次波長金屬光柵應用於相位延遲

Abstract

頻率選擇面之二維週期結構決定了其濾波特性，經由適當設計，可以使其具有雙折射晶體之特性，除了受到材料以及厚度之限制相對較小，其設計之自由度也相對較高。頻率選擇面元件係由次波長金屬光柵所構成，可直接整合至發光元件電極之製程中，並能增加發光元件效率。本篇論文旨在設計出一在可見光操作範圍內具有相位延遲片特性之金屬光柵，主要方法係利用電磁模擬方式分析金屬光柵圖形變化對其相位延遲之影響，針對微波頻段上被廣泛使用作為圓偏振片使用之蜿蜒線(meander-line)圖形，將其分為L、C、長方環作討論，然金屬在可見光波段會有色散影響，故操作頻率之改變以及幾何圖形調整並非呈現線性關係，在設計方法修正下，蜿蜒線圖形在波長為496 nm下，其場強比可達1.037，相位差為0.4745π，而L與C圖形在波長等於734 nm以及660 nm下，也可達到場強比為1，相位差分別為0.4989π以及0.434π。除蜿蜒線外，對於具有極化旋轉特性之螺旋環圖形也作一分析討論，在模擬結果中可發現螺旋環圖形之兩特徵入射偏振角度，對於不同入射偏振角度，此一螺旋圖形可產生橢圓偏振、圓偏振以及線性偏振狀態，並可產生最大值可達45度之極化旋轉角度。

In general, FSSs comprise of periodically arranged metallic patch elements or aperture elements within a metallic screen. By adjusting the period and the other geometric parameters of the metallic pattern, it can form birefringence and be used as a quarter wave plate (QWP). Compared to the traditional QWP which is made of the birefringence crystal, FSSs are not limited by the material and thickness. We propose a FSS, which consists of a sub-wavelength metallic grating, to act as a QWP. And the operation frequency is set in the visible region. The pattern we used is the meander-line (MNDL) pattern which can be decomposed into L, C and O elements. In addition to the meander-line, we also use the spiral pattern, which consists of semicircles. The design approach is using the electromagnetic simulation software, CST microwave studio, to analyze the effect of adjusting the structure and the pattern of the metallic array. First, we suppose the metallic grating as a perfect electric conductor (PEC) and design it to satisfy the criterion of QWP. Second, the dispersion relation of metallic material is added for the real situation and we modify the metallic grating by the rules derived under the PEC condition. Third, we use the cascade structure to improve the performance of the grating. The results show that the ratio of the electric field (Ay/Ax) of L and C pattern grating can attend to 1, and the phase difference are 0.4989π and 0.434π at 734nm and 660nm, respectively. When we use MNDL grating, the ratio (Ay/Ax) and phase difference are 1.037 and 0.4745π at 496nm. Different from the grating we mention above, the spiral pattern grating can produce linear, ellipse and circular polarization states by using different incident azimuth angles. The spiral pattern grating is not only used as a QWP but also used as a polarization rotator, and the maximum rotation angle can be 45°. The operation frequency is determined by total length of the spiral.