József A. Fülöp
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).
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).
THz pump—THz probe spectroscopy system:
In the TP2 system, the optical pump pulse is split into parts (Figure 2). The strongest portion (typically 50–90% of the total energy) drives the source of the THz pump pulses. Single-cycle THz pulses are generated and tightly focused to achieve a peak electric field strength up to 450 kV/cm in the sample to be investigated. The THz pulse energy at the sample is 5 μJ.
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 available sample temperature range is from 6 K to 800 K.
A small portion of the optical beam provides the sampling pulses for electro-optic sampling to measure the electric-field waveform of the THz pulses transmitted through the sample. The spectral amplitude and phase can be obtained by Fourier transformation. Figure 3 shows typical measured THz waveforms and spectra. The variable pump-probe delay enables time-resolved studies in the THz range of the processes driven by the strong THz pump pulse.
Figure 3: Left panel: Temporal waveforms of THz pump and probe pulses measured by electro-optic sampling. Right panel: Retrieved amplitude spectra of the THz pump and probe pulses.
Optical pump and probe pulses in combination with strong THz pulses:
Synchronised together, enable versatile measurements. Optical pulses of wavelengths at the fundamental (1030 nm), second harmonic (515 nm), and fourth harmonic (258 nm) of the laser frequency are available. White-light continuum pulses can be used for broadband optical probing of the sample.
Optical pulse parameters:
|Pump laser pulse|
|Central wavelength||1030 nm|
|Pulse duration||220 fs|
|Pulse energy||6 mJ|
|Repetition rate||1 kHz|
|Energy stability||< 2% (rms)|
|Second-harmonic wavelength||515 nm|
|Fourth-harmonic wavelength||258 nm|
|White-light continuum wavelength||480 nm – 700 nm|
Pump THz pulse parameters:
|Pulse energy at sample||5 μJ|
|Spectral maximum||0.3 THz – 0.6 THz|
|Useful Spectral coverage||0.1 THz – 2.5 THz|
|Peak THz electric field||450 kV/cm|
|Spectral resolution||≤ 50 GHz|
|Useful Spectral coverage||0.1 THz – 2.5 THz|
|Signal-to-noise ratio||> 300:1 (depending on measurement time)|
|Minimum sample clear aperture||2 mm|
|Temperature range||6 K – 800 K|
Please contact us for further possibilities.
The NLTSF is primarily configured for transmission-type measurements with collinear pump and probe beams.
The sample can be placed at the focus of the THz beam. It can be mounted to a cryostat to set desired sample temperatures between 6 K and 800 K.
THz pulse energy: calibrated pyroelectric detector.
Electric field waveforms of pump and probe THz pulses: electro-optic sampling.
Optical pulse spectra: spectrometer.
 Gupta, V. Gupta, A. Sharma, G. Polónyi, J. A. Fülöp, “Ultrafast nonlinear carrier dynamics in n-doped Ge at high field strengths,” 46th International Conference on Infrared, Millimeter and Terahertz Waves (IRMMW-THz), 2021, pp. 1-2, doi: 10.1109/IRMMW-THz50926.2021.9566951.
 M. C. Hoffmann, J. Hebling, H. Y. Hwang, K.-L. Yeh, K. A. Nelson, “Impact ionization in InSb probed by terahertz pump—terahertz probe spectroscopy,” Phys. Rev. B 79, 161201(R) (2009).