A schematic drawing of how a ring of exceptional points (shown in white) can be spawned from a Dirac point (a dot), and thus change the dispersion from the normal, widely known conical shape into an exotic lantern-like shape Credit: Courtesy of the researchers
Researchers create exotic states leading to new sensors and optical devices. The Dirac cone, named after Paul Dirac, started as a concept in particle and high-energy physics and has recently became important in condensed matter physics and material science. It describes aspects of graphene, suggesting the possibility of applications across various fields. Now physicists have found another 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.
This is the 1st study that relates research in exceptional points with the physical concepts of parity-time symmetry and Dirac cones. Individual exceptional points are a peculiar phenomenon unique to an unusual class of physical systems that can lead to counterintuitive phenomena. eg around these points, opaque materials may seem more transparent, and light may be transmitted only in one direction. However, the practical usefulness of these properties is limited by absorption loss introduced in the materials.
The MIT team used a photonic crystal to produce the exceptional ring. “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.” This phenomenon could enable creation of new kinds of optical systems with novel features.
“One important possible application of this work is in creating a more powerful laser system than existing technologies allow,” Soljači? says. To build a more powerful laser requires a bigger lasing area, but that introduces more unwanted “modes” for light, which compete for power, limiting the final output. “Photonic crystal surface-emitting lasers are a very promising candidate for the next generation of high-quality, high-power compact laser systems,” Soljači 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 to the perturbation strength. Normally when concentration of a substance is reduced by a million times, signal also 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 1,000 times, providing a much bigger response that can now be detected.” http://news.mit.edu/2015/exceptional-ring-sensors-optical-devices-0909
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