Subwavelength-diameter optical fibre (SDF or SDOF) is an optical fibre whose diameter is less than the wavelength of the light being propagated through the fibre. An SDOF usually consists of long thick parts (same as conventional optical fibres) at both ends, transition regions (a.k.a. tapers), where the fibre diameter gradually decreases down to the subwavelength value, and a subwavelength-diameter waist, which is the main acting part of an SDOF.

Name

There is no general agreement on how these optical elements are to be called, different groups preferring 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
• ultra-thin optical fibres
• optical nanofibre
• optical microfibres
• submicron fibre waveguides

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 synonymic to subwavelength in this case, taking into account that the majority of experiments are carried out with the light with the wavelength between 0.8 and 1.6 µm; however for other wavelengths this may not be true. All the names including the prefix nano- are somewhat misleading, since it is usually applied to objects with dimensions on the scale of nanometers or tens of nanometers (cf. nanoparticle

Nanoparticle

In nanotechnology, a particle is defined as a small object that behaves as a whole unit in terms of its transport and properties. Particles are further classified according to size : in terms of diameter, coarse particles cover a range between 10,000 and 2,500 nanometers. Fine particles are sized...

, nanotechnology

Nanotechnology

Nanotechnology is the study of manipulating matter on an atomic and molecular scale. Generally, nanotechnology deals with developing materials, devices, or other structures possessing at least one dimension sized from 1 to 100 nanometres...

). The characteristic behaviour of the SDOF—high intensity of the electromagnetic field both inside and outside the fibre, maximum confinement of light in transversal cross-section—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.

High power in the evanescent field

The main peculiarity of an SDF is that in the waist region, a significant part of the light's power propagates outside the fibre. Rigorously, this follows from the application of Maxwell's equations

Maxwell's equations

Maxwell's equations are a set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits. These fields in turn underlie modern electrical and communications technologies.Maxwell's equations...

 to a waveguide

Waveguide (optics)

An optical waveguide is a physical structure that guides electromagnetic waves in the optical spectrum. Common types of optical waveguides include optical fiber and rectangular waveguides....

 with circular cross-section. In a simplified way, this may be explained by the following. Light is guided in waveguides by total internal reflection

Total internal reflection

Total internal reflection is an optical phenomenon that happens when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary and the incident angle is...

 (TIR) occurring on the interface between the waveguide and surrounding media. During TIR, the light intensity does not fall down to zero immediately at the interface, but decreases exponentially (vanishes) in the adjacent medium (the light field outside the waveguide is called the evanescent field

Evanescent wave

An evanescent wave is a nearfield standing wave with an intensity that exhibits exponential decay with distance from the boundary at which the wave was formed. Evanescent waves are a general property of wave-equations, and can in principle occur in any context to which a wave-equation applies...

). The depth of penetration of light during TIR depends on the exact configuration, but it is usually greater than or on the order of the wavelength of light.

An SDF has a diameter which is smaller than or on the order of the wavelength of light. Since an SDF is also a waveguide and thus light propagation is physically explained by the same fundamental reasons as TIR, the light, guided by an SDF, penetrates into surrounding media (air or vacuum) to the depth of about one wavelength or more. However, while in the case of conventional waveguides this depth is very small compared to the waveguide dimensions and so only a negligible amount of energy propagates outside the waveguide, in the case of SDF the volume occupied by the evanescent field is bigger than the volume of the SDF itself. Therefore the evanescent field of an SDF contains a significant portion of the whole light energy propagating along the fibre.

Manufacturing

An SDF is usually created by tapering a commercial optical fibre. Special pulling machines accomplish the process.

An optical fibre usually consists of a core, a cladding

Cladding (fiber optics)

Cladding is one or more layers of material of lower refractive index, in intimate contact with a core material of higher refractive index. The cladding causes light to be confined to the core of the fiber by total internal reflection at the boundary between the two. Light propagation in the...

 and a protective coating. Before pulling a fibre, its coating is removed (the fibre is stripped). Then the bare fibre is fixed at two ends on the movable translation stages of the pulling machine. The middle of the fibre between the stages is then heated with a flame or a laser beam and at the same time the translation stages move in the opposite directions. The glass melts and the fibre is elongated so that its diameter decreases. The flame or laser beam usually also moves in order to obtain waist of significant length and constant thickness.

Using the described method, waists of 1...10 mm in length and diameters down to 100 nm are obtained.

Handling

Being extremely thin, an SDF is also extremely fragile. Therefore, an SDF is usually mounted onto a special frame immediately after pulling and is never detached from this frame.

Another issue is dust particles which may adsorb to the surface of an SDF. If significant laser power is coupled into the fibre, dust particles will scatter light in the evanescent field, heat up and may thermally destroy the waist. In order to prevent this, SDF are pulled and used in dust-free environments such as flowboxes or vacuum chambers.

Light propagation in SDF

Light propagation in an SDF is governed by different propagation equations as in a usual optical fibre. See and .

Research groups working with SDF

• University of Bonn, Dieter Meschede
• University College Cork, S.Nic Chormaic
• Harvard University, Eric Mazur
  Eric Mazur
  Eric Mazur is a prominent physicist and educator at Harvard University. Mazur is known for his work in experimental ultrafast optics and condensed matter physics and a national leader in science education...
  
  
• University of Mainz, Arno Rauschenbeutel
• University of Southampton, Gilberto Brambilla
• Zhejiang University, Limin Tong
• Max Planck Institute for the Science of Light, Fabio Biancalana
• University of Maryland, College Park, Atoms on SQUIDs