Polarization sensitive terahertz imaging: detection of birefringence and optical axis

Recent Advances in Birefringence Studies at THz Frequencies

“…Analogously, birefringence was reported for asymmetric hollow-core and polymer fibers [62,63]. Furthermore, polymers reinforced by aligned glass fibers were found to be birefringent [3,4,64] due to the embedded fibers representing sub-wavelength structures along one direction with regard to the relatively large THz wavelengths in the 100 μm range. Moreover, THz birefringence was observed for textiles [53], a Yucca plant leaf [2], different types of wood [65,66], but also for paper [53,65,67], which exhibits a preferential orientation of the cellulose fibers due to the production process.…”

“…Different geometries of the electrodes were investigated, such as three-contact structures with orthogonal gaps [89,90] or with tips arranged under an angle of 120° [91], as well as four-contact antennas with a single gap [92]. When multi-contact antennas are used, artifacts such as a cross-polarization response between the gaps can influence the detected THz signal [4,91,92], and a careful alignment is necessary for obtaining a good PS-detection performance [4,90].…”

“…Consequently, the extraction of the anisotropic material properties becomes more sophisticated and requires specialized methods. In this context, especially the three works presented in [2][3][4] focusing on this topic will be reviewed in more detail. But first, some general considerations about fibrous materials will be given.…”

“…5). Chapter 5.4 will especially focus on the investigation of birefringent fibrous materials by PS-THz-TDS, where in particular the three works given in [2][3][4] will be reviewed in detail. With no claim to completeness, this paper should give a comprehensive view of the field of THz birefringence measurements, also providing insight into its potential for material characterization.…”

“…With no claim to completeness, this paper should give a comprehensive view of the field of THz birefringence measurements, also providing insight into its potential for material characterization. Another wide and prosperous field is PS-THz imaging, where numerous advances have been made in the last few years (see, e.g., [4][5][6][7][8][9][10]). Although the one or other aspect will be addressed marginally in the course of this paper, PS-THz imaging is not in its main focus and is worth being reviewed elsewhere.…”