一維次波長金屬光柵在偏光調制的應用

Abstract

光柵是一種重要的光學元件，在光學上有許多應用。我們可以將其用來當做偏振片、濾波器，也可以將光柵加在發光二極體上，增加光萃取效率。 本論文主要目的在研究次波長金屬光柵的零階繞射效率，使用的軟體是GSolver及Comsol兩套商用軟體。分別對不同結構的次波長金屬光柵之零階繞射效率進行分析，包含光柵週期，光柵高度，光柵寬度，光柵間孔隙材料對繞射效率的影響。首先，先分析TM與TE波正向入射於自由空間中次波長金屬光柵的光學特性，並藉著場圖討論是由何種共振機制造成電磁波能穿透此次波長金屬光柵。接著討論當光柵在基底上時，基底材料對光柵的繞射效率特性的影響。為了更貼近發光二極體結構，分析當光柵下方具有一層極薄金屬導電層時，其繞射效率的改變。 最後，藉由最佳化金屬光柵的幾何結構參數及孔隙間填充材料，我們可以設計出:TM偏極器，在目標波長460nm處，TM波的零階穿透效率達到80%，而TE波的零階穿透效率小於10%。TE偏極器，在目標波長460nm處，TE波的零階穿透效率達到90%，TM波的零階穿透效率小於10%。建立在二氧化矽上的雙色光偏極器，在波長450nm處，TE波零階穿透效率為90%，在波長550nm處，TM波零階穿透效率為90%。具有不同孔隙材料的複合式金屬光柵，在波長460nm時，TM波零階穿透效率為78%，TE波亦可穿透，其零階穿透效率為40%。

Grating is an important type of optical element since there are lots of applications. Sub-wavelength gratings can be used as a polarizer or a filter. They can also be added gratings on light emitting diode to increase the light extraction efficiency. The purpose of this research is to study the 0th diffraction efficiency of the sub-wavelength metallic gratings. Two suites of commercial software “GSolverTM” and “ComsolTM” are used to design the sub-wavelength metallic gratings. We investigate the influences of the 0th diffraction efficiency of the sub-wavelength metallic gratings with different parameters, including grating period, grating height, grating width, and groove material. At first, we analyze the optical properties of the sub-wavelength metallic gratings in free space as the TM and TE waves are normally incident on them. By plotting the field distribution, we discuss what kind of resonant effect leads the electromagnetic wave to propagate through the sub-wavelength metallic grating. Then we put these gratings on certain substrates and discuss the influences on the diffraction efficiency. To discuss sub-wavelength gratings on light emitting diodes, we put a very thin contact metal layer between the grating and n-GaN and then analyze the 0th order diffraction efficiency. By optimizing the geometrical parameters and the groove material, we can design: for a TM wave polarizer at lambda=460nm, the 0th order transmittance of TM wave is 80% and the 0th order transmittance for TE wave is smaller than 10%. For a TE wave polarizer at lambda=460nm, the 0th order transmittance of TE wave is 90% and the 0th order transmittance for TM wave is smaller than 10%. For a dichroic Mirror on SiO2 substrate, the 0th order transmittance of TE wave at lambda=450nm is 90% and the 0th order transmittance of TM wave at lambda=550nm is 90%. For a compound grating with different groove materials, TM and TE waves can co-propagate through it with a certain ration. We design a compound grating with 78% of the 0th order transmittance of TM wave and 40% of the 0th order transmittance of TE at lambda=460nm.