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金属光阴极因其超短脉冲发射和运行寿命长的特性从而具有重要应用价值, 但是较高的功函数和较强的电子散射使其需要采用高能量紫外光子激发且光电发射量子效率极低. 本文利用Mie散射共振效应增强银纳米颗粒中的局域光学态密度, 提升光吸收率和电子的输运效率, 并利用激活层降低银的功函数, 从而增强光阴极在可见光区的量子效率. 采用时域有限差分方法分析银纳米球阵列的光学共振特性, 采用磁控溅射和退火工艺在银/氧化锡铟复合衬底上制备银纳米球, 紧接着在其表面沉积制备铯激活层, 最后在高真空腔体中测试光电发射量子效率. 实验结果表明平均粒径150 nm的银纳米球光阴极在425 nm波长的量子效率超过0.35%, 为相同激活条件下银薄膜光阴极的12倍, 峰值波长与理论计算的Mie共振波长相符合.Metallic photocathodes have drawn attention due to their outstanding performances of ultrafast photoelectric response and long operational lifetime. However, due to their high work function and the large number of scattering events, metallic photocathodes typically are driven by ultraviolet laser pulses and characterized by low intrinsic quantum efficiency (QE). In this work, a new type of Mie-type silver (Ag) nano-sphere resonant structure fabricated on an Ag/ITO composite substrate is used to enhance the photocathode QE, where Mie scattering resonance is used to enhance the local density of optical state and then to improve the light absorption and electron transporting efficiency in Ag nano-spheres. The cesium (Cs) activation layer is also used to lower the electron work function and then to excite photoemission in the visible waveband for Ag photocathode. The optical characteristics of Ag nano-sphere arrays are analyzed by using finite difference time domain method. For the investigated Ag nano-sphere array, theoretical results show that Mie-type electric dipole resonance modes can be obtained over the 400–600 nm waveband by adjusting the sphere diameter, and the large resonance-enhanced absorption can be achieved in nanospheres at the resonance wavelength. The Ag nano-spheres are fabricated on the Ag/ITO substrate by magnetron sputtering and annealing process, then the Cs activation layer is deposited on surface, and finally QE is measured in an ultra-high vacuum test apparatus. Experimental results show that over 0.35% of QE is obtained for Ag nano-sphere particle (with a diameter of 150 nm) at a wavelength of 425 nm, and the wavelength positions of QE maxima are in agreement with Mie resonance for corresponding geometry predicted from the computational model. Given these unique optoelectronic properties, Ag nanophotonic resonance structured photocathodes represent a very promising alternative to photocathodes with flat surfaces that are widely used in many applications today.
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Keywords:
- optical-resonance /
- Ag-nanospheres /
- photocathode
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[14] Kuznetsov A I, Miroshnichenko A E, Brongersma M L, Kivshar Y S, Luk’yanchuk B 2016 Science 354 aag2472
[15] Vilayurganapathy S, Nandasiri M I, Joly A G, El-Khoury P Z, Varga T, Coffey G, Schwenzer B, Pandey A, Kayani A, Hess W P, Thevuthasan S 2013 Appl. Phys. Lett. 103 161112
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Google Scholar
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图 1 光学共振增强Ag纳米结构光阴极 (a) Spicer三步光电发射物理过程; (b) Ag纳米球结构光阴极; (c) Ag薄膜光阴极; (d) ITO衬底上Ag纳米球的FDTD光学仿真设置
Fig. 1. Optical resonance enhanced Ag nano-structured photocathode: (a) Spicer’s three-step model of photoemission; (b) illustration of the Ag nano-structured photocathode; (c) illustration of the Ag film photocathode; (d) cross-section of the FDTD setup used for simulating the optical properties of the Ag nanoparticles on ITO substrate.
图 2 Ag纳米球光阴极的制备及Cs激活工艺 (a) Ag纳米球光阴极的制备工艺流程; (b)退火前Ag薄膜的SEM照片; (c)退火后Ag纳米球的SEM照片; (d) Ag纳米球表面Cs激活过程中光电流的演化过程
Fig. 2. Fabrication and activation process of the Ag nanosphere photocathode: (a) Schematics of the fabrication process for Ag nanosphere photocathode; (b) SEM image of the Ag film; (c) SEM image of the Ag nanosphere; (d) surface Cs activation process of the Ag nanosphere.
