The beam splitter is an integral part of many modern optical systems, which can divide the incident light into two or more beams with the desired direction. It is widely used in optical communication, multiplexing, interferometer and optical spectroscopy. Therefore, beam splitter has been one of the hot topics in nanophotonics. One can use the polarization state, wavelength and power of the incident light to separate the optical beam. Thus, it can be classified into three categories of beam splitters according to the different operation modes. The first category is the polarization beam splitter [1,2] which divides the incident light into two parts with independent orthogonal polarization according to the different deflection states of light. The second one is the wavelength splitter [3], which separates the incident light from different wavelengths. The third one is the power beam splitter [4,5], which divides the incident light into two or more parts according to a predetermined power ratio. Equal-power beam splitters are the most commonly used beam splitters. These beam splitters can be designed and implemented in a variety of ways and structures, such as photonic crystals [6,7], directional couplers [8], meta-gratings [9,10], anisotropic metamaterials [11], waveguides [12,13], and multimode interference structures [14,15]. Usually, the large size and heavy mass of such conventional beam splitters limit their application in the integrated miniaturization of micro-optical systems. Therefore, there has been a desire to realize beam splitters with small size, light weight and multifunction.

Metasurface is an artificial composite material consisting of periodic or non-periodically arranged subwavelength structural units [16]. It has anomalous properties compared to the conventional materials in nature, and its dielectric constant and magnetic permeability can be arbitrary values. By suitable designing of the structural units, metasurface can modulate physical quantities such as phase [17,18], amplitude [19,20] and polarization [[21], [22], [23], [24]] of electromagnetic waves as one wishes. Meanwhile, the metasurface can realize phase mutation by introducing subwavelength-scale nanoantennas on the dielectric surface, instead of relying on phase accumulation to regulate the propagation of electromagnetic waves. So far, the design of beam splitters using metasurface has become one of the popular research projects in the field of micro and nano-photonics. It is shown that there have many published works on the design of beam splitters based on metasurfaces. To obtain a specific phase gradient in metasurfaces, it is usually necessary to design a set of nanoantennas with different sizes, shapes or orientations, and arrange these antennas in large cell units with a suitable spacing distance. For example, A. Ozer et al. proposed a metasurface-based power splitter operating in transmission modes [25], and X. Zhang et al. proposed a metasurface-based power beam splitter operating in reflection modes [5]. These metasurfaces were designed with a periodic array of discrete nanoantennas adjacent to each other forming a unit cell with π phase difference. However, the phase distribution of the designed metasurfaces cannot achieve a truly continuous phase gradient, and its phase continuity is broken by the spacing distance between the nanoantennas. As a result, not all energy is transferred to the desired beam splitting direction. To solve this problem, Z. Li et al. applied trapezoidal nanoantennas to metasurfaces for anomalous refraction phenomena [26]. However, the above reported beam splitters only operated in a single transmission or reflection splitting mode, which cannot satisfy the requirement of integrated and miniaturized function. To achieve the integration of diversified functionality, a feasible method is to hybridize metamaterial with active functional materials [27], such as 2D-materials [28], phase change materials [29], and so on. Among these active materials, vanadium dioxide (VO2), as an alternative phase change material, has been widely used to tune the functionalities of metamaterials [[30], [31], [32], [33], [34]]. For example, V. Erçağlar et al. proposed an integrated VO2-based metasurface beam splitting and absorption function in the THz range [35]. However, to our knowledge, few works have suggested to obtain switchable large-angle beam splitter in the near-infrared range.

This paper theoretically proposes and numerically demonstrates a switchable equal power beam splitter with large angle based on a continuous metasurface by integrating VO2. When VO2 is in the dielectric state, the metasurface acts as a broadband power beam splitter in transmission modes with conversion efficiency of ∼100% in the wavelength range from 1175 nm to 1300 nm, and the corresponding maximum splitting angle is 120°, which matches well with the theoretical result. When VO2 is in the metallic phase, the beam splitter operates in reflection modes with conversion efficiency of ∼100%. And the maximum reflective splitting angle reaches up to 148° at the wavelength of 1440 nm. Furthermore, the influences of related geometrical parameters on the performance of beam splitter are discussed in detail. Our proposed beam splitter may find applications for switchable integrated nanophotonics devices in the near infrared region.