Spin-decoupled meta-coupler empowered multiplexing and multifunction of guided wave radiation
On the Cover of Photonics Research, Volume 11, Issue 12, 2023
Fig.1. Illustration of the working principle of spin-decoupled geometric meta-coupler for guided wave radiation manipulation excited by TE guided mode.
Fig.2. Manipulating guided wave radiation for off-chip polarization-demultiplexed focusing and hologram generation with pin-decoupled meta-coupler.
Fig.3 Experimental demonstration of polarization-demultiplexed focusing by meta-coupler based on LNOI waveguide.
Recommendation: Optical waveguides and metasurfaces offer two important platforms for light field manipulation in recent years. In photonic integrated circuits (PICs), light travels inside the waveguide in guided modes, and in-plane routing and out-of-plane coupling are realized on a tiny chip. It is highly compact but has limited free-space light controllability. On the other hand, metasurfaces, consisting of artificially designed subwavelength optical antennas, have unparalleled capability for engineering light with multiple degrees of freedom at the subwavelength scale. Here, two powerful platforms are synergically combined, and a novel spin-decoupled on-chip coupler is proposed with pure geometric metasurface, which provides a simple, efficient and multi-functional coupling interface for the interaction between guided and free-space modes.
—Zhangkai Zhou, Asisstant Editor of Photonics Research
Optical phased arrays (OPA), augmented/virtual reality (AR/VR) displays, optical communications and interconnects implemented in integrated photonics are hot topics in current research of science and technologies. In particular, efficiently radiating guided wave into free-space and flexibly controlling their wavefront is one of the key technologies. To fully exploit the benefits of integrated photonics in free-space applications, it is crucial to have an interface, also known as coupler, to flexibly manipulate light when it converts between guided and free-space modes.
Nevertheless, traditional couplers are edge couplers and grating couplers, which have limited functionality and lack complete control of light in amplitude, phase as well as polarization. Although grating arrays is an alternative which can perform more advanced functions, it suffers from large footprints and high-order diffraction loss, and has limitations in footprint, efficiency, and bandwidth. Therefore, further researches on new type of optical couplers, which can improve the efficiency and functional diversity of guided wave radiation, is of great significance for the development of chip-integrated high-performance and multifunctional optical device.
Metasurface newly emerges as an interface for light manipulation at sub-wavelength scale. Through well-designed artificial nanostructures, light can be flexibly manipulated with multiple degrees of freedom like amplitude, phase, polarization, and enabling the revolution of optical devices and technology. Previously designed metasurface are developed for controlling the wavefront of light propagating in free space, while its integration with waveguide to manipulate the guided wave radiation in a desired way is still in infancy.
Recently, professor Bin Fang's research group at China Jiliang University and Prof. Tao Li's research group at Nanjing University, collaborated to propose a new type of coupler, termed spin-decoupled geometric meta-coupler, realizing triple functions of guided wave radiation, polarization demultiplexing, and dual-channel wavefront manipulation. The relevant research results are published in Photonics Research, Volume. 11, Issue 12, 2023 (Bin Fang, Zhizhang Wang, Yantao Li, Jitao Ji, Kelei Xi, Qingqing Cheng, Fangzhou Shu, Zhongwei Jin, Zhi Hong, Chunlian Zhan, Changyu Shen, and Tao Li. Spin-decoupled meta-coupler empowered multiplexing and multifunction of guided wave radiation [J]. Photonics Research, 2023, 11(12): 2194).
It has been reported that metasurface with resonant phase or detour phase is utilized to control the wavefront of guided wave radiation, but either the design requires a lot of parameter scanning or relies on complex algorithm optimization. Fortunately, the research team found that the transverse-electric (TE) eigenmode of waveguide can be automatically decomposed into a superposition of left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) wave, hence the simple, broadband and robust geometric metasurface with Pancharatnam—Berry (PB) phase was introduced into the waveguide platform to manipulate the guided wave radiation (see Fig.1). It should be noted that there are fundamental symmetry restrictions between two circularly polarized wave phases introduced by geometric metasurface, they ingeniously encoded two different phases into a single metasurface antenna to achieve spin decoupled /independent functionalities. Stimulated by the evanescent field of guided mode, the LCP/RCP waves scattered by geometric meta-coupler will "intelligently identify" the pre-endowed phase modulation and then reconstruct the target light field with different polarizations, enabling multifunctional multiplexing.
The research team took the thin-film lithium-niobate-on-insulator (LNOI) as an ideal waveguide platform for investigations and applications, since it has been widely employed in developing versatile photonic integrated devices. They decorated several meta-couplers composed of periodically arranged silicon nano-antennas on top surface of the slab waveguide, numerically and experimentally demonstrated off-chip polarization-demultiplexed focusing and multiplexed holographic projections at the telecommunication wavelength, with polarization extinction ratio over 13.4 dB. Moreover, they performed an analysis on the guided mode distortion and broadband performance. In this work, the couplers consist of anisotropic meta-atoms with identical shapes and different in-plane orientations, simultaneously endowed with triple functions of guided wave radiation, polarization demultiplexing and dual-channel wavefront manipulation. Such coupler is simple and efficient to enrich the functionality of guided wave radiation manipulation.
The authors highlighted their investigation: this work integrates metasurface antennas with waveguide, and proposes a spin-decoupled geometric meta-coupler, which enriches the functions of traditional couplers and also expands the application scenarios of metasurface. In the future, benefited from the excellent electro-optic property of LN together with tunable metasurface, chip integrated dynamic devices for optical communications and displays could be expected, ranging from OPA to miniaturized display devices for virtual reality and augmented reality.