Wide-angle beam focusing with a tuning GeSbTe phase-change metalens

Abstract

A tuning metalens in transmission mode with GeSbTe (GST) phase-change material is proposed in this paper. Traditional single-layer metalenses with fixed geometry structure face the difficulty of focusing the wide-angle incident wave on the same focal point. Combining GST phase-change material with metasurface structure, the proposed single-layer metalens can converge wide-angel beam from −30° to 30° on a focal point with high efficiency without geometric structure variation. The unit cell of the proposed metalens consists of a GST nanopost and a silicon ring. GST phase-change material excited by a series of pulses switches among amorphous, crystalline and partially crystallized states. The phase shift controlled by each unit cell can well match the phase distribution of metalens required by focusing for different incident beams by changing the crystallization level of the GST. The simulation results show that the proposed metalens operating at 1550 nm wavelength can convert the incident plane wave to a converging spherical wave and focus it on a focal point with more than 27.68% efficiency when the incident angle changes from −30° to 30°. The focal length is in the range of 4535.4 to 4967.1 nm, in good agreement with the designed focal length of 4650 nm. Moreover, the full width at half maximum (FWHM) is from 0.581λ to 0.625λ approaching the diffraction limit of 0.6538λ. The proposed metalens is applicable for near-infrared light focusing with large view field.