Active metasurface
Metasurface, as a planar surface with subwavelength periodic structures, has versatile and powerful ability for optical beam shaping in a compact size. Active metasurface alters light propagation in time under electric signal control. This programmable device in all-solid state highly promotes performance and stabilities compared with traditional mechanical approaches. Commercialization of active metasurface brings new opportunities in miniaturization of AR/VR display and ranging tech, especially in next generation of LiDAR.
Metasurface
The word "meta" is a Greek word meaning "beyond", which means the properties are beyond the objects in nature. Metasurfaces are a novel class of functional materials designed based on unique artificial micro/nano structures in subwavelength size, so they can interact with surrounding fields as light in abnormal manners.
Abnormal optical performance of metasurface can be explained by Huygens principle. Huygens' principle states that every point on a wavefront can be considered as a new source of secondary wave. Every meta-atom on metasurface is regarded as a secondary source as the light ejected onto it, and the amplitude, phase and polarization of each secondary source highly depend on the structure of meta-atom. The interferences between secondary sources' wave results in the abnormal transmitted or reflected light. Altering the wavefront of propagating light by metasurface design paves the way to better flat optical components.
Optics become flat
Metasurface as a ROE
Most traditional lens designs use a convex or concave shape to focus or expand the incident beam, which is considered as a refractive optical element (ROE). This makes a lens with inevitable thick volume, leading to a bulky optical system. Using a metasurface to take place of traditional lens can highly decrease the thickness of optical components from millimeters to microns. Meanwhile, image distortion may occur as the light passes through the lens regions away from the optical axis. Traditional lens requires bulkier volume for additional assembly or sophisticated fabrication with higher precision. This correction can be easily done in a metasurface system during the meta-atom design process.
Metasurface as a DOE
Using phase difference to control the light highly improves the tunability of light. Diffractive optical elements (DOE) achieve this phase difference based on the different thickness of medium. There exists a trade-off between performance and cost as the fabrication process gets much more complicated with increasing stages of phase. Metasurface alters the phase of light through the in-plane structure difference, so only pattern design needs to be taken into consideration without additional cost. And the performance of metasurface reaches beyond DOE’s with more smooth phase options.
As a substitute for passive optical elements, metasurface offers a bunch of advantages including more compact size, more powerful function, more versatility, and lower cost.
From passive to active
The beam steering performance of a metasurface is highly dependent on the refractive index of metasurface materials and surrounding environment. Based on specific material design, the refractive index is controlled by input electric signal, leading to the metasurface performance change. This is considered as an active metasurface. Compared with passive metasurface, an active one extends additional designability in time scale. Therefore, it offers promising ability in multifunction and dynamic control. Especially, dynamic beam steering capability of an active metasurface highlights its opportunity in the displacement of scanning model in LiDAR system.
Make LiDAR better
One LiDAR system generates extremely accurate and precise distance data by pulsed laser beams. The market of LiDAR grows rapidly due to the increasing need from smartphone 3D sensing, robot smart locomotion, and automotive driving. The optical system, considered as the heart of a LiDAR, consists of a Laser source, beam shaping component, and a photodetector. To make a LiDAR system better in performance, it is extremely vital to design a beam shaping component in an effective, compact, and stable manner.
LiDAR system can be divided into two modes according to the work process: Flash and scanning. A flash LiDAR illuminates the entire field of view at the same time, leading to high frame rate. The distance range and resolution are limited due to the safety requirement of laser power. A scanning LiDAR illuminates part of the field of view at one time, making it possible to obtain high range and spatial resolution at a cost of time resolution.
More effective
Active metasurface provides powerful beam steering capabilities with a wider scanning angle (four times bigger than MEMS) and faster speed (Thousand times faster than moving parts). Thus, a LiDAR system combined with active metasurface results in a higher resolution and frame rate.
More compact
Active metasurface can be packaged in a small chip with control circuits, getting rid of bulky mechanical components and complex optical waveguides. Smaller size means LiDAR can be used in all kinds of desired machines , such as smartphones, floor cleaning robots and automotive driving cars.
More stable
Active metasurface completes light scanning without movable components as micro-electro-mechanical system (MEMS). Material-based scanning also makes metasurface stable to temperature variation. Therefore, a LiDAR system can overcome complex environments with superior stability and reliability.
Conclusion
Nowadays, the commercialization of both passive and active metasurfaces is in high-speed development due to their compatible fabrication and fascinating performance. Mass production of metasurface using semiconductor technology or nanoimprinting technology fulfills the market requirements on the cost, amount and integration. A mature R&D on metasurface has the ability to satisfy all kinds of optical demands with enhanced performance. Also, a LiDAR system consisting of passive and active metasurfaces will offer more accurate distance ranging due to better beam quality and more rapid scanning speed, together with the advantages of stability and miniaturization. There’s no doubt that metasurfaces will appear in our daily life, providing better performance in LiDAR and relevant systems.