We are interested in the application of arrays of electron field emitters, which can be achieved from a variety of materials, for the preparation of compact and coherent X-ray sources via inverse Compton scattering.\u00a0 Field emission of electrons is commonly achieved by applying a static electric field or optical illumination to sharp metal tips. Sub-wavelength nanostructures can provide geometry-dependent electric field enhancement for both methods. For applications in coherent X-ray sources, the field emitter arrays should be able to emit short electron pulses, typically on the femtosecond timescale, which is difficult using conventional electrical circuits. Therefore, optical triggering, whereby a femtosecond laser is used to stimulate electron emission, has been considered.<\/p>\n
We have simulated optical fields around various field emitter structures using COMSOL finite element software.\u00a0 Among them, we are particularly interested in the \u201cbullet\u201d structure illustrated in Figure 1.\u00a0 Conical tip structures are widely used to achieve both electrostatic and optical field enhancement in field emitters; however, uniform conical structures pose significant challenges in nanoscale fabrication due to their tapered geometry.\u00a0 Arrays of metallic \u201cbullet\u201d structures, as shown in Figure 1, may be fabricated with a high areal density via positive-tone electron beam lithography with a PMMA resist.<\/p>\n
We have designed a fabrication process for the preparation of arrays of optical-field emitters, based on the metallic \u201cbullet\u201d structure shown in Figure 2 (a).\u00a0 The thin SiO2<\/sub> layer shown in Figure 2 (a) is used to prevent electron emission from the bulk silicon substrate by acting as an electrically insulating barrier.\u00a0 The thickness of the SiO2<\/sub> layer has yet to be optimized. As a proof of concept, we have fabricated an array of Au nanoparticle emitters with an aspect-ratio of 1 (see Figure 2 (b)).\u00a0 Further optimization of the fabrication process is currently underway.<\/p>\n\n\t\t