不同金屬光柵於紅外光穿透率之研究

Abstract

自從1902年Wood發現電磁波在刻有光柵的金屬表面上會產生異常的反射光譜之後，對次波長金屬光柵異常穿透率的研究就沒有中斷過，但絕大數者皆只靠軟體模擬計算，較少去實地製作和量測，因此本論文將以實驗為主，藉由實驗數據中所觀察到的現象，加以分析統整之後而推導出其理論架構。 由於有時需要製作線寬小於微米級的光柵，若用傳統的光學微影術會有執行上的困難，因此我們選擇使用電子束微影術來定義樣品的線寬，再搭配反應式離子蝕刻機和熱蒸鍍系統製作出金屬光柵結構，最後再用傅氏轉換紅外線光譜儀去量測紅外光的穿透率。由於光柵是一維的，造成不同極化方向的入射光所看到的光柵是不一樣的，為了避免兩種狀況互相干擾，我們各別分為TE和TM模態去研究。首先我們討論不改變金屬的情況下，線寬、週期和入射光波長對穿透率的影響，我們先推導電磁波在光柵外、內所存在的模態，再以此去解釋我們所觀察到的現象，得到造成異常穿透率的主因為電磁波和光柵間的耦合作用。接著我們使用不同金屬去製作光柵，發現導電度高的金屬光柵與電磁波間的耦合作用較強，得知耦合作用與金屬表面的電子有關，進而推導出其理論架構。 金屬表面存在許多自由電子，當電磁波照射在金屬上時，電子會隨著其電場方向而振盪，來回振盪的電子則會提供特定頻率的電磁場，由於金屬光柵是週期性結構，造成其產生的電磁場亦為週期性，則可提供水平方向的波向量，而額外獲得水平方向波向量的電磁波在穿透光柵時所造成的影響與直接入射的電磁波不一樣，這便是金屬光柵有異常穿透率的主因。不同極化方向的入射光會讓電子振盪的方向不一樣，不同振盪方向的電子所看到的金屬結構也不一樣，因此造成其穿透率截然不同。 我們發現穿透率增強的現象用表面電漿波理論並不足以完全解釋，推測可能的原因與消散波有關，由於我們的理論架構只稍具雛形，較嚴謹完整的數學理論推導則尚在研究中，目前尚無法完全地解釋所有觀察到的現象，但我們相信我們的研究成果可以提供金屬光柵更多的應用空間！

Since 1902, Wood had found the anomalies of reflectance spectrum arose from electromagnetic wave impinging on the surface of metal grating. People never stop studying the transmission anomalies of metal grating. Most of them only simulate by software but do experiments rarely, which is not convictive to me. The main of this thesis is investigated in experiment, and we can build the theoretical model by analyzing the phenomenon which is observed from the data of experiments. Because we need to make some gratings, whose line width is under micrometer scale, it’s difficult to implement with traditional optical lithography. So we use the e-beam lithography, which is used to design the pattern, RIE and thermal evaporator to make metal gratings. Finally, infrared transmission of metal grating is measured by FTIR. Grating is one dimension, which would make different polarization directions of light encounter different structures. To avoid these situations interfere with each other, we separate into TE and TM mode to study. First, we discuss the effect of line width, period and wavelength without changing metal. The modes of electromagnetic wave can exist in or out of the grating is deduced, which are used to explain the phenomenon we observed. We find that coupling between electromagnetic wave and gratings is the cause of transmission anomalies. Then we make gratings with various metal and find that the coupling between electromagnetic wave and metal grating with high conductivity is stronger. The coupling is corresponding to the electrons of metal surface, and we build the theoretical model. There are many free electrons on the metal surface. When an electromagnetic wave impinging on metal, electrons will oscillate by the electric field, and the oscillating of electrons will provide electromagnetic field of certain frequency. The periodic structure of metal grating makes the electromagnetic field periodic, which can provide horizontal wave vector. The influences caused by electromagnetic waves which gain additional horizontal wave vector passing through gratings are different from those caused by directly impinging ones and this is why metal gratings have transmission anomalies. Different polarization directions of electromagnetic wave make electrons oscillate in different directions and encounter different metal structure, which make transmission different from each other. Transmission enhancement can’t be explained sufficiently by surface plasma theorem, and it may be correspond to the evanescent wave. But now we only have a fundamental theoretical model, a serious mathematical model is still studied. Although we can’t explain everything we observe, we believe that our research can provide more applications of metal gratings.