Plasmonic Metasurfaces for Switchable Photonic Spin–Orbit Interactions Based on Phase Change Materials

Abstract Metasurfaces with intense spin–orbit interactions (SOIs) offer an appealing platform for manipulation of polarization and wavefront. Reconfigurable beam manipulation based on switchable SOIs is highly desired in many occasions, but it remains a great challenge since most metasurfaces lack the flexibility and the optical performance is fixed once fabricated. Here, switchable SOIs are demonstrated numerically and experimentally via the combination of plasmonic metasurfaces with phase change materials (PCMs). As a proof‐of‐concept, three metadevices possessing switchable SOIs are fabricated and investigated, which enable spin Hall effect, vortex beam generation, and holography when the PCM is in the amorphous state (corresponding to the “ON” state of SOI). When the PCM changes into the crystalline state (corresponding to the “OFF” state of SOI), these phenomena disappear. Experimental measurements show that a high polarization conversion contrast between “ON” and “OFF” states is obtained within a broadband wavelength range from 8.5 to 10.5 µm. The switchable photonic SOIs proposed here may provide a promising route to design reconfigurable devices for applications such as beam steering, dynamic holographic display, and encrypted optical communications.


S1. Simulations: The permittivity of the materials
The permittivity of MgF 2 and Ge 2 Sb 2 Te 5 films used in the simulations are plotted in Figure S1 and Figure S2, respectively. These values were measured by spectroscopic ellipsometer (SENTECH SE850 and SENDIRA) for the visible and infrared spectral range. We deposited 50nm-thick GST layer on a silicon substrate to measure its permittivity (amorphous state). Then, the sample was crystallized on a bake plate for 30 min, where the temperature was set as 200 ℃. From Figure S1, we can see that the real part of the permittivity shows a large difference between the amorphous and crystalline states within a broad mid-infrared wavelength region (ε 1 ≈ 12 in amorphous state and ε 1 ≈ 25 in crystalline state at least from 4 to 16 µm). The GST possesses the low-loss dielectric properties because the imaginary parts of permittivity at both states are small compared to the real part (Im(ε)/Re(ε) < 0.1). Based on this unique dielectric properties, GST can be used to achieve high-efficiency metadevices.
The permittivity of MgF 2 film is measured by depositing on silicon substrate with a thickness of 50 nm. As shown in the Figure S2, the real part of permittivity is close to 1 and the imaginary part is close to 0, which can work as the refraction index matching layer. Due to this unique character, adding MgF 2 layer on the top of GST layer can improve the performance of metadevices. Furthermore, we also measure the permittivity of GST at visible and near-infrared (NIR) spectral range, and the properties of permittivity are similar to MIR region. So we can extend this method to other region by adjusting the geometric parameters of the unit cell. The complex permittivity of gold is described by Drude model [1] : The plasma frequency ω p and collision frequency γ are chosen to be 1.32×10 16 rad/s and 131.8 THz, respectively. ε ∞ is the relative permittivity when frequency is infinite and is set as 9.1.    Table S1.

S4. Characterization of the vortex generator
To measure the topological charge of the vortex generator, we fabricated a device with two parts of phase. While one part is used to generate the OAM and the other is to generate a spherical wavefront. The interference between the helical wavefront and the spherical wavefront occurs in output cross-polarization light when illuminated with CP light. The vortex beam can be expressed as [2,3] : where l is topological charge and φ is the azimuthal angle, ϕ 0 is the spherical phase distribution corresponding to a focal length of f: Figure S4 shows the reflected patterns of vortex generator in two states and its optical microscopy image. The topological charge of l = -6 can be identified through the interference between the vortex beam and focusing spherical wavefront in amorphous state. When the vortex generator is in crystalline state, the reflected patterns turn to be mixed and in disorder, that is, the information carried by the device is missing. The metadevice shows great potential for encrypted optical-communication. The optical microscopy image.

S5. Reconfiguration of the metadevices
To realize the reconfiguration of metadevices, several relatively mature techniques can be adopted. Two possible methods are illustrated in the Figure S5. Figure S5(a) gives a method based on phase-change electronic memory. The metasurfaces are made on the TiN electrodes (for converting the electronic energy to thermal energy).
By applying a specific electrical stimulus, the phase state can be changed, thus realizing the reconfiguration of metadevice. The second method is based on laser-direct writing, which is illustrated in the Figure S5 (b). The reamorphization of GST layer can be achieved by a single laser pulse with high power intensity, and the crystallization of GST layer can be achieved by pulse sequence with lower power intensity [4][5][6] . Additionally, different from thermal annealing, the laser pulses do not damage the gold patch-antenna. Utilizing a scan femtosecond (fs) laser, the pulse duration can be controlled by adjusting the scan speed. Figure S5. Schematics for realization of the reconfiguration of metadevices. a) Phase-change electronic memory. b) Laser-direct writing.