Coherent terahertz radiation from laser-wakefield-accelerated electron beams

Abstract

Theoretical and experimental studies are presented of coherent terahertz radiation (THz radiation, wavelength 10 - 1000 µm), emitted by electron bunches from a laser wakefield accelerator (LWFA). The studies have been performed at the Lawrence Berkeley National Laboratory in Berkeley, California. Through the nonlinear interaction of an intense laser pulse (> 1019 Wcm-2) with a helium plasma, longitudinal electric fields (˜ 10 - 100 GVm-1), which are roughly 2-3 orders of magnitude stronger than in a conventional accelerator, result in the production of relativistic electron bunches. The femtosecond bunches emit coherent THz radiation as they propagate through the plasma-vacuum boundary (coherent transition radiation). Characterization of the THz pulse parameters allows for analysis of the electron bunch charge profile. Additionally, since the THz pulse is intense and intrinsically synchronized to the laser pulse and electron bunch, unique possibilities for THz applications exist. In order to characterize the THz pulse, several measurement techniques have been applied. A THz energy detector shows that the THz energy has a quadratic dependence on electron bunch charge, as is expected for coherent radiation from an electron bunch. The polarization properties of the radiation are also found to agree with theoretical predictions. Through electro-optic sampling (EOS), the temporal THz field profile is measured with both single- and multi-shot techniques. The data shows that the coherent THz spectrum extends from 0-6 THz, while the temporal profile consists predominantly of a single-cycle field oscillation with a peak field of ?? 0.4 MVcm-1. Through comparison with a model, the electron bunches are found to have a duration of <50 fs (rms). The temporal jitter in bunch length and synchronization is <10 fs. At times, a weaker second field cycle is present in the measured THz profile, and twodimensional EOS (THz imaging) is implemented to study this double-THz-pulse waveform. The 2D field distribution shows a main THz spot with a diameter of ?? 800 µm, as well as the presence of spatial substructure related to optical aberrations. The measured substructure in both the temporal and 2D experiments indicate spatio-temporal coupling of the THz pulse at focus. A theoretical model, based on the propagation of a single-cycle pulse through an optical system with coma, confirms the coupling. Understanding of the spatiotemporal THz coupling is especially important for THz-based electron bunch analysis.