High-index dielectric nanoparticles supporting a distinct series of Mie resonances have enabled a new class of optical antennas with unprecedented functionalities. The great wealth of multipolar responses has not only brought in new physical insight but also spurred practical applications. However, how to make such a colorful resonance palette actively tunable is still elusive. Here, we demonstrate that the structured phase-change alloy Ge2Sb2Te5 (GST) can support a diverse set of multipolar Mie resonances with active tunability. By harnessing the dramatic optical contrast of GST, we realize broadband (del lambda/lambda similar to 15%) mode shifting between an electric dipole resonance and an anapole state. Active control of higher-order anapoles and multimodal tuning are also investigated, which make the structured GST serve as a multispectral optical switch with high extinction contrasts (>6 dB). With all these findings, our study provides a new direction for realizing active nanophotonic devices.

We report on experimental implementation of a ferroelectric metasurface using an x-cut KTiOPO4 (KTP) crystal for efficient manipulation of terahertz (THz) radiation. Based on the multipolar resonances that are accommodated in KTP micro-blocks in a square array, the metasurface is fabricated by precision diamond-blade dicing. Adjusting the size of the KTP micro-blocks to tailor the relative spectral positions of the anisotropic multipolar resonances, we demonstrate a subwavelength-thin THz polarizer that functions as a transparent film in the y-direction and a magnetic mirror in the z-direction with a transmission contrast of 13 dB near 0.37 THz (820 mu m). The ferroelectric-based all-dielectric metasurface will provide a versatile platform to engineer the THz waves in the far field and could potentially be combined with THz generation in the same material.

We propose ferroelectric metasurfaces based on KTiOPO4 and demonstrate the fabrication procedures. It will provide a versatile platform for THz wave manipulation and open up new possibilities to monolithically combine it with THz generation.

We propose ferroelectric metasurfaces based on KTiOPO4 and demonstrate the fabrication procedures. It will provide a versatile platform for THz wave manipulation and open up new possibilities to monolithically combine it with THz generation.

Strong near-infrared absorption in ultrathin semiconductor layers is essential for increasing the speed and efficiency of photocarrier extraction in optoelectronic devices. However, the absorption of a free-standing ultrathin film can never exceed 50% in principle. In this article, an all-dielectric germanium metasurface absorber in the near-infrared region (800-1600 nm) is proposed theoretically and experimentally. Near-unity absorption can be achieved in such a subwavelength-thin (approximate to 0.13 lambda(0)) layer of nanostructures based on the destructive interference between simultaneously excited electric and magnetic dipoles inside each element in the backward direction in combination with the destructive interference between the scattered field and the incident field in the forward direction. Its response is both polarization-independent and angle-insensitive, with over 80% absorption at an incident angle up to 28 degrees. This ultrathin and flexible design paves the way for realizing next generation optoelectronic devices aimed for high-speed photon detection and energy harvesting.

Dielectric nanoantenna-based metasurfaces have attracted wide attention for their outstanding performance in light manipulation with low loss and full phase coverage enabled by multipolar resonances. To make the metasurfaces actively tunable, we adopt a kind of phase-changing material Ge2Sb2Te5 to construct non-volatile, switchable antenna-based metasurfaces in the mid-infrared spectrum region. Our design of the metasurface can realize switching between electric and magnetic dipole resonances across a broad spectrum region through crystalline-amorphous phase transitions under fixed design. Moreover, the transmission switching contrast between different phases can be up to 30dB (-30dB), due to the shift of multipolar resonances. This reconfigurable antenna-based metasurface will pave the way for ultimate design of light modulators, deflectors, holograms and so on for future optical communication networks.