Preface 1. Wave Theory of Optical Waveguides 2. Planar Optical Waveguides 3. Optical Fibers 4. Couple Mode Theory 5. Nonlinear Optical Effects in Optical Fibers 6. Finite Element Method 7. Beam Propagation Method 8. Staircase Concatention Method 9. Planar Lightwave Circuits 10. Theorems and Formulas Appendix

Fundamentals of optical waveguides

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.

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

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

(1979). Gradient Index Optics. Optica Acta: International Journal of Optics: Vol. 26, No. 4, pp. 426-427.

Gradient Index Optics

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

A Luneburg lens with a gradient index distribution is an aberration-free and coma-free spherical lens. It has wide applications ranging from invisibility cloaks, illusion optics, and superlensing. ...

Ultra-Broadband Mode Size Converter Using On-Chip Metamaterial-Based Luneburg Lens

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.

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Subwavelength structured silicon waveguides and photonic devices

Leveraging the physical dimensions of an optical carrier (e.g., wavelength, mode, or polarization) allows significant scaling of the transmission capacity for optical communications. Here we propose a scheme for implementing on-chip silicon (de)multiplexers with simultaneous wavelength-, mode-, and polarization-division (de)multiplexing capability. The device is constructed by using cascaded subwavelength grating (SWG)-based contra-directional couplers. To verify the feasibility of the proposed structure, we perform a proof-of-concept experiment of an 8-channel (de)multiplexer with two wavelengths, two modes, and two polarizations. The insertion losses are lower than 6.6 dB and the crosstalk values are below −18.7dB at around 1540 nm and 1550 nm for all the eight channels.

Lu Sun

Papers

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

Compact Silicon Waveguide Mode Converter Employing Dielectric Metasurface Structure

Ultra-Broadband Mode Size Converter Using On-Chip Metamaterial-Based Luneburg Lens

Subwavelength structured silicon waveguides and photonic devices

Design and experimental demonstration of a silicon multi-dimensional (de)multiplexer for wavelength-, mode- and polarization-division (de)multiplexing