High‑Efficiency Visible‑Light Polarization Beam Splitter Using All‑Dielectric Metasurfaces

Introduction

Metasurfaces—two‑dimensional arrays of subwavelength nanoantennas—have emerged as powerful platforms for manipulating light phase, amplitude, and polarization with ultrathin profiles. Compared with bulk optics, metasurfaces enable compact, low‑loss devices that are easier to fabricate. They have already enabled polarization converters, holography, flat lenses, and spectral splitters.

Metallic metasurfaces originally achieved beam deflection via two π‑phase resonances or geometrical phase, but their intrinsic absorption limits transmission efficiency. All‑dielectric metasurfaces, primarily based on silicon, circumvent these losses and offer several routes to full 2π phase control: geometric phase, overlapping Mie resonances (Huygens metasurfaces), and Fabry–Pérot resonances in high‑aspect‑ratio antennas. In this work, we exploit the latter approach, treating each silicon block as a truncated waveguide whose effective index governs the transmission phase.

Polarization beam splitters (PBS) are vital in many optical architectures. Existing PBS designs employ subwavelength gratings, hybrid plasmonic couplers, or multimode interference structures. Here we propose a simple, large‑angle PBS built from a cross‑shaped silicon array that achieves equal‑power splitting for 45° incident polarization, operating efficiently at visible wavelengths.

Methods

Figure 1 illustrates the metasurface layout: a square lattice of cross‑shaped silicon blocks (height 260 nm) rests on a 1.45‑index silica substrate. The lattice constants are P_x = P_y = 200 nm. Three‑dimensional FDTD simulations (periodic boundaries in x and y, perfectly matched layers along z) model normal‑incidence plane waves from below.

Each cross block is equivalent to two orthogonal silicon bars: one with fixed width w (70 nm) and variable length along the incident polarization axis. For y‑polarized incidence, the lengths L_y (60–200 nm) tune the transmission phase while maintaining constant w. Similarly, for x‑polarized incidence, the lengths L_x are varied. Four discrete unit cells (see Fig. 2) span a 0–2π phase range with a phase step of ±π/2 between adjacent cells, creating opposite phase gradients for the two polarizations.

Schematic configuration of the cross‑shaped metasurface acting as a polarization beam splitter

Using the generalized Snell’s law

\[\frac{n_{t}\sin\theta_{t}-n_{i}\sin\theta_{i}}{\lambda_{0}}=\frac{1}{2\pi}\frac{d\Phi}{dx}\]

the anomalous refraction angle is determined by the phase gradient dΦ/dx. For y‑polarized light, dΦ = –π/2; for x‑polarized light, dΦ = +π/2. This yields a 46.8° deflection at 583 nm.

Results and Discussions

Simulations confirm the predicted 46.78° deflection for both polarizations. Figure 3a shows the electric field in the x–z plane for x‑polarized incidence, matching the theoretical angle. The far‑field intensity (Fig. 3b) yields a total transmission of 69.7 % and a deflection efficiency of 63.7 %—limited mainly by 12.5 % interface reflection, 17.8 % silicon absorption, and 6 % unwanted diffraction orders.

The electric field distributions near the metasurface in the x‑z plane under a x‑polarized and c y‑polarized incidence. Normalized far‑field intensity distributions for b x‑polarized and d y‑polarized normally incident light. The operating wavelength is 583 nm, and the transmitted angle is defined as positive (negative) value in the right (left) side of the normal.

For y‑polarized incidence, the deflection angle is –46.78°, with a deflection efficiency of 66.4 % and total transmission of 75.2 %. Neglecting silicon absorption, efficiencies could approach 90 %—on par with prior silicon metasurface PBS designs.

When a linearly polarized plane wave at 45° to the x‑axis illuminates the metasurface, the device simultaneously excites the two orthogonal resonances, generating equal‑intensity x‑ and y‑polarized outputs. Figure 4c displays the extracted electric fields; Fig. 4b shows the far‑field intensities, each 0.336 of the incident power. Thus, 46.3 % of the total transmitted energy is funneled into the +1 and –1 diffraction orders for the two polarizations, while the 0‑order accounts for only 7.4 %. Across 579–584 nm, the intensity difference between the two outputs remains below 2 %, confirming broadband equal‑power splitting.

a Working mechanism of the proposed polarization beam splitter device (front view). b Normalized far‑field intensity. c The extracted transmitted x‑polarized (left) and y‑polarized (right) electric field distributions of the designed metasurface under the normal incidence of 45° polarized light at the wavelength of 583 nm.

Periodicity effects were examined by varying the lattice constant in the orthogonal direction (190–210 nm). Phase and transmission changes were negligible (<0.05π), confirming the independence of the design from cross‑coupling and simplifying fabrication.

Conclusions

We have engineered an all‑dielectric, cross‑shaped silicon metasurface that functions as a high‑efficiency polarization beam splitter in the visible range. The device splits 45°‑polarized light into equal‑power x‑ and y‑components at 583 nm with a 46.8° deflection angle and 63–66 % deflection efficiency. Its compact, ultrathin footprint and broadband equal‑power performance make it attractive for integration into next‑generation all‑optical photonic circuits.