An Ultra-Robust Ultra-Broadband Modified Moiré Varifocal MetaLens for Terahertz Wave

Terahertz (THz) waves, known for their fingerprint spectroscopy, penetration capabilities, and biological safety, present significant potential for sensing and imaging applications. However, the lack of high-performance THz devices significantly limits the broader adoption of THz technology. A critical component for advancing THz systems is the development of lenses with broadband varifocal capabilities, which are vital for fully leveraging the broad spectral range of THz waves. Moiré varifocal metalenses (MML), which enable zooming through the relative rotation of two identical metasurfaces, have emerged as a potential solution, offering an extensive varifocal range, and an invariant optical path length. Despite these advantages, traditional MML face limitations such as an inability to achieve continuous focal adjustment, poor resilience to interference, and a narrow operating bandwidth, which present a major obstacle to their practical use. To address these limitations, we have designed a modified MML featuring a novel phase distribution that significantly improves the performance of the metalens. In a groundbreaking experiment, we demonstrated the superior performance of this MML in the THz band, using a terahertz free-electron laser (THz-FEL) with excellent beam quality and tunable bandwidth. The modified MML exhibited exceptional robustness, including continuous focal length tunability, spatial robustness against lens displacement, and an ultra-broadband range covering approximately 40% of the central wavelength. Additionally, this multifunctional THz imaging metalens offers capabilities such as broadband focusing, wide-range zooming, and fixed focal length. Our results demonstrate that the modified MML provides outstanding stability, a lightweight structure, ease of adjustment, and simple fabrication, marking a significant advancement in the development of practical, multifunctional THz optical devices. With its broad potential for applications in integrated THz systems and imaging, this lens paves the way for more versatile and higher-performance THz technologies. Furthermore, its universal design extends beyond the THz range, offering a promising approach for varifocal lens design across the entire electromagnetic spectrum.