Laser-driven acceleration of subrelativistic electrons near a nanostructured dielectric grating: From acceleration via higher spatial harmonics to necessary elements of a dielectric accelerator
Introduction
The drive towards realizing miniaturized particle accelerators capable of producing relativistic high brightness beams of charged particles has motivated the progress of the field of DLAs. These novel devices leverage the high field gradients available from commercial lasers as well as standard dielectric nanofabrication techniques to achieve acceleration gradients far exceeding those possible in microwave Linac cavities [1]. Proof of principle experiments [2], [3] as well as optimized DLA geometries [4] have shown that DLAs are indeed a viable novel accelerator with gradients exceeding 300 MeV/m. Here, we summarize the ongoing research into dielectric laser acceleration occurring in Erlangen.
The core principle used by DLAs is analogous to that used in microwave cavities. Electrons traverse a time-varying electromagnetic field patterns in such a way that those electrons whose velocity matches the phase velocity are accelerated phase synchronously. For the results discussed here, the electromagnetic field pattern is generated when a laser pulse from a long-cavity Ti:sapphire oscillator is incident upon a nanostructured fused silica grating. The grating teeth act as a phase mask, generating a near field pattern whose spatial harmonics can be used to accelerate electrons. An illustration of this mechanism is shown in Fig. 1.
The source of electrons is a DC thermal-emission SEM column. The emitted electrons have a user-controlled energy ranging from 3 to 30 keV, a transverse spot size of 70 nm and a current of 3 pA. The fused silica grating itself is fabricated by a combination of UV lithography and reactive ion etching [3] and is 50 μm long in the direction of electron propagation.
The accelerated electrons are detected with a Chevron-type micro-channel plate (MCP) downstream of a retarding field spectrometer that acts as a high pass filter for the electrons. By setting the voltage of this spectrometer higher than the injection energy divided by the electron charge, we ensure that only accelerated electrons reach the MCP. We then correlate the arrival time of accelerated electrons onto the MCP with the arrival time of a picked off portion of the incident laser pulse on an upstream photodiode. A spike in the count rate at the same time delay after the photodiode signal (e.g. shown in Fig. 2b) is then evidence of accelerated electrons.
Section snippets
Acceleration with 4th and 5th spatial harmonics
Previous work [Breuer PRL] demonstrated acceleration of 28 keV electrons with the 3rd spatial harmonic of the diffracted fields near the fused silica grating. We note that the phase velocity vph of the spatial mode can be related to the harmonic index m bywhere λ is the wavelength of the incident laser and Λ is the periodicity of the grating. Thus, higher spatial harmonics have lower phase velocities than the 3rd spatial harmonic, allowing for the acceleration of slower electrons.
Ongoing and future work
In addition to the demonstration of acceleration with higher spatial harmonics, the experimental setup described here has served as a staging ground for a wide range of recent results relevant to the development of DLAs. We have demonstrated 80 MeV/m acceleration of 25 keV electrons via use of a thulium fiber laser and Si structures, overcome the dephasing of electrons (whose velocity is increasing) with respect to the accelerating mode by introducing a tapered single grating geometry,
Acknowledgments
This research is funded in part via the ERC grant 616823 NearFieldAtto.
References (7)
- et al.
Reviews of Modern Physics
(2014) - et al.
Nature
(2013) - et al.
Physical Review Letters
(2013)
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