Ex) Article Title, Author, Keywords
New Phys.: Sae Mulli 2022; 72: 615-620
Published online August 31, 2022 https://doi.org/10.3938/NPSM.72.615
Copyright © New Physics: Sae Mulli.
Hyungyu Yu*, Hyyong Suk†
Department of Physics and Photon Science, Gwangju Institute of Science and Technology, Gwangju 61005, Korea
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License(http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
In this research, a photonic crystal for dielectric laser accelerator has been designed and analyzed by conducting finite-domain-time-differential (FDTD) simulations. The photonic crystal of dielectric materials, such as Si or SiO2, can confine electric field components of a laser pulse in space when it is designed as a cavity structure. The longitudinal electric field components can serve as a driving force for electrons. A maximum acceleration gradient is expected when the electron and acceleration field phases are matched. The phase matching can be achieved by tilting the front of the input laser pulse. In this study, a combined structure of a micro-scale prism and periodic cavities was considered to tilt the pulse front. The calculation of the acceleration field by FDTD simulations can give an energy gain of the incident electrons in dielectric laser acceleration (DLA).
Keywords: Electron beam, Electron acceleration, Dielectric laser acceleration, Photonics
Particles accelerators are important research tools in science. Diverse ways for charged particle acceleration have been used so far[1,2]. Radio-frequency (RF) accelerators, for instance, have been widely used for electron and ion acceleration. However, the acceleration gradient is limited due to electrical breakdown in acceleration structures. Hence, some methods for increasing the acceleration gradient have been extensively researched.
Dielectric laser acceleration (DLA) is one of the ambitious approaches to achieve higher acceleration gradients in accelerators[3-5]. In DLA, for example, a photonic crystal can be used to generate a very high accelerating field with a laser pulse if the laser pulse is injected into the photonic crystal. In this case, the phase matching between the electrons and the accelerating field is very important, and there are several ways to do it. One of the ways is using on-chip phase shifters. However, their structure is very complicated and may lead to a severe beam loss in acceleration. Instead of this method, the phase matching can also be achieved by pulse front tilt of the injected laser in a simpler structure. Some studies have been conducted using this method with spatial and temporal laser chirp with external diffraction gratings[7,8]. However, we achieved the phase matching with pulse-front-tilt (PFT) using micro-scale prisms and periodic cavity without external diffraction gratings. Thus, we achieved an average acceleration field of 0.28 GV/m with the structure we have designed. The details are described in this paper.
The idea of DLA is associated with the Smith-Purcell effect, so the Smith-Purcell effect is reviewed here first. The Smith-Purcell effect is a phenomenon of electromagnetic wave emission from a diffraction grating when electrons pass over the diffraction grating surface very closely. As well known, both conductor and dielectrics can be used for the grating material. The emitted electromagnetic radiation is called the Smith-Purcell radiation and its dispersion relation depends on the angle between the grating surface and the radiation direction as follows:
On the other hand, the inverse Smith-Purcell effect is the process in which an electromagnetic wave diffracted by the grating accelerates electrons. Early studies show that DLA with laminar-type holographic gratings can match the situation[10,11,12]. In this process, phase matching is one of the important concepts in DLA, which is crucial for gaining a high acceleration gradient. If a laser pulse is injected into the DLA structure with a tilted pulse front, it will generate a traveling electromagnetic wave in the longitudinal direction and can accelerate the injected electrons continuously, leading to a high energy gain. Here, the phase of the accelerating field by the input laser pulse is described by
It is also known that the phase matching condition for laser injection in the normal direction is given by
For acceleration, electrons should be injected by an injector, such as a modified scanning electron microscope (SEM) or tunneling electron microscope (TEM). Photocathode RF guns have also been used for electron injection.
The calculation of electric field components is required to know the acceleration gradient in the designed DLA structure. The finite-domain-time-differential (FDTD) simulations can show us how the input laser pulse behaves in the designed nanostructure. We can obtain time-dependent electric field components through the simulations.
For the simulations, we used the DLA structure, as shown in Fig. 1. A laser pulse is injected from the top direction in Fig. 1(a), and two laser pulses are injected from both directions in Fig. 1(b). Here, the laser field polarization is important, and the polarization direction should be in the
Here, we introduce a new component for DLA called a micro-prism. The micro-prism is a micro-scale right-angle prism fabricated with a lower refractive index than that of Si, for example, SiO2. The injected laser's wavevector can be tilted by the micro-prism, and the laser beamlets arrive at the channel at different times. The longitudinal accelerating field
We also calculated the energy gain of an electron which is injected with an energy of 30 keV when the input laser intensity is
We also investigated the effect of the cavity height
The prism angle
Based on the FDTD simulation results, we have designed an experimental setup for DLA. Fabrication of the DLA nanostructure is the most critical part in this experiment. It will be fabricated through a subtractive etching process on a silicon-on-insulator (SOI) wafer for the Si nanostructure and an additive process for the micro-prism. As the resolution of the structure is very high, the electron-beam (E-beam) lithography technique will be used for the subtractive process. The fabricated Si structure will serve as the periodic cavities and the acceleration channel. The additive process, on the other hand, will be done by atomic layer deposition (ALD) or liquid phase deposition (LPD). The micro-prism will be made by SiO2 through those processes.
For the experiment, we will use the 35 fs Ti:sapphire laser in our laboratory, which consists of a mode-locked Ti:sapphire oscillator, a Ti:sapphire regenerative amplifier, a pre-amplifier, and a pulse compressor in air. As Fig. 6 shows, the laser beam pulse will be divided into two pulses first, and then they will be injected into the DLA structure from both sides. Therefore, a strong longitudinal electric field is expected in the acceleration channel in the DLA. In this process, the focused laser spot size will be adjusted to have a maximum acceleration field, which will be higher than the injected laser field strength of
In this study, we calculated electric fields inside the DLA structure when two laser pulses were injected with tilted pulse fronts by micro-prisms. We found that transverse electric field components are not so strong in DLA, but the strength of the longitudinal acceleration field is almost twice of the injected laser field strength. The strong longitudinal acceleration field can be used for electron acceleration with an average acceleration gradient of 0.28 GeV/m.
The research was initiated at Nano Photonics Laboratory, Konkuk University, and completed at Gwangju Institute of Science and Technology (grant number: NRF-2022R1A2C3013359).