3D printed multilayer structures for high-numerical aperture achromatic lenses

Flat optics are made of nanostructures containing high-refractive index materials to produce lenses with thin form factors that function only at specific wavelengths.

Materials scientists have recently attempted to achieve achromatic lenses to uncover a tradeoff between the numerical aperture and bandwidth that limits the performance of such materials. In this work, Cheng-Feng Pan and a team of scientists in engineering product development , information technology , and computer engineering in Singapore and China proposed a new approach to design high numerical aperture, broadband, and polarization-insensitive multilayer achromatic metalenses.

The materials scientists combined topology optimization and full wavelength simulations to inversely design the metalenses using two-photon lithography . The research team demonstrated the broadband imaging performance of the engineered structures under white light and red, green, and blue narrowband illuminations. The outcomes highlighted the capacity of the 3D-printed multilayer structures to realize broadband and multifunctional meta devices. The outcomes are now published on Science Advances and are featured on the cover page of the journal.

Achromatic performance of different single lenses and multilayer lenses. (A) Schematic of the designer 3D-printed multilayer achromatic metalens (MAM). (B) Schematic of traditional and flat optics lenses [including Fresnel lens, multilevel diffractive lens (MDL), and metalens] with single layers and multilayers. (C) Evolution of focal spots 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 (works in the visible band) of various achromatic metalenses. The color bar and marker size represent the figure or merit defined as the square root of the sum of squares of efficiency, NA, and bandwidth. The gray planes indicate previous limits at bandwidth = 300 nm and NA = 0.35. The NA values of each metalens are illustrated in the legend. Credit: Science Advances , doi: 10.1126/sciadv.adj9262 Read more

Topology optimization of the MAM with different layer number and spacing distance. (A) Design model and schematic of the optimization region with indicated parameters described in the text. (B) Relations of the normalized intensity with the layer number and spacing distance. With the inverse design, the best case is located at [l, sp] = [3, 1.6 μm]. (C) Schematic of the edge rounding and surface smoothness approximations at different levels, initial design (i), level 1 by rounding the top (ii). Level 2 is generated by applying 10-nm relative tolerance interpolation to the origin height vector (iii), and level 3 is generated by applying 25-nm relative interpolation (iv). (D) Calculated FWHM (i), efficiency (ii), and position of 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 the fabricated MAM with 0.5 NA: (i) deconstructed MAM showing single, double, and triple (full) layers; (ii) enlarged view of full MAM; (iii) top view and size of the MAM; and (iv and v) sectioned MAM revealing internal structure and details of the 200-nm-wide ring structures. Credit: Science Advances , doi: 10.1126/sciadv.adj9262 Read more

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 in the USAF 1951 resolution target captured through the 0.5-NA MAM under white light and applied blue (450 nm), green (532 nm), and red (633 nm) filters. Credit: Science Advances , doi: 10.1126/sciadv.adj9262 Read more