Optical routing and signal processing devices are indispensable to meet the challenges of core-based transmission and processing capabilities of massive data. The fundamental functional blocks to control the on-chip light paths are optical switching and splitting units. Various on-chip optical devices have been reported. The most common structures are multimode interferometers (MMI) [1], [2], [3]and Mach–Zehnder interferometers [4], [5], [6]. MMIs can realize beam splitters [1], [2] and mode (de)multiplexers [3], and when assembled into MZIs, they can realize optical switches [4], [5] and optical attenuators [6]. The performance can be furtherly enhanced by utilizing subwavelength structures to tune the coupling, effective index, and dispersion. The representative devices are based on subwavelength grating (SWG). Guided wave are effectively manipulated by elaborate arrangements of subwavelength structures [7], [8], [9], [10], [11], [12], [13], [14]. For example, SWG can be used to realize broadband devices like beam splitters [7], [8], [9], [10] and optical switches [11], [12]. Besides, it can also realize waveguide sensing [13] and waveguide crossing [14]. In addition to SWG, the subwavelength structures are extensively utilized in free-form inverse-designed devices [15], [16], [17]. For instance, these devices can also be utilized to realize power splitters [15], [16] and optical switches [17]. Moreover, metasurface-assisted on-chip devices are also considered as a promising way to manipulate the guided wave. Its ability to manipulate the amplitude, phase, and polarization of guided waves has been demonstrated, which has been utilized in optical switch [18] and mode conversions [19], [20], [21], etc.

Except for the conventional devices and their transformed devices, recently another alternative has aroused called metalens-assisted on-chip device. By referring to the concept of metasurface and metalens used in free space, it achieved many innovative beam manipulation applications and offered new perspective ways to study nano-optics. By using subwavelength nanorods array to manipulate the wavefront, on-chip beam focusing [22], [23], [24] is realized, and the spatial-domain integrator [25], on-chip nanophotonic convolver [26] are also designed and demonstrated. These kinds of devices owe the advantages of low insertion loss, which is feasible for parallel and multi-stage on-chip signal processing, and have great potential in large-scale silicon photonic computational chips. However, since the materials constituting subwavelength nanorod arrays are usually composed of air or dielectric materials, the tunability is rather limited, resulting in a shortage of on-chip beam manipulation. In contrast, phase-change material (PCM), especially for Sb2S3, has low loss characteristics while offering sufficient phase shift. Thus, it has potential in tunability and can be combined with metalens-based on-chip devices to manipulate the guided wave.

In this paper, we proposed and designed the on-chip wave shaping for optical switching (beam steering) and beam splitting on the SOI platform. Owing to the low-loss characteristics of the PCM Sb2S3 at the wavelength of 1550 nm, whose refractive index is 2.712 at the amorphous state and 3.308 at the crystalline state, we introduced this material into the optical waveguide design. To satisfy the high transmission, we fixed the width of the Sb2S3 nanorods to 280 nm. Meanwhile, to meet the requirement of the 2π rad phase shift to provide enough phase gradient, we swept the length of the Sb2S3 nanorod from 0 to 4.5μm. When the Sb2S3 array switch to the amorphous state, the phase shift would cover 2π rad. When the Sb2S3 array switch to the crystalline state, the phase shift only reaches 1.7 rad. Then based on the same procedure, we fixed the width of the etched slot to 140 nm and swept the length of the etched slot from 0 to 3μm. By engineering and arranging these subwavelength structures, we first simulated the optical switches that can realize the functions of 1 × 2 and 1 × 3 optical switching. When the Sb2S3 array switches between the crystalline state and the amorphous state, it forms different phase gradients, and the light will be guided to different ports. Then we simulated the beam splitters that can realize the functions of 1 × 2 and 1 × 3 beam splitting. When we modify the phase gradient of the PCM array, the beam deflection angle will be increased as well. We provide functional demonstrations of on-chip wavefront shaping using metasurface, and this solution provides a new option to design widely used on-chip optical devices (switches, splitters), and offers larger geometric assignment flexibility. Because the constituted functional components are separate in spatial space, we can simply insert other intermediate structures to change or expand the functionalities as people did in free space optics. By this means, this is a new attempt to design the on-chip functional device in the ways of free space. We believe that this type of device has adequate prospects for applications in on-chip optical switching, beam splitting, on-chip phased arrays, and optical networks.