| 摘要: | 在Tera-Hz光通訊領域中，一個極高頻且超高效率之光調變器(light modulator)，是不可或缺的關鍵元件，本論文提出一具有此特性之光調變器—利用波導耦合(waveguide-coupled)入射光，形成一Fabry-Perot建設性干涉能量集中之表面電漿子(surface plasmons，SPs)激發光，經由外加電場調變激發光與表面電漿子之間的耦合程度，進而達成調變反射光強度的目的。因此，依據上述理論設計，我們製作出此經由衰逝全反射(attenuated total reflection，ATR) 方式調變之光波導耦合表面電漿子(waveguide-coupled surface plasmon，WCSP)光調變器，於波導耦合介電層下濺鍍一特定厚度金屬層約30?40 nm，於此金屬層下方再旋轉塗佈一層有機電光(electro-optic，E-O)高分子材料，並將此高分子層極化(poling)排列為非中心對稱結構，使得此電光高分子層結構可經由外加電場來調變而造成一階Pockels電光效應，此時波導耦合入射光與表面電漿子之耦合關係發生瞬間的大改變進而使反射光的強度產生極大的變化而達到高效率光調變的目的。 結合上述光波導耦合能量集中、衰逝全反射表面電漿子耦合以及電光高分子之電光效應等，再加上相關理論分析與多層膜反射方程式模擬比較，提出超高效率光波導耦合表面電漿子之電光調變器之最佳化設計製作準則。首先，利用Lorentzian方程式，多層膜反射式及最佳化線性資料分析(optimal linear data analysis)來對衰逝全反射的反射光譜(reflectivity spectrum)分析，可求出經由RF濺鍍之金屬層的介電常數(dielectric constant)及厚度。並利用反射光譜中的兩個波導耦合模態，而求出經由旋轉塗佈極化後之電光高分子層的折射係數(refractive index)及厚度，並配合反射光譜之最低強度(minimum intensity)，可估算其消散係數(extinction coefficient)。配合電光高分子層的一階電光效應及對量測反射光譜浮移量做分析，即可計算出此電光高分子材料的電光係數(E-O coefficient)。然而，目前所製作之電光調變器並非於最佳化的狀態，因此於30 V外加電壓時，調變光強只達1%左右，而與最佳化設計時4%的調變量還有一段距離仍待努力。 最後，我們設計一嶄新極高效率之長距離表面電漿波(long-range surface plasmon wave，LRSPW)與傳統表面電漿波(surface plasmon wave，SPW)耦合的光調變器，由於表面電漿波的激發導致入射光的能量更加集中於波導層內部，調變效率也將因此而大幅提升。採用此種方式做調變，依據模擬的結果顯示，在10V的外加電壓下將能有70%的極高效率調變量。 Due to the increasing demand for the development of light modulator with larger bandwidth and higher efficiency in the optical communication, in this thesis we present a novel ultrahigh efficiency and high dynamic response light modulator based on an external applied voltage to modulate the excitation degree of incident waveguide light in surface plasmons (SPs), known as the attenuation total reflection method (ATR method). This waveguide-coupled surface plasmon light modulator is fabricated by spin coating an organic electro-optic (E-O) polymer film onto the metal layer with suitable thickness of 30~40 nm, and then deposited a bottom metal electrode under the polymer. The molecular orientation of E-O polymer is aligned into noncentro-symmetry structure by applying contact poling process. When an external applied electric field created a linear variation of refractive index of poled E-O polymer according to the Pockels effect, the excitation degree of incident waveguide light in surface plasmons is changed and then the reflected light is modulated. Besides, we present in the thesis an optimizing design principle for efficient light modulator through the concentration of engery by waveguide coupled, surface plasmons, electro-optic effect, simulation, and theory analysis. To accurately determine the reflective index, attenuation coefficient, and the thickness of the metal layer, we analyzed the ATR spectra by techniques including Lorentzian equation analysis, Fresnel’s equation data-fitting and optimal linear data analysis. Alternatively, by using least square data-fitting techniques which employ two waveguide mode, we could calculate the refractive index and thickness of electro-optic polymer. We also estimated the attenuation coefficient of the polymer by anaylzing the reflection minimum of ATR spectrum. Finally, we calculated its actual electro-optic coefficient of this modulator by analyzing both E-O pockels effect and ATR spectrum. However, because the modulator fabricated in our experiment is not on optimum condition, the results so far show only 1% modulation in the intensity of the reflected light with driving voltage 30V. Finally, we present a novel and high efficiency electro-optic light modulators based on long range surface plasmon wave coupled with classical surface plasmon wave. Due to the excitation of surface plasmon wave, the energy is concentrated in the waveguide, so the efficiency could be improved. By simulation, it could effectively reduce driving voltage and increase the modulation index of 70 % with driving voltage 10 V. |
