介質波導之光力分析

Abstract

近年來，光對於物體所產生的力學效應吸引了科學家們的注意，已知在巨觀尺度下，光對於物體所產生的影響相當地小，可是當尺度縮小為奈米尺度時，光所產生的影響則會變得相當地大而使結構產生明顯的形變，也因為此現象的發現，光力學(Optomechanics)成為了近幾年來相當熱門的題目。 當光在兩個平行的介質波導中傳播時，波導和波導之間的散射波會互相干涉，導致表面的電磁場產生變化，進而使得兩個波導之間產生光力(optical force)，而這兩個介質波導之間所產生的力量就是本論文所要分析的目標。 本論文先利用有限元素法求出兩個平板介質波導的穩態電磁場，再將表面的電磁場帶入馬克斯威爾應力方程式，求得電磁場施加在波導表面的應力後，接著應力對表面做積分得到波導的平均受力，最後與理論值互相對照，以期望能得到相同的結果。 由於改變波導結構可以改變場的分佈，進而改變力量分佈，故接下來我們嘗試改變波導形狀以試著找出能增強力量的結構，研究結果發現在波導內挖洞確實可增強力量，因此，接下來討論各種輸入條件下挖洞後的波導能產生的力量最大值以及相關特性。 在應用方面，可以在兩個波導之間加上壓電材料，以達到利用波導的施力來驅動材料的目的，因此接下來在波導之間加上與壓電材料的介電常數相同的材料用以模擬加入之後的電磁場分佈，結果顯示加入壓電材料後其材料可受到較大的力量。

Recently, the interaction of light and matter has attracted scientists’ attention. It is well known that in macroscopic scale, radiation force is quite small. But in nanoscale, the effect of radiation force is huge. Therefore, it was observed that the deformation of object caused by radiation force showed a significantly change. Due to this phenomenon, optomechanics become very popular in recent years. Optical force exerted on a pair of parallel slab waveguides by the light propagating through them. Physically, this force comes from the interaction of the induced dipoles in the media by the electromagnetic wave. We solve the electromagnetic field by using finite element method and then use Maxwell stress tensor formulation to find out the stress distribution on the surface of a plain waveguide. The time averaged force exerted on waveguide can be calculated from the surface integral of Maxwell stress tensor. It is well known that the optical force can be changed by changing the structure of waveguide. We try to find out the maximum force by changing structure. first, we find that a new structure with holes in waveguide could enhance the optical force. Next we discuss the force distribution with different shape of holes in waveguide.

For applications, piezoelectric material can be placed between waveguides. Due to the force exerted on the piezoelectric material, the material could generate current. Next we put a material whose permittivity is the same as piezoelectric material between waveguides for simulating the field which has piezoelectric material in the model.