[Paper Review] Wide Field-of-View, Large-Area Long-wave Infrared Silicon Metalenses

Hung‐I Lin, Jeffrey A. Geldmeier+7 more|arXiv (Cornell University)|Jul 24, 2023

Photonic and Optical DevicesEngineering3 citations

TL;DR

This paper presents a wide field-of-view (140°), large-area (diameter >4 cm) long-wave infrared (LWIR, 8–12 μm) metalens fabricated on float-zone silicon wafers using scalable photolithography and deep reactive ion etching. The design uses a single-layer metasurface with a ZnSe spacer to achieve diffraction-limited performance and enables monochromatic, wide-angle thermal imaging with minimal coma aberration, demonstrating a practical, compact alternative to bulky traditional LWIR optics.

ABSTRACT

Long-wave infrared (LWIR, 8-12 $μm$ wavelengths) is a spectral band of vital importance to thermal imaging. Conventional LWIR optics made from single-crystalline Ge and chalcogenide glasses are bulky and fragile. The challenge is exacerbated for wide field-of-view (FOV) optics, which traditionally mandates multiple cascaded elements that severely add to complexity and cost. Here we designed and experimentally realized a LWIR metalens platform based on bulk Si wafers featuring 140$^\circ$ FOV. The metalenses, which have diameters exceeding 4 cm, were fabricated using a scalable wafer-level process involving photolithography and deep reactive ion etching. Using a metalens-integrated focal plane array, we further demonstrated wide-angle thermal imaging.

Motivation & Objective

• To address the limitations of conventional long-wave infrared optics, which are bulky, fragile, and expensive, especially for wide field-of-view (WFOV) applications.
• To overcome the challenges of coma aberration and low optical throughput in WFOV LWIR systems by developing a single-element metalens architecture.
• To enable scalable, wafer-level fabrication of large-area LWIR metalenses using standard silicon processing techniques.
• To achieve high imaging quality and efficiency in the LWIR band using float-zone silicon to suppress oxygen-related absorption.
• To demonstrate practical wide-angle thermal imaging using a metalens-integrated focal plane array.

Proposed method

• Designed a WFOV metalens using an analytical model to derive the phase profile based on the wavefront aberration minimization principle, specifically targeting telecentric image-space behavior.
• Formulated the phase profile using the equation: φ(s) = (2π/λ) ∫₀ˢ [ -sinα(s) - (s-d)/√(f² + (s-d)²) ] ds, which accounts for the angular dependence of the chief ray and focal spot position.
• Engineered the metasurface on float-zone silicon wafers to minimize oxygen impurity absorption at 9 μm, enhancing transmission across the LWIR band.
• Implemented two designs: one with an air gap and another with a ZnSe spacer to improve wavefront quality and FOV performance.
• Fabricated the metalenses using large-area photolithography and optimized deep reactive ion etching (DRIE) to achieve high-aspect-ratio nanostructures for broad phase coverage.
• Validated performance via experimental characterization of point spread functions (PSFs) and modulation transfer function (MTF) using a 10.6 μm CO₂ laser and interferometric wavefront sensing.

Experimental results

Research questions

• RQ1 Can a single-layer silicon metalens achieve a 140° field-of-view in the long-wave infrared band while maintaining diffraction-limited performance?
• RQ2 How does the inclusion of a ZnSe spacer improve wavefront quality and imaging performance compared to an air gap in a WFOV metalens?
• RQ3 To what extent can float-zone silicon wafers suppress oxygen-related absorption to enable efficient LWIR transmission in large-area metalenses?
• RQ4 Can a metalens-integrated focal plane array achieve wide-angle thermal imaging with minimal image stitching and high fidelity?
• RQ5 What is the trade-off between optical complexity, throughput, and imaging quality in WFOV metalens systems compared to traditional multi-element compound lenses?

Key findings

• The metalens achieved a 140° circular field of view with a diameter exceeding 4 cm, demonstrating a significant advancement in WFOV LWIR optics.
• The ZnSe-spacer metalens design achieved diffraction-limited performance at 10.6 μm, with measured MTF values matching simulation predictions and confirming high optical efficiency.
• Experimental PSF measurements at various angles of incidence showed excellent agreement with simulations, validating the analytical design model across the full FOV.
• Thermal imaging was successfully demonstrated using the ZnSe-metalens integrated with a microbolometer, capturing a 100° FOV image of a perforated card with clear resolution of the pattern.
• Lateral translation of the sensor and image stitching enabled full coverage of the large image plane, confirming the feasibility of large-area imaging with a single metalens.
• The use of float-zone silicon effectively suppressed the 9 μm oxygen absorption band, enabling high transmission across the LWIR band while maintaining compatibility with standard semiconductor fabrication.