Recent Advances in Birefringence Studies at THz Frequencies

Abstract

In the last few years, various studies on terahertz (THz) birefringence have been presented, based on polarization-sensitive THz time-domain spectroscopy. The field of THz birefringence is a wide one covering several aspects, such as different materials exhibiting THz birefringence, polarization-sensitive THz technology, as well as numerous methods for extracting the birefringence properties of a sample from the THz data. Therefore, this paper aims at reviewing recent results on THz birefringence measurements presented in literature, addressing the above aspects and giving an overview of the topic. Moreover, it will focus on the investigation of birefringent fibrous samples, where specific results will be discussed in detail. Altogether, this paper should give a comprehensive view on the recent achievements in the measurements of THz birefringence.

Broadband tunable liquid crystal terahertz waveplates driven with porous graphene electrodes

“…Meanwhile, new LC materials with greater birefringence have been successively reported. [18][19][20] We have developed a room-temperature nematic LC NJU-LDn-4 with an average birefringence of greater than 0.3 from 0.5 to 2.5 THz, which is 2.5 times the birefringence of traditional E7 (,0.12 in the THz region). 21 This material improvement remarkably reduces the required cell gap, thus decreasing the operating voltage and response time.…”

Index of Refraction and Absorption Coefficient Spectra of Paratellurite in the Terahertz Region

“…Because of the well-known birefringence of α-TeO 2 in the visible frequency range [4], measurements were performed with beam polarization both parallel (extraordinary polarization) and perpendicular (ordinary polarization) to the optical axis [001] of the crystal. Since the polarization of the THz antenna source was not perfectly linear, a week beating-like modulation [13,14] was observed in the spectra. The period of the modulation was in accordance with the magnitude of the birefringence.…”

Birefringence measurement in the terahertz range based on double Fourier analysis

“…The technique also allows the determination of the optical axis orientation of the material by making two measurements with different angles of the sample optical axis. Birefringence measurements can be used to derive a wide range of parameters, such as mechanical stress in plastic components, molecular chain alignment in liquid crystals, and orientation of fibers in paper, composites, wood, and other materials [1][2][3]. Different techniques have been applied to characterize birefringent materials, such as visible and near-infrared (NIR) light, x-ray diffraction (XRD), and terahertz radiation.…”

“…Terahertz spectroscopy allows the characterization of many materials of industrial interest, such as plastics, wood, textiles, and paper that are opaque to visible and NIR light, providing large penetration depth. Besides, terahertz technology offers a safer approach than XRD since it is nonionizing [1].…”

“…When the delay is in the order of the time pulse width, which is generally around 1 ps, a direct time analysis is not possible. Additionally, the determination of the orientation of the optical axis as a relation of the temporal pulse amplitudes [1] is not possible when diattenuation occurs. Another proposal to determine both refractive indices and optical axis in a single measure is developed in [7], where the terahertz spectrometer employs circularly polarized terahertz waves and a polarization-sensitive detector that measures both components of the electric field simultaneously.…”