From Attosecond Control of Electrons at Nano‐Objects to Laser‐Driven Electron Accelerators

Abstract

Electron emission from nano-objects, in particular from sharp tips, is not a new field at all. It is well known since the end of the 19th century that electrons can be extracted from a pointy wire if a voltage is applied to it [1]. Because of the lightning rod effect, this voltage translates into a large electric field right at the end of the wire, or better, at a tip-shaped wire. With a moderate voltage of a couple of hundred volts electric field strengths of more than a few gigavolt per meters can be reached at the tip's apex -if it is sharp and pointy. For typical metals, from these field strengths on an appreciable field emission current sets in. Fowler and Nordheim in 1928 were able to explain this effect with the newly established quantum mechanical tunneling, representing one of the first applications of the Wentzel-Kramers-Brillouin (WKB) approximation [2][3][4]. Nowadays, field emission electron sources are employed in any high-resolution electron microscope because of their superb electron beam emission characteristics [5].Similar to the lightning rod effect with DC fields, an optical field is also enhanced at the tip's apex, as introduced in Chapter 3. Here we discuss what happens if this effect generates light intensities exceeding 10 11 W cm −2 right at the tip's apex. It is convenient from the experimenter's point of view that a simple laser oscillator suffices to do so, just because of the optical field enhancement effect. Numerical tools such as the finite-difference-time-domain method allow calculating typical field enhancement factors to around 3 … 10 for a light field with a wavelength of 800 nm polarized along the tip axis. Similar to the observations using DC fields, one can generally state that the sharper the tip the higher the field enhancement.The light field at the tip apex triggers electron emission and subsequent processes that are well known in atomic physics for decades already. These include above-threshold photoemission, strong-field effects, but also effects such as electron recollision and electronic matter wave interference in the time-energy domain.The setup for the tip-based experiments is shown in Figure 6.1. A sharp metal tip -most initial experiments were done with a tip made of tungsten or gold

Dielectric Laser Accelerators: Designs, Experiments, and Applications

“…At relativistic energy, a phase-stable train of electron microbunches has been achieved using an inverse freeelectron laser, resulting in net acceleration after interaction with an optical-scale accelerator [23,24]. Sources designed for shortest electron bunches have been demonstrated using velocity bunching [144], which may also be combined with nanotip photocathode sources with demonstrated low transverse emittance [145][146][147].…”

Femtosecond few- to single-electron point-projection microscopy for nanoscale dynamic imaging

“…The realization of this time-varying field-enhancement in tungsten, gold and silver has initiated the highly active research field of ultrafast nanophotonics or femtosecond nanophysics. Themes including optical near-field sensing, subcycle carrier-envelope phase phenomena, investigations into surface plasmons and laser-driven electron accelerators have emerged [44][45][46][47] . The present work aligns itself to these efforts by looking to better quantify the spatial and temporal characteristics of femtosecond laser-driven electron microscopy to demonstrate a potential characterization method, and point to the potential of dynamic charge imaging.…”

Optimized operation of dielectric laser accelerators: Multibunch

“…Novel schemes have been presented [9,10] as well as experimental demonstrations [11][12][13][14]. While the issue of generating optically-spaced electron microbunches is beyond the scope of this study, below we discuss a phenomenon common to all such schemes-beam loading.…”

“…In order to achieve net acceleration, several research groups are considering generation of optically-spaced electron microbunches [9][10][11][12][13][14]. Novel schemes have been presented [9,10] as well as experimental demonstrations [11][12][13][14].…”