Single-mode optical wave guiding properties of silica and silicon subwavelength-diameter wires are studied with exact solutions of Maxwell’s equations. Single mode conditions, modal fields, power distribution, group velocities and waveguide dispersions are studied. It shows that air-clad subwavelength-diameter wires have interesting properties such as tight-confinement ability, enhanced evanescent fields and large waveguide dispersions that are very promising for developing future microphotonic devices with subwavelength-width structures.

As an organic optical fiber with a diameter comparable to or less than the wavelength of light, polymer nanofibers have been attracting increasing attention as a platform for manipulating light at the nanoscale. A variety of applications for polymer optical nanofibers, including waveguides, light sources and sensors, have been reported in recent years. In this article, the recent progress in the field of polymer optical nanofibers is reviewed in terms of their fabrication, characterization and applications. In particular, we focus on functionalized polymer nanofibers doped with functional materials, such as dye molecules, noble metal nanoparticles, quantum dots and rare earth ions, which greatly expand their capabilities of generating, propagating, converting and modulating light at the nanoscale.

With diameter close to or below the wavelength of guided light and high index contrast between the fiber core and the surrounding, an optical microfiber shows a variety of interesting waveguiding properties, including widely tailorable optical confinement, evanescent fields and waveguide dispersion. Among various microfiber applications, optical sensing has been attracting increasing research interest due to its possibilities of realizing miniaturized fiber optic sensors with small footprint, high sensitivity, fast response, high flexibility and low optical power consumption. Here we review recent progress in microfiber optical sensors regarding their fabrication, waveguide properties and sensing applications. Typical microfiber-based sensing structures, including biconical tapers, optical gratings, circular cavities, Mach-Zehnder interferometers and functionally coated/doped microfibers, are summarized. Categorized by sensing structures, microfiber optical sensors for refractive index, concentration, temperature, humidity, strain and current measurement in gas or liquid environments are reviewed. Finally, we conclude with an outlook for challenges and opportunities of microfiber optical sensors.

We demonstrate optical resonance from microfiber knots obtained by manipulating freestanding silica microfibers. $Q$ factors as high as 57 000 with finesse of 22 are observed in knots with sizes less than $1\mathrm{mm}$ . The free spectral range of the resonator can be easily tuned by tightening the knot structure in air. The knot resonators are highly stable in water with $Q$ factors up to 31 000 and finesse of 13. The possibility of supporting the knot resonator with a solid $\mathrm{Mg}{\mathrm{F}}_2$ substrate is also demonstrated.