1. 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 [1–25]. 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 [6–13]. 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 [14–25], 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.

Among different active control schemes, phase changing materials (PCMs), such as chalcogenides [13,21,22,26–35], vanadium dioxide [36,37], and liquid crystals [12] have played an important role in this evolution. The phase transition of PCM can be controlled by applying heat, photon or electric energy. In many studies, PCMs are adopted as a surrounding medium to take advantage of their intrinsic refractive index difference originating from phase transitions [26–32]. Recently, although they are demonstrated as components for constructing switchable metasurfaces to further increase dynamic tuning range [13,21,22,33–35], the combination of electric and magnetic dipole resonances of the high refractive index all-dielectric nanoparticles with PCM in the metasurface design has not been demonstrated yet, which will dramatically increase degrees of freedom for light manipulation.

In this study, Ge₂Sb₂Te₅ (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 ($\Delta \lambda \sim 0.6\mu m$) 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 all-dielectric 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.

2. 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 (${\epsilon }_X$) is predicted with the Lorentz-Lorentz relation [26,28]:

 

Fig. 1 (a) Mid-infrared complex refractive index of the as-deposited amorphous (0%) GST and annealed crystalline (100%) GST [26]. (b) Illustration of switching between an electric dipole and a magnetic dipole scattering behavior through amorphous-crystalline phase transitions in a single GST disk. (c) Calculated scattering cross sections of a single GST disk with d = 1100 nm, h = 400 nm in different phases (X = 0%, 50% and 100%) embedded in a homogeneous medium with n = 1.45. The calculated total scattering cross section (red dashed line) from multipole model is in good accordance with that from FDTD simulation (black solid line). It is decomposed into the contributions from an electric dipole (ED, green solid line), a magnetic dipole (MD, blue solid line), an electric quadrupole (EQ, cyan solid line) and magnetic quadrupole (MQ, magenta solid line). The arrows denote switching between an electric dipole response and a magnetic dipole response at different wavelengths through 50% phase transition.

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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].

3. Results and discussion

3.1 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), an electric quadrupole (EQ) and a magnetic quadrupole (MQ). Higher orders of multipolar resonances can be neglected in the spectrum region for different phases. The decomposition method has been discussed in detail in the previous study [39].

As shown in Fig. 1(c), when X = 0%, the magnetic and electric dipole resonances of the GST disk can be located near 3.2 μm and 3.7 μm, respectively, where the magnetic dipole resonance is optimized to appear at the dip of the electric dipole resonance. After 50% crystallization, its magnetic and electric dipole resonances are red-shifted to 3.7 μm and 4.3 μm, respectively. This implies that near the wavelength of 3.7 μm, the scattering behavior of a GST disk transforms from an electric dipole into a magnetic dipole through 50% crystallization. When the GST disk is fully crystallized, its magnetic and electric dipole resonances are further red-shifted to 4.3 μm and 4.8 μm. The switching behavior happens again near 4.3 μm. 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 an electric dipole response and a magnetic dipole response can be achieved within the wavelength range from 3.7 μm to 4.3 μm ($\Delta \lambda \sim 0.6\mu m$) for d = 1100 nm and h = 400 nm by inducing a fixed 50% phase transition.

3.2 Reconfigurable antenna-based GST metasurface

Next, we consider the optical properties of arrays of GST disks located on the glass substrate. As shown in Fig. 2(a), the overall system is under plane wave illumination propagating in z direction with y-polarized electric field. The lattice constant a is varied along with the diameter d, as a = d + g, where g is fixed at 500 nm.

 

Fig. 2 (a) Schematic of a switchable antenna-based GST metasurface located on a glass substrate, with h = 400 nm. The overall system is under plane wave illumination propagating in z direction with y-polarized electric field. Calculated transmission (b), (d), (f) and reflection (c), (e), (g) spectra of the GST metasurface with g = 500 nm in different phases (X = 0%, 50% and 100%). The white dashed lines highlight the spectral positions of induced electric (ED) and magnetic dipole (MD) resonances inside each element as a function of the disk size. For d = 1300 nm, the GST metasurface gives an electric dipole response denoted as Mode I (Mode II) when X = 0% (X = 50%) and turns into a magnetic dipole response denoted as Mode I′ (Mode II′) when X = 50% (X = 100%) at the wavelength of 3.6 μm (4.1 μm). Also, when d = 1700 nm, the metasurface can switch between the electric dipole response (Mode III) and magnetic dipole response (Mode III′) through 100% phase transition near the wavelength of 4.3 μm.

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The calculated reflection and transmission spectra of GST disk arrays at different crystallization states are plotted in Figs. 2(b)-2(g). The dependence of the spectral positions of the magnetic dipole resonance and the electric dipole resonance on the diameter is indicated by the white dashed lines. The diameter is varied from 500 nm to 2500 nm, which corresponds to the aspect ratio (d/h) changed from 1.25 to 6.25. As shown in Figs. 2(b) and 2(c), the typical suppression of backward emission, known as Kerker Condition [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 multi-level 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.

 

Fig. 3 (a) Field distribution inside each GST disk in the yz-plane. The color denotes the field intensity and the white arrows represent the electric field vectors. (b) Calculated transmission switching contrast of the metasurface with g = 500 nm and h = 400 nm between X = 50% and X = 0%. The scale bar is in log scale. (c) The transmission data of the GST metasurface with g = 500 nm, h = 400 nm, and d = 2120 nm for X = 50% and X = 0%. The transmission contrast between X = 0% and X = 50% is as high as 30 dB at the wavelength of 4.1 μm as denoted in (b). (d) 2π phase shift in transmission originating from amorphous-crystalline phase transition while maintaining T ~50% for d = 920 nm at the wavelength of 2.9 μm, denoted by the black arrow in (b).

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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 ($0<\Delta X\le 100\%$).

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.

4. Conclusions

In conclusion, a reconfigurable antenna-based metasurface consisting of square arrays of GST disks is proposed here. It can realize switching between electric dipole and magnetic dipole responses across a broad spectrum region through amorphous-crystalline phase transitions. The transmission switching contrast between different crystalline states can be up to + 30dB (−30dB) near the multipolar resonances at mid-infrared region. Furthermore, the accessible 2π phase shift originating from switching among different multipolar resonances might enable complex wavefront manipulation through spatial variation of crystallization without changing geometry or orientation of the elements, which will definitely pave the way for realizing non-volatile and reconfigurable meta-devices with a flexible and simple design.