All-optical ultrafast electron bunch compression

• Main Author: Lim, Jeremy Zhen Jie
• Other Authors: Chong Yidong
• Format: Final Year Project
• Language: English
• Published: 2018
• Subjects: DRNTU::Science::Physics
• Online Access: http://hdl.handle.net/10356/74134
• Institution: Nanyang Technological University

Ultrafast electron imaging (UEI) is a class of imaging techniques which utilizes electron bunches instead of laser pulses to probe the nature of light and matter with picometer spatial resolution and attosecond temporal resolution, making it ideal for studying the most rapid and minute of phenomena. In this thesis, we study an all-optical electron bunch compression scheme which employs counter-propagating, bi-color lasers which overlap to form a sub-luminal intensity grating which co-propagates with a quasi-monoenergetic electron bunch. The ponderomotive force exerted by the intensity grating provides sub-cycle spatio-temporal modulation on the bunch with respect to the laser fields, producing a series of equally-spaced electron bunches of attosecond duration. These bunches have been used in UEI to image processes on attosecond time resolutions. We develop a mathematical description of the compression scheme which is fully relativistic and more accurate than preceding works. In particular, we derive a closed-form approximation for the focal distance between the intensity grating-electron bunch interaction point and where the maximum compression is reached. Our expression exhibits clear dependencies on polarization type, laser pulse peak field strengths, durations, and phase shifts. The predicted scaling of focal distances with respect to the laser pulse parameters are in excellent agreement with relativistic electrodynamical simulations. We propose laser pulse parameter values which ensure high quality attosecond electron bunches at the sample position, and derive an inequality which provides order-of-magnitude estimates for the laser pulse parameters required to overcome specific kinetic energy spreads in order to achieve well-compressed bunches. Finally, we superpose counter-propagating terahertz and optical radiation of durations and intensities within the current state-of-the-art to form an intensity grating which co-propagates with 4.45 MeV electrons. This configuration leverages on relativistic electron energies to suppress space charge expansion arising from Coulomb repulsion - an inherent challenge for UEI which have previously restricted the realization of attosecond electron bunches to the single-electron regime at non-relativistic energies. We demonstrate using ab initio simulations, which include space charge effects, that up to 40 fC of charge contained within a full width half maximum duration of 300 attoseconds can be produced with this scheme. In addition to relativistic, single-shot UEI, the resulting bunches could potentially be used for high brightness, coherent radiation production.