利用金屬長條結構設計週期超穎材料應用於極化偏轉與負折射率

Abstract

我們提出由兩層分別具有四個金屬長條狀結構所形成的旋光性超穎材料做為一種可以使線性極化光偏轉的極化偏轉器，並且此超穎材料在左、右圓極化光的激發下於特定微波頻率具有負折射率，此負折射率的產生不需要同時為負值的介電係數與導磁係數。 由於位在基板上下方之金屬結構在入射光傳播方向上結構的不對稱姓，此超穎材料在左、右圓極化光的激發下之穿透率與穿透係數有很大的差別，進而造成當線性極化光入射時，其極化平面在角度上的偏轉與產生不同的極化狀態。 在特定頻率下，此材料設計在線性極化之入射光下，可以讓其於穿透材料後仍保持其極化狀態下並且具備大角度之極化偏轉。 藉由觀察此超穎材料在左、右圓極化光入射下的穿透率曲線可以發現其對於這兩種極化光源之穿透率在兩個共振頻率下具有很大的差別，這兩個共振頻率分別對應到電性與磁性上的響應。 此外，我們也探討了不同長度之長條狀結構與由四個金屬長條狀所構成的正方形之邊長對於此旋光性超穎材料在光學上的影響。

We propose the four-bar shaped chiral metamaterial which can be used as a polarization rotator for linear polarized incident wave and exhibits negative refractive index for both left and right circular polarized waves (LCP and RCP) without the need of simultaneous negative permittivity and permeability. Owing to the lack of structure symmetry along the propagation direction of incident wave, large differences between the transmission and transmission coefficients for RCP and LCP occur, resulting in the rotation of the polarization plane and circular dichroism. At certain frequencies, large polarization rotation angle for pure optical activity is achieved. The two frequencies where large difference between transmission for RCP and LCP occurs correspond to a magnetic response and electric response. Furthermore, the influence of different arm length and side length of the square formed by the bars on the performance of the chiral metamaterial is as well investigated.