The growing maturity of nanofabrication has ushered massive sophisticated optical structures available on a photonic chip. The integration of subwavelength-structured metasurfaces and metamaterials on the canonical building block of optical waveguides is gradually reshaping the landscape of photonic integrated circuits, giving rise to numerous meta-waveguides with unprecedented strength in controlling guided electromagnetic waves. Here, we review recent advances in meta-structured waveguides that synergize various functional subwavelength photonic architectures with diverse waveguide platforms, such as dielectric or plasmonic waveguides and optical fibers. Foundational results and representative applications are comprehensively summarized. Brief physical models with explicit design tutorials, either physical intuition-based design methods or computer algorithms-based inverse designs, are cataloged as well. We highlight how meta-optics can infuse new degrees of freedom to waveguide-based devices and systems, by enhancing light-matter interaction strength to drastically boost device performance, or offering a versatile designer media for manipulating light in nanoscale to enable novel functionalities. We further discuss current challenges and outline emerging opportunities of this vibrant field for various applications in photonic integrated circuits, biomedical sensing, artificial intelligence and beyond.

Optical meta-waveguides for integrated photonics and beyond.

Research on two-dimensional designer optical structures, or metasurfaces, has mainly focused on controlling the wavefronts of light propagating in free space. Here, we show that gradient metasurface structures consisting of phased arrays of plasmonic or dielectric nanoantennas can be used to control guided waves via strong optical scattering at subwavelength intervals. Based on this design principle, we experimentally demonstrate waveguide mode converters, polarization rotators and waveguide devices supporting asymmetric optical power transmission. We also demonstrate all-dielectric on-chip polarization rotators based on phased arrays of Mie resonators with negligible insertion losses. Our gradient metasurfaces can enable small-footprint, broadband and low-loss photonic integrated devices.

Controlling propagation and coupling of waveguide modes using phase-gradient metasurfaces

Silicon Photonic Platform for Passive Waveguide Devices: Materials, Fabrication, and Applications

The Kerr nonlinear optical performance of silicon nanowire waveguides integrated with 2D layered graphene oxide (GO) films is theoretically studied and optimized based on experimentally measured linear and nonlinear optical parameters of the GO films. The strong mode overlap between the silicon nanowires and highly nonlinear GO films yields a significantly enhanced Kerr nonlinearity for the hybrid waveguides. A detailed analysis for the influence of waveguide geometry and GO film thickness on the propagation loss, nonlinear parameter, and nonlinear figure of merit (FOM) is performed. The results show that the effective nonlinear parameter and nonlinear FOM can be increased by up to ∼52 and ∼79 times relative to bare silicon nanowires, respectively. Self-phase modulation (SPM)-induced spectral broadening of optical pulses is used as a benchmark to evaluate the nonlinear performance, examining the trade-off between enhancing Kerr nonlinearity and minimizing loss. By optimizing the device parameters to balance this, a high spectral broadening factor of 27.8 can be achieved – more than 6 times that achieved in previous experiments. Finally, the influence of pulse chirp, material anisotropy, and the interplay between saturable absorption and SPM is also discussed, together with the comparison between the spectral broadening after going through GO-coated and graphene-coated silicon waveguides. These results provide useful guidance for optimizing the Kerr nonlinear optical performance of silicon waveguides integrated with 2D layered GO films.

Optimizing the Kerr Nonlinear Optical Performance of Silicon Waveguides Integrated With 2D Graphene Oxide Films

Integrated circuits (ICs) and optoelectronic chips are the foundation stones of the modern information society. The IC industry has been driven by the so-called “Moore’s law” in the past 60 years, and now has entered the post Moore’s law era. In this paper, we review the recent progress of ICs and optoelectronic chips. The research status, technical challenges and development trend of devices, chips and integrated technologies of typical IC and optoelectronic chips are focused on. The main contents include the development law of IC and optoelectronic chip technology, the IC design and processing technology, emerging memory and chip architecture, brain-like chip structure and its mechanism, heterogeneous integration, quantum chip technology, silicon photonics chip technology, integrated microwave photonic chip, and optoelectronic hybrid integrated chip.

