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 2π and 4π 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 2π rad FZP generates a strong focal spot at 0 Vpp and switches off at higher voltages. In contrast, the 4π 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.