Metaphotonics for Advanced Imaging Techniques—Electrically Tunable Varifocal Metalens and 3D Flash LiDAR

Metasurface science is further expanding field of optics and photonics by providing alternative device platform of ultra-compact and multifunctional flat optical devices. Along with the advances in design principles for metasurfaces, the metasurface can realize multi-functionality within a single optical element, in which multiple degrees of freedoms of light can be modulated simultaneously at will.<br/>In this talk, I will introduce metasurface-driven advanced imaging techniques. First, I will show an electrically tunable varifocal metalens that operates at visible wavelengths [1]. By combining a spin-multiplexed metalens with a liquid crystal (LC) cell, we are able to successfully demonstrate active switching between focal planes on the scale of milliseconds. A single meta-atom contains two degrees of freedom of phase information (i.e. geometric and propagation phase). Thanks to its dual phase modulation, a spin-dependent bifocal function is realized. The bifocal metalens is integrated with LC modulator where a circular polarization state of passing through metasurface is determined according to the applied electric bias. Within a millisecond, a focal point can be switched and also it enables electrically-tunable zoom change. Secondly, I will propose a point cloud generating metasurface for advanced 3D depth imaging or LiDAR application [2]. The proposed flash-type LiDAR is able to spread 10K dots over 2π full space with over 80% diffraction efficiency. The unit meta-atom is rigorously designed via convolution theorem at the reciprocal space to gain equal intensity point cloud. With an aid of stereo matching method, we simply capture distorted 3D point clouds and reconstruct 3D object (or depth information of the object). Such beam scanning-free LiDAR method has a variety of advantages like compactness, robustness and faster data processing over scanning-type LiDAR systems.<br/>We believe such metasurface-driven advanced imaging techniques will further accelerate real-life applications such as ultracompact focus-tunable camera systems, indoor LiDAR sensor for face/object/motion recognition [3], advanced augmented/virtual/mixed reality displays [4].<br/>[1] I. Kim* <i>et al</i>., <i>Advanced Science </i>8, 2102646 (2021)<br/>[2] G. Kim <i>et al</i>., <i>Nature Nanotechnology </i>(submitted)<br/>[3] I. Kim <i>et al</i>., <i>Nature Nanotechnology </i>16, 508 (2021)<br/>[4] I. Kim <i>et al</i>., <i>Advanced Materials </i>32, 2070378 (2020)