High nonlinearity, wide transparency range and optical quality allowed potassium titanyl phosphate (KTiOPO₄, KTP) crystals to be used in a wide range of nonlinear applications. The success of KTP usage in the visible and infrared (IR) ranges drives interest in applying it at longer wavelengths, that is, in the terahertz (THz) range. We use THz optical properties of KTP crystals measured by terahertz time-domain spectrometer (THz-TDS) and refractive index approximated in the form of Sellmeier equations to investigate KTP application possibilities for IR-to-THz and THz-to-THz wave conversion. As a result, phase matching for s − f → f and s − f → s types of difference frequency generation (DFG) of Ti:Sapphire laser (at the wavelengths of 0.65, 0.8 and 1.1 µm) is found possible, as well as second harmonic generation (SHG) of THz waves by f + s→f type of interaction in the XZ principle plane of the crystal. Terahertz wave generation by phase-matched parametric processes in KTP demonstrates evident advantages in comparison with that of widely used MgO-doped LiNbO₃ crystals.

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Nuclear spin isomers of water molecules, i.e., ortho- and para-H${}_{2}$O, are quantum states of one of the most important molecules. Presently, our understanding of the water isomer properties, especially in the gas phase, remains insufficient. Scientific and practical applications of water spin isomers require the development of efficient methods of their detection in gaseous samples. In this paper, we apply the method of wideband terahertz time-domain spectroscopy to study rotational absorption spectra of ortho and para water isomers and measure their relative content in the ambient atmosphere. We find the ortho/para ratio to be 3.03 ± 0.03 close to the expected value of 3 when using the measurement spectral range 0.15–1.05 THz.

One of the key elements of terahertz time-domain spectrometers is the optical delay line. Usually it consists of a motorized translation stage and a corner reflector mounted on its top. Errors in the positioning of the translation stage lead to various distortions of the measured waveform of terahertz pulses and, therefore, terahertz spectra. In this paper, the accuracy of position measurements is improved by using an optical encoder. Three types of systematic errors are found: Increasing and periodic offsets of the translation stage position, as well as a drift of its initial position in a series of consecutive measurements. The influence of the detected errors on the measured terahertz spectra is studied and correction methods are proposed.