Broadband Achromatic Metalens for the Short-Wave Infrared

TL;DR

This work tackles the challenge of broadband achromatic focusing in the short-wave infrared (SWIR) band (1.8–2.3 μm) by designing a metalens on a CaF2 substrate that uses a single-bar silicon nanocell array coupled with geometric phase tuning and Pancharatnam-Berry (PB) phase engineering. A multi-wavelength phase library is generated and optimized to achieve continuous 0–2π phase control across the band, aiming to suppress chromatic aberration and maintain a stable focus. Finite-difference time-domain simulations show the metalens maintains a stable focal position with a total focal-length variation within 6% of the designed length, while delivering moderate focusing efficiency (20–38%) and high transmission (~83%), albeit with polarization purity DoP degrading toward the long-wavelength edge. The results highlight a practical, scalable approach to broadband SWIR optics with potential impact on quantum communication and sensing, and outline fabrication feasibility and trade-offs for polarization control that guide future optimizations.

Abstract

The 1.8-2.3 μm band lies within the short-wavelength infrared (SWIR) region and serves as a key operational window for a wide range of applications, including quantum sensing, molecular spectroscopy, and free-space quantum and classical optical communication. Despite its significance, optical devices operating in this band still face two major challenges: chromatic aberration across the wide spectral range and the difficulty of integration due to bulky optical elements. Metalenses are composed of subwavelength nanostructures that locally control the phase and group delay of light, enabling precise wavefront shaping and broadband dispersion compensation. These capabilities make them highly promising for use in infrared optical systems, particularly in applications such as focusing and imaging for compact integrated devices. In this study, we propose a metalens design based on a CaF2 substrate, where each nanocell consists of a single-bar silicon structure. These nanocells are periodically arranged with a 900 nm period, enabling precise control of dispersion and phase. By systematically finetuning the bar length and width, the design enables simultaneous dispersion compensation and phase modulation, achieving stable focusing performance over a broad spectral range. Finite-Difference Time-Domain (FDTD) simulations demonstrate that the metalens design achieves effective suppression of chromatic aberration in the 1800-2300 nm range, maintaining a stable focal position throughout the bandwidth with variation within 6% of the focal length. This design offers a compact, broadband, and high-performance approach for beam collimation and wavefront shaping in the SWIR band, with promising potential for applications in quantum communication and sensing systems.

Figures (12)

• Figure 1: Timeline of representative metalens developments across the near-infrared (NIR), short-wavelength infrared (SWIR), and mid-infrared (MIR) regions. Each point marks a key study identified by its publication year and operating wavelength. The error bars indicate the range of operating wavelengths, respectively. The diagram compiles results from multiple works Chen2012Wang2015Yu2019Zhang2020Xiao2023Zou2025Zhang2016Guo2017Ou2020Yue2023Xu2025. Notably, fewer works have been reported in the SWIR band of 1.8-2.3 $\mu$m, which is the focus of this study.
• Figure 2: Schematic diagram showing the ideal wavefront distribution for a metalens. To focus parallel incident light to a single focal point at distance $f$, the metasurface must introduce a spatially varying phase delay to compensate for the optical path difference between the center and off-axis positions. The required phase compensation at each radial distance $R$ from the optical axis is given by $\Delta = \sqrt{R^2 + f^2} - f$, ensuring that all rays interfere constructively at the focus Chen2018.
• Figure 3: Calculated phase distribution of the metalens along the radial direction for three different design wavelengths ($\lambda_{\text{min}} = 1.8\,\mu\text{m}$, $\lambda_{\text{mid}} = 2.05\,\mu\text{m}$, and $\lambda_{\text{max}} = 2.3\,\mu\text{m}$). The curves indicate the required phase compensation to achieve focusing at the designed focal length, showing the variation of phase with respect to wavelength.
• Figure 4: Unitcell profile and structure of Metalens. (a) Schematic of the unit cell geometry used in the metalens design. The rectangular dielectric nanopillar has a length l, width w, and height h, and is placed on a dielectric substrate. (b) Top view of the metalens layout, showing the spatial arrangement of rectangular meta-atoms with varying geometries across the aperture.(c) Perspective view of the complete metalens composed of anisotropic nanopillars, where each unit is tailored to locally modulate the transmitted wavefront.
• Figure 5: Phase response maps of the unit cell under different wavelengths (1800 nm to 2300 nm ). The phase distribution is plotted as a function of nanostructure length and width, showing a smooth phase variation across the SWIR band and achieving full 0-2$\pi$ modulation.