Phase-Change Metasurface by U-Shaped Atoms for Photonic Switch with High Contrast Ratio

: Currently, diverse metasurfaces act as exotic platforms enabling versatile wave regulations in deep-subwavelength level for ultracompact integration. To address the existing issues of passive nature and low-efﬁciency in wave controls, one type of metasurface for active phase tuning is proposed in this paper by integrating the phase-change dielectric of Ge 2 Sb 2 Te 5 into the of U-shaped meta-atoms. Speciﬁcally, the phase-change-based hybrid design of Ge 2 Sb 2 Te 5 -integrated metalens switch is demonstrated and numerically conﬁrmed with switchable focusing. The well-deﬁned metal-insulator-metal (MIM) setup is used to enable high-efﬁciency reﬂective wavefront tunig and practical Ge 2 Sb 2 Te 5 phase transition. Upon the phase transition between the amorphous and crystalline states of Ge 2 Sb 2 Te 5 , the cross-polarized component of reﬂected waves in the given wavelength range is switched “on” (maximized) for as-designed geometric phase plus meta-lensing or “off” (minimized) for no lensing with ultra-high contrast ratio of ~36:1. As a result, such hybrid design of phase-change metasurface may provide a promising route for active photonic device with compact integration.

Therefore, this paper proposes one type of PCM-integrated active metasurface to exhibit the planar-lensing-based middle-IR photonic switching with ultrahigh contrast ratio up to 36:1. The switchable behaviors of cross-polarization and geometric phase generation for well-defined meta-lensing are numerically confirmed. The chalcogenide alloy of Ge 2 Sb 2 Te 5 that assumes large contrast of optical constant (e.g., refractive indices) upon amorphous-and-crystalline phase transition in middle-IR range is embedded into the reflective metal-insulator-metal (MIM) architecture to construct the active meta-lens with switchable focusing. Herein, the Ge 2 Sb 2 Te 5 phase transition essentially determines the actively switchable behaviors and potentially occurs at possible external excitations of thermal heating or electrical or optical pulses, which had been intensively explored previously [25,[37][38][39], and proved experimentally viable with proper device integration based on the MIM architecture [40,41]. Further, for high optical efficiency in phase and amplitude modulation, the high-density U-shaped antenna array with intrinsic high fill factor is employed atop the MIM meta-atom architecture due to its advantages in compactness and relatively high optical efficiency. In a given wide wavelength range between 9.5 µm and 10.5 µm, the meta-lens can be switched "on" with the maximized cross-polarization for as-designed geometric phase profile for normal beam focusing or "off" with minimized cross-polarization with no beam focusing upon Ge 2 Sb 2 Te 5 phase transition between amorphous and crystalline states.

