ACHIP: Electron acceleration with laser light
Accelerators on a chip: Dielectric laser accelerators
Particle accelerators are exciting research tools that provide energetic high brightness charged particle beams that can be used to probe physical phenomena otherwise inaccessible. However, the enormous cost and user demand of many of the radio-frequency based accelerators limits their availability. To provide an accelerator accessible on the university-lab scale, a novel accelerator design must be developed.
One such design, whose development is funded by the Gordon and Betty Moore Foundation (press release), is the Dielectric Laser Accelerator or DLA for short. DLAs leverage both the GV/m electromagnetic fields of commercially available lasers and the advanced nanofabrication techniques of dielectric materials developed in the semiconductor industry. The large available electromagnetic fields are used to create acceleration gradients that exceed those in radio frequency accelerators by a factor of 100. Even though DLAs are orders of magnitude smaller than their RF brethren, their accelerating gradients allow for DLAs to impart similar energy gains to charged particles. However, instead of imparting these energy gains over meters, DLAs impart these energy gains over millimeters, potentially enabling a university lab-scale particle accelerator.
The DLAs tested at FAU operate in a phase-synchronous scheme, as follows. An incident laser pulse impinges upon a nanofabricated accelerating structure, exciting travelling near-field modes. Electrons, propagating in close proximity to the structure, are accelerated when their velocity matches the phase velocity of one of the traveling modes, and they are injected into these fields at a particular time. This synchronicity condition and general principle is similar to that employed in large-scale RF accelerators, and the structures are designed to satisfy this condition (see video produced by our SLAC partners on the dual-grating approach).
A laser impinges upon the silicon grating (with the structural period and grating height d indictated) from above, exciting the travelling wave mode depicted by the blue and red regions in the bottom image that shows a cross section of two periods of the accelerator. An electron, depicted by the green circle and travelling to the right, surfs the travelling wave, also moving to the right. (Image: FAU/Joshua McNeur)
Already there have been repeated confirmations that this principle of acceleration works over a wide range of electron velocities (from 15% to 100% of the speed of light) and lasers [1,2,3,4]. The ACHIP collaboration, see below, aims to extend the success of DLAs towards the realization of the analogy of an accelerator beamline. Multiple stages of dielectric-laser based acceleration, focusing, and diagnostics are being developed and tested. Specifically, auxiliary dielectric elements can be integrated [4,5] to additionally provide spatial focusing, steering, and bunching of electron pulses, important elements making up an essential toolbox in any particle accelerator.
A laser-triggered electron cathode appropriate for operation with DLAs will eventually be incorporated with the multiple stages, resulting in a beamline where electrons are generated via laser-triggered emission, and then alternatingly accelerated, collimated, and diagnosed with sequential DLA-based devices. The complete accelerator can be schematically illustrated as shown below:
The resulting beam may be used as a high brightness light source via the incorporation of an element that wiggles the electron beam transverse to its direction of motion, creating photons as the beam alternatingly curves upwards and downwards. The compact size of such a beamline and its various components allows for many exciting applications, ranging from handheld MeV electron sources for tumor irradiation to table-top Free Electron Lasers .
Current efforts at the Chair of Laser Physics
In our recent publication in Optics Letters, we show how a dielectric laser accelerator (DLA) can be more efficient. We use silicon dual-pillar structures that provide an easily-fabricated symmetric near-field geometry. Equipped with a distributed Bragg reflector (DBR), these structures can accelerate electrons with a gradient 57% larger than their counterparts without a DBR. This improvement in acceleration efficiency is achieved by placing an on-chip DBR on one side of the structure, as shown in the image below. The DBR reflects about 99% of the incident light. The reflected light from the DBR interferes constructively with the incident laser light, yielding a stronger electric field in between the pillars where electrons are injected. This mimics a double-sided laser illumination without having the complexity of adding a second laser beam path. In this letter we demonstrate a maximum acceleration gradient of 133 MeV/m using such structures.
Our structures can be used to explore other components of a miniaturized dielectric-based particle accelerator for electron beam focusing and micro-bunching purposes. DLAs could ultimately lead to a compact laser-driven particle accelerator for variety of applications from low energy radiation therapy devices to high energy particle colliders.
Phase space dynamics and control in dielectric laser accelerators
Future technologies necessary to drive forward the development of dielectric laser accelerators require precise control of the phase space the electrons occupy.
