Generation of Tightly Focused Cylindrical Vector Beams with Dual-Channel Transmissive Metasurfaces

Abstract

Tightly focusing cylindrical vector beams (CVBs) are widely used in diverse applications from particle trapping to high-resolution microscopy. Recently, planar metasurfaces have been proposed as promising alternatives to traditional bulky optical devices for tightly focused CVB generation. However, few metasurfaces allow the generation and independent phase control of different CVBs. In this paper, cascaded dual-channel transmissive metasurfaces that can be used to simultaneously generate different CVBs and independently control their phases are designed. The polarization and phase of the transmitted waves are controlled independently via only two geometric parameters. Under $x$- and $y$-polarized wave incidences, the radially polarized beam (RPB) and azimuthally polarized beam (APB) are generated by a first metasurface. In addition, a second metasurface is used to produce different focal fields in different focal planes under RPB and APB excitations. In such a proposed approach, the phase responses of the RPB and APB are distinct, which is significantly different from existing designs. A regular polygonlike focus and a lateral displacement of the focal spot are, respectively, realized in different focal planes at 7.8 GHz through different channels. Our approach improves the flexibility of focusing, allowing the generation of focal arrays at different locations within the same focal plane. Since the electromagnetic energy can be confined to a small specific zone in the microwave band, the proposed method can provide the basis for a number of important applications in the medical domain, such as the accurate hyperthermia treatment of large area tumors.