[Paper Review] High quality factor silicon-on-lithium niobate metasurfaces for electro-optically reconfigurable wavefront shaping

Elissa Klopfer, Sahil Dagli+3 more|arXiv (Cornell University)|Oct 8, 2021

Metamaterials and Metasurfaces Applications46 references84 citations

TL;DR

This paper proposes a high-quality-factor (high-Q) silicon-on-lithium niobate metasurface with individually addressable nanobars for electro-optically reconfigurable wavefront shaping. By leveraging guided mode resonances in periodically patterned nanobars and transparent conducting oxide contacts, the platform achieves full 2π phase tunability with ±25 V bias, enabling high-efficiency beamsteering (up to 86%) and beamsplitting (93% efficiency) across wide angular ranges, including 51° beamsteering and 18–31° tunable beamsteering.

ABSTRACT

Dynamically reconfigurable metasurfaces promise compact and lightweight spatial light modulation for many applications, including LiDAR, AR/VR, and LiFi systems. Here, we design and computationally investigate high quality factor silicon-on-lithium niobate metasurfaces with electrically-driven, independent control of its constituent nanobars for full phase tunability with high tuning efficiency. Free-space light couples to guided modes within each nanobar via periodic perturbations, generating quality factors exceeding 30,000, while maintaining bar spacing <$\lambda$/1.5. We achieve nearly 2$\pi$ phase variation with an applied bias not exceeding $\pm$ 25 V, maintaining reflection efficiency above 91%. Using full-field simulations, we demonstrate a high angle, 51\deg, switchable beamsplitter with a diffracted efficiency of 93%, and an angle-tunable beamsteerer, spanning 18-31\deg, with up to 86% efficiency, all using the same metasurface device. Our platform provides a foundation for highly efficient wavefront shaping devices with a wide dynamic tuning range capable of generating nearly any transfer function.

Motivation & Objective

• To develop a dynamically reconfigurable metasurface platform for compact, high-efficiency wavefront shaping.
• To overcome limitations in existing metasurfaces, such as low diffraction efficiency, poor directivity, and narrow field of view.
• To enable full 2π phase tunability through individually addressable nanobars with low-voltage electro-optic control.
• To achieve high Q factors (>30,000) and maintain subwavelength spacing (<λ/1.5) for high-performance wavefront control.
• To demonstrate multifunctionality by switching between beamsplitting and beamsteering with a single device.

Proposed method

• The metasurface uses silicon-on-lithium niobate nanobars with periodic notches to excite high-Q guided mode resonances (GMRs).
• Transparent conducting oxide (ITO) contacts are applied to each nanobar for individual electrical addressing and electro-optic phase modulation.
• The device operates in reflection with a gold mirror 610 nm below the bottom ITO contact to maximize reflectance and minimize absorption.
• Nano-fins are added on either side of each nanobar to suppress crosstalk and coupling between adjacent resonators.
• Full-field finite element simulations (COMSOL) model electrostatic and electromagnetic responses to predict phase shifts, diffraction efficiency, and beam steering.
• A supercell approach with 2–5 bars is used to simulate beamsteering and beamsplitting functions under varying voltage configurations.

Experimental results

Research questions

• RQ1 Can high-Q nanobars in a silicon-on-lithium niobate platform enable full 2π phase tunability with low voltage (<±25 V)?
• RQ2 Can individually addressable nanobars achieve high diffraction efficiency (>85%) and high directivity in beamsteering and beamsplitting functions?
• RQ3 How does the inclusion of nano-fins affect crosstalk and coupling between resonators in subwavelength arrays?
• RQ4 What is the maximum beamsteering angle and efficiency achievable with a single reconfigurable metasurface?
• RQ5 To what extent do fabrication-induced surface losses affect phase control and reflectance in high-Q metasurfaces?

Key findings

• The metasurface achieves quality factors exceeding 30,000 while maintaining bar spacing <λ/1.5, enabling high Q and subwavelength operation.
• A 51° beamsteering angle is demonstrated with 93% diffraction efficiency using a 2-bar supercell and ±21 V bias.
• A tunable beamsteerer spans 18–31° with up to 86% efficiency using a 3-bar supercell and ±23 V bias.
• Phase variation across the resonance is preserved even with simulated surface losses (k up to 0.02), indicating robustness to fabrication imperfections.
• Extraneous diffraction orders (±2nd and ±3rd) are negligible, contributing less than 5% of total power in larger supercells.
• Pixelating the metasurface enables near-continuous beamsteering by varying unit cell dimensions across adjacent pixels.