Demonstration of sub-luminal propagation of single-cycle terahertz pulses for particle acceleration

The sub-luminal phase velocity of electromagnetic waves in free space is generally unobtainable, being closely linked to forbidden faster than light group velocities. The requirement of sub-luminal phase-velocity in laser-driven particle acceleration schemes imposes a limit on the total acceleration achievable in free space, and necessitates the use of dispersive structures or waveguides for extending the field-particle interaction. We demonstrate a travelling source approach that overcomes the sub-luminal propagation limits. The approach exploits ultrafast optical sources with slow group velocity propagation, and a group-to-phase front conversion through nonlinear optical interaction. The concept is demonstrated with two terahertz generation processes, nonlinear optical rectification and current-surge rectification. We report measurements of longitudinally polarised single-cycle electric fields with phase and group velocity between 0.77c and 1.75c. The ability to scale to multi-megavolt-per-metre field strengths is demonstrated. Our approach paves the way towards the realisation of cheap and compact particle accelerators with femtosecond scale control of particles

Single-shot, femtosecond resolution, terahertz pulse measurement without a femtosecond probe via electro-optic transposition

We report a technique for the single-shot measurement of single-cycle THz radiation without the need for ultrashort laser pulses. The nonlinear mixing between a narrow linewidth laser and a THz pulse encodes the THz temporal profile into the envelope of a new optical pulse. The preservation of temporal and spectral information in this process enables retrieval of the THz pulse from a standard FROG measurement of the new optical signal. A system has been built which includes a broadband OPA for amplification of the low energy transposed pulse to a level compatible with single-shot FROG methods. The technique should prove essential in the characterization of ultrashort THz pulses that are generated with large shot-to-shot jitter

Spatial and Temporal Field Evolution of Evanescent Single-Cycle THz Pulses

The temporal and spatial evolution of single-cycle THz pulses travelling across a dielectric boundary with effective surface velocity below c has been examined. The resulting subluminal evanescent waves in the near field vacuum region are potential drivers of particle accelerators. The electro-optic temporal and spatial imaging of the THz pulses emitted from the surface were measured in the near field, as a function of distance from the dielectric boundary, and reveal both temporal broadening and field decay that is dependent on the effective velocity of the wave in the plane of the boundary

Competition between polaron pair formation and singlet fission observed in amorphous rubrene films

In this paper, we investigate excited state dynamics in amorphous rubrene vacuum sublimed films. We report the direct observation of singlet fission in amorphous rubrene films. We have determined the fission rate to be >2.5 x 10(12) s(-1). Simultaneously, we observe strong polaron pair absorption and propose that polaron pair formation could be competing with singlet fission. Another possible conclusion from our experiments could be that two triplets from singlet fission might arise via polaron pairs. In either case, polaron pairs play an important role in singlet fission in an amorphous rubrene film. We also observe that triplets created by singlet fission fuse to regenerate a singlet, giving delayed fluorescence (DF) scaling linearly with initial laser energy (i.e., one singlet gives two triplets and two triplets give back one singlet). This is a strong evidence of S-1(n) -> 2T(1). We did not observe substantial temperature dependence of DF decay curve shape, indicating that triplet migration in amorphous rubrene films is not hopping limited and that triplets undergo fusion before their migration

Measurement of interchain and intrachain exciton hopping barriers in luminescent polymer

The integrated photoluminescence intensity in thin films of 'Super Yellow' copolymer has been analyzed using a Mott-like temperature dependence. This has enabled us to observe contributions from two emission channels, indicative of exciton recombination proceeding from two distinct origins. At high temperature, interchain thermally activated exciton energy transfer and migration dominates, resulting in large scale quenching of the integrated emission intensity and hence the photoluminescence quantum yield. However, at relatively low temperature, an additional increase of the integrated emission intensity occurs. This new channel of emission has been attributed to recombination from excitons where intrachain exciton energy transfer between adjacent subunits of the copolymer backbone becomes hindered. The activation energy barriers that control both of these emission channels have been obtained and are correlated with chain backbone degrees of freedom

Terahertz-driven acceleration of a relativistic 35 MeV electron beam

We will present the first results from the CLARA research facility at Daresbury Laboratory demonstrating terahertz-driven acceleration of a relativistic 35 MeV electron beam. A polarization-tailored, frequency-tunable, narrowband terahertz source was used to directly excite the longitudinal accelerating mode of a dielectric-lined waveguide structure for collinear phase-velocity-matched THz-electron interaction

Demonstration of sub-luminal propagation of single-cycle terahertz pulses for particle acceleration

Research data relating to the journal paper 'Demonstration of sub-luminal propagation of single-cycle terahertz pulses for particle acceleration' published in Nature Communications 2017

Design, specifications, and first beam measurements of the compact linear accelerator for research and applications front end

The compact linear accelerator for research and applications (CLARA) is an ultrabright electron beam test facility being developed at STFC Daresbury Laboratory. The ultimate aim of CLARA is to test advanced free electron laser (FEL) schemes that can later be implemented on existing and future short-wavelength FELs. In addition, CLARA is a unique facility to provide a high-quality electron beam to test novel concepts and ideas in a wide range of disciplines and to function as a technology demonstrator for a future United Kingdom x-ray FEL facility. CLARA is being built in three phases; the first phase, or front end (FE), comprises an S-band rf photoinjector, a linac, and an S-bend merging with the existing versatile electron linear accelerator beam line; the second phase will complete the acceleration to full beam energy of 250 MeV and also incorporate a separate beam line for use of electrons at 250 MeV; and the third phase will include the FEL section. The CLARA FE was commissioned during 2018, and the facility was later made available for user experiments. Significant advancements have been made in developing high-level software and a simulation framework for start-to-end simulations. The high-level software has been successfully used for unmanned rf conditioning and for characterization of the electron beam. This paper describes the design of the CLARA FE, performance of technical systems, high-level software developments, preliminary results of measured beam parameters, and plans for improvements and upgrades. © 2020 authors. Published by the American Physical Society