One avenue toward this is to modulate the longitudinal phase-space via ballistic bunching. In a first interaction (red), a sinusoidal energy modulation is imprinted on the electrons. Since the electrons are not relativistic (1/3 of the speed of light in our experiments), the difference in speed of individual electrons causes them to bunch in space during a defined drift distance. This causes the formation of a train of microbunched electrons. The longitudinal density modulation is probed via a second interaction (blue). There, the electrons experience a phase dependent energy modulation. Combined with realistic numerical models, this allows us to retrieve the density distribution of the electrons, or in other words – the microbunch length.
Short microbunches are necessary to inject electrons at a precise phase into phase sensitive DLA devices using, for example, alternating phase focusing (APF). These short electron pulses – in the range of roughly a hundred attoseconds, are furthermore interesting tools for ultrafast physics, to probe, for example, electronic states in condensed matter physics.
Alternating Phase Focusing (APF)
One issue when accelerating a particle beam is the constant defocusing in the transverse direction. In classical accelerators, this is countered by refocusing the beam, either utilizing external quadrupoles or focusing modes within the accelerator itself. The experimental demonstration of electron beam focusing using a curved dielectric grating  opened the possibility of transferring some of these techniques to dielectric laser accelerators. The theoretical exploration of new focusing methods for DLA was pioneered at TU Darmstadt: in simulation, an implementation of alternating phase focusing, a well known technique in classical particle accelerators, was made in DLAs. In our joint publication  we present a theoretical work on how to accelerate an electron beam from 83 keV to 1 MeV while, at the same time, confining most of the electrons so that they are not lost in the long acceleration channel. Today we focus on the experimental demonstration of APF in dielectric laser accelerators. Below, we present an SEM image of the first fabricated structure, which is currently being tested.
Nanocrystalline diamond-coated nanotips
The requirements of the electron source are very strict for dielectric accelerators. The geometry of the dielectric structures has direct consequences for the required beam quality of the electrons. To prevent the electrons from colliding with the structure, the beam size and divergence must be as small as possible (typically approx. 100 nm beam waist and 1 mrad divergence). In addition, the electron pulse duration should be shorter than the laser pulse duration (sub-ps) and the number of electrons per pulse should be as high as possible in order to accelerate as many electrons as possible. Because of these high demands on the electron source, we are also doing research on new electron sources that meet these requirements. A possible candidate is a tungsten tip coated with nanocrystalline diamond.
Diamond is a highly interesting material for electron sources because it is not only mechanically robust, chemically inert and thermally conductive, but also has a negative electron affinity – if the surface is saturated with hydrogen. Due to this special feature, diamond has a low work function and can show a significantly higher quantum efficiency than materials with positive electron affinity – i.e. almost any other materials. After fabrication and characterization  of diamond-coated tungsten tips, we also investigated the photoemission of electrons with ultrashort laser pulses . The high photon density of femtosecond laser pulses allows efficient electron emission by multiphoton absorption. We measure the number of photons required per emitted electron at different wavelengths and can thus identify specific emission channels in the complex band diagram of the heterostructure.
In addition, we characterize the potential application of these coated tips as ultrafast electron sources. We observe that the lower bound of the brightness is already comparable to the sources used today (e.g ultrafast SEM/TEM) and the stability is even better.
Ponderomotive interaction of laser fields and electron and attosecond electron pulse train generation
An alternative solution to electron pulse bunching using nanostructures involves the second-order all-optical ponderomotive scheme, in which two crossed laser beams generate an optical wave, either standing or co-propagating with the electron pulse. Free electrons impinging on such a transvers intensity grating will predominantly diffract as in the Kapitza-Dirac effect, or strongly disperse in a longitudinal grating . In the latter case, a relatively long (~100 fs) electron pulse may be structured into a train of attosecond bunches with lengths below 300 as . Such highly-bunched electron trains may be interesting not only for acceleration, but also for the ultrafast time-resolved probing of structural and chemical changes in atoms, molecules, and solids.
Very recently, we have achieved very similar results also in a nearfield scheme, so with the help of the photonic accelerator structures.
