Electrically Tunable Optical Metasurfaces for Dynamic Polarization Conversion

Abstract

Dynamic control over the polarization of light is highly desirable in many optical applications, including optical communications, laser science, three-dimensional displays, among others. Conventional methods for polarization control are often based on bulky optical elements. To achieve highly integrated optical devices, metasurfaces, which have been intensively studied in recent years, hold great promises to replace conventional optical elements for a variety of optical functions. In this work, we demonstrate electrically tunable optical metasurfaces for dynamic polarization conversion at visible frequencies. By exploring both the geometric and propagation phase tuning capabilities, rapid and reversible polarization rotation up to 90° is achieved for linearly polarized light. The dynamic functionality is imparted by liquid crystals, which serve as a thin surrounding medium with electrically tunable refractive indices for the metasurface antennas. Furthermore, we expand our concept to demonstrate electrically tunable metasurfaces for dynamic holography and holographic information generation with independently controlled multiple pixels.

Keywords: electrical control; holography; metasurfaces; polarization conversion; visible frequencies.

Figure 1

Working principle of the electrically tunable optical metasurface for polarization conversion. (a) An array of gold nanorods embedded in a PC403 (green) layer resides on an ITO-coated SiO₂/Si substrate, which serves as the bottom electrode. The alternating rows are covered by high-birefringence LCs and PMMA (blue) trenches, respectively. The metasurface is encapsulated in a LC cell. A polyimide alignment layer is rubbed along the direction of the PMMA trenches (x-axis). The LC cell is covered by an ITO-coated quartz superstrate as the top electrode. The incident LP light is polarized along the x-axis. The LC molecules change their orientations, when the circuit is switched on/off. The polarization of the reflected beam is along the y- and x-axes, when the circuit is off and on, respectively. (b) Top-view and (c) side-view of the metasurface supercell. Linear polarization generation through superposition of LCP and RCP light under an x-polarized light illumination. The PMMA thickness is t. The gold nanorods in the neighboring rows are covered by two dielectric materials with refractive indices of n₁ and n₂, respectively. (d) Simulated polarization states of the reflected beam for n₂ = 1.25, 1.5, 1.75, and 2, while n₁ is fixed at 1.5. The rotation angle is defined by the angle formed between the polarization direction of the reflected beam and the x-axis.

Figure 2

Dynamic polarization control. (a) SEM image of the metasurface sample with PMMA coatings on selective rows. The light gray areas correspond to the coated PMMA trenches. (b) Polarization states of the reflected LP light at V = 0 and 20 V, respectively. The incident light at 633 nm is polarized along the x-axis. (c) Rotation angle of the linear polarization in dependence on the applied voltage V. The polarization states for 60° (green) and 30° (red) rotations are depicted in the insets. (d) Cycling performance of the dynamic metasurface for polarization rotation. The output intensity is detected after the reflected light passes through a polarizer polarized along the y-axis.

Figure 3

Dynamic holography based on polarization rotation. (a) SEM image of the metasurface sample. (b) Optical setup for characterization of the metasurface holography. P1 and P2 represent polarizers. HWP represents a half wave plate. (c) Detected light power in dependence on the applied voltage. (d) Holographic patterns at the on and off states, corresponding to V = 0 or 50 V, respectively. White arrows indicate the polarization directions of the holograms.

Figure 4

Holographic information generation with independently controlled metasurface pixels. (a) SEM image of the sample. Two addressable metasurface pixels (M1 and M2) are controlled via two independent ITO electrodes. Insets: schematics of the reconstructed holographic patterns from M1 and M2. (b) Holographic information at four different states is generated by electrically controlling the two independent metasurface pixels.