二維高斯光束照射到不同幾何物體產生之光力矩討論

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，已知在宏觀尺度下，光對於物體所產生的影響相當地小，但是當尺度縮小為微米以下尺度時，光會有較明顯的影響，因此光力學(Optomechanics)成為了近幾年來相當熱門的題目。 當光打在介質波導時，如果波導幾何形狀對稱，將導致左右結構產生之力矩互相抵消，進而使得整體所得淨力矩趨近於零，而為了使正負力矩抵消現象減小，必須改變幾何形狀，藉此提高整體結構淨力矩。 本論文藉由有限元素法模擬各種介質波導的穩態電磁場，再將表面之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，接著對表面應力與該點至中心之距離外積後積分，求得波導的平均力矩.。 我們嘗試改變波導形狀來改變場的分佈，進而改變力量分佈來求得最大力矩之結構，研究結果發現在波導內有空隙、不對稱及直接受光照面確實可增強力矩，但如果結構旋轉一圈之平均力矩之最大值又會與固定角度下最大值可能有所不同，而實務上平均力矩最大化較為重要，最大力矩可以當作初始的推動或是克服較難旋轉的點。 綜觀而論，不對稱形狀與受光面之盡可能垂直是整體結構上無論單點與旋轉一圈下之平均力矩都有最好的表現。

Recently, the mechanical effects of light on objects greatly attract many scientists. To be more specific, though the effect of light on objects at macroscopic scale is unobvious, we can find obvious influence on the objects if we observe in the scale below micron. Therefore, Optomechanics has become a very popular topic in recent years. However, the light strikes the dielectric waveguide with the waveguide geometry is symmetrical, the moments generated by the left and right structures will cancel each other out and the overall net torque will approach zero. Thus, in order to reduce the positive and negative moment cancellation phenomenon, we must change the geometry and increase the net torque of the overall structure. In this Thesis, we use the finite element method to simulate the steady-state electromagnetic field of various dielectric waveguides and then substitute the electromagnetic field of the surface into the Maxwell stress equation to obtain the stress applied by the electromagnetic field on the waveguide surface and finally calculate the integrated of the surface stress and the point-to-center for the average torque of the waveguide is obtained. We try to change the shape of the waveguide to impel of the distribution of the field, and then observe the different of the force distribution to find the structure of the maximum moment. The results show that it can indeed enhance the torque by the voids in the waveguide, the asymmetry and the directly illuminated surface. Yet the structure rotates with the maximum value of the average moment, it may be different from the maximum value at a fixed angle, and the rotation of the light receiving surface may be greatly affected. In summary, the asymmetry shape and the light-receiving surface are as perpendicular as possible to the overall structure will obtain the best performance of the average moment.