| 摘要: | 近年來，表面電漿極化子已經被廣泛的研究與應用在不同的領域，包括：生醫感測、高密度資訊存取、高解析顯微鏡、高效率太陽能電池等等。另一方面，由於表面電漿極化子波導可突破光的繞射極限，使得積體光路重新獲得更多的注意，因此不論是在實驗上或理論上皆有大量研究的人力投入表面電漿極化子波導的研究。本論文利用「時域有限差分法」模擬電磁波與材料的交互作用，研究以電漿子建構之波導結構的線性與非線性傳播特性，進而評估電漿子波導元件應用於各式光通訊元件之可行性。 在線性傳播方面，提出「耦合脊狀電漿子波導」、「耦合電漿子波導陣列」與「異質電漿子波導」作為導波、極化分離與波長分波多工器之被動元件。其中「耦合脊狀電漿子波導」的傳播距離可達169μm，在光通訊波段（1550nm）打破世界記錄。此外，基於相同之耦合結構，彎曲波導與雙通道方耦合器亦被研究，目的在測詴次波長積體光路的可行性。研究結果顯示，可利用「耦合脊狀電漿子波導」建構次波長尺度之光元件。其次，提出「耦合電漿子波導陣列」作為極化分離器，將TE 與TM 模態於空間中分離至不同的輸出埠，材料與結構的最佳化過程，將極化分離器之插入損耗、消光比與操作頻寬優化為1dB、20dB 與450nm。此極化分離器可實現在100nm×2000nm 的小面積上；最後，提出以「異質電漿子波導」作為波長分波元件，此元件的角色散可達2.1°/nm，為傳統棱鏡的200 倍，並且可與以光子晶體所形成的棱鏡相比。此元件有機會應用於近場天線。 非線性方面，提出「耦合非線性電漿子波導陣列」作為光限制器，光限制器由耦合的金屬波導陣列埋藏於非線性波導所構成。利用具有光學克爾效應或雙光子吸收之材料，可將光限制器分為兩類：(金/克爾/金)波導陣列與(金/砷化鎵/金)波導陣列。(金/克爾/金)波導陣列的非線性吸收機制可歸因於電漿子的色散關係之改變，由於強場造成電漿子的傳播常數增大，進而增加電漿子的吸收；(金/砷化鎵/金)波導陣列的非線性吸收機制為雙光子吸收與雙光子吸收所誘發之自由載子吸收，利用金屬陣列的高穿透率與表面能量的增強效應提升非線性介質的吸收。此波導陣列的線性穿透可高於85.18﹪，其等效模場面積是輸入波導的一半，因此能提升功率密度，並且降低光限制閥值至42.69GW/cm2 ， 以此建構之波導陣列的表面積為300nm×500nm，有利於高密度積體光路之實現。另一方面，由於自由載子的響應速度較慢，對於邏輯閘的高速響應之要求，是不利的。根據本研究之評估，當光脈衝寬度為10fs，甚至功率密度為89GW/cm2 時，雙光子吸收所誘發的自由載子效應僅為1﹪的穿透率變化。 基於電漿子波導的元件，可將尺寸進一步的縮小至0.1至1 倍的操作波長(0.15~1.5 微米)，所以這樣的尺寸能夠完全滿足高密度積體光路的需求。然而，實際上礙於金屬本質的歐姆損耗，電漿子波導無法全面地取代介電質波導，作為積體光路的基本元件。但是卻能夠成功的取代一些功能性的元件，例如：次波長波導、極化光束分離器、角色散元件與光限制器。 In recent years, surface plasmon polariton (SPP) has been widely investigated and applied to various fields such as biomedicine sensors, high-density data storage and access, super-resoultion microscopy, and high-efficiency solar cell etc. On the other hand, optical integrated circuits have regained more attentions due to the capability of metallic waveguide structures to overcome diffraction limit. As a consequence, there is a significiant growth on experiments and theories devoted to the research of SPP. In this thesis, finite-difference time-domain method (FDTD) was utilized for the simulation of the interaction between electromagnetic waves and structured materials. The linear and nonlinear propagation characteristics of proposed plasmonic waveguide structures were investigated, and the feasibility of using plasmonic waveguide devices to optical communications is assessed. In the aspect of linear propagation, coupled rib plasmonic waveguides (CRPW), coupled plasmonic waveguides array (CPWA), and hetero-plasmonic waveguides (HPW) were proposed for wave-guding, polarization beam splitting, and wavelength-division multiplexer, respectively. The proposed CRPW was predicted to have a propagation length of 169μm at wavelength of 1550nm, knocking down presnt world record. In addition, bent waveguide and dual channel directional coupler basing on the same structure were investigated, aiming at assessing the feasibility of subwavelength optical integrated circuits. The results reveal that the CRPW can be utilized to construt subwavelength optical devices. Secondly, CPWA was proposed as a polarization beam splitter (PBS) that can separate TE and TM mode in spatially distinct output ports. After optimization of structural and material parameters, the obtainable insertion loss, extinction ratio, and the operational bandwidth are 1dB, 20dB, and 450nm, respectively. It is realizable on a chip size as small as 100nm×2000nm. Finally, a HPW was proposed as an angular wavelength-division multiplexer. The resolving power is estimated to be 2.1°/nm, which is higher by 200 times than a conventional prism and comparable to those made of photonic crystals. This device is potentially applicable to near field antenna. In the aspect of nonlinear propagation, coupled nonlinear plasmonic waveguides array (CNPWA) was proposed as optical limiters constructed by metal array embedded in nonlinear materials. According to optical Kerr effect (OKE) and two-photon absorption (TPA), the optical limiters can be classified by Au/Kerr/Au waveguides array and Au/GaAs/Au waveguides array. The nonlinear absorption of Au/Kerr/Au waveguides array is due to the variation of plasmonic dispersion, which using strong fields to enhance the propagation constant of plasmonic, and then to increase the absorption of plasmonic. The nonlinear absorptions of Au/GaAs/Au waveguides array, which utilizes high transmittance and surface enhanced energy effect to enhance the absorption of nonlinear material, include TPA and TPA induced free carrier absorption (FCA). The linear transmittance is higher than 85.18﹪, and the effective modal area is a half of the introduced waveguide, which result in a upgraded optical intensity, and the optical limiting threshold is shrunk down to 42.69GW/cm2. Moreover, the area of Au/GaAs/Au waveguides array is 300nm×500nm, which is advantage for the realization of highly dense optical integrated circuits. On the other hand, the TPA induced free carrier is disadvantage for a highly speeding demand of logic gate due to the slower response time. According to the assessments of this investigation, while optical pulsewidth is 10fs, and even the intensity is 89GW/cm2, the variation of transmission caused by TPA induced FCA is only ~ 1﹪. Devices based on plasmonic waveguides can be further shrink down to ~ 0.1 to 1 times of the operating wavelength, which thereby satisfying the demand of highly dense optical integrated circuits. Practically, however, the plasmonic waveguides may not be able to replace all dielectric waveguides completely as the fundamental components in OICs due to the intrinsic ohmic loss of metallic materials. Nevertheless, we have shown that plasmonic waveguides could substitute some functional devices, such as subwavelength waveguide, polarization beam splitter, angular dispersive device, and optical limiter. |
