Reconfigurable all-dielectric antenna-based metasurface driven by multipolar resonances

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. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement


Introduction
Metasurfaces consisting of two-dimensional arrangement of meta-atoms are hoped-for flat and compact devices to locally manipulate amplitude, phase and polarization of incident light .The recently proposed all-dielectric antenna-based metasurfaces as counterparts to the metallic ones show low loss and full phase coverage, which can be mainly classified into two categories accordingly.One is repeated periodic arrays controlling unidirectional emission, harmonic generation or even enhanced absorption [6][7][8][9][10][11][12][13].The other is spatial variation of geometries or orientations of the meta-atoms for wavefront or polarization control, such as deflectors, achromatic lens, holograms and so on [14][15][16][17][18][19][20][21][22][23][24][25], which dramatically expand the functionality of conventional optical devices.However, most of the developed all-dielectric antenna-based metasurfaces so far are passive devices and their performance is fixed after fabrication.To further increase the degrees of freedom for light manipulation, there is an urgent need for developing active all-dielectric antenna-based metasurfaces to include a wealth of dynamic, such as switchable, tunable or reconfigurable functionalities, which is usually achieved by applying heat, electrostatic and magnetic forces, and stretching to the metasurface.
In this study, Ge 2 Sb 2 Te 5 (GST) is adopted for constructing an all-dielectric antenna-based metasurface in the mid-infrared region (2.5~5 μm).GST nanostructures can be fabricated in its amorphous phase by magnetron sputtering and transform gradually into crystalline phase when annealed around 160 °C.The reversible reamorphization process can then take place under fast annealing over 640 °C.The refractive index of GST in both amorphous and crystalline states is high and distinct and its phase is stable after removing the stimuli.Moreover, by controlling the specific annealing temperature and time, even semi-crystalline state with distinct optical properties can be obtained after annealing, which makes GST a good candidate for constructing multi-level switchable all-dielectric antenna-based metasurfaces driven by multipolar resonances.In our design, it can realize switching between the electric and magnetic dipole resonances across a broad spectrum region ( 0.6 m λ μ Δ  ) through 50% crystallization transition.The transmission switching contrast between different phases is demonstrated to be up to 30dB (−30dB) near the multipolar resonances.The combination of electric and magnetic dipole resonances of the high refractive index alldielectric nanoparticles with PCM can dramatically increase degrees of freedom for light manipulation, which is an essential step towards practical applications of all-dielectric metasurfaces in the near future.

Simulation methods
The complex refractive index of GST in the mid-infrared region (2.5~5 μm) is shown in Fig. 1(a) [26], where GST shows high refractive index with low dispersion at both states (n > 4) and the refractive index difference is as large as n ≈2 under amorphous-crystalline phase transition.GST is almost lossless at amorphous state and a bit lossy at crystalline state.For the intermediate state with a crystallization X, its dielectric constant ( X ε ) is predicted with the Lorentz-Lorentz relation [26,28]: where c ε and a ε are dielectric constants of GST at the crystalline state (X = 100%) and the amorphous (X = 0%) state.
The design is modeled by the finite-difference-time-domain method (FDTD Solutions, version 8.15.697).Considering ease of fabrication and more degrees of freedom for light manipulation, GST disks are adopted as elements in the metasurface design.The multi-level switchable scattering properties of a single GST disk embedded in a homogeneous medium (n = 1.45) are first calculated both numerically and analytically.Perfect matched layers (PML) boundary conditions are used in the FDTD simulation [38].Next, GST disks are put onto a glass substrate (n = 1.45) forming square arrays for studying the response of a GST metasurface.The thickness of the GST disk is h = 400 nm and the gap between adjacent disks is g = 500 nm, where g = a -d (a represents the periodicity of the metasurface and d corresponds to the diameter of the GST disk).PML boundary conditions are used in the propagation direction of the incident light and periodic boundary conditions are used in the directions normal to the propagation direction in the FDTD simulation [25].

