Enhanced sensitivity at higher-order exceptional points

Abstract

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.

PT symmetry breaking in multilayers with resonant loss and gain locks light propagation direction

“…Transmissions and reflections and corresponding propagation directions are indicated for the waves incident from left-hand (T LG and R LG ) and right-hand (T GL and R GL ) sides. [24,25]. Third, PT symmetry breaking plays an important role in laser physics, offering new types of lasers [26][27][28] and antilasers [29] based on the effect of coherent perfect absorption [30,31].…”

Electromagnetic Impurity-Immunity Induced by Parity-Time Symmetry

“…In this sense, the impurity-immunity phenomenon is associated with the coalescence of EPs. The coalescence and high order of EPs [63][64][65][66] are interesting phenomena with many important applications such as enhanced sensitivity. Here, we have demonstrated another interesting consequence of the coalescence of EP in the transmission behavior of a PT-symmetric system with ENZ medium.…”

Observation of bulk Fermi arc and polarization half charge from paired exceptional points

“…One such example is the emergence of a new class of degeneracies, commonly referred to as exceptional points (EPs), where two or more resonances of a system coalesce in both eigenvalues and eigenfunctions [26][27][28]. So far, isolated EPs in parameter space [29][30][31][32][33][34][35] and continuous rings of EPs in momentum space [36][37][38] have been studied across different wave systems due to their intriguing properties, such as unconventional transmission/reflection [39][40][41], relations to parity-time symmetry [42][43][44][45][46][47][48], as well as their unique applications in sensing [49,50] and single-mode lasing [51][52][53].Here, we theoretically design and experimentally realize a new configuration of isolated EP pairs in momentum space, which allows us to reveal the unique topological signatures of EPs in the band structure and far-field polarization, and to extend topological band theory into the realm of non-Hermitian systems. Specifically, we demonstrate that a Dirac point (DP) with nontrivial Berry phase can split into a pair of EPs [54][55][56] when radiation loss-a form of non-Hermiticity-is added to a 2D-periodic photonic crystal (PhC) structure.…”