Fig. 1. Characterization of silk-based optical waveguides. (a) SEM image of a spider-silk-based metallic dielectric fiber^[29]; (b) stress‒strain curve (left) and Young's modulus measurement result (right) of a spider-silk-based metallic dielectric fiber^[29]; (c) genetically engineered spider silk protein fabrication of an optical waveguide^[37]; (d) SEM image and its enlarged image of a genetically recombinant spider silk optical waveguide^[37]; (e) schematic diagram of biology in situ mineralization spinning to prepare a silk optical fiber waveguide^[31]; (f) SEM image of RSF/CaCO₃ fiber^[31]; (g) strength‒strain curves of RSF, RSF/CaCO₃ fiber, and natural silk fiber^[31]; (h) comparison of integrated performance of RSF, RSF/CaCO₃ fiber, and natural silk fiber^[31]

Fig. 2. Cellulose-based optical waveguide. (a) Schematic diagram of biocompatible optical fibers made from regenerated cellulose and recombinant cellulose combined with a solution of the spider silk protein eADF 4 (C16) ^[57]; (b) SEM image of fiber tip prepared by the method in Fig. 2(a) ^[57]; (c) microscopic image of the tip of a fiber with a core‒cladding structure using RC as the core material and eADF 4 (C16)-CBD as the cladding^[57]; (d) coaxial cellulose acetate regeneration cellulose fiber preparation process^[61]; (e) SEM image of cross-section of coaxial fibers prepared by the method in Fig. 2(d) ^[61]; (f) preparation and characterization of transparent fibers based on natural cellulose fibers and epoxy resins^[62]; (g) schematic diagram of lotus root filament microfibers used for environmentally friendly optical waveguides and biosensing^[63]; (h) photographs of natural filaments extracted by hand directly from the lotus root^[63]; (i) SEM image of lotus root microfilaments^[63]; (j) optical micrographs of monofilament microfibers with swiftly coupled light from a tapered optical fiber at wavelengths of 655, 532, and 440 nm, respectively^[63]

Fig. 3. Characterization of optical waveguide properties of AIE and other materials. (a) Aggregation-induced luminescence^[66]; (b) schematic diagram of the main mechanism of AIE^[66]; (c) fluorescence micrographs of MeDHQB-I (left) and MeDHQB-Ⅱ (right) under 365 nm UV irradiation^[74]; (d) fluorescence microscope image of AIE microfibers irradiated by 365 nm UV light^[76]; (e) near-field coupling of AIE microfibers with 473, 532, and 655 nm lasers coupled at different positions^[76], where scale bar is 50 μm; (f) optical microscopy image of two crossed ice microfibers with a diameter of 3 μm^[76]; (g) optical microscopy images of elastically bending IMFs^[78]; (h) tapered end-face image of IMFs^[78]; (i) schematic diagram of launching light into an IMF by the evanescent wave coupling method^[78]; (j) microscopy image of light guided by IMFs of different wavelengths (4.4 μm diameter and 200 μm length) ^[78]; (k) schematic of the preparation of OIMFs^[82]; (l) microscopic image of nanoclusters Pt₁Ag₁₈ (orange) and Au_xAg_19-x (red) under visible light and 405 nm laser irradiation^[87]; (m) nanoclusters Pt₁Ag₁₈ and Au_xAg_19-x in an experimental setup for optical waveguide devices^[87]

Fig. 4. Fiber optical waveguides for sensing applications. (a) Polarization experiment setup for water sensing with polarization transition on Poincaré sphere of output light when the filaments are exposed to a small amount of water vapor and a nonpolar gas^[89]; (b) experimental arrangement of a spider-silk-based metal-medium fiber optic sensor^[29]; (c) scheme of a pH sensing system for a lotus-root filament microfiber^[63]; (d) TE WGM spectra of an ice microfiber^[78]; (e) simulation of TE₂₆ WGM in Fig. 4(d) on an ice microfiber electric field distribution on the ice microfiber^[78]

Fig. 5. Biomedical and photonic device applications of fiber optical waveguides. (a) Schematic of the system of lotus root silk microfiber for real-time monitoring of Helicobacter pylori activity^[63]; (b) upconversion of nanoparticles decorating spider silk as a single-cell thermometer^[91]; (c) variation of light penetration length of a recombinant spider silk optical waveguide inserted into a muscle^[37]; (d) light guided by a silk waveguide in a tissue, inset shows that the core of the waveguide emits a green light after the incision is closed^[50]; (e) silk optical fiber light guided through a chicken breast in an end view, with bright lines showing light confined in the core of the fiber^[92]; (f) microscopic images of a spider silk waveguide integrated on a photonic chip in different views^[28],where RD is a photon reservoir-disk; (g) graphical illustration of a dark blue aggregation-induced-emission microfiber as a waveguide source to illuminate a variety of emissions^[76]; (h) illumination of white-light emission on a microscale using a dark blue AIE fiber as a waveguide light^[76]