Non-Hermitian Sensing in Photonics and Electronics: A Review

Abstract

Recently, non-Hermitian Hamiltonians have gained a lot of interest, especially in optics and electronics. In particular, the existence of real eigenvalues of non-Hermitian systems has opened a wide set of possibilities, especially, but not only, for sensing applications, exploiting the physics of exceptional points. In particular, the square root dependence of the eigenvalue splitting on different design parameters, exhibited by 2 × 2 non-Hermitian Hamiltonian matrices at the exceptional point, paved the way to the integration of high-performance sensors. The square root dependence of the eigenfrequencies on the design parameters is the reason for a theoretically infinite sensitivity in the proximity of the exceptional point. Recently, higher-order exceptional points have demonstrated the possibility of achieving the nth root dependence of the eigenfrequency splitting on perturbations. However, the exceptional sensitivity to external parameters is, at the same time, the major drawback of non-Hermitian configurations, leading to the high influence of noise. In this review, the basic principles of PT-symmetric and anti-PT-symmetric Hamiltonians will be shown, both in photonics and in electronics. The influence of noise on non-Hermitian configurations will be investigated and the newest solutions to overcome these problems will be illustrated. Finally, an overview of the newest outstanding results in sensing applications of non-Hermitian photonics and electronics will be provided.

Uncovering exceptional contours in non-Hermitian hyperbolic lattices

“…Given the recent experimental realisation of hyperbolic lattices [8,9] using circuit quantum electrodynamics and the simulation of non-Hermitian phases using optical and electrical circuit elements [28][29][30], engineering non-Hermitian lattices in hyperbolic geometry is an experimentally viable endeavour. The abundance of EPs can have potential applications in designing non-Hermitian sensors operating on the scaling of eigenvalues near EPs [30][31][32]. We hope our work motivates further theoretical and experimental explorations of non-Hermitian hyperbolic matter.…”

Hall conductance of a non-Hermitian Weyl semimetal

“…Recently, NH topological phases have drawn considerable attention of the research community ranging from photonics to condensed matter physics [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45]. Dissipation in both classical and quantum mechanical systems is quite common, and this may lead to NH loss and gain.…”

Parity-time symmetry transition and exceptional points in terahertz metal–graphene hybrid metasurface with switchable transmission and reflection characteristics

“…10 More strikingly, an exceptional point (EP) was generated during the PT symmetry phase transition, whereby the eigenvalues and eigenstates degenerate simultaneously, thus resulting in some peculiar physical phenomena, such as unidirectional reflectionless, [11][12][13] asymmetric diffraction, 14 vortex generation, 15 polarization state transition, [16][17][18] chiral mode switching, 19 and sensing enhancement. [20][21][22] With thinner thickness, lower loss, and more flexible modulation than three-dimensional metamaterials, metasurfaces have become a favored platform for the study of PT symmetry by a wide range of researchers. Lawrence et al first observed PT symmetry breaking and EP in polarization space in a terahertz transmission metasurface composed of two resonant rings with mutually orthogonal openings.…”

“…10 More strikingly, an exceptional point (EP) was generated during the PT symmetry phase transition, whereby the eigenvalues and eigenstates degenerate simultaneously, thus resulting in some peculiar physical phenomena, such as unidirectional reflectionless, 11–13 asymmetric diffraction, 14 vortex generation, 15 polarization state transition, 16–18 chiral mode switching, 19 and sensing enhancement. 20–22…”