光学共振增强的表面铯激活银纳米结构光阴极

Optical resonance enhanced Cs activated nano-structured Ag photocathode

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)nanosphere 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.