Planar optics are made of nanostructures containing high-refractive index materials, and their lenses are thin and can only operate at specific wavelengths.
Recently, materials scientists have attempted to design aberration-free lenses to address the trade-off between numerical aperture and bandwidth, which limits the performance of these materials. In the fields of engineering product development, information technology, and computer engineering in Singapore and China, Pan Chengfeng and his team have proposed a new approach to design multilayer aberration-free metal lenses with high numerical aperture, broadband and polarization insensitivity.
Materials scientists used a combination of topology optimization and full-wavelength simulation to reverse-engineer these metal lenses, fabricated using two-photon lithography. The research team demonstrated the broadband imaging performance of an engineered structure under white light and narrowband illumination of red, green, and blue.
The results highlight the ability of 3D printed multilayer structures to achieve broadband and multifunctional components. These ultra-compact, ultra-thin, and lightweight structures make them ideal for making excellent metal lenses for imaging systems. However, most metal lenses are patterned with high refractive index materials to provide good optical control, but the strong light makes broadband implementation challenging.
Physicists cite the Abbe number as an excellent performance in lens design, representing a dispersion-free transparent material that is commonly used for high refractive index materials and as a formula for achieving highly efficient focusing lenses. The research team used 3D printing to solve the manufacturing challenges behind multilayer aberration-free metal lenses. The nanoscale 3D printing method allows for the patterning of multilayer lenses in a single lithography step, allowing for rapid prototyping of complex structures. Using two-photon polymerization, scientists have achieved a variety of three-dimensional designs, including complex microlenses, gradient index lenses, and diffractive lenses.
In this work, Pan and colleagues used topology optimization to achieve aberration-free lens behavior. They quickly achieved a stable, multi-layered, high-resolution structure.
The resulting multi-layer, aberration-free metal lenses demonstrate an unprecedented level of efficient performance, combining the benefits of nanoscale high-resolution 3D printing to create metal lenses with superior performance, providing a new paradigm for the design and manufacture of versatile broadband optical components and devices. The main difference between a multilayer metal lens and a multilayer diffractive lens is the size of the smallest feature size.
For example, while the smallest feature size can be designed to fit a specific size, full-wavelength simulations are required to account for interlayer interactions and scattering. By using filtering and binarization steps, the researchers translate the designed structure into a real one.
The team used the NanoScale GmbH Photonics Specialty 3D Printing System for topology optimization and cross-linked the liquid resin using a scanning focused beam to form a nanoscale solid voxel. The scientists optimized the manufacturing method to achieve a prototype close to the normal design and evaluated it by placing it on a resolution target that was three times the focal length of the objective.
Engineered metal lenses perform well under white light and can be used in chromatic-aberration imaging applications, demonstrating the unmatched ability of metal lenses to eliminate chromatic aberration. The scientists optimized the parameters and demonstrated that the multilayer aberration-free metal lens has high focusing efficiency, broadband performance, and topology optimization, resulting in a precise realization of the nanoscale characteristics of the design.
Pan Chengfeng and his research team developed a multilayer metal lens system and treated it as a chromatic-free corrector and focusing element. The results show that the stacked metasurfaces based on low refractive index materials overcome the limitations of single-layer planar optics and extend the performance of metal lenses to broadband functions while maintaining a high numerical aperture. The use of higher resolution 3D printing methods and high refractive index resins will help to achieve enhanced and versatile optical systems with a broadband response range beyond the visible range of the near- or mid-infrared range.