A new publication from Opto-Electronic Advances; DOI 10.29026/oea.2024.240085 , discusses how cascaded metasurfaces enable adaptive aberration corrections for focus scanning.
Optical researchers have long dreamed of the perfect control over the focus of light, since this technology is particularly promising in areas such as capturing detailed images of living organisms, manipulating materials with precision using lasers, and using light to move tiny objects, a process known as optical tweezers.
Yet, this pursuit is not without its challenges. One such challenge is the occurrence of scanning aberrations, which can be thought of as the light's focal point straying off course from its intended target, leading to a loss in the clarity and brightness of the light. This affects the accuracy and responsiveness of optical scanning systems.
The conventional wisdom to correct these aberrations has been to employ additional optical components and intricate control mechanisms. While these methods have proven successful, they come with the drawback of being bulky and cumbersome. This approach is at odds with the prevailing movement towards systems that are sleek, compact, and seamlessly integrated into our technological landscape.
A breakthrough in this field comes in the form of metasurfaces, innovative 2D materials that consist of tiny structures called meta-atoms, each designed to interact with light in specific ways. These metasurfaces can dynamically alter light's properties, potentially replacing the need for bulky and complex systems.
However, despite the promise of metasurfaces, there is a gap in developing devices that can effectively manage aberrations at higher frequencies, such as terahertz, infrared, and visible light. The challenge lies in finding a method that can efficiently adjust to these frequencies, which are crucial for precise light manipulation.
Recently, cascaded metasurfaces have emerged, offering a simpler way to control light by rotating layers of metasurfaces to change its properties. This method is more cost-effective, but it still struggles with the precision needed for high-precision optical manipulations due to its global phase-tuning mechanism. However, such cascaded metasurfaces still grapple with the challenge of making precise, localized adjustments needed for high-precision optical manipulations. Their global phase-tuning mechanism, while innovative, makes it difficult to fine-tune the light's focus for dynamic wave generation. The search for a solution that can overcome these limitations continues, as researchers strive to perfect the art of light manipulation.
The research group of Prof. Shiyi Xiao from Shanghai University proposes employing two cascaded transparent metasurfaces to achieve adaptive aberration corrections coordinated with focus scanning. Such adaptive aberration correction is realized by simply rotating two cascaded metasurfaces with carefully designed phase distributions, without the necessity for any additional optical elements or external control algorithms. They develop a generic parameter-solving method to optimize phase-profile parameters for moving a focal spot on any custom-designed curved surface, resulting in enhanced focus quality (both intensity and shape) during scanning. To validate this concept, two meta-devices operating at the terahertz frequency range were engineered and fabricated. Experimental results demonstrate that the first meta-device dynamically scans the focal spot on a planar surface, achieving an average scanning aberration of 1.18% within the scanning range of ±30°, which is a 5.56-fold improvement compared with a hyperbolic scanning lens. Meanwhile, the second meta-device is demonstrated to scan two focal points on a planar surface and a conical surface, exhibiting 2.5% and 4.6% average scanning aberrations respectively. This approach is generic enough to extend to the development of other high-precision optical devices for versatile purposes, which opens the gateway toward dynamic meta-devices capable of adaptive fine-tuning to meet high-precision demands, with potential applications in laser processing, lithography, and optical tweezers.
Keywords: terahertz / focus scanning / aberration correction / dielectric metasurface
