Micaceous layer silicate clay minerals are attractive materials for applications involving non-linear optics because of their low cost and ability to form well ordered, platy aggregates, but such applications require precise knowledge of the dielectric behavior of the clay. The purpose of the present study was to use Terahertz time-domain spectroscopy (THz-TDS) to determine the dielectric properties of certain cleavable layered clay minerals, including muscovite, vermiculite, phlogopite, and biotite. The samples were characterized by X-ray diffraction and Fourier transform infrared spectroscopy as well as chemical analysis by Energy dispersive X-ray spectroscopy. The THz frequency window investigated was the far-infrared region of 3.3 to ∼40.0 cm⁻¹ corresponding to 0.1 and 1.2 THz, respectively. The samples were selected so that the hydrated form of the interlayer cation, e.g. Mg²⁺ present in the interlayer gallery of vermiculite, could be compared to species such as phlogopite, biotite, and muscovite with the dehydrated form of interlayer cations such as K⁺ or Na⁺. The frequency-dependent complex index of refraction of these natural materials was determined to vary between 2.50 and 2.80. The presence of water in the interlayer space of vermiculite was reflected in the detection of increased values of the absorption index in comparison with the muscovite, phlogopite, and biotite.

While the third harmonic of a monochromatic pump field with a frequency ${\omega }_0$ is generated exactly at the frequency $3{\omega }_0$, frequency tripling of broadband ultrashort pump pulses in extended nonlinear media tends to generate isolated spectral peaks substantially shifted from $3{\omega }_0$. We demonstrate this phenomenon by studying nonlinear spectral transformations of femtosecond Cr:forsterite laser pulses in multimode photonic-crystal fibers. Third-harmonic generation is shown to map adjacent guided modes of the third harmonic onto a manifold of spectral peaks within a $150\text{−}\mathrm{THz}$ range around $3{\omega }_0$. The spectral shifts and the widths of these peaks are controlled by the phase and group-velocity mismatch between the pump field and third-harmonic modes, as well as the length of nonlinear-optical interaction and broadening of the pump spectrum, allowing the spectral content of the third-harmonic signal to be engineered by tailoring the fiber dispersion.