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In this paper, we theoretically and experimentally demonstrated photothermal nonlinearities of both forward and backward scattering intensities from quasi-perfect absorbing silicon-based metasurface with only λ/7 thickness. The metasurface is efficiently heated up by photothermal effect under laser irradiation, which in turn modulates the scattering spectra via thermo-optical effect. Under a few milliwatt continuous-wave excitation at the resonance wavelength of the metasurface, backward scattering cross-section doubles, and forward scattering cross-section reduces to half. Our study opens up the all-optical dynamical control of the scattering directionality, which would be applicable to silicon photonic devices.

Perfect absorbers based on all-dielectric metasurfaces exhibit great potential in photodetection, photovoltaics, and imaging applications. This study proposes and demonstrates an all-dielectric broadband absorber comprising subwavelength-thick nanopillar Mie resonators in the visible light range. This nanopillar functions as a perfect absorber based on degenerate critical coupling with a characteristic “degenerate critical length.” At this length, the nanopillars are capable of achieving perfect absorption. Beyond this length, the peak of perfect absorption is not affected with further increases in the length of the nanopillars. Hence, this study realizes broadband absorption via the stacking of amorphous silicon and germanium nanopillars with the same width at different peak absorption wavelengths. The absorption spectra are almost independent of the order of the stacked structures; hence, the stacked nanopillars in the specific stacking order can behave as a vertical photon sorter, sorting photons based on the wavelength. This study provides a systematic route to the realization of broadband absorbers with vertical photon sorting capability via the vertical stacking of nanopillars.

Benefited from the well-known Mie resonance, a plethora of physical phenomena and applications are attracting attention in current research on dielectric-based nanophotonics. High-index dielectric metastructures are favorable to enhance light-matter interaction in nanoscale with advantages such as low loss, optical magnetism, and multipolar responses, which are superior to their plasmonic counterpart. In this review, we highlight the important role played by Mie resonance-based multipolar and multimodal interaction in nanophotonics, introducing the concept of “multipole and multimode engineering” in artificially engineered dielectric-based metastructures and providing an overview of the recent progress of this fast-developing area. The scope of multipole and multimode engineering is restricted not only in multipolar interferences of meta-atom and meta-molecule but also in the nontrivial intermodal coupling (Fano resonance and bound states in the continuum), in the collective mode and the surface lattice mode appearing via periodic meta-lattices and aperiodic meta-assembly, in chiral enhancement via chiral and achiral dielectric metastructures, and in Mie resonance-mediated hybrid structures (Mie-plasmon and Mie-exciton). Detailed examples and the underlying physics of this area are discussed in-depth, in order to lead the multifunctional metastructures for novel applications in the future.

To overcome the absorption limit of an ultra-thin layer, all-dielectric perfect absorbers of dipole modes can be realized based on degenerate critical coupling. Here, we propose theoretically a cross-shaped silicon perfect absorber of quadrupole modes in the visible region and demonstrate that the degenerate critical coupling applies to higher-order modes.

We design a wideband all-dielectric perfect absorber of nanopillar Mie resonators based on degenerate critical coupling. In addition, the nanopillar perfect absorber is found to have a characteristic “degenerate critical length” beyond which the absorption peak is almost unaffected by increasing length. Based on the existence of the degenerate critical length, we develop a broadband dielectric quasi-perfect absorber by stacking nanopillar Mie resonators of different materials that selectively absorb incident light like photon sorters. The 1300 nm-thick absorber achieves an average absorptivity of above 94% from 300 to 1000 nm and has great potential for photodetection and photovoltaic applications.

Metasurfaces have attracted widespread interest owing to their ability to control light at the nanoscale level. However, the optical response of dipole mode-based metasurfaces is sensitive to changes in the resonator period and the light incidence angle; thus, the device performance typically degrades in practical applications owing to the presence of non-normal incident light. Here, we study cross-shaped Mie resonators based on quadrupole modes, whose optical response is almost independent of the variations in period and incidence angle. Based on this property of quadrupole modes, we propose a Huygens’ metasurface with stable transmissivity at different incident angles.

Perfect absorbers based on metasurfaces are extensively studied owing to their engineered optical properties.

supplementary materials

Silicon metasurfaces have been attracting interest in the sensing field because of their ability to support magnetic Mie resonance, low optical heating, and CMOS-compatible fabrication processes. Herein, we demonstrate that the sensitivity of the magnetic dipole (MD) mode for nanodisk Mie resonators (as high as 385 nm/RIU) is similar to the sensitivity of plasmonic metasurfaces and greater than that of the electric dipole (ED) mode of nanodisk Mie resonators. We also engineer the thickness of Mie resonators to achieve an MD-mode linewidth as small as 0.56 nm and a figure of merit greater than 160 RIU−1. The measured sensitivity of the MD mode is more accurate than that of the ED mode, which is more prone than the MD mode to measurement errors arising from the partial filling of the liquid. Our study paves the way for the development of MD-mode-based dielectric biosensors for use in personal healthcare and medical diagnosis.

In principle, the absorbance of a free-standing ultra-thin film is limited to 50%. To overcome this limitation, an all-dielectric perfect absorber is proposed herein based on the concept of degenerate critical coupling (DCC) of quadrupole modes. We study the absorbance of a dielectric elliptic cylinder and find that perfect absorption can be achieved by spectrally overlapping peaks of electric and magnetic quadrupole modes. This suggests that the DCC method can be extended to the quadrupole modes beyond dipole modes. Such an all-dielectric perfect absorber can be used in photodetectors, optical filters, and optical modulators mediated by the photothermal effect.

Fano resonances are produced by integrating an electric split ring resonator (eSRR) and an I shaped resonator (ISR) into a composite metasurface. By adjusting the length of the ISR the degree of asymmetry in the line shape can be controlled and even made to be symmetric. Metasurfaces of the composite structures are fabricated, and their transmission is measured with terahertz time-domain spectroscopy. The measurements show good agreement with finite difference time domain (FDTD) simulations.

The absorbance of a free-standing ultrathin layer is limited to 50%; we overcome this limitation by numerically investigating a wavelength-selective perfect absorber based on Mie resonance and degenerate critical coupling. We extend the wavelength of close-to-unity absorbance to the entire visible region by controlling the radiative loss and intrinsic loss. Radiative loss can be controlled by embedding the Mie resonator into a thin film with the defined refractive index. Meanwhile, intrinsic loss can be controlled by addition of a dielectric cap with a higher extinction coefficient on the Mie resonator. Such all-dielectric perfect absorbers can be applied to efficient photodetectors, imaging sensor pixels, or all-optical switching devices mediated by the photothermal effect.

Fano resonances are observed in a composite metamaterial that consists of an electric split ring resonator eSRR and an I-shaped resonator ISR. By adjusting the length of the ISR the degree of asymmetry in the line shape of the composite metamaterial can be controlled and even made to be symmetric. In contrast to other methods to create Fano resonances, the individual modes of the eSRR and ISR have the same symmetry and are not evanescently coupled to each other. The transmission is simulated using the finite difference time domain method and a coupled oscillator model is used to obtain nominal values of the Fano asymmetry factor q. Composite metamaterials and individual eSRR and ISR metamaterials are fabricated, and their transmission is measured with terahertz time-domain spectroscopy.