Strange physics turns off laser (Nature Communications)

Posted on by Catherine Zandonella

By Steve Schultz, School of Engineering Office of Communications

An electron microscope image shows two lasers placed just two microns apart from each other. (Image source: Turecki lab)
An electron microscope image shows two lasers placed just two microns apart from each other. (Image source: Turecki lab)

Inspired by anomalies that arise in certain mathematical equations, researchers have demonstrated a laser system that paradoxically turns off when more power is added rather than becoming continuously brighter.

The finding by a team of researchers at Vienna University of Technology and Princeton University, could lead to new ways to manipulate the interaction of electronics and light, an important tool in modern communications networks and high-speed information processing.

The researchers published their results June 13 in the journal Nature Communications.

Their system involves two tiny lasers, each one-tenth of a millimeter in diameter, or about the width of a human hair. The two are nearly touching, separated by a distance 50 times smaller than the lasers themselves. One is pumped with electric current until it starts to emit light, as is normal for lasers. Power is then added slowly to the other, but instead of it also turning on and emitting even more light, the whole system shuts off.

“This is not the normal interference that we know,” said Hakan Türeci, assistant professor of electrical engineering at Princeton, referring to the common phenomenon of light waves or sound waves from two sources cancelling each other.  Instead, he said, the cancellation arises from the careful distribution of energy loss within an overall system that is being amplified.

Interactions between two lasers
Manipulating minute areas of gain and loss within individual lasers (shown as peaks and valleys in the image), researchers were able to create paradoxical interactions between two nearby lasers.(Image source: Turecki lab)

“Loss is something you normally are trying to avoid,” Türeci said. “In this case, we take advantage of it and it gives us a different dimension we can use – a new tool – in controlling optical systems.”

The research grows out of Türeci’s longstanding work on mathematical models that describe the behavior of lasers. In 2008, he established a mathematical framework for understanding the unique properties and complex interactions that are possible in extremely small lasers – devices with features measured in micrometers or nanometers. Different from conventional desk-top lasers, these devices fit on a computer chip.

That work opened the door to manipulating gain or loss (the amplification or loss of an energy input) within a laser system. In particular, it allowed researchers to judiciously control the spatial distribution of gain and loss within a single system, with one tiny sub-area amplifying light and an immediately adjacent area absorbing the generated light.

Türeci and his collaborators are now using similar ideas to pursue counterintuitive ideas for using distribution of gain and loss to make micro-lasers more efficient.

The researchers’ ideas for taking advantage of loss derive from their study of mathematical constructs called “non-Hermitian” matrices in which a normally symmetric table of values becomes asymmetric. Türeci said the work is related to certain ideas of quantum physics in which the fundamental symmetries of time and space in nature can break down even though the equations used to describe the system continue to maintain perfect symmetry.

Over the past several years, Türeci and his collaborators at Vienna worked to show how the mathematical anomalies at the heart of this work, called “exceptional points,” could be manifested in an actual system. In 2012 (Ref. 3), the team published a paper in the journal Physical Review Letters demonstrating computer simulations of a laser system that shuts off as energy is being added. In the current Nature Communications paper, the researchers created an experimental realization of their theory using a light source known as a quantum cascade laser.

The researchers report in the article that results could be of particular value in creating “lab-on-a-chip” devices – instruments that pack tiny optical devices onto a single computer chip. Understanding how multiple optical devices interact could provide ways to manipulate their performance electronically in previously unforeseen ways. Taking advantage of the way loss and gain are distributed within tightly coupled laser systems could lead to new types of highly accurate sensors, the researchers said.

“Our approach provides a whole new set of levers to create unforeseen and useful behaviors,” Türeci said.

The work at Vienna, including creation and demonstration of the actual device, was led by Stefan Rotter at Vienna along with Martin Brandstetter, Matthias Liertzer, C. Deutsch, P. Klang, J. Schöberl, G. Strasser and K. Unterrainer. Türeci participated in the development of the mathematical models underlying the phenomena. The work on the 2012 computer simulation of the system also included Li Ge, who was a post-doctoral researcher at Princeton at the time and is now an assistant professor at City University of New York.

The work was funded by the Vienna Science and Technology Fund and the Austrian Science Fund, as well as by the National Science Foundation through a major grant for the Mid-Infrared Technologies for Health and the Environment Center based at Princeton and by the Defense Advanced Research Projects Agency.

Read the abstract.

