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How to spawn an “exceptional ring”

Researchers create exotic states that could lead to new kinds of sensors and optical devices.

David L. Chandler | MIT News Office
September 9, 2015

 A Diagram of Frequency vs. Momentum in Two Dimensions. Image courtesy of Bo Zhen.

The Dirac cone, named after British physicist Paul Dirac, started as a concept in particle and high-energy physics and has recently became important in research in condensed matter physics and material science. Since its realization in graphene, a two dimensional form of carbon whose discovery resulted in the 2010 Nobel Prize in physics, the Dirac cone has never ceased to be a source of amazing research results and crucial applications across various fields.

Now, physicists at MIT have found another unusual phenomenon produced by the Dirac cone — it can spawn a “ring of exceptional points.” This connects two fields of research in physics and may have applications in building powerful lasers, precise optical sensors and other devices.

The results are published this week in the journal Nature by MIT postdoc Bo Zhen, Yale University postdoc Chia Wei Hsu, MIT physics professors Marin Soljacic and John Joannopoulos, and five others.

This work represents “the first experimental demonstration of a ring of exceptional points,” Zhen says, “and it is the first study that relates research in exceptional points and parity-time symmetry to the field of Dirac cones.”

Individual exceptional points are a peculiar phenomenon unique to non-Hermitian systems, which can lead to counterintuitive phenomena. For example, around these points, systems containing more opaque materials may seem more transparent; light will be transmitted only in one direction but not the reversed direction; etc. However, the practicality of these fascinating demonstrations is limited by the material absorption loss introduced in this way of studying parity-time symmetric systems.

This new ring of exceptional points takes a completely different route, making them potentially more practical, the researchers say.

“Instead of absorption loss, we adopt a different loss mechanism – radiation loss – which does not affect the device performance,” Zhen says, “In fact, radiation loss is useful and is necessary in devices like lasers.” The team used a slab of nano-engineered material called a photonic crystal to produce the exceptional ring.

This phenomenon could enable creation of new kinds of optical systems with novel features due to the exotic properties of exceptional points.

“One important possible application of this work is in creating a more powerful laser system than existing technologies allow,” Soljacic says. To build a more powerful laser, it will naturally require a bigger lasing area. However, a bigger lasing area introduces more unwanted “modes” that compete with the current lasing mode for the input power, resulting in a limit of the final output power.

“Photonic crystal surface emitting lasers are a very promising candidate for the next generation of high-quality, high-power compact laser systems,” Soljacic says, “and we estimate we can improve the output power limit of such lasers by a factor of at least 10.”

“Our system could also be used for high-precision detectors for biological or chemical materials, because of its extreme sensitivity,” Hsu says. This improved sensitivity is due to another exotic property of the exceptional points: their response to perturbations is not linear with respect to the perturbation strength.

Normally, Hsu says, it becomes very difficult to detect a substance when its concentration is low. When the concentration of the target substance is reduced by a million times, the overall signal decreases by a million times, which can make it too small to detect. “But at an exceptional point, it’s not linear anymore,” Hsu says, “and the signal goes down by only a thousand times, providing a much bigger response that can now be detected.”

Demetrios Christodoulides, a professor of optics and photonics at the University of Central Florida who was not involved in this work, says, “This represents the first observation of an exceptional ring in a 2-D crystal associated with a two-dimensional band. The MIT work opens up a number of opportunities … in particular, around exceptional points where systems are known on many occasions to behave in a peculiar fashion.”

The research team also included Yuichi Igarashi of NEC Corp. in Japan and, MIT research scientist Ling Lu, postdoc Ido Kaminer, Harvard graduate student Adi Pick, and Song-Liang Chua at DSO National Laboratory in Singapore. The work was supported in part by the Army Research Office through MIT’s Institute for Soldier Nanotechnologies, the National Science Foundation, and the Department of Energy.

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A version of this article also appears on the MIT News website. Click here to go to that site.