THz spectroscopy and material analysis
THz spectroscopy is an analysis method that is use to investigate the sample in the THz frequency range. It is used to determine the change of a THz-wave impinging on the sample after its transmission through the sample, or its reflection from the sample, due to it material parameters. The measurements are typically performed by THz time-domain spectroscopy, TDS.
Considering the attenuation of the wave after the transmission through the samples material, it is discernible which frequencies are strongly absorbed and can penetrate sample poorly and which frequencies are weakly absorbed and can penetrate the sample easily. By displaying this information as a function of frequency, an absorption spectrum can be derived. Besides the investigation of amplitude respectively intensity data, THz-TDS also allows the evaluation of the phase information of the wave’s electric field.
Especially in the THz range, many materials show a distinctive shape of the absorption spectrum, that equals an unique “fingerprint” of the material. Therefore, THz spectroscopy can be used as a detection method for hazardous materials, like explosives or poisonous gases, the investigation of pharmaceutical products and their composition, and for the recognition of illegal substances like narcotics. In these cases the use of THz radiation is particular advantageous, since most comment wrapping and packaging materials (plastics, paper, textiles) are transparent in that frequency range, which allows the analysis of sealed samples and the penetration of optical nontransparent materials.
Basis for the detection of a material is the knowledge of its spectral characteristic, as part of a “fingerprint database”, which enables a comparison and thereby the recognition of a substance. Hence, the measurement and acquisition of the spectral information of know as well as novel material is an essential research filed, in the context of THz spectroscopy. Besides the investigation of absorption properties, the main task is the determination of further material parameters from the measured data. These material parameters, like the complex refractive index or dielectric constant, can one the one hand be use as a general description of the spectral characteristics of a material, independent from the samples geometry. On the other hand, they provide further inside in the physical mechanisms that determine those material properties. To this end new and improved calculation methods for the extraction of material parameters from measured data are investigated and developed, that can be applied to a broad field of applications, like complex multilayer systems, and at the same time provide an improved robustness to the influence of noise effects.
The spectroscopic investigation of materials in the THz range is not limited to substances with absorption characteristics but can also be applied to evaluate the spectral properties of all kind of materials, e.g. dielectric materials that are used as a substrate medium for other materials and structures, so that these parameters can be used in further design and analysis processes. Furthermore, the conductivity of dopes semiconductors and conductors can also be determined, which allow the deduction of properties like the carrier density and mobility. Also in these applications, new materials require the adaptation and expansion of existing calculation methods to new sample and measurement conditions.
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