變焦超穎透鏡製程技術優化與成像品質量測

Abstract

超穎介面為平面次波長尺寸的人造奈米結構，透過操控空間區域內的電磁波特性如相位、偏振、振幅等結構去滿足所需的光學應用，可避免傳統透鏡常受限於自然界中折射率而無法達到極小尺寸的情況，且效率的提高與改善使得越來越多相關微型化光電元件的應用。 製造超穎透鏡的技術中，微影為主要且具有較成熟的製程技術，本篇論文透過E-beam電子束直寫出奈米等級的圖案，無須使用任何光罩，其最小線寬可達50 nm，進而製造出深寬比將近13的奈米柱結構，另外我們運用雙光阻塗佈對光敏感度的差異，經過曝光顯影後去呈現上寬下窄的mushroom結構，這個做法有利於後續金屬在保留所需線寬下，不易鍍到下層側壁，在lift-off時可有效剝離金屬層。此外藉由結合扭曲向列型液晶(TN-LC)去變換入射光的極化方向，我們也量測不同極化方向下分別的聚焦能力，再結合1951 USAF空間分辨率測試版以測試經過超穎透鏡後的成像品質。在實驗中驗證了兩個焦點的焦距和聚焦效率皆與模擬結果十分相近，未來也期待能運用在更多光學範疇上。

Metasurfaces are planar nanostructured interfaces that have used increasingly as miniature optical devices since their efficiency has recently been improved. Conventional lighting devices are often limited by refractive index of the natural material while their distribution is determined by the lens shape [25]. Metalens provide a new opportunity for optical applicaions due to the strong capability in manipulating the propagation of light by its phase, polarization and amplitude at a subwavelength scale, metalenses can control the electromagnetic waves to achieve an entire 2π phase shift. For the fabrication techniques of metasurfaces, lithography is the most mature and dominant method. In this paper, we selected electron beam lithography method that producing pattern features by a serial writing without the use of masks to fabricate our metalens. The minimum physical size can be as small as 50 nm, and the maximum aspect ratio is near 13. To increase the tolerance and make the sidewall vertical and smooth, we coated the double photoresist in the process. After the exposure and development, the different light sensitivity of the photoresist presents a mushroom-type. This type is beneficial to the quality of the whole metalens since the metal layer is easier to strip while in the lift-off process. Otherwise, our experiment also demonstrates the focusing ability in different polarization of incident lights and a 1951 USAF resolution chart is used to measure the resolution and magnification of the proposed setup. Since our experiment shows that the two focuses’ focal length and focusing efficiencies are approximate to the simulation results., we also look forward to applying it to more optical fields in the future.