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 10вЂ‰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 650вЂ“1000вЂ‰nm with an average efficiency of 77.1%вЂ“88.5% and a numerical aperture of 0.24вЂ“0.1.

A large-scale fabrication technology has been developed to produce titanium dioxide nanopillars with record-high aspect ratios featuring pillar heights of 1.5вЂ‰Вµm and ~90В° 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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