The NLTSF (Nonlinear Terahertz Spectroscopy Facility) is driven by a multi-mJ femtosecond pump laser, enabling THz pump—THz probe measurements up to 450 kV/cm peak electric field and from 6 K to 800 K sample temperatures at 1 kHz repetition rate. Optical spectroscopy and electrooptical sampling is available.
The NLTSF (Nonlinear Terahertz Spectroscopy Facility) consists of two main units: a multi-mJ femtosecond pump laser and the THz pump—THz probe system. It enables time-resolved studies of THz-induced phenomena by using a strong THz pulse to trigger changes in the sample and a weaker THz pulse to detect these changes.
Intense THz pulses can drive selected degrees of freedom in matter into regimes far beyond the small-perturbation limit. This feature has enabled new applications that can roughly be classified in terms of nonlinear spectroscopy (providing insights into the nature of the driven mode) and materials control (driving the system into a target state). THz radiation enables resonant excitation of fundamental motions such as phonons, electron intraband transport, and magnons. Driving these modes to large amplitudes can enable new insights into their properties. A time-delayed probe pulse of suitable photon energy probes the response of the system.
The NLTSF (Figure 1) consists of two main units: a multi-mJ femtosecond pump laser and the THz pump—THz probe system. It enables time-resolved studies of THz-induced phenomena by using a strong THz pulse to trigger changes in the sample and a weaker THz pulse to detect these changes. Additional measurement capabilities use optical pump or probe pulses in combination with THz pulses. A broad temperature range from 6 K to 800 K is available for the sample in investigation.
Figure 1: 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 NLTSF system (Figure 2). The laser 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 both the THz pump and the THz probe pulses.
Figure 2: Block scheme of the Nonlinear THz Spectroscopy Facility (NLTSF).
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.
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.
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: