Liquid Crystal Spiral Phase Plate (LC SPP)
Fig. 1 Helical wave front.
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An optical vortex is an optical beam that possesses helical wave front as shown in Fig.1. A convenient way to obtain an optical vortex is to imprint a helical phase shift to a beam with plane wave front. Spiral phase plate is such a plate that can imprint the helical phase shift to a beam passing through it.
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As the refractive index of a liquid crystal phase plate changes with the driving voltage, so it is possible to make a liquid crystal spiral phase plate by applying azimuthally dependent driving voltage to a liquid crystal phase plate. The azimuthally dependent driving voltage can be achieved by some specially designed electrodes such as that shown in Fig. 2. The liquid crystal spiral phase plate (LC SPP) was fabricated as shown in Fig. 3, and connected as shown in Fig. 4.
The multiple of the phase shift of 2π circling the beam axes of an optical vortex is called azimuthal index. Optical vortices with azimuthal index from 1 to 6 as shown in Fig. 6 were realized by use of the LC SPP.
Fig. 6
For more details about this work pls refer to the publication on optics express and my Ph.D thesis.