Fundamental and Scheme
Different from traditional volumetric metamaterials, the 2D metasurfaces are constructed of atoms with different geometries for anisotropic amplitude and phase control. Herein, the high-density U-shaped atoms with relatively large fill factor (that is defined as the ratio of resonator area to that of a periodic cell in the 2D lattice) are used in a metal-insulator-metal (MIM) architecture due to the relatively high optical efficiency in amplitude modulations, as shown in Figure 1. the LCP component, whereas tLL and tRR denote the ratios of co-polarization from LCP to LCP and RCP to RCP. As for a local U-shaped antenna rotated by an angle θ with respect to the x-axis, shown in Figure 1a, the reflected (output) LCP and RCP component [ELCPout, ERCPout] where the new Jones matrix for a rotated atom can be derived by applying the rotation transformation as  (3) and (4), the first term denotes the copolarized part whereas the second term denotes the cross-polarized part. Obviously, the latter indicates an extra phase variation of Φ = −2θ for RCP-to-LCP cross-polarization. Similarly, the extra phase shift for LCP-to-RCP cross-polarization is Φ = 2θ. Therefore, any local phase distribution can be reshaped by the meta-atoms array in a pixel-wise and subwavelength manner. Further, to construct a reflective meta-lens based on the MIM architecture in Figure 1a, the parabolic phase profile needs to be constructed in the (x, y) plane, namely, At the incidence of linearly or circularly polarized (i.e., LP or CP) waves, the amplitude and phase of polarized components along different directions are tuned in an anisotropic and localized manner, as can be expressed by the Jones matrix below Herein, the Jones matrix JCP for CP wave incidence is where t LR denote the cross-polarization ratio of left circularly polarized (LCP) component to right circularly polarized (RCP) component and t RL denote that of RCP component to the LCP component, whereas t LL and t RR denote the ratios of co-polarization from LCP to LCP and RCP to RCP. As for a local U-shaped antenna rotated by an angle θ with respect to the x-axis, shown in Figure 1a, the reflected (output) LCP and RCP component [E LCPout , E RCPout ] at incidence (input) of LCP or RCP waves [E LCPin , E RCPin ] can be expressed as where the new Jones matrix for a rotated atom can be derived by applying the rotation transformation as As a result, taking the LCP output component E LCP out as an example, namely, E LCPout = E LCPin t LL + E RCPin t LL e −2θ by Equations (3) and (4), the first term denotes the co-polarized part whereas the second term denotes the cross-polarized part. Obviously, the latter indicates an extra phase variation of Φ = −2θ for RCP-to-LCP cross-polarization. Similarly, the extra phase shift for LCP-to-RCP cross-polarization is Φ = 2θ. Therefore, any local phase distribution can be reshaped by the meta-atoms array in a pixel-wise and sub-wavelength manner.
Further, to construct a reflective meta-lens based on the MIM architecture in Figure 1a, the parabolic phase profile needs to be constructed in the (x, y) plane, namely, where k 0 = 2π/λ is the free-space wave vector, f is the focal length as-designed. Therefore, ultimate surface layout for lensing is constructed by encoding the phase distributions in Equation (5) into the spatially distributed U-shaped antenna with varied angles by Φ = 2θ, as shown in Figure 1b. As a result, the proposed MIM-based device works in a Fabry-Pérot resonant mode and provides the maximized cross-polarization to produce phase profile as-designed by Equation (5) for a given wavelength range when the sandwiched Ge 2 Sb 2 Te 5 film is in the amorphous state. On the contrary, when the Ge 2 Sb 2 Te 5 film undergoes a phase change to crystalline state, the cross-polarization for geometric phase generation is minimized or switched off to leave only the co-polarized component for normal specular reflection in the same wavelength range. Namely, such MIM-based meta-lens provides a photonic switch that works between "on" and "off" states for switchable lensing.
Herein, the Ge 2 Sb 2 Te 5 phase change occurs at possible excitations of thermal heating, electrical stimuli or laser pulses. Essentially, the MIM setup of meta-atom design combines the advantages of reflective focusing with high optical efficiency and utility for phase transition of Ge 2 Sb 2 Te 5 . In special, as is recently confirmed for the similar MIM setup [41], thermal heating only triggers the one-way amorphous-to-crystalline phase change due to high temperature (above the melting point of underneath Cu) for reamorphization. Therefore, laser pulses or electrical stimuli can be introduced by proper optoelectronic design, e.g., electrical excitations in the well-known framework of phase-change memory for further compact device integration. In particular, the electrical heating by indium tin oxide (ITO) microheaters turns out to be a viable way for electrically active control, in which the diffusion barrier usually needs to be involved for practical device fabrication and long-term stability [38].