Recent progress of integrated circuits and optoelectronic chips

DOI: 10.1002/adom.201801191 plasmonic or dielectric nanoantennas.[20] Mode conversions between the trans-electric (TE) modes TE0, TE1, and TE2 were achieved with a length of ≈10 μm at 1550 nm using silicon nanoantennas on LiNbO3 or Si3N4 waveguides by simulations.[20] A nanoscale mode converter was theoretically proposed[11] and experimentally demonstrated[21] by introducing a periodic perturbation in its effective refractive index along the propagation direction and a graded effective index profile along its transverse direction. Mode conversion between TE0 and TE1 modes was successfully achieved with a length of 23 μm at 1550 nm. However, no experimental results on the crosstalk of the TE mode conversion were provided in the previous reports.[20,21] It is highly desired to miniaturize the device footprint and realize a compact waveguide mode converter. In this article, we propose and demonstrate a compact silicon waveguide mode converter based on all-dielectric metasurface structure with a tilted subwavelength periodic perturbation. The mode coupling coefficient changes along the propagation direction, which can help the mode conversion process. Our proposed waveguide mode converter can be scaled to realize arbitrary waveguide mode conversion. As examples, we experimentally demonstrate two silicon waveguide mode converters employing all-dielectric metasurface structures that can convert TE0 mode to TE1 mode and TE0 mode to TE2 mode, respectively. The mode conversion lengths are 5.75 and 6.736 μm for the TE0-to-TE1 and TE0-to-TE2 waveguide mode converters, respectively, which are shorter than previously reported results.[20,21] Mode demultiplexers are cascaded after the metasurface mode converter to measure the crosstalk values and conversion losses. The insertion losses are lower than 1 dB and the crosstalk values are below −10 dB in a wavelength range of 20 nm for the TE0-to-TE1 and TE0-to-TE2 waveguide mode conversions. Figure 1a depicts the 3D view of the proposed waveguide mode converter based on dielectric metasurface structure with a tilted subwavelength periodic perturbation. Propagation of optical field in a perturbed dielectric structure can be described by the coupled mode theory. Attributed to the perturbation on the silicon waveguide, one waveguide mode can be coupled into another mode, and the amplitude of each waveguide mode along the propagation direction is determined by a set of differential equations[11,25]

Compact Silicon Waveguide Mode Converter Employing Dielectric Metasurface Structure

Abstract Subwavelength structures such as subwavelength gratings (SWGs) and subwavelength metamaterials are capable of tailoring the optical properties of materials and controlling the flow of light at the nanoscale. The effective indices of the subwavelength structured strip and slab waveguides can be changed in a wide range by choosing an appropriate duty cycle or a filling factor of silicon, which provides an effective method to manipulate the optical field and achieve effective index matching for functional devices. Recent advances in nanofabrication techniques have made it possible to implement subwavelength structures in silicon strip and slab waveguides. Here we review various approaches used to design subwavelength structures and achieve exotic optical responses and discuss how these structures can be used to realize high-performance silicon photonic devices. Both one-dimensional SWG devices and two-dimensional subwavelength metamaterial devices are covered in this review, including subwavelength structure–based polarization handling devices, mode manipulation devices, and building blocks for integrated optical interconnects. Perspectives on subwavelength structured silicon photonic devices are also discussed.

/pdf/subwavelength-structured-silicon-waveguides-and-photonic-3zml374num.pdf

Subwavelength structured silicon waveguides and photonic devices

Valley‐Hall photonic topological insulators (VPTIs) provide an intriguing approach to suppress backscattering, enhance the robustness of transport, and shrink the footprints of the topological devices. However, previous works focused on the transmission characteristics of the symmetric topological waveguides with few reports on functional devices involving edge mode coupling, which hinders the implementation of various functional devices such as optical couplers/splitters, switches, filters, and etc. In this paper, a new method to manipulate the mode field distributions in topological waveguides is demonstrated by introducing asymmetric edge states, thus the mode coupling with an arbitrary coupling ratio can be achieved between topological waveguides. As an example, a topological power splitter with a 33:67 splitting ratio is demonstrated experimentally. In addition, to verify the robust transmission property of the topological photonic devices, a high‐speed data transmission experiment for a topological power splitter is performed and a high data rate of 128 Gb s−1 is achieved. These results promise a new method to manipulate the mode field distribution in topological waveguides and devices that may find wide applications in diverse fields such as optical communications, nanophotonics, and quantum information processing.

Asymmetric Topological Valley Edge States on Silicon‐On‐Insulator Platform

Electro-optic modulators are key components in data communication, microwave photonics, and quantum photonics. Modulation bandwidth, energy efficiency, and device dimension are crucial metrics of modulators. Here, we provide an important direction for the miniaturization of electro-optic modulators by reporting on ultracompact topological modulators. A topological interface state in a one-dimensional lattice is implemented on a thin-film lithium-niobate integrated platform. Due to the strong optical confinement of the interface state and the peaking enhancement of the electro-optic response, a topological cavity with a size of 1.6 × 140 μm2 enables a large modulation bandwidth of 104 GHz. The first topological modulator exhibits the most compact device size compared to reported LN modulators with bandwidths above 28 GHz, to the best of our knowledge. 100 Gb/s non-return-to-zero and 100 Gb/s four-level pulse amplitude modulation signals are generated. The switching energy is 5.4 fJ/bit, owing to the small electro-optic mode volume and low capacitance. The topological modulator accelerates the response time of topological photonic devices from the microsecond order to the picosecond order and provides an essential foundation for the implementation of large-scale lithium-niobate photonic integrated circuits. 

High-speed electro-optic modulation in topological interface states of a one-dimensional lattice.

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