An ultra-broadband frequency-domain terahertz measurement system based on frequency conversion via DAST crystal with an optimized phase-matching condition

Abstract

By applying the frequency conversion technique to 4-dimethylamino-N-methyl-4-stilbazolium tosylate crystal, a monochromatic terahertz (THz) measurement system, including both generation and detection, has been developed over quite a broad frequency band, from 1.85 to 30 THz. In the case of frequency upconversion detection of THz waves, for the first time, we used gratings instead of filters to tackle the tough phase-matching conditions for broadband operations. By synchronizing the rotation of two gratings to extract the frequency upconverted signal, the infrared (IR) pumping beam can be tuned freely over 300 nm with decent diffraction efficiency and sufficient isolation between the weak frequency upconversion signal and the strong IR pumping beam to be realized. Such a large tuning range has overcome the limit of commercial filters with a fixed passband, while such a high optical density value has been beyond the limit of commercial tunable filters. Consequently, the proposed frequency domain system gives the largest THz frequency band. Unlike THz time-domain spectroscopy systems in which a fs laser is applied and broadband THz pulses are applied, our system works based on a ns laser and it can function at a single THz frequency with random frequency access ability from pulse to pulse.

Organic Crystals for THz Photonics

“…The most often used dual-wavelength source is a nanosecond (in some cases picosecond) optical parametric oscillator that generates two closely spaced pump wavelengths that are possibly in the phase-matching range of a particular NLO crystal employed. In many of the reported investigations, DAST has been used as a THz-wave generation crystal [106,[134][135][136][137][138][139][140][141][142][143][144][145][146][147][148][149][150][151], while more recently other organic NLO crystals such as BNA [103,106], DSTMS [73] and OH1 [89,91,95,97,152] have been employed, with different benefits and differences in particular with respect to the achievable efficiencies, THz frequency ranges and optimal pump wavelengths of the dual-wavelength source.…”

Phonon modes and local polar distortions in nanoclusters of Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3crystals probed by terahertz reflectance spectra and Raman scattering

“…In contrast to the well-studied ultrahigh dielectric and piezoelectric properties, a quantitative understanding of the complex dielectric behavior in relaxors has not been fully investigated. Compared with the inelastic neutron scattering (INS), which probes couples with a relative short-wavelength comparable to the nanoclusters' size, the THz probe couples with very long-wavelength phonons, which can detect the effectively homogeneous medium averaged over the nanoclusters [17][18][19]. The coherent length for the THz activity determined by the range of interatomic forces is very short, which makes the local polar distortions in polar clusters observable from the THz spectra.…”