Simulations and Verifications
As a proof of concept, the meta-atom model is designed as in Figure 1 and numerically simulated using the finite difference time domain (FDTD) method to confirm the switchable meta-lensing at normal incidence of RCP waves. For the MIM structure in Figure 1a, the antenna parameters are set to be l = 2.88 µm and w = 0.6 µm with the thickness of 0.12 µm.
Here, the Ge 2 Sb 2 Te 5 film should be adequately thick to cause appreciable spectral shift to switch off the resonance and cross-polarization at given wavelength range upon phase transition, but not too thick for reversible triggering by possible optical or electrical pulses. Therefore, also confirmed by recent studies using the similar MIM setup [1,40], the Ge 2 Sb 2 Te 5 film thickness is chosen as 0.6 µm for reversible phase change under electrical or optical stimuli and enough for photonic switching, and h MgF2 = 0.15 µm and h Au = 0.12 µm, and the atom pitch is Λ = 4.2 µm, shown in Figure 1a. Optical constant of Ge 2 Sb 2 Te 5 is cited from previous work [40,41]. After calculations, the reflectance of co-polarized and cross-polarized components and total reflected waves are extracted.
As shown in Figure 2a, for amorphous state of Ge 2 Sb 2 Te 5 , the cross-polarized reflectance (close to 80%) is maximized and becomes dominant over that of co-polarization (less than 10%) in total components of reflected waves in a broad range between 9.5~10.5 µm (around the resonance wavelength at 9.8 µm), whereas for the crystallized Ge 2 Sb 2 Te 5 ( Figure 2b) after amorphous-to-crystalline phase change, the cross-polarized reflectance of this device is then suppressed close to zero, and the co-polarized component becomes maximized to almost equals to the total reflection. Obviously, the cross-polarized reflectance between two states of Ge 2 Sb 2 Te 5 assumes an ultra-high contrast.
Considering cross-polarized component further contributing to as-designed metalensing for amorphous state of Ge 2 Sb 2 Te 5 , the contrast of focused intensity is even higher. Therefore, such ultrahigh contrast ratio of reflectance enables a sensitive lensing switch for bistable active control. For the geometric phase produced by U-shaped antenna, shown in Figure 2c, the continuous rotation angle from zero to 180 degree produces the spatially distributed phase variations covering a full range of [0 2π].
Further, to characterize the cross-polarization efficiency for geometric phase, the polarization conversion ratio (PCR) is defined as cross-polarized reflectance over total reflection, namely, PCR = Rcross/(Rcross + Rco). Similarly, PCR spectrum extracted in Figure 2d also assumes an obvious high contrast between the amorphous and crystalline states of Ge 2 Sb 2 Te 5 in the chosen range of range 9.5~10.5 µm. In particular, at the resonance point of 9.8 µm, the largest PCR contrast ratio attains to be 98% versus 2.7% (~36:1). Further, to characterize the cross-polarization efficiency for geometric phase, the polarization conversion ratio (PCR) is defined as cross-polarized reflectance over total reflection, namely, PCR = Rcross/(Rcross + Rco). Similarly, PCR spectrum extracted in Figure 2d also assumes an obvious high contrast between the amorphous and crystalline states of Ge2Sb2Te5 in the chosen range of range 9.5~10.5 μm. In particular, at the resonance point of 9.8 μm, the largest PCR contrast ratio attains to be 98% versus 2.7% (~36:1).

