以具全極化二維週期奈米結構之「金屬-介電質-金屬」吸收體實現電漿子增強之光電轉換;Polarization-insensitive two-dimensional periodic metallic absorbers in structured metal-insulator-metal con guration for plasmon-enhanced photoelectric conversion

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題名:	以具全極化二維週期奈米結構之「金屬-介電質-金屬」吸收體實現電漿子增強之光電轉換;Polarization-insensitive two-dimensional periodic metallic absorbers in structured metal-insulator-metal con guration for plasmon-enhanced photoelectric conversion
作者:	馬林甘;Karthickraj, Muthuramalingam
貢獻者:	光電科學與工程學系
關鍵詞:	Plasmonics;MIM;solar energy conversion;Plasmonics;MIM;solar energy conversion
日期:	2016-10-19
上傳時間:	2017-01-23 16:33:42 (UTC+8)
出版者:	國立中央大學
摘要:	本研究針對以數種尺寸之奈米六角柱週期陣列置於金屬-介電質-金屬結構(metal- insulator-metal, MIM)所形成之新式金屬吸收體，其在可見光光譜範圍內之寬頻入射電磁波吸收及電漿子(plasmon)增強之光電轉換進行相關之探討。於反射頻譜中觀察到，以金為材料之單一六角柱其分別置於二氧化矽(SiO2)-金與二氧化矽-銀(Ag)結構之上時，其共振波長(resonance wavelength, λres)隨正六角柱邊長增加而近乎線性紅移。金奈米結構-二氧化矽-金之吸收體於入射自由波長(free space wavelength, λ0)大於500 nm所產生之寬頻入射電磁波吸收主要源自於底層金膜之材料吸收。數值模擬結果顯示該吸收體於垂直入射下具有與入射電磁光偏振(polarization)不相依之特性，且其吸收頻譜(λ0 = [300, 1100] nm)於入射角(incidence angle)小於40◦之範圍內幾乎恆定。對於垂直入射之橫向磁場(transverse magnetic, TM)電磁波，金-二氧化矽-金與金-二氧化矽-銀吸收體之波長平均(λ0 = [400, 700] nm)之總吸收率(absorptance)分別為91.63%及82.31%。此外，與以金為底層金屬膜之吸收體相比，於波長範圍λ0 = [400, 550] nm內，改以銀為底層金屬膜促使上層金六角柱之吸收率增加2.5倍，其主要源自於反射波增益之局域性表面電漿子(reflected wave-enhanced localized surface plasmons)與間隙電漿子(gap plasmon)共振。於多層結構之鋁(Al)-二氧化鈦(TiO2)-銀之吸收體，電漿子增強之光電轉換效應主要產生於六角柱之邊緣以及側壁，同時可於六角柱間之中層鋁金屬膜內所產生。對於最佳化之結構，於可見光範圍內對TM電磁波之吸收率其大於60%之頻寬約為293 nm。於本研究中，透過電子束微影(electron beam lithography)製作最佳化之具奈米六角柱之鋁-二氧化鈦-銀之吸收體並針對其外部量子效率(external quantum efficiency, EQE)進行量測。透過量測該元件於入射光波長為633 nm之光激發下所產生之光電流，可推得該元件之外部量子效率與響應率(responsivity)分別為0.0568%與0.2899mA/W。; In this research, novel metallic absorbers with multi-sized nanohexagons arranged peri- odically in metal-insulator-metal (MIM) configuration for broadband optical absorption and plasmon-enhanced photoelectric conversion at visible frequencies are numerically and experimentally explored. The resonance wavelength (λres) of a single gold (Au) hexagon supported by silica film on Au or silver (Ag) bottom layer shifts approximately linearly towards longer wavelengths in the reflectance spectrum with an increasing side length. Broadband absorption for about λ0 < 500 nm in nanostructured Au-SiO2-Au absorbers is mainly due to material absorption of the Au bottom layer. The design is shown to be polarization insensitive at normal incidence and the absorptance spectrum (300 nm-1100 nm) is nearly independent of the incident angle up to about 40◦. The wavelength-averaged total absorptance of the Au-SiO2-Au and Au-SiO2-Ag absorbers are about 91.63% and 82.31%, respectively, for transverse magnetic (TM) wave at normal incidence for λ0 = [400, 700] nm. In addition, the absorptance within top Au hexagons is enhanced up to 2.5 times for λ0 = [400, 550] nm with the Ag bottom layer mainly because of reflected wave-enhanced localized surface plasmons and gap plasmon resonances. In the multilayered aluminum (Al)-titanium oxide (TiO2)-Ag absorbers, plasmon- enhanced photoelectric conversion is found to achieved mainly at edges and side walls of the hexagons and also in the mid Al layer between hexagons. The simulated bandwidth for the absorptance greater than 60% in the fabricated sub-optimum structure is about 293 nm for the TM wave at visible wavelengths. The optimum Al-TiO2-Ag absorbers with nanohexagons is fabricated using electron beam lithography and the external quantum efficiency (EQE) is measured. The calculated EQE and responsivity from the measured photocurrent of the fabricated device is 0.0568% and 0.2899 mA/W at λ0 = 633 nm under the bias of 0.5 V.
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