Deviation of optical constants extracted in terahertz transmission spectroscopy

Dielectric property of MoS(2) crystal in terahertz and visible regions

Two-dimensional materials such as MoS₂ have attracted much attention in recent years due to their fascinating optoelectronic properties. The dielectric response of MoS₂ crystal in both the terahertz (THz) and visible regions is studied in this work. Time-domain THz spectroscopy is employed for the THz property investigation. The real and imaginary parts of the complex dielectric constant of MoS₂ crystal are found to follow a Drude model, which is due to the intrinsic carrier absorption. In the visible region, ellipsometry is used to investigate the dielectric response. The general trend of the complex dielectric constant is found to be described with a Lorentz model, while two remarkable dielectric response peaks are observed to be located at 1.85 and 2.03 eV, which has been attributed to the splitting arising from the combined effect of interlayer coupling and spin-orbit coupling. This work can be the research foundation for future optoelectronic applications with MoS₂.

Anisotropic terahertz dielectric responses of sodium nitrate crystals

Terahertz (THz) spectroscopy has become an effective tool to characterize the low-frequency rotational and vibrational modes of molecules. In addition, novel THz dielectric responses and optical properties on the basis of molecular rotation and vibration have attracted lots of attention because of their potential application in THz devices. In this paper, the dielectric response of low-symmetric sodium nitrate crystals in the frequency range of 0.2-1.5 THz was experimentally demonstrated. Four absorption bands at 0.23, 0.47, 0.92, and 1.15 THz were observed in the dielectric spectra and were tentatively ascribed to the rotational motion of nitrate ions. Based on the molecular rotation mechanism, the dielectric anisotropy and dielectric resonance of the crystal were discussed in detail.

Using modified Kramers-Kronig relations to test transmission spectra of porous media in THz-TDS

We show that modified Kramers-Kronig relations provide a useful tool to test the validity of the complex refractive index extracted from transmission terahertz spectra of porous matrices containing pharmaceutical materials. The role of scattering of terahertz radiation is qualitatively considered as a reason for the observed discrepancy between experimental data and the values extracted from the inverted complex refractive index. As an example we present an analysis of the terahertz spectra of carbamazepine and lactose alpha-monohydrate.

Wiener bounds for complex permittivity in terahertz spectroscopy: case study of two-phase pharmaceutical tablets

The terahertz measurement technique has become popular in the field of pharmaceutical technology for tablet quality inspection. Spectral data obtained from the tablets is based on the utilization of Fresnel's formulas for an ideal slab. However, a tablet is a porous medium. Hence, in the THz gap one has to assume that a tablet constitutes at least an effective medium if the Fresnel theory is applied in quantitative permittivity spectra analysis. Hence, it is suggested that one should consider instead of the permittivity of homogeneous media the concept of effective permittivity in the THz terminology of porous tablets. Usually the fill factor of a component of a tablet is known but not the detailed bulk structure. Nevertheless, it is possible to estimate the complex effective permittivity of a tablet with the aid of so-called Wiener bounds. The idea of this article is to present a modification of Wiener bounds applied to the estimation of the real and imaginary part of the permittivity of the pure component of a tablet. As an example, the effective complex permittivity of a starch acetate tablet is considered.