消色差超穎透鏡陣列之光場影像

Abstract

視覺是生物感知周遭環境訊息的最重要的系統，與人類的一雙眼睛相比，昆蟲的視覺系統是由一個陣列的微小眼睛所組成的，又稱作複眼，如此的視覺系統具有較大的視野以及能夠估計物體深度的優勢，如此的特色已經吸引了人類的科技嘗試去發展類似的光學成像系統，例如光場相機。利用微透鏡陣列，我們可以將光束的位置以及行走(角度)的方向擷取出來，得到四維的光場資訊，再利用演算法得到物體的深度資訊，相較於一般的成像系統，只能得到光的二維位置資訊，四維的光場技術不僅可以得到物體的深度資訊，還能做到先拍照後聚焦的成像的功能。然而目前微透鏡陣列的製作技術上，例如電子顯微術、紫外光顯微術、光阻融化或是奈米壓印技術等等，越微小的微透鏡越無法精準的控制材料表面的曲率與品質的好壞，並且是無法做到任意的聚焦長度跟數值孔徑以及帶有色差與球差的缺點，這樣的問題目前是無法輕易地解決的。 如今新崛起的奈米光學技術，二微的超穎材料 - 超穎介面驚人的快速發展出許多非常輕薄短小的尖端光學元件，例如偏振轉換、非線性元件、全像片、超穎透鏡等等，但是源自於奈米天線共振以及繞射光學導致的嚴重色差使得超穎透鏡的實用性面臨一大考驗，然而近年來如此的可見光色差已經透過集體共振單元的特性得以消除，這項突破性的發展導致超穎透鏡的成像應用快速崛起。 在這篇論文中，我們利用非常輕薄的氮化鎵消色差超穎透鏡陣列(AMLA)去捕捉四維的光束資訊，並且利用演算法去得到不同成像面的影像以及物體的深度資訊，並且可以進一步地得到物體速度的資訊，消色差超穎透鏡陣列相較於傳統的微透鏡陣列具有許多優勢，例如消色差、無球差、品質穩定、焦距與數值孔徑可以任意設計、未來可直接與CMOS CCD直接以半導體製程整合的優勢。

Vision is the most important system for living creature to perceiving surrounding environmental information. Compared with a human eyes, the insect's visual system is composed of an array of tiny eyes, also known as compound eyes. Such a visual system has a large field of view and the advantage of estimating the depth of the object. Such features have attracted human technology to try to develop similar optical imaging systems, such as light field cameras. The light field image records the position and direction information of light rays distributing in the target scene which can be captured by the use of microlens array. Compared with the conventional imaging system, four-dimensional light field imaging system can provide not only the two-dimensional intensity but also two-dimensional momentum information of light which enables the scene to be reconstructed with refocusing images and depth of objects. However, it is not easy to obtain the precise shaping and low defect microlens array or different forms (convex or concave) or arbitrary numerical aperture (NA) at one microlens array by most of fabrication processes such as electron beam lithography, UV- lithography, photoresist melting, nanoimprinting lithography and so on. Metasurfaces, the two-dimensional metamaterials, have appeared as one of the most rapidly growing fields of nanophotonics. They have attracted extensive research interest because their exceptional optical properties and compact size can provide technical solutions for cutting-edge optical applications, such as imaging, polarization conversion, nonlinear components, and hologram. Recently, the chromatic aberrations of metasurface, resulting from the resonance of nanoantennas and the intrinsic dispersion of constructive materials, has been eliminated in visible region by using incorporating an integrated-resonant unit element. It gives rise to a burst of upsurge of imaging applications by using metalens. Here, we propose a light field imaging system with an ultra-compact and flat GaN achromatic metalens array without spherical aberration to acquire four-dimensional light field information. Using this platform and rendering algorithm, we can get the reconstructed scene by a series of images with arbitrary focusing depths slice-by-slice and depth of objects. Compared with microlens array, the advantages of our metalens array are achromatism, spherical aberration free, focal length and numerical aperture can be arbitrarily designed, and can directly integrate with CMOS CCD by semiconductor fabrication process.