THz spectroscopy is an analytical method used to determine the properties of a sample in the terahertz frequency range. This method determines how the THz wave incident on the sample changes at each individual frequency after passing through or reflecting off the sample, based on the material’s properties. Typically, THz time-domain spectroscopy (THz-TDS) systems are used for these measurements.

For example, by examining the attenuation of the wave after it passes through the sample material, one can determine which frequencies penetrate the sample poorly and are therefore strongly absorbed, and which frequencies penetrate it well and are weakly absorbed. Plotted against frequency, this yields an absorption spectrum of the analyzed material. In addition to analyzing amplitude or intensity values, TDS measurement technology also allows for the evaluation of phase information of the wave’s electric field.

Many substances exhibit a characteristic absorption profile, particularly in the terahertz range, which corresponds to a unique “fingerprint” of the material. Therefore, THz spectroscopy can be used to detect hazardous substances such as explosives or toxic gases, to analyze pharmaceutical products and their composition, and to identify prohibited substances such as drugs. A particular advantage here is that the most common packaging materials (plastics, paper, textiles) are transparent to THz radiation, making it possible to analyze sealed samples and penetrate optically opaque media.

The basis for detecting a material is knowledge of its spectral characteristics as part of a "fingerprint database", enabling comparison and identification. Therefore, characterizing the properties of both known and novel materials is one of the core research areas of THz spectroscopy. The goal is to use the collected data not only to determine the absorption properties but also to identify other material properties of the sample, which, on the one hand, provide universally applicable descriptions of the sample’s spectral characteristics that are independent of the sample geometry (complex refractive index or dielectric constant). On the other hand, they provide further insights into the physical mechanisms underlying these material properties.

Spectroscopic analysis of materials in the THz range is not limited to the investigation of substances with absorption characteristics. For example, the dielectric properties of materials that serve as substrate media for other materials and structures can be determined over a broad frequency range. Furthermore, the conductivities of doped semiconductors and conductive materials can be determined, and conclusions can then be drawn regarding their charge carrier density and mobility.