Subwavelength-diameter optical fibre

A subwavelength-diameter optical fibre (SDF or SDOF) is an optical fibre whose diameter is less than the wavelength of the light being propagated through it. An SDF usually consists of long thick parts (same as conventional optical fibres) at both ends, transition regions (tapers) where the fibre diameter gradually decreases down to the subwavelength value, and a subwavelength-diameter waist, which is the main acting part. Due to such a strong geometrical confinement, the guided electromagnetic field in an SDF is restricted to a single transverse spatial mode called fundamental.

Name

There is no general agreement on how these optical elements are to be named; different groups prefer to emphasize different properties of such fibres, sometimes even using different terms. The names in use include subwavelength waveguide, subwavelength optical wire, subwavelength-diameter silica wire, subwavelength diameter fibre taper, (photonic) wire waveguide, photonic wire, photonic nanowire, optical nanowires, optical fibre nanowires, tapered (optical) fibre, fibre taper, submicron-diameter silica fibre, ultrathin optical fibres, optical nanofibre, optical microfibres, submicron fibre waveguides, micro/nano optical wires (MNOW).

The term waveguide can be applied not only to fibres, but also to other waveguiding structures such as silicon photonic subwavelength waveguides. The term submicron is often synonymous to subwavelength, as the majority of experiments are carried out using light with a wavelength between 0.5 and 1.6 μm. All the names with the prefix nano- are somewhat misleading, since it is usually applied to objects with dimensions on the scale of nanometers (e.g., nanoparticle, nanotechnology). The characteristic behaviour of the SDF appears when the fibre diameter is about half of the wavelength of light. That is why the term subwavelength is the most appropriate for these objects.