ER
To illustrate the resonance mechanism of F-P mode and U-shaped antenna for switchable cross-polarization, the electric field intensity distributions are extracted in Figure 3. Clearly, at the RCP incidence of λ = 9.8 μm for the amorphous Ge2Sb2Te5 (Figure 3a), the intensity profile indicates highly confined E-field by resonant meta-atom of antenna leading to the maximized cross-polarization for as-designed geometric phase. However, crystallization of Ge2Sb2Te5 (Figure 3b) leads to almost no E-field confinement or resonance and thus minimized cross-polarization with no geometric phase. Further, at the incidence of x-and y-polarized wave at λ = 9.8 μm for amorphous Ge2Sb2Te5, the electric To illustrate the resonance mechanism of F-P mode and U-shaped antenna for switchable cross-polarization, the electric field intensity distributions are extracted in Figure 3. Clearly, at the RCP incidence of λ = 9.8 µm for the amorphous Ge 2 Sb 2 Te 5 (Figure 3a), the intensity profile indicates highly confined E-field by resonant meta-atom of antenna leading to the maximized cross-polarization for as-designed geometric phase. However, crystallization of Ge 2 Sb 2 Te 5 (Figure 3b) leads to almost no E-field confinement or resonance and thus minimized cross-polarization with no geometric phase. Further, at the incidence of xand y-polarized wave at λ = 9.8 µm for amorphous Ge 2 Sb 2 Te 5 , the electric displacement profiles in Figure 3c,d also reveal the symmetric and asymmetric modes of resonant U-shaped antenna leading to high cross-polarization or PCR (~98%). As a result, FDTD calculations of the whole meta-device designed in Figure 1b give rise to switchable focusing in two states of Ge2Sb2Te5. As shown in Figure 4, at three wavelengths of 10 μm, 10.5 μm, and 11 μm selected from the spectral range with high PCR contrast in Figure 2d, the extracted xoz intensity profiles obviously demonstrate the "on" and "off" states of our metalens-based photonic switch with switchable focusing. The calculated focal lengths in Figure 4a-c are 67.8 μm, 65 μm, and 60.7 μm, which agrees well with both Equation (5) and the generalized Snell's law, since larger wavelengths lead to smaller deflection angles as well as smaller focal lengths. As a result, FDTD calculations of the whole meta-device designed in Figure 1b give rise to switchable focusing in two states of Ge 2 Sb 2 Te 5 . As shown in Figure 4, at three wavelengths of 10 µm, 10.5 µm, and 11 µm selected from the spectral range with high PCR contrast in Figure 2d, the extracted xoz intensity profiles obviously demonstrate the "on" and "off" states of our metalens-based photonic switch with switchable focusing. The calculated focal lengths in Figure 4a-c are 67.8 µm, 65 µm, and 60.7 µm, which agrees well with both Equation (5) and the generalized Snell's law, since larger wavelengths lead to smaller deflection angles as well as smaller focal lengths.

Conclusions
In conclusion, to address the issue of passive nature and low optical efficiency that exists in most of current metasurface devices, one type of PCM-based planar meta-lens is proposed and numerically verified with actively switchable focusing by integrating the U-shaped meta-atoms and chalcogenide alloy of Ge2Sb2Te5 into the reflective MIM architecture. By distributing the U-shaped antenna array with intrinsically high fill factor pixelwise, the phase profile can be reshaped as designed for the switchable lensing with high optical efficiency and ultrahigh contrast ratio up to 36:1. The switchable behavior occurs upon a conditional phase transition of Ge2Sb2Te5 between the amorphous and crystalline states, which can be optically or electrically triggered in practical device integration, e.g., based on the previously reported MIM architecture [1,41]. At the amorphous state of the embedded Ge2Sb2Te5 layer, the reflected cross-polarized component is maximized (i.e., switched "on") for the as-designed geometric phase to define the normal focusing, or minimized (switched "off") with no focusing when the Ge2Sb2Te5 layer is switched to a crystalline state. As a result, our scheme potentially paves a promising way to construct the metasurface-based active photonic devices, especially for ultracompact integration.

Conclusions
In conclusion, to address the issue of passive nature and low optical efficiency that exists in most of current metasurface devices, one type of PCM-based planar meta-lens is proposed and numerically verified with actively switchable focusing by integrating the U-shaped meta-atoms and chalcogenide alloy of Ge 2 Sb 2 Te 5 into the reflective MIM architecture. By distributing the U-shaped antenna array with intrinsically high fill factor pixel-wise, the phase profile can be reshaped as designed for the switchable lensing with high optical efficiency and ultrahigh contrast ratio up to 36:1. The switchable behavior occurs upon a conditional phase transition of Ge 2 Sb 2 Te 5 between the amorphous and crystalline states, which can be optically or electrically triggered in practical device integration, e.g., based on the previously reported MIM architecture [1,41]. At the amorphous state of the embedded Ge 2 Sb 2 Te 5 layer, the reflected cross-polarized component is maximized (i.e., switched "on") for the as-designed geometric phase to define the normal focusing, or minimized (switched "off") with no focusing when the Ge 2 Sb 2 Te 5 layer is switched to a crystalline state. As a result, our scheme potentially paves a promising way to construct the metasurface-based active photonic devices, especially for ultracompact integration.