Abstract
Liquid crystal (LC) is a promising candidate for terahertz (THz) devices. Recently, LC has been introduced to generate THz vortex beams. However, the efficiency is intensely dependent on the incident wavelength, and the transformed THz vortex beam is usually mixed with the residual component. Thus, a separating process is indispensable. Here, we introduce a gradient blazed phase, and propose a THz LC forked polarization grating that can simultaneously generate and separate pure THz vortices with opposite circular polarization. The specific LC gradient-rotation directors are implemented by a photoalignment technique. The generated THz vortex beams are characterized with a THz imaging system, verifying features of polarization controllability. This work may pave a practical road towards generating, separating and polarizing THz vortex beams, and may prompt applications in THz communications, sensing and imaging.
Highlights
Vortex beams, which feature a spiral wavefront, have gained considerable interest in recent years [1,2]
We introduce a gradient blazed phase into the vortex phase, and propose a THz Liquid crystal (LC)
The generated THz vortex beams are characterized with a THz imaging system
Summary
Vortex beams, which feature a spiral wavefront, have gained considerable interest in recent years [1,2]. In contrast to the spin angular momentum (SAM) related to circular polarization, a vortex beam carries orbital angular momentum (OAM) [3]. OAM is quantized as mhper photon, where m is the topological charge, indicating how many twists the light performs in one wavelength. OAM adds another degree of freedom in the manipulation of light. The OAM-induced torque and abundant eigenstates lead to numerous applications, such as optical tweezers [4], optical communications [5,6], quantum entanglement [7], and informatics [8,9]. The concept of the vortex beam has been extended to much broader spectra; the terahertz (THz) range
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