Contact person: Prof. József Fülöp, email: jozsef.fulop_at_eli-alps.hu
The Nonlinear Terahertz Spectroscopy Facility (NLTSF) consists of two main units: (i) a multi-mJ femtosecond pump laser and (ii) the THz pump—THz probe (TP2) system. TP2 enables time-resolved studies of THz-induced phenomena by using a strong THz pulse to initiate changes in the sample and a weaker THz pulse to detect these. Additional measurement capabilities enable to use optical pump or probe pulses in combination with THz pulses. A broad temperature range from 6 K to 800 K will be available for the sample.
Fig. 1. Block scheme of the Nonlinear THz Spectroscopy Facility (NLTSF).
Pump Laser. The cryogenically cooled Yb:CaF2 femtosecond pump laser, operating at 1030 nm wavelength, drives the THz sources of the system. The pulse energy is 6 mJ and the repetition rate is 1 kHz. The 200 fs laser pulse duration is sufficiently short to support a bandwidth of 0.1 THz to 2.5 THz for the THz pump and the THz probe pulses.
The THz pump—THz probe (TP2) spectroscopy system. In the TP2 system (Fig. 1), the optical pump pulse is split into parts. The strongest portion (typically 50–90% of the total energy) drives the source of the THz pump pulses. Single-cycle THz pulses of 10 μJ energy are generated and tightly focused to achieve a peak electric field strength up to 450 kV/cm in the sample to be investigated.
Another part of the optical pump (typically 10–40% of the total energy) generates the THz probe pulses, which propagate through the sample collinearly with the THz pump. The sample can be cryogenically cooled to 100 K. The available sample temperature range is currently being extended from 6 K to 800 K.
A small portion of the optical beam provides the sampling pulses for electro-optic sampling (EOS) to measure the electric-field waveform of the THz pulses transmitted through the (excited) sample. The related spectral amplitude and phase for the full bandwidth is obtained by Fourier transformation. Examples of measured waveform and spectra are shown in Fig. 2 in page 3. The variable pump-probe delay enables time-resolved studies in the THz range of the processes induced by the strong THz pump pulse.
Optical pump and probe pulses in combination with strong THz pulses, synchronized together, enable more versatile measurements. Optical pulses of wavelengths at the fundamental (1030 nm), second-harmonic (515 nm), and fourth-harmonic (258 nm, in development) of the laser frequency are available. White-light continuum pulses can be used for broadband optical probing of the sample.
Optical pulse parameters:
Pump THz pulse parameters:
Fig. 2. Left panel: Temporal waveforms of THz pump and probe pulses measured by electro-optic sampling.
Right panel: Retrieved spectra of the THz pump and probe pulses.