Switchable multipolar scattering behavior of a single GST disk
To clarify the switchable multipolar resonant properties of individual GST disks with different crystallizations (conceptually illustrated in Fig. 1(b)), the multipolar decompositions of a single GST disk with d = 1100 nm, h = 400 nm, embedded in the homogeneous medium with a refractive index of 1.45 are conducted analytically.The total scattering cross section of a GST disk is decomposed into the contributions from an electric dipole (ED), a magnetic dipole (MD), multipolar re decomposition  The lattice c at 500 nm.ic quadrupole ( um region for e previous stud as-deposited amor on of switching be ough amorphouscross sections of a 0%, 50% and 100% scattering cross se from FDTD simu ctric dipole (ED, g drupole (EQ, cya rows denote switc different wavelen and electric d pectively, whe electric dipole ces are red-shi of 3.7 μm, the o a magnetic zed, its magne μm.The switc avior of GST in g between an e the wavelength nm by induci ce T disks located ne wave illumi constant a is (MQ).Higher r different pha dy [39].rphous (0%) GST etween an electric -crystalline phase a single GST disk %) embedded in a ection (red dashed ulation (black solid green solid line), a an solid line) and ching between an ngths through 50%   [6,7], can be obtained at about 2.9 μm for amorphous state when d = 920 nm (d/h = 2.3), resulting from the destructive interference between the induced electric and magnetic dipole resonances inside each GST disk in the backward direction.For the GST disk arrays at 50% or 100% crystallization state, the crossing of the two resonances is observed with broader spectral width at longer wavelength, originating from both larger real part and imaginary part of refractive index.
As predicted by the scattering behavior of single GST disks at different phases, the multilevel switchable resonant behavior is maintained in the metasurface design.For example, for d = 1300 nm and X = 0%, the GST metasurface gives an electric dipole response denoted as Mode I at the wavelength of 3.6 μm in Figs.2(b) and 2(c), which turns into a magnetic dipole response denoted as Mode I′ after 50% (X = 50%) phase transition, as shown in Figs.2(d) and 2(e).Moreover, when X = 50%, the metasurface also gives an electric dipole response (Mode II) near the wavelength of 4.1 μm.By further 50% crystallization, the metasurface turns into supporting magnetic dipole resonances (Mode II′) at the same wavelength, as shown in Figs.2(f) and 2(g).Considering the low-dispersion behavior of GST in the wavelength range from 3 μm to 5 μm as shown in Fig. 1(a), the switching between electric dipole and magnetic dipole responses can be achieved within the wavelength range from 3.6 μm to 4.1 μm for d = 1300 nm, g = 500 nm and h = 400 nm by inducing a fixed 50% phase transition.Also, when d = 1700 nm, the metasurface can switch between the electric dipole response (Mode III) and the magnetic dipole response (Mode III′) through 100% phase transition near the wavelength of 4.3 μm.
To prove the validity of analysis, the field distribution inside each GST disk is calculated in FDTD simulation.Here, Mode I and Mode I′ are illustrated representatively in Fig. 3(a).It is obvious that Mode I is an electric dipole resonance with the enhancement of field intensity as large as 52 and Mode I′ is a magnetic dipole resonance with a typical displacement current loop formed inside the disk, where the magnetic field intensity enhancement is up to 214.
Since the optical properties of GST are distinct between its amorphous and crystalline states, switchable properties are expected for both the amplitude and the phase response of the system through phase transitions.The calculated switching contrast of transmission (T X = 50% /T X = 0% ) between X = 50% and X = 0% is shown in Fig. 3(b).The dips and peaks in the transmission switching contrast are attributed to the shift of multipolar resonances.For example, when d = 2120 nm, the transmission switching contrast can be −30 dB at the magnetic resonances of X = 50% as denoted in Fig. 3(b) while maintaining T X = 0% > 0.7 at the wavelength of 4.1 μm, as shown in Fig. 3(c).The transmission switching contrast at electric dipole resonances for both states is much lower compared with that at magnetic dipole resonances mentioned above.Similar transmission switching behavior can be expected in the whole spectral region by inducing different amount of phase transitions ( 0 1 0 0 % X < Δ ≤ ).Moreover, it is worth noticing that when d = 920 nm, the Kerker Condition is fulfilled at the wavelength of 2.9 μm at the amorphous state, where high transmission is maintained at the crossing of the electric and magnetic dipole resonances.During crystallization, the high transmission behavior is maintained and transmission phase experiences 2π shift while the resonances are red-shifted away from 2.9 μm, as shown in Fig. 3(d).The accessible 2π phase shift here will enable complex wavefront manipulation, such as tunable focusing or beam steering through spatial variation of crystallization without changing geometry or orientation of the elements.
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3. 2
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3 .
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op W. L. Barnes, and ution inside each G arrows represent he metasurface wit is in log scale.(c) and d = 2120 nm X = 50% is as high n transmission ori 0% for d = d I. R. Hooper, "Pl GST disk in the yz the electric field v th g = 500 nm and The transmission m for X = 50% and h as 30 dB at the w iginating from am m lasmonic meta-ato yz-plane.The color vectors.(b) Calcu d h = 400 nm betw data of the GST m d X = 0%.The tran wavelength of 4.1 morphous-crystallin gth of 2.