Achromatic performance of different single lenses and multicoated lenses. (A) Schematic of designer 3D printed multilayer achromatic metals (MAMs). (B) Schematic of traditional and flat optical lenses (including Fresnel lens, multilevel diffractive lens (MDL), and metals) with single and multilayers. (C) Evolution of focal points at wavelengths of 400, 533, and 800 nm when additional layers are added (results are from an optimized three-layer design of 0.5-NA MAM). (D) The efficiencies, numerical apertures, and bandwidths (operates in the visible band) of various achromatic metallic lenses. The color bar and marker size represent the figure or merit defined as the square root of the sum of the squares of efficiency, NA, and bandwidth. The gray planes indicate the previous limits at bandwidth = 300 nm and NA = 0.35. The NA values of each metal are shown in the legend. Credit: Scientists progressdoi: 10.1126/sciadv.adj9262
Flat optics are made of nanostructures containing high refractive index materials to produce lenses with fine form factors that operate only at specific wavelengths.
Materials scientists have recently attempted to create achromatic lenses to uncover a tradeoff between numerical aperture and bandwidth that limits the performance of these materials. In this work, Cheng-Feng Pan and a team of engineering product development, information technology, and computer engineering scientists in Singapore and China proposed a new approach to design high-performance multi-layer achromatic metal lenses. numerical aperture, broadband and polarization insensitive.
Materials scientists combined topology optimization and full-wavelength simulations to inversely engineer the metal lenses using two-photon lithography. The research team demonstrated the broadband imaging performance of artificial structures under white light and narrow-band red, green, and blue illuminations.
The results highlighted the ability of 3D printed multilayer structures to realize broadband and multifunctional metadevices. The results are now published on Scientists progress and appear on the cover page of the magazine.
Imaging performance
Recent advances in micro- and macro-scale metal lenses have proven important in achieving remarkable imaging performance suitable for a variety of applications in bright field imaging, bioanalysis, medicine and quantum technologies. For example, achromatic lenses feature broadband responses to capture color information, to expand the design possibilities and application scenarios of photonic devices.
Such constructions are ultra-compact, ultra-thin, lightweight and well suited to fabricate compelling metal lenses for imaging systems. Most metal lenses, however, are designed in high refractive index materials to provide good optical control, with strong light making broadband implementation difficult.
Physicists have shown the Abbe number as a figure of merit in lens design to represent a dispersion-free transparent material commonly used for high refractive index materials and as a formula for making a high-efficiency focusing lens.
MAM topology optimization with different layer numbers and spacing distances. (A) Design model and schematic of the optimization region with the indicated parameters described in the text. (B) Relationships of normalized intensity with layer number and spacing distance. With the inverse plan, the best case is at (l, sp) = (3, 1.6 μm). (C) Schematic of edge rounding and surface smoothing approximations at different levels, initial design (i), level 1 rounding the top (ii). Level 2 is generated by applying a 10 nm relative tolerance interpolation to the original height vector (iii), and level 3 is generated by applying a 25 nm relative interpolation (iv). (D) Calculated FWHM (i), efficiency (ii) and position of the maximum focal intensity along the propagation axis (iii) for different levels. The efficiency (ii) is calculated at the focal plane corresponding to the maximum focal intensity. (E) Tilted-view SEM images of MAM fabricated with 0.5 NA: (i) deconstructed MAM showing single, double, and triple (full) layers; (ii) enlarged view of the complete MAM; (iii) top view and size of the MAM; and (iv and v) sectioned MAM revealing the internal structure and details of the 200 nm wide ring structures. Credit: Scientists progressdoi: 10.1126/sciadv.adj9262
The 3D printing method
The research team addressed the manufacturing challenges underlying multilayer achromatic metallic lenses using three-dimensional printing. The nanoscale 3D printing method made it possible to pattern a multilayer lens in a single lithographic step to rapidly prototype complex structures. Using two-photon polymerization, scientists have achieved various 3D designs, including complex microlenses, gradient index lenses, and diffractive lenses.
In this work, Pan and colleagues used topology optimization to achieve achromatic lens behavior. They quickly obtained a stable, multilayered, high-resolution structure.
The resulting multi-layered achromatic metal lenses showed previously unknown levels of effective performance to integrate the benefits of high-resolution 3D printing at the nanoscale to create metal lenses with exceptional performance to inspire a new paradigm for designing and manufacturing multifunctional broadband optical elements and devices.
Design of multilayer achromatic metallic lenses and experimental results
Focusing efficiency and imaging performance of MAM. (A) Comparison of experiment and simulated broadband focusing efficiencies for MAMs with NA of 0.5 and 0.7 at the same focal plane defined by NA. (B) Comparison of experiment and simulated broadband FWHM for MAMs with NA of 0.5 and 0.7 at the same focal plane defined by NA. (C) Optical images of the number “3” in group 6, element 3 of the USAF 1951 resolution target captured via the MAM 0.5-NA under white light and applied to blue (450 nm), green (532 nm) and red (633 nm) filters. Credit: Scientists progressdoi: 10.1126/sciadv.adj9262
The main difference between multilevel metal lenses and multilevel diffractive lenses is the size of the smallest element.
For example, although the minimum feature size can be designed to fit a specific dimension, full-wave simulations are necessary to account for interactions and diffusion between layers. Using filtering and binarization steps, the researchers transformed the designed structure into a real construct.
The team subjected the samples to topology optimization and formed them using the professional Nanoscale GmbH photonic 3D printing system, with a galvo-scanning focused beam to induce cross-linking of a liquid resin into a voxel nanoscale solid at the focal point.
The scientists optimized the manufacturing method to obtain a prototype close to the normal design and evaluated the imaging quality of the product by placing it on a resolution target with a spacing distance of three times the focal length of the lenses.
The designed metal lenses performed well under white light for achromatic imaging applications to show the unmatched ability of metal lenses to eliminate chromatic aberrations. The scientists optimized the parameters to show how the multilayer achromatic metal lenses exhibited high focusing efficiency with broadband performance and topology optimization to precisely realize the designed metal lenses with nanoscale features.
Outlook
In this way, Cheng-Feng Pan and the research team developed a multi-layer metal system and considered each layer as an achromatic corrector and focusing element. The results showed how stacked metasurfaces based on low refractive index materials overcame the limitations of single-layer flat optics to extend the performance of metal lenses to broadband functions while preserving high numerical aperture.
The use of higher resolution 3D printing methods and high refractive index resins will contribute to an increased multifunctional optical system that operates with a broadband response range beyond the visible range to contain an infrared range close or average.
More information:
Cheng-Feng Pan et al, 3D printed multilayer structures for high numerical aperture achromatic metallic lenses, Scientists progress (2023). DOI: 10.1126/sciadv.adj9262
Ren Jie Lin et al, Achromatic metal grating for color light field imaging, Nature Nanotechnology (2019). DOI: 10.1038/s41565-018-0347-0
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