Abstract
Abstract Photonic nanostructures with gain and loss have long been of interest in the context of diverse scattering anomalies and light-shaping phenomena. Here, we investigate the scattering coefficients of simple gain-doped diffractive metasurfaces, revealing pairs of scattering anomalies surrounded by phase vortices in frequency–momentum space. These result from an interplay between resonant gain, radiative loss, and interference effects in the vicinity of Rayleigh anomalies. We find similar vortices and singular points of giant amplification in angle-resolved reflectivity spectra of prism-coupled gain slabs. Our findings could be of interest for gain-induced wavefront shaping by all-dielectric metasurfaces, possibly employing gain coefficients as low as ∼50 cm−1.
Highlights
Electromagnetic metasurfaces are two-dimensional (2D) arrays of scatterers used to control amplitude, phase, and polarization of reflected, transmitted, and diffracted electromagnetic waves [1,2,3,4]
Extensive research has been devoted to combining metasurfaces with gain media, with the main focus on distributed feedback lasing and diffractive outcoupling in plasmonic and dielectric nanoparticle arrays [5]
We theoretically demonstrate the existence of scattering anomalies embedded in the band structure of simple diffractive arrays of identical dielectric nanoparticles (Figure 1, left) that are intrinsically weakly scattering, but imbued with a weak frequency-dependent gain
Summary
Electromagnetic metasurfaces are two-dimensional (2D) arrays of scatterers used to control amplitude, phase, and polarization of reflected, transmitted, and diffracted electromagnetic waves [1,2,3,4]. We theoretically demonstrate the existence of scattering anomalies embedded in the band structure of simple diffractive arrays of identical dielectric nanoparticles (Figure 1, left) that are intrinsically weakly scattering, but imbued with a weak frequency-dependent gain We study this system using a Green function method [24,25,26,27,28], which includes the coherent retarded electrodynamic coupling between all particles, and radiative damping as the essential ingredient. Each particle is a weak scatterer with response dominated by radiative loss, and their optical response is significantly modified in arrays, where the interplay of gain, loss, and interference gives rise to scattering anomalies These anomalies resemble sharp resonances associated with lasing thresholds, studied over the past decades in various gain-doped dielectric structures [31,32,33,34,35,36,37,38,39,40,41]. The scattering of light is determined by the nanoparticle’s dynamic polarizability αdyn, which is obtained from αstat by adding radiation loss [10, 43, 44]: α−dy1n α−st1at
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