Non-Hermitian and topological photonics: optics at an exceptional point

Abstract

In the past few years, concepts from non-Hermitian (NH) physics, originally developed within the context of quantum field theories, have been successfully deployed over a wide range of physical settings where wave dynamics are known to play a key role. In optics, a special class of NH Hamiltonians – which respects parity-time symmetry – has been intensely pursued along several fronts. What makes this family of systems so intriguing is the prospect of phase transitions and NH singularities that can in turn lead to a plethora of counterintuitive phenomena. Quite recently, these ideas have permeated several other fields of science and technology in a quest to achieve new behaviors and functionalities in nonconservative environments that would have otherwise been impossible in standard Hermitian arrangements. Here, we provide an overview of recent advancements in these emerging fields, with emphasis on photonic NH platforms, exceptional point dynamics, and the very promising interplay between non-Hermiticity and topological physics.

Highlighting photonics: looking into the next decade

“…Subsequently, many surprising experiments and fundamental findings blossomed in the study of non-Hermitian photonic systems, including for example PT-symmetric microring lasers and whisperinggallery microcavities [75][76][77], and the use of the extreme sensitivity near exceptional points for sensing [78][79][80]. Ever since these important experimental demonstrations, non-Hermitian physics has turned into one of the fast growing and important research frontiers in the field of photonics and beyond [81][82][83][84].…”

Enhanced avionic sensing based on Wigner’s cusp anomalies

“…The latter is an important metric in the characteristics of the sensor, and it is defined as the ratio between the maximum and the minimum acceleration that an accelerometer can measure. A prominent example of such sensing protocols is resonant systems operating near Nth-order exceptional point degeneracies (EPDs) (8,9), where a small perturbation  ≪ 1 activates an inherent sublinear response ∼ N √ ─  ≫  in resonant splitting (10)(11)(12)(13). Its implementation using microresonators (14,15) led to the realization of a class of EPD-based avionic devices such as EPD gyroscopes (16,17).…”

Electrically Pumped Topological Insulator Lasers

“…In this respect, photonics has made it possible to study some of the intriguing aspects of topological physics by providing access to synthetic dimensions [20][21][22][23] and higher order topological insulators [24][25][26] . Also, photonics allows to study topological behaviors arising due to non-Hermiticity (primarily gain) and nonlinearity that have no immediate counterparts in condensed matter [27][28][29][30][31][32][33] . In return, topological physics has inspired unique light transport schemes that may have important ramifications in integrated photonics [34][35] , quantum optics [36][37] , and laser science [38][39][40][41][42][43][44][45][46][47][48][49][50] .…”