Enhanced sensitivity at higher-order exceptional points

The response of a ternary, parity–time-symmetric system that exhibits a third-order exceptional point increases as a function of the cube-root of induced perturbations. Exceptional points, exceptional optics Recent insights into open (non-Hermitian) physical systems have led to a new range of optical systems in which, counter-intuitively, loss is introduced. By careful tuning of loss and gain, certain degeneracies called 'exceptional points' emerge, which have intriguing properties that can be harnessed, for example, in new types of lasers, one-way optical waveguides and topological effects. Two papers in this issue demonstrate the high sensitivity of such non-Hermitian degeneracies to external perturbations, which can be used for precision sensing and detection. Weijian Chen et al . report sensing of nanoparticles with exceptional points generated in a silicon dioxide micro-toroid resonator. Hossein Hodaei et al . generated a higher-order exceptional point by coupling three micro-rings made from a semiconductor laser material. This third-order exceptional point has an even higher, cube-root (rather than square-root) dependence on perturbations. The two papers together provide a new route to ultraprecise chip-based sensing systems. 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 1 , 2 , 3 . 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 4 , unidirectional invisibility 5 , 6 , band merging 7 , 8 , topological chirality 9 , 10 and laser mode selectivity 11 , 12 . 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 13 . 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 14 , 15 , 16 , their exp