An intense Terahertz source for SwissFEL

Terahertz radiation (0.1-10 THz), located between the optical and radio frequency range, is suited to explore fundamental physical phenomena and to drive novel applications in condensed matter physics, biology, surface chemistry and others. Ultrashort and intense Terahertz pulses are of particular interest in view of the future hard x-ray free electron laser at PSI, since a series of resonant modes (magnons, phonons, electromagnons) can be coherently excited by THz and tracked by the femtosecond x-ray pulses. A high-field Terahertz transient provide a novel tool to control and investigate collective motions since it can be understood as "cold" stimulus: indeed the photon energy in the THz range is not more than a few meV, and therefore orders of magnitudes lower than the equivalent photon of a conventional laser (e.g. Ti:sapphire laser h=1.6 eV). While the latter heats electronic system significantly, the terahertz driving field excites only the THz sensitive port while leaving other degrees of freedom unexcited.
Intense coherent radiation in the Terahertz gap (0.1-15 THz) has been a challenge to produce in the past due to the lack of adequate emitters. Several schemes have been employed but none satisfied the SwissFEL requests of a reliable high-field, single-cycle pulse with a spectrum spanning the full THz gap. To access significantly higher frequencies we have recently developed a compact and powerful laser-driven THz source. The radiation is generated by nonlinear optical rectification of a mid-infrared femtosecond laser pulses in organic salt crystals like DAST [1], OH1 [2] and DSTMS [3]. The generation mechanism of THz radiation is understood as phase-matched nonlinear (2) process which permits the generation of multi-octave-spanning THz radiation. The difference frequency between components carrying the same absolute phase leads to intrinsically carrier phase stable pulses, which is essential for the envisaged field-sensitive experiments. The table-top Terahertz source delivers single-cycle pulses at currently record-high peak field strength of 1.5 MV/cm and 0.5 Tesla and lasts only one optical cycle (Fig. 1). The high conversion efficiency of one percent provides pulse energies of up to 40 µJ. Most interestingly our scheme offers pulse shaping capabilities to form highly asymmetric field shapes, as shown in figure 1b, by directly modifying the absolute phase of the pulse. Such asymmetric field shapes offer new opportunities like the ultrafast switching of magnetic domain which our group is currently investigating.
For SwissFEL, such Terahertz electric transients are also required for the arrival time and the temporal reconstruction of the femtosecond FEL x-ray pulses. This diagnostic is based on the principle of a conventional streak camera, but this time with the THz transient as streaking field. This allows online reconstruction of the femtosecond hard x-ray pulse and measurement of the pulse arrival time with respect to the optical pump laser. The development of reliable and powerful laser-based THz source is a prerequisite towards the realization of such an essential online x-ray pulse diagnostics at SwissFEL.


[1] C. P. Hauri, C. Ruchert, F. Ardana and C. Vicario “Strong-field single-cycle THz pulse generated in organic crystal,” Appl. Phys. Lett. 99, 161116 (2011).
[2] C. Ruchert, C. Vicario and C. P. Hauri, “Scaling Sub-mm single-cycle transients towards MV/cm fields via optical rectification in organic crystal OH1,” Opt. Lett. 37, 899 (2012).
[3] C. Ruchert, C. Vicario and C. P. Hauri, Spatio-temporal focusing dynamics of intense supercontinuum THz pulses,“ Phys. Rev. Lett. 110,123902 (2013)