[Paper Review] Broadband switchable terahertz half-/quarter-wave plate based on a graphene-metal hybrid metasurface

Xiaoqing Luo, Juan Luo+2 more|arXiv (Cornell University)|Aug 20, 2021

Metamaterials and Metasurfaces Applications31 references4 citations

TL;DR

This paper proposes a broadband, dynamically switchable terahertz half-wave plate (HWP) and quarter-wave plate (QWP) using a graphene-metal hybrid metasurface. By tuning the graphene Fermi energy from 0 eV to 1 eV, the device switches between HWP (polarization conversion ratio >97% across 0.7–1.3 THz) and QWP (ellipticity >0.92 across 0.78–1.33 THz), achieving a shared operational bandwidth of 0.52 THz with a 50% relative bandwidth—significantly broader than prior graphene-based designs.

ABSTRACT

Metasurfaces incorporating graphene hold great promise for dynamic manipulation of terahertz waves. However, it remains challenging to design a broadband graphene-based terahertz metasurface with switchable functionality of half-wave plate (HWP) and quarter-wave plate (QWP). Here, we propose a graphene-metal hybrid metasurface for achieving broadband switchable HWP/QWP in the terahertz regime. Simulation results show that, by varying the Fermi energy of graphene from 0 eV to 1 eV, the function of the reflective metasurface can be switched from an HWP with polarization conversion ratio exceeding 97% over a wide band ranging from 0.7 THz to 1.3 THz, to a QWP with ellipticity above 0.92 over 0.78-1.33 THz. The sharing bandwidth reaches up to 0.52 THz and the relative bandwidth is as high as 50%. We expect this broadband and dynamically switchable terahertz HWP/QWP will find applications in terahertz sensing, imaging, and telecommunications.

Motivation & Objective

• To address the challenge of designing broadband, dynamically tunable terahertz waveplates using graphene-based metasurfaces.
• To overcome the narrow bandwidth limitations of existing graphene-based HWP/QWP devices reported in the literature.
• To achieve simultaneous, switchable operation as both HWP and QWP over a shared, wide frequency band.
• To demonstrate a hybrid metasurface design that enables dynamic polarization control through tunable graphene Fermi energy.
• To provide a platform for practical terahertz applications in sensing, imaging, and telecommunications.

Proposed method

• The metasurface unit cell consists of perpendicular gold and graphene stripes on a thick gold film with a polyimide dielectric spacer, forming a metal-insulator-metal structure.
• The device operates in reflection mode, with incident linearly polarized terahertz waves at 45° to the x-axis to excite orthogonal polarization states.
• The Fermi energy of graphene is tuned from 0 eV to 1 eV via electrical gating, altering the surface impedance and enabling dynamic switching between HWP and QWP functions.
• The polarization conversion ratio (PCR) and ellipticity are calculated from the reflection coefficients |r_xy|, |r_yy|, and their phase difference ΔΦ = ΔΦ_yy − ΔΦ_xy.
• The broadband performance is attributed to the superposition of multiple resonances—both magnetic and electric—induced by surface currents on the gold and graphene stripes and the underlying gold film.
• Simulations are performed using full-wave electromagnetic modeling to analyze reflection amplitudes, phases, and Stokes parameters across the terahertz band.

Experimental results

Research questions

• RQ1 Can a graphene-metal hybrid metasurface achieve broadband, switchable HWP and QWP functionality in the terahertz band?
• RQ2 What is the achievable bandwidth and relative bandwidth for switchable HWP/QWP operation using graphene tuning?
• RQ3 How does the Fermi energy tuning of graphene enable dynamic switching between HWP and QWP modes?
• RQ4 What physical mechanisms underlie the broadband performance of the metasurface?
• RQ5 Can the metasurface maintain high polarization conversion efficiency and ellipticity across a wide frequency range?

Key findings

• The metasurface achieves a polarization conversion ratio exceeding 97% for the HWP function across the 0.7–1.3 THz band.
• For the QWP function, the ellipticity exceeds 0.92 across the 0.78–1.33 THz frequency range.
• The shared operational bandwidth for both HWP and QWP functions spans 0.78–1.3 THz, yielding a total bandwidth of 0.52 THz.
• The relative bandwidth of 50% is significantly higher than previous graphene-based HWP/QWP designs, which report relative bandwidths of 6% to 22%.
• The broadband performance is attributed to the superposition of multiple resonances, including both magnetic and electric resonances, as confirmed by surface current distributions at key frequencies.
• The device enables dynamic switching between HWP and QWP modes by tuning the graphene Fermi energy from 0 eV to 1 eV, demonstrating full reconfigurability in a single device.