Coherent single-cycle pulses with MV/cm field strengths from a relativistic transition radiation light source

Abstract

Terahertz (THz) pulses with energies up to 100 μJ and corresponding electric fields up to 1 MV=cm were generated by coherent transition radiation from 500 MeV electron bunches at the free-electron laser Freie-Elektronen-Laser in Hamburg (FLASH). The pulses were characterized in the time domain by electro-optical sampling by a synchronized femtosecond laser with jitter of less than 100 fs. High THz field strengths and quality of synchronization with an optical laser will enable observation of nonlinear THz phenomena. © 2011 Optical Society of America OCIS codes: 120.6200, 140.2600, 320.7110.Intense single-cycle terahertz (THz) pulses are increasingly being used to observe extreme nonlinear optical phenomena at high field strengths [1][2][3]. These pulses can be obtained on the tabletop by optical rectification of femtosecond laser pulses and can reach pulse energies on the level of tens of microjoules [4]. Even higher pulse energies and fields can potentially be reached by accelerator-based sources [5,6]. Coherent transition radiation (CTR) is emitted when an electron bunch crosses the boundary between two media [7,8]. The energy of the pulses scales with the average line charge density ("peak current") of the bunch squared and their frequency content covers the THz up to the IR region of the electromagnetic spectrum, depending on the electron bunch duration. Since the driving linear accelerators for highgain free-electron lasers (FELs) are built to boost the peak current to several kiloampere, they are well suited to drive powerful CTR sources. A number of high-gain xray FEL facilities are now developing a complementary THz research program with short broadband THz pulses. At the soft x-ray FEL Freie-Elektronen-Laser in Hamburg (FLASH) [9], transition radiation is generated from collision of electron bunches with energies up to 1:2 GeV with a metal screen and then guided through an evacuated beam line [10] with 20 m length into a user accessible lab. While the main purpose of this beam line is highresolution diagnostics of the longitudinal electron bunch characteristics [11], it can also serve as a powerful and broadband source for experiments with subpicosecond single-cycle pulses. The pulses obtained have energies exceeding 100 μJ and span a frequency band of 200 GHz to 100 THz. Here we determine the temporal structure and the electric field of the THz pulse below 10 THz directly in the time domain using electro-optic sampling with a femtosecond laser synchronized to the electron bunches. The availability of optical probe lasers synchronized to this intense THz source enables THz pump optical probe measurements.The CTR source at FLASH is located in a straight section between the superconducting linear accelerator and the undulator. The last electron bunch within a sequence of up to 800 bunches with a spacing of 1 μs can be deflected onto an off-axis CTR target by a fast kicker magnet without interfering with routine FEL user operation at the x-ray undulator downstream. The CTR radiation is produced on a metal ...

Intense terahertz pulses from SLAC electron beams using coherent transition radiation

“…Several accelerator-based sources of THz radiation using these approaches have been constructed. [1][2][3][4][5][6][7][8][9][10][11] In CTR from a thin metallic foil, surface currents that match boundary conditions on the foil produce the fields of a collapsing dipole as the electrons approach, and an expanding dipole as they leave, generating an impulse of dipole radiation on both sides. 12 The temporal shape has a long-wavelength cutoff due to the finite size of the foil and apertures.…”

Characterization of the THz radiation source at the Frascati linear accelerator

“…Indeed, these machines, recently developed for producing high brightness electron beams (HBEB) for Free-Electron Lasers (FELs) emission, are designed to work in the sub-ps regime down to 10−50 fs. 9 Linac-based coherent THz radiation from relativistic electron bunches is a potentially revolutionary source [10][11][12] in different field of science ranging from THz pump and probe experiments to molecular spectroscopy and imaging. 13 The study of ultra-fast and nonlinear phenomena can profit of peak electric fields greater than 1−10 MV/cm 14 and intense singlecycle and multi-cycles THz radiation.…”

Demonstration of a low-frequency three-dimensional terahertz bullet with extreme brightness

“…However, a large range of THz applications, ranging from ultrafast domain switching [9][10][11] to particle acceleration 12 , requires a giant leap in brightness to match the theoretically predicted needs in field strength being on the order of a few GV m À 1 . Intense THz radiation is mainly generated by employing electron accelerators [13][14][15] and laser-based systems. Although the former has been ahead in terms of intensity, the availability and versatility of lasers are dominating time-resolved THz high-field science.…”