THz Generation
ZnTe crystals
In modern THz time-domain spectroscopy (THz-TDS), the common approach is THz pulses generation by optical rectification (OR)of ultrashort laser pulses and then the detection by free space electro-optic sampling (FEOS) in nonlinear crystals of special orientation.
In optical rectification, the bandwidth of the incident powerful laser pulse is converted into the bandwidth of THz emission, while both the optical and THz signal co-propagate through the nonlinear crystal.
In FEOS, both THz and weak probe laser pulses co-propagate through the nonlinear crystal, leading to theTHz field-induced phase retardation of the specially prepolarized probe laser pulse. This phase retardation is proportional to the electric field strength of the detected THz signal.
Optical contacted ZnTe crystals
10x10x(1+0.01)mm
Nonlinear crystals like ZnTe, with <110> crystal orientation are able to be applied in OR and FEOS at normal incidence. However, the crystals of <100> orientation do not possess nonlinear properties which are needed for OR and FEOS, although their linear THz and optical properties are identical to that of <110>-oriented crystals.The requirements for a successful THz generation or detection in such a nonlinear crystal-based THz-TDS spectrometer is phase matching between the generating (detecting) optical pulse and generated (detected) THz signal. Nevertheless, the nonlinear crystals suitable for THz spectroscopy applications have strong optical phonon resonances in the THz range, the strong dispersion of THz refractive index limits the phase-matching frequency range.
Thick nonlinear crystals provide THz-optical phase matching around a narrow frequency band.They support only a fraction of the bandwidth of the generating (detecting) laser pulse, since optical and THz signals experience larger walk-off over long co-propagation distances. But the generated (detected) peak signal strength is generally high for long co-propagation distance.
Thin nonlinear crystals provide good THz-optical phase matching within the full bandwidth of the generating (detecting) laser pulse, but the generated (detected) signal strength is usually small, because the signal strength is proportional to the THz-optical co-propagation distances.
In order to provide a broad-band phase matching in THz generation and detection and keep the frequency resolution high enough at the same time,DIEN TECH successfully developed refractive combined ZnTe crystal- a 10µm thickness (110) ZnTe crystal on a (100)ZnTe substract. In such crystals the THz-optical co-propagation is only crucial within the<110> part of the crystal, and the multiple reflections have to span the full combined crystal thickness.