图 3 Ag纳米球的Mie共振特性 (a)真空中和Ag/ITO衬底上直径D = 120 nm的Ag纳米球的Qsca, Qabs和Qext谱; (b), (c)分别为真空中和Ag/ITO衬底上Ag纳米球中心截面在电偶极子共振波长处(分别为439和505 nm)的归一化磁场强度和磁场线分布; (d) Ag纳米球中心截面在电四极子共振波长处(350 nm)的归一化磁场强度和磁场线分布
Fig. 3. Mie resonance characteristics of Ag nanospheres: (a) Light scattering, absorption and extinction efficiency (Qsca, abs, ext) spectra of the Ag nanosphere (D = 120 nm) in air and on substrates; (b) and (c) are the normalized magnetic field intensity (|H|2, color) and lines (white lines) in vertical crosscuts through the center of the Ag nanosphere in air and on Ag/ITO substrate with the electric dipole resonance wavelength of 439 and 505 nm, respectively; (d) the normalized magnetic field intensity (|H|2, color) and lines (white lines) in vertical crosscuts through the center of the Ag nanosphere with the electric quadrupole resonance wavelength of 350 nm.
图 4 Ag/ITO衬底上二维正方形排列的Ag纳米球阵列光吸收率ηa (a) P = 400 nm时, ηa随纳米球直径D与波长的关系; (b) D = 120 nm时, ηa随纳米球间距P与波长的关系
Fig. 4. ηa spectra of the square arranged Ag nanosphere array on Ag/ITO substrate: (a) Dependence of ηa
spectra upon D when P = 400 nm; (b) dependence of ηa spectra upon P when D = 120 nm. 
图 5 Ag纳米球阵列与薄膜的光吸收谱 (a) 测试结果; (b) 仿真结果
Fig. 5. Light absorption spectrums of the Ag nanosphere array and Ag film: (a) Measured results. (b) simulated results.
图 6 (a)实验测得的Ag纳米球阵列与薄膜光阴极光电发射量子效率; (b)理论计算的Ag纳米球阵列结构光阴极的光吸收谱
Fig. 6. (a) Measured photoemission quantum efficiency of the Ag nanosphere array and film photocathode; (b) calculated light absorption spectra of the Ag nanosphere array photocathode.
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[1] Zhang Y, Qian y, Feng C, Shi F, Cheng H, Zou J, Zhang J, Zhang X 2017 Opt. Mater. Express 7 3456
Google Scholar
[2] 邓文娟, 彭新村, 邹继军, 江少涛, 郭栋, 张益军, 常本康 2014 63 167902
Google Scholar
Deng W J, Peng X C, Zou J J, Jiang S T, Guo D, Zhang Y J, Chang B K 2014 Acta Phys. Sin. 63 167902
Google Scholar
[3] Karkare S, Boulet L, Cultrera L, Dunham B, Liu X, Schaff W, Bazarov I 2014 Phys. Rev. Lett. 112 097601
Google Scholar
[4] Hernandez G C, Poelker M, Hansknecht J 2016 IEEE Trans. Dielectr. Electr. Insul. 23 418
Google Scholar
[5] Xiang R, Teichert J 2015 Phys. Procedia 77 58
Google Scholar
[6] Pasmans P L E M, van Vugt D C, van Lieshout J P, Brussaard G J H, Luiten O J 2016 Phys. Rev. Accel. Beams 19 103403
Google Scholar
[7] Musumeci P, Moody J T, England R J, Rosenzweig J B, Tran T 2008 Phys. Rev. Lett. 100 244801
Google Scholar
[8] Xiang R, Arnold A, Michel P, Murcek P, Teichert J, Lu P, Vennekate H 2016 Proceedings of the 7th International Particle Accelerator Conference Busan, Korea, May 8−13, 2016 p3928
[9] Barday R, Burrill A, Jankowiak A, Kamps T, Knobloch J, Kugeler O, Matveenko A, Neumann A, Schmeißer M, Völker J, Kneisel P, Nietubyc R, Schubert S, Smedley J, Sekutowicz J, Will I 2013 Phys. Rev. ST Accel. Beams 16 123402
