Orbital angular momentum (OAM), one fundamental property of light, has been of great interest over the past decades. An ideal OAM generator, fully compatible with existing physical dimensions (wavelength and polarization) of light, would offer the distinct features of broadband, polarization diversity, and ultra-compact footprint. Here, we propose, design, fabricate, and demonstrate an ultra-compact chip-scale broadband polarization diversity OAM generator on a silicon platform with a 3.6 × 3.6 μm2 footprint. The silicon OAM chip is formed by introducing a subwavelength surface structure (superposed holographic fork gratings) on top of a silicon waveguide, coupling the in-plane waveguide mode to the out-plane free-space OAM mode. We demonstrate in theory and experiment the broadband generation of polarization diversity OAM modes (x-/y-polarized OAM+1/OAM−1) from 1500 to 1630 nm with high purity and efficiency. The demonstrations of an ultra-compact broadband polarization diversity OAM generator may open up new perspectives for OAM-assisted N-dimensional optical multiplexing communications/interconnects and high-dimensional quantum communication systems.

Chiral light-matter interaction enables new fundamental researches and applications of light. The interaction has traditionally faced challenges in low directionality and efficiency based on spin-orbit interaction of light in microscopic waveguides. It is pivotal to exploit photonic integrated circuits to efficiently engineer photonic chiral behavior. Here, we present ultra-directional high-efficient chiral coupling in silicon photonic circuits based on low-order to high-order mode conversion and interference. We show that the directionality of chiral coupling, in principle, can approach ±1 with circular polarization inputs, benefited from the underlying mechanism of complete destructive and constructive interference. The chiral coupling efficiency can exceed 70%, with negligible scattering to non-guided modes, much higher than conventional coupling mechanisms. Moreover, the chiral silicon photonic circuits can function as a perfect 3-dB power splitter for arbitrarily linear polarization inputs, and also open up the possibility of on-chip chirality determination to further flourish the development of chiral optics.

Compact and broadband manipulation of spatial modes is important in applications exploiting the space domain of light waves. Here, we demonstrate chip-scale generation and synthesization of ultrabroadband orbital angular momentum (OAM) modes (twisted light) on a silicon platform. By introducing a subwavelength holographic fork grating on top of a silicon waveguide, the in-plane guided mode is converted to the free-space OAM mode. Inputs from both sides of the waveguide enable the synthesization of OAM modes. We also characterize wavelength-dependent emission efficiency, offset angle, and purity with favorable performance. The chip-scale ultrabroadband OAM generator and synthesizer may find potential applications in multidimensional optical communications and quantum key distribution.