Radiationless optical modes in metasurfaces: recent progress and applications

Abstract

Non-radiative optical modes attracted enormous attention in optics due to strong light confinement and giant Q-factor at its spectral position. The destructive interference of multipoles leads to zero net-radiation and strong field trapping. Such radiationless states disappear in the far-field, localize enhanced near-field and can be excited in nano-structures. On the other hand, the optical modes turn out to be completely confined due to no losses at discrete point in the radiation continuum, such states result in infinite Q-factor and lifetime. The radiationless states provide a suitable platform for enhanced light matter interaction, lasing, and boost nonlinear processes at the state regime. These modes are widely investigated in different material configurations for various applications in both linear and nonlinear metasurfaces which are briefly discussed in this review.

Fig. 1. Recent advances in BIC and Anapole states in TMDs, all-dielectric, plasmonic and hybrid metasurfaces.

BIC in TMDs metasurface. Reproduced with permissions. Copyright © 2021, American Chemical Society. K-space emission and lasing from BIC hosted dielectric structure. Reproduced with permissions. Copyright © 2018, Springer Nature Limited. Near field intensity in plasmonic structure based BIC. Reproduced with permissions. Copyright © 2020, American Chemical Society. Anapole mode in TMDs. Reproduced with permissions. Copyright © 2019, Springer Nature Limited. Plasmonic based metasurface. Reproduced with permissions. Copyright © 2018, American Chemical Society. Dielectric metasurface. Reproduced with permissions. Copyright © 2018, American Chemical Society. Second harmonic generation from WS₂ coupled to dielectric (hybrid) q-BIC structure. Adapted with permissions. Copyright © 2020, American Chemical Society

Fig. 2. BIC in few-layers TMD based metasurface and dielectric-metal-dielectric stacked structure.

a Schematic of TMD based metasurface. b Magnetic field profiles of q-BIC state at corresponding wavelengths, c Transmission spectra at different symmetry breaking cuts. Reproduced with permissions. Copyright © 2021, American Chemical Society. d, e Reflection spectra for dimer height h as a function of wavelengths under x- and y-polarized normal incidence. The formation of BIC is highlighted by a circle in two indicated branches of states. f The q-factor a function of dimer height h for the quasi-BIC mode with different incorporated losses. Adapted with permissions. Copyright © 2020, American Chemical Society

Fig. 3. The mechanism of anapole in dielectric metasurface and demonstration in TMD metasurface.

a Illustration of anapole excitation (toroidal dipole linked with circulating magnetic field accompanied by electric dipole). The identical radiation patterns of toroidal and electric dipoles can destructive interference results in total scattering cancellation. Reproduced with permissions. Copyright © 2015, Springer Nature Limited. b Experimental observation of anapole mode in WS₂ resonator of different diameters. c, d Experimental and theoretical normalized scattering spectra, e Normalized dark-field scattering map for WS₂ nanodisks (experimental) of disk radius vs wavelength. The spectra show a linear dispersion attributed to anapole mode as indicated by green dotted line. f The electric field distribution profile shows an anapole-like feature, calculated theoretically and normalized to the free space. Reproduced with permissions. Copyright © 2019, Springer Nature Limited

Fig. 4. THG from anapole hosted plasmonic and dielectric metasurfaces.

a–i Illustration of current excited in meta-atom on the top of perfect electric conductor substrate ii, iii. Anapole current and field distribution, this work facilitate boosting nonlinear efficiency. Reproduced with permissions. Copyright © 2018, Springer Nature Limited b–i. Direction of toroidal dipole moment and formation of charge current configuration in two resonators in plasmonic nonlinear metasurface ii–iii. Top view and side view, and iv. SEM image of metasurface. Reproduced with permissions. Copyright © 2019, American Chemical Society

Fig. 5. BIC in perovskite and TMD based metasurfaces.

a Spin valley locked emission in perovskite metasurface. Reproduced with permissions. Copyright © 2023, Springer Nature Limited. b A monolayer TMD has been coupled with Bragg gratings to achieve 100 meV photonic bandgap and Rabi splitting of 70 meV in BIC inspired structure. Reproduced with permissions. Copyright © 2023, Springer Nature Limited. c Two beam pumping experiment for ultra-fast control of the quasi-BIC micro-laser. The insets are far-field emission pattern under symmetric and asymmetric excitation from the metasurface. With one applied beam the structure sustain the symmetry and result in a donut shaped and the normalized ratio is approximately zero. Applying the second beam reduce the symmetry. The metasurface switch vortex lasing to linearly polarized beam and back to vortex with switching time of 1.5 p.s. Reproduced with permissions. Copyright © 2020, The American Association for the Advancement of Scienc

Fig. 6. Nonlinear efficiencies from BIC-hosted metasurfaces.

a–i Schematic of BIC hosting metasurface generating third to eleventh harmonics. ii Transmission spectra with respect to pump wavelength. iii Top view of unit cells with broken symmetry, and iv. Q-factor as a function of asymmetry parameter values. Quasi-BIC appears with broken symmetry and high harmonic generation related to make best use of the electric field in the dielectric resonators. For resonant metasurface the nearfield enhancement is the interplay of non-radiative and radiative losses. The field enhancement is not maximum when the radiative losses are proportional to the coupling strength between mode and pump where the structure losses more energy. The field enhancement is maximum in critical coupling regime when non-radiative and radiative losses are comparable. Reproduced with permissions. Copyright © 2022, American Chemical Society. b THG reported from a BIC hosting metasurface and its control via asymmetry parameters. In this work it is shown that how the non-radiative and radiative losses control the THG intensity. The structure configurations have been tuned to critical coupling regime to exploit the maximum nonlinear frequency conversion efficiency. Reproduced with permissions. Copyright © 2019, American Chemical Society