以多重奈米柱表面電漿超穎介面設計寬頻高效率之極化分光器

Abstract

偏振測量技術是一種常見的光學分析手段，偏振可以揭示一些原本不可見的特徵，例如結構應力、雙折射性、旋光性等，然而在偵測過程中往往只能記錄強度，卻丟失了相位的資訊，因此我們必須在量測前先使用極化分光器將正交極化光倆倆分離，為了確定全史托克斯參數(能完整描述任意光的偏振態)，至少需要進行四種獨立的偏振測量，這使得光學系統的體積通常龐大。近年來人們透過對超穎表面適當的設計，可有效控制結構的相位響應以實現對電磁波的行為操控，並由於超穎表面為次波長尺寸，相較傳統光學元件展現了輕薄的優勢。 基於以上原因，本篇論文我們將利用相位梯度電漿子超穎表面設計兩個極化分光器，分別量測偏振態x/y和±45°，設計中心波長為730 nm，並且嘗試藉由積體共振單元的設計，利用其複雜的耦合機制達成高效率、寬頻的極化分光器，成功將x/y極化分光器的效率由50%提升至70%，頻寬由130 nm提升至280 nm；±45°極化分光器效率則由50%提升至60%，頻寬由125 nm提升至240 nm。我們進一步考慮了兩組極化分光器在y方向交替排列出現的高階繞射，為了解決這個問題，我們基於最佳化與傅氏光學之計算獲得只沿著x方向重新排列的相位分佈，並通過全波模擬驗證了此種解決方式依然能同時分離四組對應的偏振光，其總效率為50%且四種極化對比度高於90%。

Measurement techniques involving polarization information is a common optical analysis method, which reveals invisible features, such as structural stress, birefringence, optical rotation and so on. However, only the intensity can be recorded without the phase information in the detection process. Therefore, we must use polarization beam splitter to separate the orthogonally polarized light during the measurement. To determine the full Stokes parameter, which can fully describe the polarization state of light, at least four individual polarization measurements are required. However, this makes the optical system usually bulky. In recent years, the technology of phase control has been realized through the appropriate design of metasurfaces, which enable us to fully manipulate the behavior of electromagnetic waves. As the metasurfaces are subwavelength, they exhibit ultra-thin and light-weight property compared to the traditional optical components. Based on the above reasons, in this thesis we use the phase-gradient plasmonic metasurfaces to design two polarization beam splitters to measure x/y and ±45° polarization states at the design wavelength of 730 nm. We try to achieve a broadband and high-efficiency polarization beam splitter via the complex coupling mechanism of the multi-rod metasurfaces. We have successfully increased the efficiency of the x/y polarization beam splitter from 50% to 70% in a bandwidth from 130 nm to 280 nm. The efficiency of the ±45° polarization beam splitter has been enhanced from 50% to 60%, and the bandwidth was improved from 125 nm to 240 nm. In addition, we consider the higher-order diffraction of two polarization beam splitters arranged alternately in the y-direction. In order to solve this problem, we rearranged the phase distribution with a variation only along the x-direction through optimal designs based on the theory of Fourier optics. This solution is verified by the full-wave simulation that the rearranged phase polarization splitter can still separate four corresponding polarized lights simultaneously with a total efficiency of 50%, and four polarization contrasts are higher than 90%.