Optical properties of lithium triborate (LBO) in the terahertz regime (0.2–2 THz) were characterized using broadband terahertz time-domain spectroscopy. The frequency dependence of refractive index and absorption coefficient of the LBO crystal was experimentally investigated over the temperature range of 77–297 K, which the experimental results indicated that LBO has very low optical absorption coefficient at terahertz frequencies especially for the beam polarization along the crystal’s principal dielectric axis X. Moreover, a giant birefringence was observed, and the refractive index difference between the axis X and Z gradually decreased with decreasing temperature, which is attributed to the behavior of the TO phonon modes of B1 and B2 symmetries at low frequencies at different temperatures. As potential applications, LBO can be exemplarily used as terahertz wave shapers, beam splitters, terahertz wave plates, circular polarizers and other polarization devices.

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Using the Delude model. we theoretically calculate the dispersion of conductivity with frequency in the orthogonal direction of the two-dimensional black phosphorus (2D BP) <i>x</i> and <i>y</i> direction in the THz band. We find that the conductivity in the <i>x</i> direction is more sensitive to the electron doping concentration. The difference between 2D BP conductivities in both directions leads to the difference in dielectric constant which in turn can modulate light in different polarization directions. Using 2D BP to polarize the THz wave, the 2D BP-SiO₂ periodic sandwich structure is designed. The three-dimensional electromagnetic field simulation software CST Microwave Studio can be used to calculate the regulation characteristics of this structure to THz wave. It is found that this structure has different polarization directions, and the incident THz wave has different absorption. By changing the thickness of the underlying SiO₂ layer in the structure it is found that the absorption rate of this structure also changes accordingly. When the polarization direction of the THz pulse is parallel to the <i>x</i> axis, the absorption rate first increases and then decreases. When <i>d</i>₅ = 9.5 μm, the absorption rate reaches 93% near 3.86 THz; when the polarization direction of the THz pulse is parallel to the <i>y</i> axis, the absorption rate gradually increases. The absorption peak has a significant red shift.