Monolithic‐Integrated Multiplexed Devices Based on Metasurface‐Driven Guided Waves

Functionality Expansion of Guided Mode Radiation via On-Chip Metasurfaces

On-chip metasurfaces play a crucial role in bridging the guided mode and free-space light, enabling full control over the wavefront of scattered free-space light in an optimally compact manner. Recently, researchers have introduced various methods and on-chip metasurfaces to engineer the radiation of guided modes, but the total functions that a single metasurface can achieve are still relatively limited. In this work, we propose a novel on-chip metasurface design that can multiplex up to four distinct functions. We can efficiently control the polarization state, phase, angular momentum, and beam profile of the radiated waves by tailoring the geometry of V-shaped nanoantennas integrated on a slab waveguide. We demonstrate several innovative on-chip metasurfaces for switchable focusing/defocusing and for multifunctional generators of orbital angular momentum beams. Our on-chip metasurface design is expected to advance modern integrated photonics, offering applications in optical data storage, optical interconnection, augmented reality, and virtual reality.

On-chip non-uniform geometric metasurface for multi-channel wavefront manipulations

Metasurfaces integrated with waveguides have been recently explored as a means to control the conversion between guided modes and radiation modes for versatile functionalities. However, most efforts have been limited to constructing a single free-space wavefront using guided waves, which hinders the functional diversity and requires a complex configuration. Here, a new, to the best of our knowledge, type of non-uniformly arranged geometric metasurface enabling independent multi-channel wavefront engineering of guided wave radiation is ingeniously proposed. By endowing three structural degrees of freedom into a meta-atom, two mechanisms (the Pancharatnam-Berry phase and the detour phase) of the metasurface are perfectly joined together, giving rise to three phase degrees of freedom to manipulate. Therefore, an on-chip polarization demultiplexed metalens, a wavelength-multiplexed metalens, and RGB-colored holography with an improved information capacity are successively demonstrated. Our results enrich the functionalities of an on-chip metasurface and imply the prospect of advancements in multiplexing optical imaging, augmented reality (AR) holographic displays, and information encryption.

Metasurfaces integrated with a single-mode waveguide array for off-chip wavefront shaping

Integration of metasurfaces and SOI (silicon-on-insulator) chips can leverage the advantages of both metamaterials and silicon photonics, enabling novel light shaping functionalities in planar, compact devices that are compatible with CMOS (complementary metal-oxide-semiconductor) production. To facilitate light extraction from a two-dimensional metasurface vertically into free space, the established approach is to use a wide waveguide. However, the multi-modal feature of such wide waveguides can render the device vulnerable to mode distortion. Here, we propose a different approach, where an array of narrow, single-mode waveguides is used instead of a wide, multi-mode waveguide. This approach tolerates nano-scatterers with a relatively high scattering efficiency, for example Si nanopillars that are in direct contact with the waveguides. Two example devices are designed and numerically studied as demonstrations: the first being a beam deflector that deflects light into the same direction regardless of the direction of input light, and the second being a light-focusing metalens. This work shows a straightforward approach of metasurface-SOI chip integration, which could be useful for emerging applications such as metalens arrays and neural probes that require off-chip light shaping from relatively small metasurfaces.

Leaky-wave metasurfaces for integrated photonics

Metasurfaces have been rapidly advancing our command over the many degrees of freedom of light; however, so far, they have been mostly limited to manipulating light in free space. Metasurfaces integrated on top of guided-wave photonic systems have been explored to control the scattering of light off-chip with enhanced functionalities-namely, the point-by-point manipulation of amplitude, phase or polarization. However, these efforts have so far been limited to controlling one or two optical degrees of freedom at best, as well as device configurations much more complex compared with conventional grating couplers. Here we introduce leaky-wave metasurfaces, which are based on symmetry-broken photonic crystal slabs that support quasi-bound states in the continuum. This platform has a compact form factor equivalent to the one of grating couplers, but it provides full command over the amplitude, phase and polarization (four optical degrees of freedom) across large apertures. We present devices for phase and amplitude control at a fixed polarization state, and devices controlling all the four optical degrees of freedom for operation at a wavelength of 1.55 μm. Merging the fields of guided and free-space optics through the hybrid nature of quasi-bound states in the continuum, our leaky-wave metasurfaces may find applications in imaging, communications, augmented reality, quantum optics, LIDAR and integrated photonic systems.

