On-chip OAM Mode Demultiplexing

In recent years, there has been increasing attention on utilizing terahertz (THz) waves above 100 GHz as carrier waves for data transmission, in response to the evolving needs of next-generation wireless communication development. Simultaneously, modulation-demodulation and channel multiplexing technologies related to communication have become focal points of research in terahertz regime. Multiplexing primarily target applications in terahertz communications requiring high-speed data transmission and large-capacity communication scenarios. Within multiplexing technologies, wave frequency, spin direction, and orbital angular momentum can correspond to independent channels, thereby enhancing data transmission efficiency in communication systems. One implementation method of spatial mode division multiplexing involves utilizing orthogonal orbital angular momentum (OAM) modes to carry different information. Presently, demultiplexer devices for OAM modes in THz frequencies typically convert high-order modes into Gaussian-based modes in free space. To enable more integrated demultiplexer terminals in mode division multiplexing links, it is essential to propose a more compact OAM mode demultiplexing on-chip solution.

Therefore, the research team at Terahertz Center for Tianjin University proposed a demultiplexing solution that utilizes metasurfaces to convert spatial OAM modes into on-chip modes. Firstly, they theoretically achieved coupling arbitrary spatial OAM mode to focused surface plasmon modes utilizing holographic design principles. Then, by arranging subwavelength metallic slits, they obtained the phase distribution necessary for the metasurface to achieve complex wavefront modulation functions. The designed device can simultaneously manipulate multi-channel OAM modes with circular polarization states, and the corresponding surface plasmons waves will be efficiently guided in different directions and focused. In experiments, the fabricated metasurface was characterized using a terahertz near-field time-domain spectroscopy system, achieving low crosstalk and high isolation mode demultiplexing of seven channels. Relevant research results were recently published in Photonics Research, Volume 12, Issue 5, 2024. [Xiaohan Jiang, Wanying Liu, Quan Xu, Yuanhao Lang, Yikai Fu, Fan Huang, Haitao Dai, Yanfeng Li, Xueqian Zhang, Jianqiang Gu, Jiaguang Han, Weili Zhang, "On-chip terahertz orbital angular momentum demultiplexer," Photonics Res. 12, 1044 (2024)]

Figure 1 Results for the seven-channel OAM demultiplexer. (a) Calculated and measured surface plasmon field intensity distributions. (b) Extracted intensities at each focal spot. (c) Frequency spectra at each focal spot.

The energy transmission of surface plasmon modes is depicted in Figure 1(a), where the propagation directions for each channel are evenly distributed at the same angular interval. In the figure, these foci correspond to left-handed circularly polarized (LCP) vortices with l = –6, –4, –2, 0, +2, +4, +6. The operating frequency of the on-chip demultiplexer is designed at 0.75 THz. Through computation and near-field detection of the E_z field component, it is observed that the numerical computation results exhibit good consistency with experiment performances. The surface plasmons will propagate and focus along the predetermined directions, after generated from the central excitation region. From further analysis and comparison of the electric field intensity and frequency domain spectrum at each focal spot, employing geometric phase wavefront modulation methods can minimize crosstalk in OAM mode demultiplexing. In addition, the average full width at half maximum (FWHM) in the frequency spectrum is 23 µm, indicating excellent frequency response characteristics.

The metasurfaces designed based on holographic principles can achieve arbitrary OAM mode demultiplexing. This design method offers two main advantages: firstly, the propagation direction and focal position of surface plasmons can be customized as demanded; secondly, other dimensions can be integrated into on-chip devices during the design process, such as wavelength or polarization. The results presented in this study demonstrate the feasibility and universality of OAM mode demultiplexing from spatial to on-chip domains. The holographic design theory employed here holds significance for the design of other on-chip coupling devices in terahertz frequencies. The proposed metasurfaces, as integrated and compact functional devices, are expected to advance on-chip information processing and terahertz communications.

In the next step, the team will further optimize the efficiency of the on-chip demultiplexer to better match practical application requirements. Firstly, the energy distribution of each channel will be enhanced through algorithmic optimization. Secondly, coupling the focused surface waves into waveguides to establish a complete transmission link will reduce energy dissipation. Additionally, the team will delve into the physical mechanisms of multi-dimensional surface plasmon coupling and improve design methodologies for on-chip devices, paving the road to high-performance on-chip demultiplexing technology.