Third-Harmonic Generation in Photonic Topological Metasurfaces

Abstract

We study nonlinear effects in two-dimensional photonic metasurfaces supporting topologicallyprotected helical edge states at the nanoscale. We observe strong third-harmonic generation mediated by optical nonlinearities boosted by multipolar Mie resonances of silicon nanoparticles. Variation of the pump-beam wavelength enables independent high-contrast imaging of either bulk modes or spin-momentum-locked edge states. We demonstrate topology-driven tunable localization of the generated harmonic fields and map the pseudospin-dependent unidirectional waveguiding of the edge states bypassing sharp corners. Our observations establish dielectric metasurfaces as a promising platform for the robust generation and transport of photons in topological photonic nanostructures.Topological photonics describes optical structures with the properties analogous to electronic topological insulators [1]. These systems are distinguished by bulk band gaps that host disorder-robust states localized at edges or interfaces and provide a novel approach for designing non-reciprocal or localized modes for optical isolators, photonic-crystal waveguides, and lasers. Since the original demonstration of backscattering-immune photonic topological edge states with the use of a gyrotropic microwave photonic crystal under a strong magnetic field [2], there has been a concerted effort towards realizing topological photonics at the nanoscale. Recently suggested optical designs compatible with non-magnetic alldielectric structures [3][4][5][6][7][8] are now emerging as a promising platform for quantum and nonlinear topological photonics [9][10][11][12].Stimulated by the progress in nanofabrication techniques, a new favourable ground for topological photonics based on dielectric nanoparticles with high refractive index has recently emerged [13,14]. Strong optical resonances and low Ohmic losses make it feasible for practical implementation of topological order for light at subwavelength scales. The underlying conceptual framework is to use arrays of meta-atoms with judiciously engineered shape and lattice structure, with topologically nontrivial features arising from pseudospin degrees of freedom. It bridges fundamental physics of topological phases with resonant nanophotonics and multipolar electrodynamics [15,16]. Topological metasurfaces could form a ground for a new class of ultra-thin devices with functionalities based on novel physical principles through engineering light-matter interactions in synthetic photonic potentials [17]. Their pseudospindependent physics may be useful for manipulation of internal degrees of freedom of light such as polarization and angular momentum.However, the experimental characterization of topological photonic structures becomes much more challenging at the nanoscale. Most implementations so far have been limited to indirect probing of topological states such as transmission spectra [11,[18][19][20], which cannot provide spatially-resolved information about the edge modes and suffer from input/output coupling losses. ...

Topological phases in ring resonators: recent progress and future prospects

“…Recently, several studies have proposed the use of topological modes supported by domain walls of topological photonic crystals as a means of constructing novel classes of ring resonators [143][144][145][146]. Light confinement is typically weaker than the standard approach based on integrated photonic ridge waveguides, meaning larger resonator sizes are required.…”

Near- and Far-Field Excitation of Topological Plasmonic Metasurfaces

“…A number of experimental works have investigated the directionality of these edge states under far-field excitation in a range of frequency regimes [17,21,22]. In each of these experiments, purely unidirectional modes are observed.…”

“…This result is true for any bosonic breathing honeycomb lattice [16], since it is rooted in time-reversal symmetry and the absence of Kramer's degeneracy for bosons as opposed to fermions. There have been a range of experimental investigations on the breathing honeycomb photonic crystal [17][18][19][20], including the observation of edge modes in the visible regime [21,22]. Despite this, comprehensive theoretical studies of the directional excitation of pseudospin edge modes in the breathing honeycomb lattice based on an analysis of their inhomogeneous electromagnetic angular momentum are scarce; in particular, modelling with a circularly polarised incident beam, as used in recent experiments [17,21,22] are needed.…”

Multipolar second-harmonic generation from high-Q quasi-BIC states in subwavelength resonators

“…Due to the strong near-field confinement and tailorable field distributions in the subwavelength regime, optically resonant dielectric nanostructures offer powerful tools to facilitate various nonlinear processes including nonlinear frequency conversion, wave mixing, and ultrafast all-optical switching [4,[6][7][8][9][10][11]. Resonant mechanisms of light localization in dielectric nanostructures, such as magnetic dipole resonance [12][13][14], nonradiating anapole state [15][16][17], magnetic Fano resonance [18,19], and topologically protected edge states [20,21], have been widely utilized for applications of nonlinear nanophotonics, such as nanoscale light sources [22], imaging [23], sensing, and advanced optoelectronic devices [3].…”