Experiments on DLAs

(1) Dielectric Laser Acceleration of Relativistic Electrons

These proposed DLA structures have been theoretically and numerically demonstrated to generate high accelerating gradients of up to GV/m. However, no successful experimental demonstrations have been observed except with grating-based structures. In 2013, a milestone experiment demonstrating dielectric laser acceleration of relativistic electrons in a fused-quartz dual-grating structure was reported by SLAC [47]. In this experiment, an accelerating gradient of up to 300 MeV/m was observed in a fused silica dual-grating structure. A Ti:Sapphire laser system operating at a wavelength of λ = 800 nm, pulse energy

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up to 319 μJ and pulse duration of 2 ps was used as the pumping power and highly relativistic 60 MeV electron bunches were used as the particle source. The interaction length of laser and electrons was estimated at 563 ± 104 grating periods (one grating period was equal to 800 nm). The cross section of a DLA structure and the relevant experimental set up are illustrated in Figure 18.

Figure 18. (a) SEM image of a dual-grating DLA structure, and (b) experimental setup [47].

In this experiment, the dual-grating structure has a vacuum channel gap of only 400 nm, which makes beam loading a very challenging task. Particle tracking simulation shows that only 2.2% of the 60 MeV beam can pass through the vacuum channel and be modulated by the laser field. Those electrons travelling through the structure substrate and grating pillars suffer significant energy loss due to collisional straggling [90] in the dielectric material, so they were excluded from Figure 19. This shows only the energy distribution for modulated electrons, with the laser on and off. It can be seen that the laser field efficiently modifies the bunch energy distribution, which is in good agreement with simulation results. In addition, the electron bunch has a RMS length of 129 ± 9 μm, which is much longer than the laser wavelength of 800 nm. Electrons therefore sampled all phases of the laser field, causing some electrons to gain energy from acceleration while some were decelerated.

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Figure 19. Experimental and simulation results of bunch energy distribution with the laser on and off [47].

It is then followed by another experiment carried out by SLAC in 2016 [48]. A dual- grating structure with the same geometry was also used for this experiment. A Ti:Sapphire laser system operating at a wavelength of λ = 800 nm and pulse duration of 64₋₇+11 _{fs was}

used to illuminate a dual-grating structure. The only difference from the earlier experiment is that a femtosecond laser system was used in order to generate an accelerating gradient higher than 300 MV/m. Figure 20 shows the measured and simulated energy distribution for those electrons transmitted through the vacuum channel of the dual-grating structure. The measured maximum energy gain is 24 ± 1.1 keV, corresponding to an accelerating gradient of G = 690 ± 110 MV/m which is a record gradient for DLA experiments to date.

Figure 20. Measured, fitted, and simulated electron beam energy distribution, with the laser off and on [48]. Electrons which traversed the structure substrate and grating pillars were observed to suffer a mean energy loss of 300 keV, and are not shown.

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(2) Dielectric Laser Acceleration of Non-relativistic Electrons

In order to realize a particle accelerator ‘on a chip’, synchonized acceleration between the non-relativistic electrons emitted from the injector and the exciting laser pulses should be achieved. This requires that the transit time of the electrons across a period of the grating structure must be equal to the laser period. Based on this, three experiments have so far been performed for acceleration of non-relativistic electrons in grating-based structures. In 2013, a milestone experiment successfully demonstrated acceleration of non-relativistic electrons in the vicinity of a fused-quartz single-grating structure [51]. In this experiment, a laser beam with a centre wavelength of 787 nm and a pulse duration of 110 fs was used to illuminate a single-grating structure located on top of a mesa, as shown in Figure 21. The grating period of 750 nm was chosen so that the third spatial harmonics travelled synchronously with 28 keV electrons due to a trade-off between the available accelerating gradient and fabrication limitations on the grating period (𝜆_p> 0.6 μm). A maximum accelerating gradient of 25 MV/m was observed, which was already comparable with state-of-the-art conventional RF cavity-based accelerators. The parameter dependencies were investigated in detail and had excellent agreement with numerical simulations.

Figure 21. (a) Schematic of experimental setup for acceleration of 28 keV electrons at a fused-quartz single- grating structure, where the electron beam and laser are represented in blue and red colors, respectively; (b) SEM image of a single-grating structure which is located on the top of a mesa with a width of 25 μm [91].

After this, similar successful demonstrations were reported for the acceleration of sub- 100 keV electrons by researchers from Stanford University [52], [53]. As shown in Figure 22, a 5 nJ, 130 fs mode-locked Ti-sapphire laser with 907 nm wavelength was used to acclerate 96.3 keV electrons in a silicon single-grating structure with a period of 490 nm. In this situation, the phase synchronicity was achieved between the first spatial harmonics and the electrons. Due to higher accelerating efficiency for the first spatial harmonics, an

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accelerating gradient of 218 MV/m [52], higher than the previous 25 MV/m, was obtained. The single-grating structure was then replaced by a dual-pillar structure for acceleration of 96.3 keV and 86.5 keV electrons using the same laser system, which can be seen in Figure 23. Because the dual-pillar structures have a higher accelerating efficiency than the single- grating structures, an accelerating gradient of 370 MV/m [53] was observed, even higher than the 218 MV/m seen earlier. Such a dual-pillar structure can be integrated with a Bragg reflector, which will be studied in Chapter 6.

Figure 22. Schematic of experimental setup for acceleration of 96.3 keV electrons at a silicon single-grating structure [52].

Figure 23. Schematic of experimental setup for acceleration of sub-100 keV electrons at a silicon dual-pillar structure [53].

With the mature lithographic techniques and availability of compact and efficient fiber laser technology, the successful demonstrations of relativistic and non-relativistic electron

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acceleration discussed above may pave the way towards an all-optical compact particle accelerator ‘on a chip’.

(3) Damage Threshold Measurement

One major advantage of DLAs is their ability to support accelerating fields 2 orders of magnitude higher than those achieved in conventional RF cavity-based accelerators. The key to sustaining such high accelerating gradients is the much higher damage threshold of dielectric materials compared with metals. The appropriate dielectric material should therefore be chosen to maximize the achievable accelerating gradient. Experimental studies [92]–[94] have been carried out to investigate the damage threshold for different dielectric materials. As shown in Figure 24, researchers at SLAC demonstrated that the quartz material has a damage threshold of 4 J/cm2 _{for a 1 ps pulsed laser. Lenzner et al. [95] found that the}

damage threshold for a 1 ps pulsed laser is about twice as large as for a 100 fs pulsed laser, which is also supported by Breuer’s measurement [92]. This indicates that the damage threshold for quartz material is 2 J/cm2 _{for 100 fs laser pulses, implying that a peak electric}

field of the order of 10 GV/m can be sustained in the quartz surface. Quartz material is therefore usually chosen for DLA structures, as has been demonstrated in many DLAs experiments [47], [48], [51]. Figure 24 also shows that silicon has a much lower damage threhold (0.18 J/cm2_{) than quartz, corresponding to a peak field of about 3 GV/m. However,}

silicon may be still chosen to create a resonant-like cavity for many DLA structures, due to its having a higher refractive index [53], [82] than quartz. In addition, silicon can be specifically designed to take advantage of the mass-production techniques of the computer chip industry and so offers a far less expensive way to build a partcle accelerator ‘on a chip’.

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Figure 24. The laser damage threshold of a variety of optical materials, including copper for comparison [94]. Each of these measurements was conducted with a 1 ps, 800 nm, 600 Hz Ti:Sapphire laser.