This paper studies the terahertz optical properties of nonlinear potassium titanyl phosphate crystals with different conductivities in the spectral range of 0.2 to 2.6 THz. The observed properties are characterized by several absorption lines lying along different optical axes which represent the relevant potassium sublattice phonon modes. The peculiarities of these absorption lines are attributed to the structural order of potassium ions.

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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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Dispersion of refractive index and absorption coefficients in flux-grown high-resistivity KTiOPO4 crystals between 0.2–2.5 THz are verified at room temperature by a THz-TDS. Measured dispersion components nx, ny and nz are approximated for the first time in the form of Sellmeier equations. Phase matching for down-conversion into the THz range under a visible and near IR pump is found possible only in the principle plane by and types of three-wave interactions. Low frequency THz generation is favorable due to the low absorption coefficient down to 0.2 cm−1, below 0.5 THz.

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The effect of a magnetic field on the enhancement of the efficiency of terahertz (THz) generation on the surface of narrow-gap semiconductors is analyzed. A novel small-size THz generator is proposed. It consists of two permanent magnets with opposite magnetization placed on a yoke forming Kittel structure. The construction provides magnetic field concentration in the active zone of a semiconductor placed closely on top of the magnets. Its capabilities of converting femtosecond pulses of the first and second harmonics of an erbium fiber laser are investigated by THz time-domain spectrometer. -InAs and -InSb semiconductors in the magnetic field of 0.8 T prove to be the best THz generators at 775-and 1550-nm pump wavelengths correspondingly.

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