M. Brandstetter, M. Liertzer, C. Deutsch,P. Klang,J. Schöberl,H. E. Türeci,G. Strasser,K. Unterrainer & S. Rotter. Reversing the pump dependence of a laser at an exceptional point. Nature Communications 13 June 2014. DOI:10.1038/ncomms5034

Science 2 May 2008. DOI: 10.1126/science.1155311

Physical Review Letters 24 April 2012. DOI:10.1103/PhysRevLett.108.173901

 

Quantum computing moves forward (Science)

Posted on by Catherine Zandonella

By Catherine Zandonella, Office of the Dean for Research

New technologies that exploit quantum behavior for computing and other applications are closer than ever to being realized due to recent advances, according to a review article published this week in the journal Science.

Science_cover
A silicon chip levitates individual atoms used in quantum information processing. Photo: Curt Suplee and Emily Edwards, Joint Quantum Institute and University of Maryland. Credit: Science.

These advances could enable the creation of immensely powerful computers as well as other applications, such as highly sensitive detectors capable of probing biological systems. “We are really excited about the possibilities of new semiconductor materials and new experimental systems that have become available in the last decade,” said Jason Petta, one of the authors of the report and an associate professor of physics at Princeton University.

Petta co-authored the article with David Awschalom of the University of Chicago, Lee Basset of the University of California-Santa Barbara, Andrew Dzurak of the University of New South Wales and Evelyn Hu of Harvard University.

Two significant breakthroughs are enabling this forward progress, Petta said in an interview. The first is the ability to control quantum units of information, known as quantum bits, at room temperature. Until recently, temperatures near absolute zero were required, but new diamond-based materials allow spin qubits to be operated on a table top, at room temperature. Diamond-based sensors could be used to image single molecules, as demonstrated earlier this year by Awschalom and researchers at Stanford University and IBM Research (Science, 2013).

The second big development is the ability to control these quantum bits, or qubits, for several seconds before they lapse into classical behavior, a feat achieved by Dzurak’s team (Nature, 2010) as well as Princeton researchers led by Stephen Lyon, professor of electrical engineering (Nature Materials, 2012). The development of highly pure forms of silicon, the same material used in today’s classical computers, has enabled researchers to control a quantum mechanical property known as “spin”. At Princeton, Lyon and his team demonstrated the control of spin in billions of electrons, a state known as coherence, for several seconds by using highly pure silicon-28.

Quantum-based technologies exploit the physical rules that govern very small particles — such as atoms and electrons — rather than the classical physics evident in everyday life. New technologies based on “spintronics” rather than electron charge, as is currently used, would be much more powerful than current technologies.

In quantum-based systems, the direction of the spin (either up or down) serves as the basic unit of information, which is analogous to the 0 or 1 bit in a classical computing system. Unlike our classical world, an electron spin can assume both a 0 and 1 at the same time, a feat called entanglement, which greatly enhances the ability to do computations.

A remaining challenge is to find ways to transmit quantum information over long distances. Petta is exploring how to do this with collaborator Andrew Houck, associate professor of electrical engineering at Princeton. Last fall in the journal Nature, the team published a study demonstrating the coupling of a spin qubit to a particle of light, known as a photon, which acts as a shuttle for the quantum information.

Yet another remaining hurdle is to scale up the number of qubits from a handful to hundreds, according to the researchers. Single quantum bits have been made using a variety of materials, including electronic and nuclear spins, as well as superconductors.

Some of the most exciting applications are in new sensing and imaging technologies rather than in computing, said Petta. “Most people agree that building a real quantum computer that can factor large numbers is still a long ways out,” he said. “However, there has been a change in the way we think about quantum mechanics – now we are thinking about quantum-enabled technologies, such as using a spin qubit as a sensitive magnetic field detector to probe biological systems.”

Read the abstract.

Awschalom D.D., Bassett L.C., Dzurak A.S., Hu E.L. & Petta J.R. (2013). Quantum spintronics: engineering and manipulating atom-like spins in semiconductors. Science 339 (6124) 1174-1179. PMID:

The research at Princeton University was supported by the Alfred P. Sloan Foundation, the David and Lucile Packard Foundation, US Army Research Office grant W911NF-08-1-0189, DARPA QuEST award HR0011-09-1-0007 and the US National Science Foundation through the Princeton Center for Complex Materials (DMR-0819860) and CAREER award DMR-0846341.

How do bacteria clog medical devices? Very quickly. (PNAS)

Posted on by Catherine Zandonella

stone-figure-2D_540A new study has examined how bacteria clog medical devices, and the result isn’t pretty. The microbes join to create slimy ribbons that tangle and trap other passing bacteria, creating a full blockage in a startlingly short period of time.