Metasurfaces on silicon photonic waveguides for simultaneous emission phase and amplitude control

Chip-scale photonic systems that manipulate free-space emission have recently attracted attention for applications such as free-space optical communications and solid-state LiDAR. Silicon photonics, as a leading platform for chip-scale integration, needs to offer more versatile control of free-space emission. Here we integrate metasurfaces on silicon photonic waveguides to generate free-space emission with controlled phase and amplitude profiles. We demonstrate experimentally structured beams, including a focused Gaussian beam and a Hermite-Gaussian TEM₁₀ beam, as well as holographic image projections. Our approach is monolithic and CMOS-compatible. The simultaneous phase and amplitude control enable more faithful generation of structured beams and speckle-reduced projection of holographic images.

Metasurface around the side surface of an optical fiber for light focusing

Optical fibers integrated with metasurfaces have drawn tremendous interest in recent years due to the great potential for revolutionizing and functionalizing traditional optics. However, in most cases, metasurfaces have been placed on the fiber end-facet where the area is quite limited. Here, by dressing a series of identical dielectric rings around the side surface of the microfiber and adjusting their positions along the microfiber axis, we extracted guided waves into free-space radiation with continuously controllable phase shift and achieved circular-arc-shaped line focusing. We demonstrated that the off-fiber foci could be rotated around the fiber axis by tuning the polarization of the guided waves. In addition, we demonstrated that the shape of the focus could be further tuned by introducing symmetry breaking into the dielectric rings. Our study provides a new dimension for the design of optical fiber devices decorated with metasurfaces.

Meta-Optics-Empowered Switchable Integrated Mode Converter Based on the Adjoint Method

Monolithic integrated mode converters with high integration are essential to photonic integrated circuits (PICs), and they are widely used in next-generation optical communications and complex quantum systems. It is expected that PICs will become more miniaturized, multifunctional, and intelligent with the development of micro/nano-technology. The increase in design space makes it difficult to realize high-performance device design based on traditional parameter sweeping or heuristic design, especially in the optimal design of reconfigurable PIC devices. Combining the mode coupling theory and adjoint calculation method, we proposed a design method for a switchable mode converter. The device could realize the transmission of TE0 mode and the conversion from TE0 to TE1 mode with a footprint of 0.9 × 7.5 μm² based on the phase change materials (PCMs). We also found that the mode purity could reach 78.2% in both states at the working wavelength of 1.55 μm. The designed method will provide a new impetus for programmable photonic integrated devices and find broad application prospects in communication, optical neural networks, and sensing.

Metasurface-Dressed Two-Dimensional on-Chip Waveguide for Free-Space Light Field Manipulation

We show that a metasurface-coated two-dimensional (2D) slab waveguide enables the generation of arbitrary complex light fields by combining the extreme versatility and freedom on the wavefront control of optical metasurfaces with the compactness of photonic integrated circuits. We demonstrated off-chip 2D focusing and holographic projection with our metasurface-dressed photonic integrated devices. This technology holds the potential for many other optical applications requiring 2D light field manipulation with full on-chip integration, such as solid-state LiDAR and near-eye AR/VR displays.

Bifocal focusing and polarization demultiplexing by a guided wave-driven metasurface

Metasurfaces have shown extraordinary light-manipulation abilities, however, most of them deal with free-space waves. It is highly desirable to develop a guided wave-driven metasurface which can extract the in-plane guided modes in the waveguide and mold it into the desired out-of-plane free-space modes. In this paper, an all-dielectric guided wave-driven metasurface, composed of an array of silicon meta-atoms on top of a silicon nitride waveguide, is proposed and simulatively demonstrated. When directly driven by fundamental transverse electric (TE₀₀) and fundamental transverse magnetic (TM₀₀) guided modes at operation wavelength 1.55 µm, the guided wave-driven metasurface converts them into y-polarized and x-polarized free-space light, respectively, and focuses them at different focal points, with polarization extinction ratio over 27 dB, thus simultaneously realizing triple functions of coupling guided modes to free-space waves, bifocal metalens and polarization demultiplexing. Our work offers an alternate way to control light across photonic integrated devices and free-space platforms.