC) material is inexpensive and highly efficient at absorbing radioactive metal cations, including cesium and strontium, toxic elements released into the environment when the Fukushima plant melted down after an earthquake and tsunami in March 2011. OMC can easily trap common radioactive elements found in water floods from oil extraction, such as uranium, thorium and radium, said Rice chemist James Tour, who led the project with Ayrat Dimiev, a former postdoctoral researcher in his lab and now a research professor at Kazan Federal University.
The material makes good use of the porous nature of two specific sources of carbon, Tour said. One is an inexpensive, coke-derived powder known as C-seal F, used by the oil industry as an additive to drilling fluids. The other is a naturally occurring, carbon-heavy mineral called shungite found mainly in Russia.
The results appear this month in Carbon.
Tour and researchers at Lomonosov Moscow State University had already demonstrated a method to remove radionuclides from water using graphene oxide as a sorbent, as reported in Solvent Extraction and Ion Exchange late last year, but the new research suggests OMC is easier and far less expensive to process.
Treating the carbon particles with oxidizing chemicals increased their surface areas and “decorated” them with the oxygen molecules needed to adsorb the toxic metals. The particles were between 10- and 80-μm wide. While graphene oxide excelled at removing strontium, Tour said, the two types of OMC were better at extracting cesium, which he said has been the hardest element to remove from water stored at Fukushima. The OMC was also much easier and less expensive to synthesize and to use in a standard filtration system, he said.
“We know we can use graphene oxide to trap the light radioactive elements of relevance to the Fukushima cleanup, namely cesium and strontium,” Tour said. “But in the second study, we learned we can move from graphene oxide, which remains more expensive and harder to make, to really cheap oxidized coke and related carbons to trap these elements.”
While other materials used for remediation of radioactive waste need to be stored with the waste they capture, carbon presents a distinct advantage, he said. “Carbon that has captured the elements can be burned in a nuclear incinerator, leaving only a very small amount of radioactive ash that’s much easier to store,” Tour said.
“Just passing contaminated water through OMC filters will extract the radioactive elements and permit safe discharge to the ocean,” he said. “This could be a major advance for the cleanup effort at Fukushima.”
Co-authors of the paper are Artur Khannanov, Vadim Nekljudov, Bulat Gareev and Airat Kiiamov, all of Kazan Federal University. Tour is the T.T. and W.F. Chao Chair in Chemistry as well as a professor of computer science and of materials science and nanoengineering at Rice. The Russian Government Program of Competitive Growth of Kazan Federal University supported the research.
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