Furthermore, emission is stable for all wavelengths and bunch charges investigated. Depending on the laser wavelength and intensity, excitation across the direct band gap or direct emission into vacuum dominates, which is identified by the number of photons needed to emit one electron. Based on the band diagram of this heterostructure, we propose an emission model, which explains the femtosecond laser-induced multiphoton emission from the ultraviolet to the infrared (235-1932 nm). The crystallites have a columnar shape, are 20 nm in size and have graphitic grain boundaries. Electron energy loss spectroscopy and electron diffraction in a high resolution transmission electron microscope confirm conformal diamond coating. We report a reliable fabrication recipe, which is based on dip-seeding of electrochemically etched tungsten in nanodiamond suspension, dry-blowing with pressurized nitrogen and microwave plasma-enhanced chemical vapor deposition of diamond. The tungsten substrate was chosen as a metallic contact which can easily be fabricated in the desired needle shape. Not only does this significantly lower the work function, it also boosts the photoelectron yield as the photoexcited carriers can be emitted into vacuum even after thermalizing to the conduction band minimum. Furthermore, the diamond surface exhibits negative electron affnity if it is terminated by hydrogen. These properties promise a robust emitter which can withstand the high laser intensity of femtosecond laser pulses. As emitter material, we chose diamond as it is not only mechanically and chemically robust, but also exhibits high thermal conductivity.
We demonstrated electron acceleration using optical near fields in the vicinity of a periodic nanostructure with energies as low as 9.6 keV. The lower the injection energy is, the more compact the injector unit can be built due to reduced electrostatic field breakdown.
This leads to a small source emittance, which is one of the key parameters for realizing a photonic particle accelerator on a chip, especially at low injection energies. Another major advantage of the needle shape is the small effective electron source size both in dc and laser-triggered emission. We measured up to 2.92 V/nm at a 20 nm sized tungsten emitter and achieved quantitative agreement between experiment and simulation. Measuring electron deflection allows us to determine the 3D electrostatic field with nanometer resolution, even during dc field emission. To better understand the electrostatic field distribution at nanometer-sized emitters, we developed a differential phase contrast technique in a transmission electron microscope. These large fields and the resulting acceleration of emitted electrons are crucial for mitigating space charge effects and achieving high beam brightness during high current density operation. This geometry results in a large DC field enhancement at the emitter apex, which enables operation at GV/m electrostatic fields. The main goal of this work was to invent, fabricate and characterize a robust high brightness emitter designed for femtosecond laser-triggered operation: We present nanodiamond-coated tungsten needle tips. The high brightness Schottky and cold field emitters suffer from emission instability when triggered by femtosecond laser pulses.
However, the electron sources used in state of the art ultrafast electron microscopes and our photonic laser acceleration setup were developed for dc operation. The temporal and spatial resolution of the latter is limited by the electron pulse duration and beam emittance, respectively. Electron beams are powerful tools with applications ranging from treating cancer to recording ultrafast microscopic movies.
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