Research
Dielectric Laser Accelerators
The working principle of dielectric laser accelerators (DLAs) is that laser pulses interact with electron beams in the vicinity of dielectric nanostructures. The strong electric field in the ultrafast laser can accelerate the electrons, leading to high acceleration gradient, which is about one to two orders of magnitude higher than conventional microwave-cavity accelerators. Moreover, the DLAs and the laser power delivery system have the potential to be integrated on a chip, resulting in compact accelerators. With a high acceleration gradient and a compact size, DLAs have wide range of applications in scientific research, industry, and medical diagnosis and treatment. To learn more about DLAs, welcome to read the Physics Today article and our recent review article.
The accelerator-on-a-chip (ACHIP) collaboration focuses on researching and developing DLAs to build an accelerator on a chip. As a member of ACHIP collaboration, I mainly contributed to the design of DLAs and the on-chip power delivery waveguide system, as well as studying the classical and quantum applications related to DLAs.
Design of a multi-channel photonic crystal DLA
One challenge of DLAs is the low electron current, limited by the sub-micron width of the electron channel. To increase the electron current, we studied a photonic crystal DLA architecture with multiple electron channels. By investigating the band structure and eigenmodes of the underlying photonic crystal, we proposed a design where the electromagnetic fields inside different channels are almost identical. (Published in Photonics Research, 2020)
Design of a tapered slot-waveguide DLA for sub-relativistic electrons
A challenge of long-distance sub-relativistic acceleration is the dephasing, which arises when the velocities of the accelerated particles increase and become mismatched with the phase velocity of the accelerating field. To address this problem, we proposed a tapered slot-waveguide DLA. This structure supports a guided mode with a longitudinal electrical field in the vacuum slot, which co-propagates and accelerates the electrons. We provided the design formalism for the tapering of the slot waveguide. (Published in Optics Express, 2018)
Electron pulse compression with optical beat note
Compressing electron pulses is important in many applications of electron beams. Terahertz (THz) electron compressors have been demonstrated to compress electron pulses from pico-second to femto-second timescales. However, the generation efficiency of THz waves is typically low. DLAs provide new potential solutions to the electron pulse compression. In this study, we propose to use optical beat notes to compress electron pulses. The beat frequency is chosen to match the initial electron pulse duration. Such interaction field can be generated in DLAs illuminated by two laser pulses with a controlled central frequency difference. This proposed compressor bypasses the difficulty associated with the low efficiency in generating THz waves. (Published in PRL, 2021, Editors' Suggestion, arXiv version)
Quantum entanglement and modulation enhancement of free-electron-bound-electron interaction
The interaction between the free electron and the near-field of ultrafast laser enables the modulation of free-electron wave function, which brings new ingredients into the electron-matter interaction. Inspired by Gover and Yariv’s work on free-electron—bound-electron resonant interaction, we applied the quantum mechanical treatment and obtained the scattering matrix of such interaction. We obtained the perturbation in the two-level system due to the interaction and the electron energy loss spectrum after the interaction. We found that the modulated free electron can be a probe to measure the atomic coherence and enhance the electron-atom interaction. Moreover, we found that distantly separated two-level atoms can be entangled through interacting with the same free electron. We also studied the interaction between a two-level system and a dilute electron beam and obtained the effective dynamics of the two-level system. (Published in PRL, 2021, arXiv version)
Connection of temporal coupled-mode-theory formalisms for a resonant optical system and its time-reversal conjugate
The resonance phenomenon is ubiquitous in photonics. To design a functional nanophotonic device, it is crucial to engineer its resonance properties. Temporal coupled-mode theory (TCMT) intuitively describes the dynamics of the resonant system, where the resonant mode interacts with propagation waves and dominantly influences the scattering properties of the system. It links the measurable scattering matrix to the resonances, which are easier to control in design and experiments.
The formalism of TCMT is strongly constrained by various symmetry constraints, such as time-reversal symmetry, energy conservation, and Lorentz reciprocity. For these three symmetries, the presence of any two implies the third, and previous studies usually assumed the presence of all three. To extend TCMT for lossy or nonreciprocal systems, we studied the question about the constraints on TCMT for systems where only one of the three symmetries is present. In deriving these constraints, we utilized the connections of the physical properties between the original system and its time-reversal conjugate. We also found some nontrivial implications of the relation between the resonant properties of a physical system and its time-reversal conjugate and validated these implications numerically. (published in PRA, 2019)
Design principles of apodized grating couplers
The grating coupler is a crucial component in integrated photonics. It couples light between the optical fiber and the integrated photonic circuits. The main objective of the grating coupler design is to increase the coupling efficiency. A uniform grating has an exponential scattering intensity distribution, which mismatches the Gaussian-like distribution of a fiber mode. To increase the coupling efficiency, the grating couplers must be apodized, i.e., with spatially changing geometric parameters.
A well-established description of the apodized grating coupler is the position-dependent scattering strength. The optimal scattering strength is a function of the target output light distribution. With this model, many apodized grating couplers with high efficiency have been demonstrated. However, the optimal scattering strength predicted by this model may not be achieved with the fabrication technology. In practice, the scattering strength can neither be arbitrarily large due to the finite scattering strength of the grating, nor be arbitrarily small, due to the minimal feature size.
In this study, we extended the previous model by including the constraints of upper and lower bounds of the scattering strength and provided a formalism to determine the globally optimal scattering strength under these constraints. We applied our model to both standard and complex coupling problems. In the problem of coupling to a vortex beam, our design obtained significantly higher efficiency than existing designs. (Published in JLT, arXiv version)
