Abstract-Magnetic-field tailoring of the terahertz polarization emitted from a spintronic source

Applied Physics Letters

M. T. Hibberd, D. S. Lake, N. A. B. Johansson, T. Thomson,  S. P. Jamison, D. M. Graham

(a) Mechanism of THz generation from a spintronic source. A laser pump pulse launches a spin-polarized current (jS) in the ferromagnetic (FM) layer that is converted to a transverse charge current (jC) in the non-ferromagnetic (NM) layer, generating a THz pulse polarized perpendicular to the applied magnetic field (Bmag). (b)–(e) Schematic diagrams of the two permanent magnets on either edge of a spintronic source, showing the magnetic field lines when oriented with (b) aligned and (d) opposing polarity. The corresponding THz electric field lines are given in (c) and (e), respectively.

https://aip.scitation.org/doi/abs/10.1063/1.5055736

We demonstrate a method to create arbitrary terahertz (THz) polarization profiles by exploiting the magnetic field-dependent emission process of a spintronic source. As a proof-of-concept, we show that by applying a specific magnetic field pattern to the source, it is possible to generate a quadrupole-like THz polarization profile. Experimental measurements of the electric field at the focus of the THz beam revealed a polarity flip in the transverse profile of the quadrupole-like mode with a resulting strong, on-axis longitudinal component of 17.7 kV cm−1. This represents an order of magnitude increase in the longitudinal component for the quadrupole-like profile compared to a linear polarization, showing an example of how the magnetic field patterning of a spintronic source can be exploited to obtain desirable THz polarization properties. This unique ability to generate any desired THz polarization profile opens up possibilities for schemes such as rotatable polarization spectroscopy and for efficient mode coupling in various waveguide designs. Furthermore, the strong longitudinal fields that can be generated have applications in areas including intra-subband spectroscopy of semiconductors, non-diffraction limited THz imaging, and particle-beam acceleration.

Abstract-Electron-Terahertz Interaction in Dielectric-Lined Waveguide Structures for Electron Manipulation

A. L. Healy, G. Burt, S. P. Jamison

https://arxiv.org/abs/1802.00632

Terahertz-driven dielectric-lined waveguides (DLWs) have uses in electron manipulation; in particular deflection, acceleration, and focussing. A rectangular DLW has been optimised for deflection of 100 keV electrons using a THz pulse with a centre frequency 0.5 THz. Electron-THz interaction and the effect of electron bunch injection timing on maximising deflection is presented. DLWs and corrugated waveguides are compared to discuss relative advantages and disadvantages.

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

D. A. Walsh, D. S. Lake, E. W. Snedden, M. J. Cliffe, D. M. Graham,  S. P. Jamison

https://www.nature.com/articles/s41467-017-00490-y?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+ncomms%2Frss%2Fcurrent+%28Nature+Communications+-+current%29

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-megawatt-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.