A new publication from Opto-Electronic Sciences; DOI 10.29026/oes.2025.240024 , discusses generation of terahertz complex vector light fields on a metasurface driven by surface waves.
With the rapid development of information and communication technologies, especially in the context of 5G, 6G networks, artificial intelligence, and the Internet of Things, the development of on-chip optical control devices with high bandwidth, high speed, low power consumption, and miniaturization has become increasingly important. However, traditional optical devices often face issues such as large size, low efficiency, and limited control capabilities. Metasurfaces, as a new type of optical device, are composed of a series of ultra-thin subwavelength artificial atoms arranged in a specific manner, enabling unusual effects such as anomalous reflection/refraction, planar prisms, holographic imaging, and surface wave excitation. In particular, recent work has proposed using on-chip surface waves as an excitation source, employing metasurfaces to efficiently decouple surface waves and achieve wavefront control in free space, thus opening new avenues for on-chip optical applications. However, previous work has primarily focused on phase control, and achieving joint control of phase, amplitude, and polarization to realize more flexible light field control remains a significant challenge.
This paper proposes a general method for designing ultra-compact on-chip optical devices that can efficiently generate pre-designed complex wavefront vector beams (VOFs) under surface wave (SW) excitation, with experimental verification in the terahertz (THz) frequency range.
For reflective metasurface devices with linear geometric phase, when illuminated by linearly polarized light in the vertical direction, the scattered field will simultaneously contain both spin-related and spin-independent anomalous and normal modes (as shown in Fig. 1a). As the incident angle increases, one of the anomalous modes and normal modes, after being manipulated by the metasurface, both have their reflection angles gradually increase. When the incident wave is an on-chip surface wave, the mode "surviving" in free space is a specific circularly polarized light, and both the radiation angle and polarization state of this mode can be arbitrarily controlled by precisely designing the phase gradient of the metasurface (Fig. 1b, c).
Building on the above concepts, researchers have further proposed the idea of designing composite metasurfaces to radiate complex vector light fields. The traditional single "artificial atom" is expanded into a 2×2 "artificial molecule," where the different subunits (blue and purple) have independent rotation angles and directions. Under the illumination of surface waves, these subunits can simultaneously radiate left-handed circular polarization (LCP) and right-handed circular polarization (RCP) components. By controlling the local phase and polarization components through interference effects, specific wavefronts and polarization distributions of vector beams can be constructed on a macroscopic scale (Fig. 1d).
To achieve this concept, researchers have developed a universal design method that decomposes the target vector light field into a sum of different wave vectors and circular polarization basis vectors. Through the mapping relationship between the target total field and the artificial atoms, the design parameters of the composite metasurface are determined, ultimately completing the design of the prototype device (Fig. 2a). For example, the researchers developed a terahertz device that generates a radially polarized Bessel beam under surface wave excitation. Using full-wave simulation and near-field scanning, the light field morphology was demonstrated in different planes and polarization directions, showing excellent agreement, thereby verifying the device's outstanding performance (Fig. 2b-g). This research provides a new approach for achieving highly integrated on-chip terahertz devices, with broad application prospects in fields such as biosensing, high-speed communication, lidar, and augmented and virtual reality (AR/VR).
Keywords: surface waves / vector beam / multi-pixel metasurface / terahertz / ultrathin and high-efficiency
