Mission Statement

The TeraFERMI beamline uses Coherent Transition Radiation to produce sub-ps, single-cycle, broadband THz pulses to be used as a pump beam for nonlinear studies and THz control of matter. TeraFERMI allows operating with THz pulses reaching up to 50 microJoule at sample position, with electric fields exceeding 5 MV/cm, and a spectral span from 0.3 to 5 THz. 
 

The spectral range covered by TeraFERMI is rich in basic excitations, thus providing several possibilities to control material's properties.

THz pulses directly interact with free electrons, plasmons, phonons, electro-magnons, polarons, hydrogen bondings and van-der-Waals modes between others. Moreover the THz pulses from TeraFERMI can also be employed to nonlinearly excite Higgs modes in superconductors. The repetition rate of 50 Hz allows the materials under study to relax to their equilibrium temperature between successive THz pulses, thereby avoiding unwanted heating effects. This is particularly relevant for the study of biological materials.

The direct interaction with free electrons allows modulating the THz transmission properties of 2D materials through the saturable absorption phenomenon, or the tuning of plasmon resonances as in Bi2Se3 ribbons. The nonlinear interaction of THz light with hydrogen bonding also modifies the THz transparency of H2O. In a polar semiconductors as TiO2, the presence of polarons allows using a THz excitation to transiently change the response at near-infrared frequencies

The experimental station presently allows performing experiments exploiting 4 different set-ups:

• THz fluence dependent spectroscopy: saturable absorption and harmonics generation
• THz-pump/NIR-probe (780 or 1560 nm): coupling of the THz degrees of freedom with higher energy excitations
• THz-pump/THz-probe: dynamical evolution of the THz spectrum
• THz irradiation: THz field-induce irreversible changes in matter (resistive switching, biological samples)

In the near future, the TeraFERMI portfolio will also allow performing new classes of experiments:

1. Longitudinal Spectroscopy. The TeraFERMI source presents a radial polarization profile. The focusing of radially polarized beams with high numerical aperture results in the efficient generation of longitudinal fields. This makes TeraFERMI a unique source of strong THz longitudinal fields. We are presently developing a platform for linear and non-linear THz spectroscopy. Possible applications range from free space electron acceleration, optical pumping of THz quantum-cascade-lasers, transverse properties of 2D materials to the efficient illumination of nanoscopic tips 
2. Single-shot spectroscopy. The present THz spectrometer is based on a standard scanning electro-optic-sampling detection system, which may however be affected by fluctuations from the source. The development of a single-shot THz spectrometer will not only allow a quicker characterization of the samples, but will also be used to perform so-called covariance spectroscopy. The concept of covariance spectroscopy is based on considering the transmission  measurement from each single pulse, as a repetition of the same experiment but under different excitation conditions, thereby leveraging on beam fluctuations. The nonlinear properties of the sample can thus be extracted from the spectral correlations between the various single-shot measurements.

3. THz-Field Induced Second Harmonic generation (TFISH). Use SHG to probe a symmetry breaking induced by the TeraFERMI strong THz fields. TFISH is a new promising technique which has been recently applied for the study of ferroelectricity and of polar liquids. TFISH will be made available thanks to the integration of the infrastructures of T-ReX (Coherent Legend Elite DUO) with TeraFERMI. This will also allow implementing THz-pump/laser probe with low-harmonics (400nm, 266 nm), or supercontinuum in the VIS-IR. Probe in the infrared is possible with an OPA.