Achromatic Flat Subwavelength Grating Lens Over Whole Visible Bandwidths

One of the major challenges for emerging planar subwavelength micro lens is the significant chromatic behavior due to phase mismatch of subwavelength phase shifters. In this letter, the achromatic micro grating lens covering the whole visible wavelength is demonstrated by utilizing relatively low index contrast gratings. Based on the unique chromatic phase shift behavior of polymer nanostructure, we have designed and fabricated a broadband subwavelength achromatic microlens that can cover 250 nm of visible bandwidths (from 435 to 685 nm) with focal shift less than 5%. Our works also represent the first time to design, fabricate, and characterize micrograting lens ( $7~\mu$ m in size) promising for compact integrated nanophotonic devices on chip. In addition, there are more advantages of using a polymer-based microlens, such as easy fabrication on flexible substrate and potential applications, including imaging, spectroscopy, lithography, laser fabrication, and future integrated wearable devices.