Ultrahigh Numerical Aperture Silicon Metalens at Visible Wavelengths

High Numerical Aperture (NA) Metalens is among the promising optical elements for ultrathin microscope objective [1, 2]. Meanwhile, crystalline silicon (c-Si) has been shown to be used in the high transmission metasurfaces at visible wavelength because of its low loss and low cost [3, 4].
Here, we demonstrate an oil front-immersion metalens with NA = 1.48, a bandwidth of 211 nm and a focusing efficiency of 48% at 532 nm wavelength, base on a 500 nm thick c-Si on sapphire [2] (compared with 50% for back-immersion TiO2 metalenses with NA = 1.1 [1]). Thereby demonstrating the highest NA of any metalens at visible wavelengths reported to the best of our knowledge. We further implement our metalens onto the laser scanning confocal microscopy as the immersion objective to demonstrate the imaging quality.
We note that this fabrication process is fully compatible with mature CMOS technologies. We envision this design also to be beneficial for immersion metalens doublets, thereby pushing metasurfaces into practical applications such as confocal microscopy and achromatic lenses.
References
[1] W.T. Chen, et. al., Nano Letters, 17, 3188, 2017
[2] H. Liang, et. al., Nano Letters, 18, 4460, 2018
[3] Z. Zhou, et. al., ACS Photonics, 4, 544, 2017
[4] D. Sell, et. al., ACS Photonics, 3, 1919, 2016