The finding could help shape strategies for preventing clogging of devices such as stents — which are implanted in the body to keep open blood vessels and passages — as well as water filters and other items that are susceptible to contamination. The research was published in Proceedings of the National Academy of Sciences.

stone-figure-2D_540
Click on the image to view movie. Over a period of about 40 hours, bacterial cells (green) flowed through a channel, forming a green biofilm on the walls. Over the next ten hours, researchers sent red bacterial cells through the channel. The red cells became stuck in the sticky biofilm and began to form thin red streamers. Once stuck, these streamers in turn trapped additional cells, leading to rapid clogging. (Image source: Knut Drescher)

Using time-lapse imaging, researchers at Princeton University monitored fluid flow in narrow tubes or pores similar to those used in water filters and medical devices. Unlike previous studies, the Princeton experiment more closely mimicked the natural features of the devices, using rough rather than smooth surfaces and pressure-driven fluid instead of non-moving fluid.

The team of biologists and engineers introduced a small number of bacteria known to be common contaminants of medical devices. Over a period of about 40 hours, the researchers observed that some of the microbes — dyed green for visibility — attached to the inner wall of the tube and began to multiply, eventually forming a slimy coating called a biofilm. These films consist of thousands of individual cells held together by a sort of biological glue.

Over the next several hours, the researchers sent additional microbes, dyed red, into the tube. These red cells became stuck to the biofilm-coated walls, where the force of the flowing liquid shaped the trapped cells into streamers that rippled in the liquid like flags rippling in a breeze. During this time, the fluid flow slowed only slightly.

At about 55 hours into the experiment, the biofilm streamers tangled with each other, forming a net-like barrier that trapped additional bacterial cells, creating a larger barrier which in turn ensnared more cells. Within an hour, the entire tube became blocked and the fluid flow stopped.

The study was conducted by lead author Knut Drescher with assistance from technician Yi Shen. Drescher is a postdoctoral research associate working with Bonnie Bassler, Princeton’s Squibb Professor in Molecular Biology and a Howard Hughes Medical Institute Investigator, and Howard Stone, Princeton’s Donald R. Dixon ’69 and Elizabeth W. Dixon Professor of Mechanical and Aerospace Engineering.

“For me the surprise was how quickly the biofilm streamers caused complete clogging,” said Stone. “There was no warning that something bad was about to happen.”

By constructing their own controlled environment, the researchers demonstrated that rough surfaces and pressure driven flow are characteristics of nature and need to be taken into account experimentally. The researchers used stents, soil-based filters and water filters to prove that the biofilm streams indeed form in real scenarios and likely explain why devices fail.

The work also allowed the researchers to explore which bacterial genes contribute to biofilm streamer formation. Previous studies, conducted under non-realistic conditions, identified several genes involved in formation of the biofilm streamers. The Princeton researchers found that some of those previously identified genes were not needed for biofilm streamer formation in the more realistic habitat.

Read the abstract.

Drescher, Knut, Yi Shen, Bonnie L. Bassler, and Howard A. Stone. 2013. Biofilm streamers cause catastrophic disruption of flow with consequences for environmental and medical systems. Proceedings of the National Academy of Sciences. Published online February 11.

This work was supported by the Howard Hughes Medical
Institute, National Institutes of Health grant 5R01GM065859, National Science Foundation (NSF) grant MCB-0343821, NSF grant MCB-1119232, and the Human Frontier Science Program.

Water filters made with copper could remove bacteria at lower cost (Journal of Applied Physics)

Posted on by Catherine Zandonella

Porous ceramic water filters are often coated with colloidal silver, which prevents the growth of microbes trapped in the micro- and nano-scale pores of the filter. Other metals such as copper and zinc have also been shown to exhibit anti-microbial activity. Princeton University’s Wole Soboyejo and colleagues used atomic force microscopy (AFM) measurements to study the adhesion interaction between Escherichia coli (E. coli) bacteria and colloidal silver, silver nanoparticles, and copper nanoparticles, as well as the interactions of the bacteria and the three different types of metal to porous clay-based ceramic surfaces.

As reported in the May 24, 2012 issue of Journal of Applied Physics, of the three antimicrobial metals studied the silver nanoparticles had the highest affinity for E. coli bacteria. The colloidal silver had the highest affinity for a porous ceramic surface and is therefore the least likely to leach into the filtrate. However, since the adhesion between colloidal silver and E. coli is in the same range as the adhesion between copper and the bacteria, copper may have potential as a less expensive disinfectant coating for ceramic water filters.

Source: American Institute of Physics

A Universal Method to Produce Low–Work Function Electrodes for Organic Electronics (Science)

Posted on by Catherine Zandonella

Read the abstract: A Universal Method to Produce Low–Work Function Electrodes for Organic Electronics. Zhou, Y et al. Science 20 April 2012: 327-332. Work by researchers at Georgia Institute of Technology in collaboration with Princeton University’s Antoine Kahn.