Strange physics turns off laser

June 17, 2014, Princeton University
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. Credit: Vienna University of Technology

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 Communications1.

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.

An electron microscope image shows two lasers placed just two microns apart from each other. Credit: Vienna University of Technology
"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 2, 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 . 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 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 .

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.

Explore further: Chemical sensor on a chip

More information: 1. Nature Communications 13 June 2014. DOI: 10.1038/ncomms5034

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

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

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4.5 / 5 (2) Jun 17, 2014
I like how this "related story" from 2012 was linked:
5 / 5 (1) Jun 17, 2014
I think it is cool that what they predicted two years ago ended up being proven with experimental data. Science at its finest.
1 / 5 (5) Jun 17, 2014
I think it is cool that what they predicted two years ago ended up being proven with experimental data. Science at its finest.
Actually this finding has been made accidentally - after all, in the same way, as many similar counterintuitive findings ("When we saw that the two lasers can switch each other off completely, due to the coupling between them, we knew: either we made a mistake or this is a spectacular result"). Yes, this is really the science at its finest, but the naive kids who believe in power of theory will never learn from its history. Many findings are made just because the scientists do realize, they actually don't fit the theory. Just this type of unexpected findings introduces the actual breakthroughs. The confirming of theory is nice and all, but as Feynman has said: "The actual research is, if you don't know what you're actually doing. All the rest is just a stamp collection".
1 / 5 (3) Jun 17, 2014
The fact, that such opinion gets immediately downvoted at public forum just indicates, the mediocre attitude based on paradigm of incremental progress based on confirmation of hypothesis is firmly rooted in the contemporary science. Such a psychology simply ignores if not dismisses every finding, which doesn't fit the established theory. Whereas in healthy science just these findings should get the highest attention.

Why this paradigm is so firmly rooted? Well, because it plays well with lobby of theorists and high school teachers who just based their social credit on building and teaching of theories and because this way of research enables to prolong it arbitrarily. The new findings are accepted just only when all other options for their explanation with existing theories are depleted. The scientists have absolutely no problem with such attitude, until their money are going: once the problem is finally solved, then the whole research ends.
5 / 5 (1) Jun 17, 2014
Jantoo, you are stretching it a little bit. If you see there is a ton of experimental research done both at institutions and private entities, far more than theory and everybody understands that reality. Theoretical understanding extends and forecasts and helps understand complex situations put together by multiple simple situations.

Yes, some direction to experiments come from theoretical backing which may or may be the best way. However, the flip side is not good either.

I do totally agree with the schools lack of experimental approach to teaching.

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