In recent years, with the rapid development of electromagnetic regulation technology, near-field focusing technology has been extensively researched by scholars in fields such as biomedical [1], sensing detection [2], microwave imaging [3], and RFID (radio frequency identification) [4]. Near-field focusing refers to the technique of converging incident electromagnetic waves into a specified area to achieve constructive superposition. In the microwave range, there are several methods to achieve near-field focusing of electromagnetic waves: 1. Dielectric Lens [5]: This method focuses the incident waves by utilizing the phase accumulation of electromagnetic waves in isotropic media. However, it has disadvantages such as high losses and manufacturing difficulties. 2. Spherical Array Antennae [6]: By setting each antenna equidistant to the focal point, electromagnetic waves are converged at the focus. However, spherical array antennas are bulky and not convenient for practical use. 3. Planar Array Antennae [7]: This method achieves focusing electromagnetic waves by controlling the phase-shifting network loaded at the front end of the antenna. Compared to the previous methods, planar array antennas are smaller and more convenient to use, but the loaded phase-shifting network is complex, significantly increasing the design difficulty. Metasurfaces, as two-dimensional artificial electromagnetic structures, exhibit extraordinary capabilities in controlling electromagnetic waves, freely manipulating characteristics such as amplitude, phase, frequency, and polarization at subwavelength scales [8,9]. Compared to traditional optical devices, they offer advantages such as smaller size, low profile, and lower cost [[10], [11], [12]], providing more advanced approaches and solutions for achieving near-field focusing of electromagnetic waves. With the rapid development of metasurfaces in the electromagnetic field, metasurfaces have been widely applied in areas such as radar antennae [13], electromagnetic stealth [14], and holographic imaging [15]. In 2014, Cui proposed the concept of coded metasurfaces [16], which discretizes and encodes the phase response of metasurfaces, introducing a binary coding method similar to that of computers to design metasurface arrays. Coded metasurfaces not only expand the application range of metasurfaces but also further simplify their design and fabrication. Scholars have designed various microwave devices based on coded metasurfaces, achieving functions including polarization conversion [17], anomalous refraction [18], generation of orbital angular momentum [19], and RCS reduction [20].

However, as modern microwave devices become more integrated and miniaturized, the limitations of conventional passive metasurfaces, such as a limited number of channels and single functionality, have become increasingly apparent. Although active devices can be integrated into metasurface units to flexibly control the electromagnetic response of metasurfaces [21], this approach typically requires additional control devices, increasing the complexity and power consumption. In order to expand the functionality and information capacity of metasurfaces without introducing active devices, it is possible to carefully design the metasurface structure by combining characteristics such as electromagnetic wave polarization [22] and frequency [23], thereby achieving multifunctional or multichannel passive metasurface designs. Long Li proposed a design for a dual-polarized near-field focusing reflective metasurface for wireless power transfer (WPT) systems [22], which can achieve single-focus and dual-focus focusing in the near field under the incident of x-polarized and y-polarized waves at 10 GHz. In Ref. [24], a three-layer near-field focusing transmission-type metasurface based on polarization multiplexing is proposed, which can focus incident waves at at different positions under the incident of 15.5 GHz x-polarized and y-polarized wave, and its maximum focusing efficiency reaches 30%. Zheng Sen and others proposed a T-shaped reflection unit structure composed of two dipoles based on the polarization characteristics of the beam, which generated Orbital Angular Momentum (OAM) beams at 18 GHz and 32 GHz [25].In Ref. [26], the authors designed a transmissive metasurface based on aperture and patch coupling to generate dual-mode converging OAM beams, capable of producing converging OAM waves of modes l = +1 and l = −2 under x-polarized and y-polarized wave incidence, respectively. The simulation efficiency of the metasurface exceeded 85%. In Ref. [27], a multifocal metasurface lens was designed using circular polarization multiplexing, capable of generating multiple longitudinal (or transverse) foci under the illumination of left-handed and right-handed circularly polarized terahertz waves. In the field of near-field focusing, polarization-multiplexed metasurfaces can expand the focusing channels while flexibly adjusting the energy distribution of the foci [28]. However, an arbitrarily polarized electromagnetic wave propagating in the same direction can only be decomposed into two orthogonally polarized electromagnetic waves, hence most polarization-multiplexed focusing metasurfaces have only two independent focusing channels. How to further expand the focusing channels of metasurface lenses and enhance the integration level of metasurface lenses remains a question worthy of investigation.

This paper proposes a four-channel near-field focusing coding metasurface lens based on frequency -polarization multiplexing, consisting of a central cross-shaped metal patch and four corner right-angle metal patches in the unit. By combining frequency multiplexing with polarization multiplexing, the dual-polarization structures of two frequency bands are cleverly integrated into the same unit, enabling the metasurface to independently control the x-polarized and y-polarized incident waves at 15 GHz and 23 GHz. Furthermore, combining the principles of metasurface focusing and phase superposition, the phase distribution and untis distribution of the metasurface lens are calculated. The resulting metasurface array can achieve four different types of electromagnetic wave focusing functions in four transmission channels. Under the incidence of x-polarized and y-polarized waves at 15 GHz, single-point and dual-point focusing are achieved, respectively, with the focus located on the z = 70 mm plane; under the incidence of x-polarized and y-polarized waves at 23 GHz, single-point focusing with different focal positions is achieved, with the focus located on the z = 80 mm plane.