Electrically switchable continuous phase liquid crystal Fresnel zone plate

We present the design, fabrication, and characterization of continuous phase Fresnel zone plates (FZPs) using two-photon polymerization direct laser writing in a polymerizable nematic liquid crystal (LC) confined between glass substrates. Unlike conventional binary LC diffractive elements, our devices exhibit a smooth, continuous three-dimensional phase profile. Two devices were demonstrated with wrapped phase profiles of 2 π and 4 π radians, respectively. Polarized optical microscopy and digital holographic microscopy confirm that the polymerized regions follow the intended spatially varying phase distribution. Far field measurements show that the 2 π rad FZP generates a strong focal spot at 0 Vpp and switches off at higher voltages. In contrast, the 4 π rad FZP exhibits varifocal behavior, switching between two focal lengths: 24 mm at 0 Vpp and 48 mm at an intermediate voltage of 2.1 Vpp. At higher voltages, the focus disappears entirely. Compared to a binary FZP of equal size and focal length, the continuous phase design nearly doubles the focusing efficiency and enables switchable, compact, vari-focal, and energy-efficient optical components. This approach offers new opportunities for advanced applications such as augmented and virtual reality, adaptive optics, and other next-generation photonic systems.