"Control free-electron—light interaction for nanophotonic particle accelerators and quantum applications"

Abstract:

Electron accelerators are important in studying fundamental science, research, medical diagnosis and treatment, and industrial processing. In dielectric laser accelerators (DLAs), laser pulses interact with electron beams in the vicinity of dielectric nanostructures and can accelerate the electrons. DLAs have the potential to provide acceleration gradients at least one order of magnitude higher than conventional radio-frequency accelerators, due to the high damage threshold of dielectric materials, with a compact size.

Key challenges of DLAs include the integration of the optical power delivery waveguide with the DLAs, extending the acceleration distance, and increasing the current throughput. In this presentation, we show examples of how photonic design and control address these challenges. In a compact DLA system, the electrons generated by on-chip sources are sub-relativistic. A major challenge of long-distance sub-relativistic acceleration is dephasing, when the velocities of the accelerated particles increase and become mismatched with the phase velocity of the accelerating field. To solve the dephasing, we designed a tapered slot waveguide DLA where the phase velocity of the slot waveguide matches the electron velocity continuously. The slot waveguide supports a guided mode that has a longitudinal electric field in the vacuum slot and co-propagates with the electrons. The waveguide width is changed gradually along the particle trajectory, such that the phase velocity is matched with the velocity of the accelerated electrons. Moreover, light can be coupled into the slot waveguide through grating couplers. Another challenge is the low current throughput due to the sub-wavelength narrow channel. To increase the DLA current throughput for science and medical applications, we introduced a photonic crystal DLA that has multiple electron channels. Through engineering the band structure of the underlying photonic crystal, we can make the acceleration field in different channels to be almost identical. The photonic crystal DLAs can increase the current throughput by orders of magnitude.

The development of DLAs opens new opportunities in both classical and quantum applications. Modern electron microscopy requires high temporal resolution to probe ultrafast physics. DLAs can generate a train of micro-bunches of sub-femtosecond duration. Here, we propose the compression of picosecond electron pulses using DLAs, where the energy modulation is generated by the optical beat note. Its performance is comparable to terahertz electron compression, while being more efficient and compact. In the quantum region, DLAs can modulate the wavefunction of free electrons, as in photon-induced nearfield electron microscopy [8, 9]. We studied how resonant modulation of the free electron can enhance the interaction between the free electron and a two-level atom and probe the atomic coherence. We also found that distant identical atoms can be entangled by interacting with the same free electron. Furthermore, a large coupling between free electrons and photons is generally desired for many free-electron quantum optics applications. We derived an upper bound for the coupling coefficient describing the free-electron—photo interaction. The upper bound depends on the interaction length, the optical medium, the free-electron velocity, and the separation between the free electron and the optical medium. It can provide guidance to reach the strong coupling between free electrons and photons.

Looking forward, to make free electron beams and free-electron radiation sources widely accessible, it is important to further integrate the optical system and the free-electron components in a compact platform. To explore the ultimate quantum capabilities of free electron probes, it is crucial to develop systems that can enhance the quantum signals of free-electron—matter interactions.