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Exceptional points make for exceptional sensors

Research by Professors Mercedeh Khajavikhan and Demetri Christodoulides and graduate student Hossein Hodaei on enhanced sensitivity at exceptional points, which was recently published in Nature (Nature 548, 187, 2017), has generated great interest in the scientific community. An article in News and Views "Applied physics: Optical sensing gets exceptional" presented this research, and another published in the October 1 issue of Physics Today entitled “Exceptional points make for exceptional sensors” highlighted this new discovery. Johana Miller writes in Physics Today: “For at least half a century, exceptional points have been studied as mathematical curiosities. Then, in 2014, Jan Wiersig of the University of Magdeburg in Germany proposed that [this] could be put to good use for ultrasensitive measurements of weak signals. Now two groups have created the first proof-of-principle exceptional-point sensors.”  Khajavikhan and her colleagues used two and three coupled resonators in parity-time symmetric configurations to create second- and third-order exceptional points. They found, as expected, that the frequency splitting around the degeneracy scales with the square root and cube root of the perturbation magnitude, which opens the door to even higher detection sensitivity.

Excerpts from the Nature paper  

Non-Hermitian degeneracies, also known as exceptional points, have recently emerged as a new way to engineer the response of open physical systems, that is, those that interact with the environment. They correspond to points in parameter space at which the eigenvalues of the underlying system and the corresponding eigenvectors simultaneously coalesce. In optics, the abrupt nature of the phase transitions that are encountered around exceptional points has been shown to lead to many intriguing phenomena, such as loss-induced transparency, unidirectional invisibility, band merging, topological chirality and laser mode selectivity. Recently, it has been shown that the bifurcation properties of second-order non-Hermitian degeneracies can provide a means of enhancing the sensitivity (frequency shifts) of resonant optical structures to external perturbations. Of particular interest is the use of even higher-order exceptional points (greater than second order), which in principle could further amplify the effect of perturbations, leading to even greater sensitivity. Although a growing number of theoretical studies have been devoted to such higher-order degeneracies, their experimental demonstration in the optical domain has so far remained elusive. Here we report the observation of higher-order exceptional points in a coupled cavity arrangement—specifically, a ternary, parity–time-symmetric photonic laser molecule—with a carefully tailored gain–loss distribution. We study the system in the spectral domain and find that the frequency response associated with this system follows a cube-root dependence on induced perturbations in the refractive index. Our work paves the way for utilizing non-Hermitian degeneracies in fields including photonics, optomechanics, microwaves and atomic physics.

Posted Friday, October 6, 2017

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