Design and modeling of a transmission and reflection switchable micro-focusing Fresnel device based on phase-change materials.

In this paper, a switchable micro-focusing Fresnel device based on phase-change materials (PCMs) is proposed, which can selectively display the functions of transmission and reflection without the use of mechanical adjustment on micro scale. The switchable function is realized by combining Fresnel structure with PCM. A four-level switchable Fresnel device consisting of a typical PCM GeSbTe (GST-326) is designed to focus light into a focal length of 30 µm at wavelength of 3.1 µm. The optical performance of the switchable device has been analyzed by using finite-difference time-domain (FDTD) method, showing bright convergence point near pre-designed focal length with focusing efficiencies larger than 18%, depth of focus (DOF) less than 4.65 µm and the full width at half-maximum (FWHM) not larger than 1.30 µm. Furthermore, by precisely manipulating the variation of PCM thickness, we also obtain a device that possesses the characteristics of a transmission-reflection focusing beam splitter. The devices show good potential for the combination of traditional binary optical devices and PCM to produce new functions, and provides a promising innovative approach for miniature focal length switching device.