[10] Camino B, Noakes T C Q, Surman M, Seddon E A, Harrison N M 2016 Comput. Mater. Sci. 122 331
Google Scholar
[11] Lorusso A 2013 Appl. Phys. A 110 869
Google Scholar
[12] Dowell D H, Bazarov I, Dunham B, Harkay K, Hernandez-Garcia C, Legg R, Padmore H, Rao T, Smedley J, Wan W 2010 Nucl. Instrum. Methods A 622 685
Google Scholar
[13] Sheldon M T, Groep J, Brown A M, Polman A, Atwater H A 2014 Science 346 828
Google Scholar
[14] Kuznetsov A I, Miroshnichenko A E, Brongersma M L, Kivshar Y S, Luk’yanchuk B 2016 Science 354 aag2472
[15] Vilayurganapathy S, Nandasiri M I, Joly A G, El-Khoury P Z, Varga T, Coffey G, Schwenzer B, Pandey A, Kayani A, Hess W P, Thevuthasan S 2013 Appl. Phys. Lett. 103 161112
Google Scholar
[16] An C, Zhu R, Xu J, Liu Y, Hu X, Zhang J, Yu D 2018 AIP Adv. 8 055225
Google Scholar
[17] Li R K, To H, Andonian G, Feng J, Polyakov A, Scoby C M, Thompson K, Wan W, Padmore H A, Musumeci P 2013 Phys. Rev. Lett. 110 074801
Google Scholar
[18] Polyakov A, Senft C, Thompson K F, Feng J, Cabrini S, Schuck P J, Padmore H A, Peppernick S J, Hess W P 2013 Phys. Rev. Lett. 110 076802
Google Scholar
[19] Polyakov A, Cabrini S, Dhuey S, Harteneck B, Schuck P J, Padmore H A 2011 Appl. Phys. Lett. 98 203104
Google Scholar
[20] Baryshev S V, Antipov S, Kanareykin A D, Techlabs E, Savina M R, Zinovev A V, Thimsen E 2014 Proceedings of the 5th International Particle Accelerator Conference Dresden, Germany, June 15−20, 2014 p739
[21] Nolle E L, Khavin Y B, Schelev M Y 2005 Proc. of SPIE 5580 424
Google Scholar
[22] Nolle E L and Schelev M Y 2004 Tech. Phys. Lett. 30 304
Google Scholar
[23] Droubay T C, Chambers S A, Joly A G, Hess W P, Németh K, Harkay K C, Spentzouris L 2014 Phys. Rev. Lett. 112 067601
Google Scholar
[24] König T, Simon G H, Rust H -P, Heyde M 2009 J. Phys. Chem. C 113 11301
[25] He W, Vilayurganapathy S, Joly A G, Droubay T C, Chambers S A 2013 Appl. Phys. Lett. 102 071604
Google Scholar
[26] Ge X, Zou J, Deng W, Peng X, Wang W, Jiang S, Ding X, Chen Z, Zhang Y, Chang B 2015 Mater. Res. Express 2 095015
Google Scholar
[27] Spicer W E 1977 Appl. Phys. 12 115
Google Scholar
[28] Evlyukhin A B, Reinhardt C, Seidel A, Luk’yanchuk B S, Chichkov B N 2010 Phys. Rev. B 82 045404
Google Scholar
[29] 江智宇, 王子仪, 王金金, 张荣君, 郑玉祥, 陈良尧, 王松有 2016 65 207802
Google Scholar
Jiang Z Y, Wang Z Y, Wang J J, Zhang R J, Zheng Y X, Chen L Y, Wang S Y 2016 Acta Phys. Sin. 65 207802
Google Scholar
[30] Huang Y, Ringe E, Hou M, Ma L, Zhang Z 2015 AIP Adv. 5 107221
Google Scholar
[31] Groep J, Center A P 2013 Opt. Express 21 26285
Google Scholar
[32] Spinelli P, Verschuuren M A, Polman A 2012 Nat. Commun. 3 692
Google Scholar
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