The latter can be manipulated with optimizing the waveguide mode by choosing suitable effective refractive index n eff and thickness h 26, 27, 28. The first term can be controlled by properly designing the resonant response, the Pancharatnam−Berry (PB) phase, or the propagation phase 24, 25, 26, 27. To eliminate the chromatic aberration, both the phase \(\varphi \left(r,\) around the central frequency ω d must be properly controlled in the Taylor expansion of (1). Where r, ω, and F are the radial coordinates, angular frequency, and the focal length.
To gain ultimate control over the phase of the propagating light, both the metalens’ spatial and frequency-dependent phase profile must be controlled. In order to enable practical applications, several metasurface’ characteristics must be carefully designed and optimized.
The replacement of conventional bulk lenses with all-dielectric achromatic metalenses could address the long-standing challenge in on-board biomedical imaging. Very recently, the chromatic aberration of metalenses have been successfully tackled too, making them attractive for active imaging 17, 18, 19, 20, 21, 22, 23. High focusing efficiency and large numerical apertures (NA) have been demonstrated for a single wavelength soon after the invention of metalens 13, 14, 15, 16. Metasurface-based devices uniquely focus incident light to a diffraction-limited spot utilizing thin, flatform structures by precisely tailoring the wavefront 7, 8, 9, 10, 11, 12. All-dielectric metalens that is a two-dimensional metamaterial consisting of a large number of dielectric nano-antennas 4, 5, 6 offers intrinsic advantages for the realization of miniaturized integrated lens. Consequently, the development of integrated optical lens has been a crucial step towards compact microrobots with on-board functionalities as well as other biomedical imaging techniques such as nano-optical endoscopy for high-resolution optical coherence tomography 1, 2, 3. The main restriction for the microrobot size is in fact their on-board optical lenses. Even though significant progress has been made in miniaturizing mobile untethered robots, their size for active imaging ranges from about 1 to 10 mm. Miniaturized untethered robots have been intensively studied due to their potential to bring disruptive advances to medical diagnostics and biological studies. This research paves a solid step towards practical applications of flat photonics. The demonstrated metalens exhibits dramatically increased group delay range, and the spectral range of achromatism is substantially extended to the wavelength range of 650–1000 nm with an average efficiency of 77.1%–88.5% and a numerical aperture of 0.24–0.1. A large-scale fabrication technology has been developed to produce titanium dioxide nanopillars with record-high aspect ratios featuring pillar heights of 1.5 µm and ~90° vertical sidewalls. Herein, we experimentally demonstrate a polarization-insensitive, broadband titanium dioxide achromatic metalens for applications in the near-infrared biological imaging. Consequently, metalens-based optical imaging in the biological transparency window has so far been severely limited. In turn, the recently proposed achromatic metalens utilizing transparent, high-index materials such as titanium dioxide has been restricted by the small thickness and showed relatively low focusing efficiency at longer wavelengths. Even though significant progress has been made, the efficiency of technologically relevant silicon metalenses is limited by the intrinsic material loss above the bandgap.
Over the past years, broadband achromatic metalenses have been intensively studied due to their great potential for applications in consumer and industry products.
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