Accelerator on a Chip International Program — ACHIP
ACHIP, generously funded by the Gordon and Betty Moore Foundation and commenced on November 2015, is headed by Stanford and our group at FAU. See here for the Stanford ACHIP web site. Next to Stanford and FAU, the following groups are members of ACHIP:
DESY (R. Assmann, I. Hartl)
EPFL (L. Rivkin)
Hamburg University (F. Kaertner)
PSI (R. Ischebeck)
Purdue University (M. Qi)
SLAC (J. England, S. Tantawi)
Stanford University (B. Byer, S. Fan, J. Harris, O. Solgaard, J. Vuckovic)
Tech-X (B. Cowan)
TU Darmstadt (O. Boine-Frankenheim)
UCLA (P. Musumeci)
Industrial affiliate: Hamamatsu Photonics
The members of the Accelerator on a Chip International Program, ACHIP (Erlangen, 2016)
Please find our full list of publications here.
 J. Breuer and P. Hommelhoff, “Laser-Based Acceleration of Nonrelativistic Electrons at a Dielectric Structure,” Physical Review Letters 111, 134803 (2013)
 E. A. Peralta, K. Soong, R. J. England, E. R. Colby, Z. Wu, B. Montazeri, C. McGuinness, J. McNeur, K. J. Leedle, Walz, E. B. Sozer, B. Cowan, B. Schwartz, G. Travish R. L. Byer, “Demonstration of Electron Acceleration in a Laser-Driven Dielectric Micro-Structure,” Nature 503, 7474 (2013).
 K. J. Leedle, A. Ceballos, H. Deng, O. Solgaard, R. Pease, R.L. Byer, J. Harris, “Dielectric Laser Acceleration of sub-100 keV Electrons with Silicon Dual Pillar grating Structures,” Optics Letters 40 18 (2015).
 J. McNeur, M. Kozak, D. Ehberger, N. Schönenberger, A. Tafel, A. Li, P. Hommelhoff, “A miniaturized electron source based on dielectric laser accelerator operation at higher spatial harmonics and a nanotip photoemitter” J. Phys. B: At. Mol. Opt. Phys. 49 034006 (2016).
 J. McNeur, M. Kozák, N. Schönenberger, K. J. Leedle, H. Deng, A. Ceballos, H. Hoogland, A. Ruehl, I. Hartl, R. Holzwarth, O. Solgaard, J. S. Harris, R. L. Byer, and P. Hommelhoff, “Elements of a dielectric laser accelerator,” Optica 5, 687-690 (2018).
 R. J. England, R. J. Noble, K. Bane, D.H. Dowell, C. Ng, J.E. Spencer, S. Tantawi, Z. Wu, R. L. Byer, E. Peralta, K. Soong, C. Chang, B. Montazeri, S.J. Wolf, B. Cowan, J. Dawson, W. Gai, P. Hommelhoff, Y. Huang, C. Jing, C. McGuiness, R. B. Palmer, B. Naranjo, J. Rosenzweig, G. Travish, A. Mizrahi, L. Schachter, C. Sears, G. R. Werner, R. B. Yoder. “Dielectric Laser Accelerators,” Rev. Mod. Phys. 86, 1337 (2014).
 P. Yousefi, N. Schönenberger, J. Mcneur, M. Kozák, U. Niedermayer, and P. Hommelhoff, “Dielectric laser electron acceleration in a dual pillar grating with a distributed Bragg reflector,” Opt. Lett. 44, 1520-1523 (2019).
 U. Niedermayer, T. Egenolf, O. Boine-Frankenheim, and P. Hommelhoff, „Alternating-Phase Focusing for Dielectric-Laser Acceleration“ ,Phys. Rev. Lett. 121, 214801, (2018).
 A. Tafel, M. Wu, E. Spiecker, P. Hommelhoff, J. Ristein, “Fabrication and structural characterization of diamond-coated tungsten tips,” Diamond and Related Materials, 97 107446 (2019).
 A. Tafel, S. Meier, J. Ristein, and P. Hommelhoff, “Femtosecond Laser-Induced Electron Emission from Nanodiamond-Coated Tungsten Needle Tips,” Phys. Rev. Lett. 123, 146802 (2019).
 M. Kozák, T. Eckstein, N. Schönenberger & P. Hommelhoff „Inelastic ponderomotive scattering of electrons at a high-intensity optical travelling wave in vacuum,“ Nat. Phys. 14, 121 (2017).
 M. Kozák, N. Schönenberger, and P. Hommelhoff, “Ponderomotive Generation and Detection of Attosecond Free-Electron Pulse Trains,”, Phys. Rev. Lett. 120, 103203 (2018).
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