Multifunctional metalens generation via bilayer phase-change metasurfaces at near-infrared wavelength

Multifunctional metalens with tunable focus and intensity is powerfully attractive for compact planar optics. However, the limited degree of freedom in the single-layer metasurface design makes it difficult to satisfy more complicated functionalities, which restricts the application scenarios of metalens. In this paper, a multifunctional metalens with three operating modes is designed at near-infrared wavelength by combining the phase-change material antimony selenide (Sb2Se3) with the bilayer Pancharatnam-Berry (P-B) phase. By reversibly transforming the upper and lower Sb2Se3 nanofins between the crystalline and amorphous states, the metalens can achieve arbitrary switching among dual-focus F1 and F2, single-focus F1, and single-focus F3. Moreover, when the upper Sb2Se3 nanofin is remained in the amorphous state, a bifocal metalens with tunable relative intensity is achieved by controlling the crystallization state of the lower Sb2Se3 nanofin, and the total focusing efficiency of the bifocal metalens is above 86%. Similarly, when the upper Sb2Se3 nanofin is in the crystalline state, a single-focus metalens with adjustable intensity is realized via adjusting the crystallization fraction of the lower Sb2Se3 nanofin. Regardless of whether the metalens operates in bifocal or single focal mode, the simulated focusing efficiency is higher than 53% and the full-width at half-maximum (FWHM) of each focus approaches its diffraction-limited value. The proposed multifunctional metalens can enable wide potential applications in biomedical imaging, optical communication systems, multifunctional devices, and so on.