Anomalies in light scattering

Abstract

Scattering of electromagnetic waves lies at the heart of most experimental techniques over nearly the entire electromagnetic spectrum, ranging from radio waves to optics and X-rays. Hence, deep insight into the basics of scattering theory and understanding the peculiar features of electromagnetic scattering is necessary for the correct interpretation of experimental data and an understanding of the underlying physics. Recently, a broad spectrum of exotic scattering phenomena attainable in suitably engineered structures has been predicted and demonstrated.Examples include bound states in the continuum, exceptional points in -symmetrical non-Hermitian systems, coherent perfect absorption, virtual perfect absorption, nontrivial lasing, nonradiating sources and cloaking, and others. In this paper, we establish a unified description of such exotic scattering phenomena and show that the origin of all these effects can be traced back to the properties of the underlying scattering matrix. two or three dimensions. Maxwell's equations describing spatial and temporal evolution of electric ( , ) t Er and magnetic ( , ) t Hr fields for an arbitrary system are

Coherent Perfect Diffraction in Metagratings

“…[ 24–30 ] The idea can be traced back to the concept of coherent perfect absorption (CPA), which refers to an optical device that can absorb all incident energy provided that it corresponds to a scattering matrix eigenstate with zero eigenvalue. [ 31 ] In a two‐port system, this corresponds to an excitation with two counter‐propagating coherent beams with proper amplitude and relative phase, which interfere constructively within the structure. [ 24 ] Based on this concept, coherent wave manipulation has been proposed in various systems, finding potential applications even down to the single‐photon level.…”

Gain-induced scattering anomalies of diffractive metasurfaces

“…For better understanding of the anomalies reported here, it is useful to place them among the known scattering phenomena. Krasnok et al [14] have recently provided a thorough classification of diverse scattering anomalies on the basis of zeros and poles in the S-matrix Hamiltonians and scattering coefficients, using the notion of complex eigenfrequencies rather than complex eigenpolarizabilities. The divergence of scattering coefficients presented in our work corresponds exactly to the scenario in which a pole in the scattering coefficient intersects with the real frequency axis due to the balance between the gain of the system's eigenmode and its outcoupling loss.…”

“…For instance, properly engineered gain and loss could overcome efficiency barriers of metasurfaces for wavefront transformation imposed by impedance mismatch [8,9], mitigate constraints on electric and magnetic optical response of matter [10,11], and control the scattering by small ensembles of nano-objects [12,13]. The interplay between gain and loss gives rise to many scattering anomalies [14], such as unidirectional invisibility, coherent perfect absorber-lasers, as well as manifestations of parity-time (PT) symmetry [15][16][17], offering a platform for active control of light propagation [18][19][20] and for enhanced light-matter interactions [21][22][23].…”

High-order exceptional points in supersymmetric arrays

“…The exceptional point (EP) in a non-Hermitian system occurs when eigenstates coalesce [1][2][3], and usually associates with the non-Hermitian phase transition [4,5]. In a parity-time (PT ) symmetric non-Hermitian coupled system, the PT symmetry of eigenstates spontaneously breaks at the EP [6][7][8][9][10][11][12][13][14][15][16], which determines the exact PTsymmetric phase and the broken PT -symmetric phase in this system.…”