A.Perucchi Nonlinear THz studies at the TeraFERMI beamline

Nonlinear THz studies at the TeraFERMI beamline

N.Adhlakha1 , P. Di Pietro1 , F. Piccirilli2 , S. Lupi2.3 , and A. Perucchi1 , 1Elettra - Sincrotrone Trieste, Trieste, 34149 Italy 2CNR-IOM, Trieste, 34149 Italy 3Dipartimento di Fisica, Università La Sapienza, Roma, 00185 Italy

Abstract—TeraFERMI is the THz beamline of the FERMI Free Electron Laser. The beamline is based on a Coherent Transition Radiation source providing intense and broadband THz pulses in the MV/cm range. These fields are used to achieve THz control of matter and to push materials well into their nonlinear regime.

I.INTRODUCTION ermi is the seeded Free Electron Laser (FEL) in Trieste (Italy) operating in the 100 – 4 nm wavelength range and F open to external users since 2012. TeraFERMI is the THz beamline of the FERMI FEL, extracting THz light (0.3-10 THz) from the beam dump section by a Coherent Transition Radiation source. A 33m-long beamline transports the THz pulses in the experimental hall, where the beam is delivered to a standard optical table. The high peak power (from MW to GW) that can be stored in one single pulse makes TeraFERMI a suitable source for nonlinear THz spectroscopy studies, from condensed to soft matter.

II. RESULTS

The beamline performances have been first characterized with the help of calibrated pyroelectric detectors, and a homemade step-scan Michelson interferometer. During standard user operation conditions the beamline produces THz pulses whose energy ranges from 15 to 60 µJ per pulse, with a bunch charge of ~700 pC, and a bunch length of about 1 ps, as measured at the end of the LINAC. This results in energies at sample up to 35 µJ per pulse. The spectrum, normally extends up to 4 THz. However, upon optimization of the electron beam specific for TeraFERMI, energies at source up to 100 µJ, and a spectral extent up to 12 THz were measured.

In order to characterize also the time-structure of the THz pulses, we have performed Electro-Optic-Sampling measurements (EOS) with the help of an IR fs laser source synchronized to the Fermi Optical Master Clock [1].

An example of EOS measurement during standard beamtime operations is shown in Fig.1. Interestingly, the full energy profile has a duration of about 2 ps, with a main sharp peak containing 60% of the energy of the full pulse. The measured THz peak can be fitted with a gaussian profile, corresponding to a Dt=112 fs. The power spectrum related to this electric field shape is reported in Fig. 1c). The spectrum extends up to about 4.5 THz, roughly corresponding to the maximum detectable signal in a ZnTe EOS crystal, due to the presence of an optical phonon centered at 5.3 THz.

III. SUMMARY

TeraFERMI produces broadband high peak power THz pulses that can be used to perform non-linear THz spectroscopy. In the case of life sciences, TeraFERMI can be used to investigate the non-linear response of fast solvation dynamics

Fig. 1. a) Representative EOS measurement, acquired by averaging 10 shots for each position of the delay line. b) Intensity time profile obtained by evaluating the square of the EOS profile shown in a). The red curve is a Gaussian fit to the main peak. c) Intensity spectrum obtained by calculating the magnitude squared of the Fourier Transform of the spectrum in a).The arrow indicates the dip in the spectrum at about 1 THz which may result from the interference of a long wavelength (~0.9 THz) and shorter wavelength (~2 THz) components.

of biological matter caused by THz-induced structural changes. This can be achieved by either performing fluencedependent THz absorption measurements or through timeresolved pump-probe spectroscopies. In the latter case a synchronized IR source is also available at TeraFERMI, thus allowing to investigate novel material’s properties as the socalled THz Kerr effect (TKE) [2], through THz-pump/IRprobe schemes.

REFERENCES

[1]. N. Adhlakha et al., “The TeraFERMI Electro-Optic Sampling Set-Up for Fluence-Dependent Spectroscopic Measurements”, Condensed Matter, 5,-1 (2020) [2] P. Zalden et al, “Molecular polarizability anisotropy of liquid water revealed by terahertz-induced transient orientation”, Nat. Comm. 9:214 (2018)

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