Fig. 1. (Color online) (a) Strip, slot, ridge, and subwavelength waveguides. (b) A plasmonic slot waveguide. (c) A hybrid waveguide^[18]. Copyright 2016, The Optical Society. (d) A photonic crystal waveguide. (e) A hybrid plasmonic cap waveguide^[28]. Copyright 2009, The Optical Society.

Fig. 2. (Color online) (a) A slab waveguide and its s- and p-polarized modal profiles. (b) A strip waveguide with its RI distributions of both s and p polarizations. (c) Modal profiles |E| of the TE and TM modes in a strip waveguide. (d) A ridge waveguide. (e) Modal profiles |E| of TE and TM continuous modes. The RI distributions of the ridge waveguide with n_sc > n_ps (f) and n_sc < n_ps (g). (h) Modal profiles of the TE and TM modes in a thin-ridge waveguide with n_sc < n_ps. (i) Schematically illustration of leakage channels.

Fig. 3. (Color online) (a) Schematically illustration of a thin-ridge silicon waveguide. (b) The propagation loss of the TM mode as a function of the waveguide width^[72]. Copyright 2009, IEEE. (c) Modal profiles of the TM mode at waveguides with “magic width” and “anti-magic width”^[72]. Copyright 2009, IEEE. (d) Planar view and modal coupling diagram of a bend thin-ridge waveguide^[81]. Copyright 2010, The Optical Society. (e) The propagation loss of the TM mode in a bent waveguide as a function of the waveguide width and radius^[81]. Copyright 2010, The Optical Society.

Fig. 4. (Color online) (a) A fabricated silicon thin-ridge waveguide and the scattering field views of waveguides with “magic width” and “non-magic width”^[66]. Copyright 2016, IEEE. (b) The measured propagation loss of the waveguide with different widths^[66]. Copyright 2016, IEEE. (c) Modal profiles of the TE and TM modes in a waveguide with “anti-magic” structural parameters^[82]. Copyright 2010, The Optical Society. (d) The measured results for the transmissions of the TE and TM modes^[82]. Copyright 2010, The Optical Society.

Fig. 5. (Color online) (a) Schematic illustration of a silicon thin-ridge waveguide supporting BIC resonances and its simulated transmission spectra as a function of waveguide width and incident angle^[77]. Copyright 2018, Chinese Laser Press. (b) A fabricated silicon waveguide supporting BIC resonances and its measured resonance^[76]. Copyright 2019, John Wiley & Sons. (c) A fabricated metagrating waveguide and its measured BIC resonance for applications in sensing^[83]. Copyright 2020, John Wiley & Sons. (d) A flat-top filter based on BIC waveguides and its simulated transmission and reflection spectra^[60]. Copyright 2019, Chinese Laser Press.

Fig. 6. (Color online) (a) Schematic illustration of a straight hybrid waveguide^[56]. (b) RI distributions of s and p polarizations. (c) Modal profiles of a TM mode in waveguides with BIC and non-BIC parameters^[56]. (d) Fabricated straight waveguides and measured propagation loss as a function of waveguide width^[56]. (e) Fabricated bent waveguides and measured propagation loss as functions of waveguide width and bending radius^[56]. Copyright 2019, The Optical Society.

Fig. 7. (Color online) (a) Measured transmission spectrum of the fabricated microring cavity with structural parameters satisfying the BIC condition^[56]. (b) Optical microscope image of the fabricated directional coupler and the corresponding measured spectrum^[56]. (c) Optical microscope image of the fabricated MZI and the corresponding measured spectrum^[56]. Copyright 2019, The Optical Society.

Fig. 8. (Color online) (a) An optical microscope image of the fabricated mode (de)multiplexer integrated with EO modulators^[55]. (b) Propagation loss and effective RIs of the different orders of TM modes as a function of the waveguide width w^[55]. (c) Normalized spectra of light transmission for a fabricated mode (de)multiplexer^[55]. (d) Measured modulated signals for each order of modes^[55]. Copyright 2020, Springer Nature Limited.

Fig. 9. (Color online) (a) Schematic illustration of AO cavity modulation^[59]. (b) Measured AO modulation signals with a frequency higher than 4 GHz^[59]. (c) Measured AO-induced transparency and absorption^[59]. Copyright 2020, Light: Science & Applications. (d) Schematic illustration of AO waveguide modulation^[61]. (e) Measured frequency shifts^[61]. Copyright 2021, ACS.

Fig. 10. (Color online) (a) Schematic illustration of a 2D material integrated with a BIC waveguide^[63]. (b) A fabricated hybrid graphene thermo-optic modulator with BICs and its measured spectra^[63]. (c) A fabricated hybrid graphene photodetector with BICs and its measured optoelectrical response^[63]. (d) A fabricated hybrid graphene EO modulation with BIC and its measured EO response^[63]. Copyright 2019, John Wiley & Sons. (e) A fabricated hybrid PtSe₂ photodetector with BICs and its measured optoelectrical response^[62]. Copyright 2020, ACS.