High-refractive index materials are incorporated into nanostructures to create lenses with tiny form factors that are only functional at particular wavelengths, which is how flat optics are formed.
Recently, efforts have been made by materials scientists to create achromatic lenses in an effort to determine the trade-off between bandwidth and numerical aperture that restricts the capabilities of these materials. In this paper, Cheng-Feng Pan and a group of scientists from China and Singapore with backgrounds in computer engineering, information technology, and engineering product development proposed a novel method for creating multilayer achromatic metalenses with high numerical aperture, bandwidth, and insensitivity to polarization.
The materials scientists used two-photon lithography to inversely design the metalenses by combining topological optimization with full wavelength simulations. The study team used red, green, and blue narrowband illuminations along with white light to illustrate the built structures’ broadband imaging capabilities.
The results demonstrated how 3D-printed layered structures could be used to create broadband and multipurpose meta devices. The results have been published on Science Advances and are showcased on the journal’s cover page.
Visual performance
Recent advancements in both macro- and micro-scale metalenses have demonstrated the importance of achieving exceptional imaging performance appropriate for a range of applications in quantum technologies, bioanalysis, light-field imaging, and medicine. For example, broadband responses are shown by achromatic lenses to collect color information, hence increasing the design options and application scenarios for photonic devices.
These structures are incredibly lightweight, thin, and compact, making them ideal for creating powerful metal lenses for imaging systems. Broadband implementation is difficult since most metalenses are patterned in high refractive index materials, which offer superb optical control but have a strong light.
In lens design, physicists have demonstrated the Abbe number’s value as a formula to achieve a high-efficiency focusing lens and as a figure of merit to indicate a dispersion-free transparent material that is frequently employed for high-refractive index materials.
The process of 3D printing
By adopting three-dimensional printing, the study team was able to overcome the fabrication hurdles that underlie multilayer achromatic metalenses. Complex structures might be quickly prototyped by patterning a multilayer lens in a single lithographic step thanks to the nanoscale 3D printing technique. The researchers created a range of 3D shapes, such as diffractive lenses, gradient index lenses, and intricate microlenses, using two-photon polymerization.
In this work, achromatic lensing behavior was achieved through topology optimization by Pan and colleagues. They swiftly produced a multilayer, stable, and well resolved structure.
The resultant multilayer achromatic metalenses demonstrated previously unheard-of levels of effective performance, combining the benefits of high-resolution 3D printing at the nanoscale to produce metalenses with remarkable performance that will spur the development of novel approaches to the design and manufacture of multifunctional broadband optical elements and devices.
Creating multilayer achromatic metal lenses and the results of the experiments
The size of the smallest feature is the main distinction between multilayer diffractive lenses and multilevel metalens.
For example, the minimum feature size can be tailored to fit a particular dimension, but full-wave simulations are needed to take scattering and interlayer interactions into consideration. The scientists created a genuine build out of the intended structure by using filtering and binarization techniques.
The samples were created utilizing the Nanoscale GmbH photonic professional 3D printing system, with a galvo-scanned focused beam utilized to induce crosslinking of a liquid resin into a nanoscale solid voxel at the focal site. The samples were then subjected to topological optimization.
The scientists placed the product on a resolution target that was three times the focal length away from the objectives in order to evaluate the imaging quality. They then improved the fabrication process to produce a prototype that was nearly identical to the original design.
The modified metalens demonstrated its unparalleled ability to eliminate chromatic aberrations by performing effectively under white light for achromatic imaging applications. The multilayer achromatic metalenses demonstrated high focusing efficiency with broadband performance and topological optimization to precisely realize the designed metalenses with nanoscale features, as demonstrated by the scientists’ optimization of the parameters.
Prospects
Cheng-Feng Pan and the research group created a multilayer metalens system in this manner, treating each layer as a focusing element and achromatic corrector. The outcomes demonstrated how the low-refractive index materials-based stacked metasurfaces overcome the limitations of single-layer flat optics to enable the metalenses to operate in broadband functions while maintaining a high numerical aperture.
High refractive index resins and higher resolution 3D printing techniques will help create a more multipurpose optical system that can operate in the near or mid-infrared spectrum and has a broadband response range that extends beyond the visible spectrum.