nanofibers (original) (raw)

Definition: optical fibers with transverse dimensions below one micrometer

Alternative terms: photonic nanowires, optical nanowires

Category: article belongs to category fiber optics and waveguides fiber optics and waveguides

Related: fiberstapered fiberswaveguidescavitieswavelength

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Contents

What are Nanofibers?

Optical nanofibers, also called photonic nanowires, are optical fibers with diameters in the range from tens to a few hundreds of nanometers. This means that the diameter is often well below the optical wavelength. The alternative term sub-wavelength fibers emphasizes this important aspect. Such nanowires can have peculiar mechanical and optical properties.

Properties of Nanofibers

Due to the large refractive index difference between fiber and air, the numerical aperture is very high, and the effective mode area is very small. For precise calculations of the mode properties, full vectorial models are required, as the paraxial approximation is not fulfilled.

Silica nanowires have an exceptional mechanical strength, allowing for bending with radii of a few micrometers. The high numerical aperture keeps the bend losses low even for such tight bending. Tightly coiled fibers can be used for miniature fiber resonators.

Light that is guided in nanofibers can experience strong nonlinearities due to the small effective mode area, and is associated with significant evanescent fields just outside the fiber surface. For fiber diameters below ≈ 0.6 μm (in the case of silica fibers), the mode radius of guided light increases as the fiber diameter is further decreased [5], essentially because the “guiding power” of a thinner fiber becomes weaker. Most of the optical power then propagates in the evanescent field outside the fiber, and bend sensitivity becomes high.

Fabrication

A variety of techniques can be used to fabricate optical nanofibers. Particularly low-loss nanofibers [8] are obtained by tapering of larger optical fibers (mostly silica fibers), i.e. by heating and stretching them over a flame (flame brushing). To keep the losses at a low level, the taper transition should be very smooth (adiabatic tapering). However, even for a constant fiber diameter, the losses become very high when the diameter is too small.

With totally different techniques, one can fabricate semiconductor nanowires [15].

Applications

Although optical nanowires are a fairly new area of research, various possible applications have been identified and in some cases demonstrated. Some examples are:

Frequently Asked Questions

What are optical nanofibers?

Optical nanofibers, also known as photonic nanowires, are optical fibers with diameters of tens to hundreds of nanometers, which is often smaller than the wavelength of the guided light.

How are low-loss nanofibers fabricated?

They are typically produced by tapering standard optical fibers, a process of heating and stretching them over a flame. To maintain low optical losses, the transition region must be very smooth (adiabatic tapering).

What is special about light guiding in nanofibers?

Due to the small core and high refractive index contrast with air, light is strongly confined, leading to a small effective mode area and strong nonlinear effects. A significant portion of the light travels in an evanescent field just outside the fiber surface.

Why does the mode size increase for very thin nanofibers?

For very small diameters (e.g., below 0.6 Îźm for silica), the guiding power of the fiber becomes too weak to tightly confine the light, causing the mode field to expand significantly into the surrounding air.

What are some applications of optical nanofibers?

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Bibliography

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[4] L. Tong et al., “Subwavelength-diameter silica wires for low-loss optical wave guiding”, Nature 426, 816 (2003); doi:10.1038/nature02193
[5] L. Tong et al., “Single-mode guiding properties of subwavelength-diameter silica and silicon wire waveguides”, Opt. Express 12 (6), 1025 (2004); doi:10.1364/OPEX.12.001025
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[13] M. A. Foster et al., “Nonlinear optics in photonic nanowires”, Opt. Express 16 (2), 1300 (2008); doi:10.1364/OE.16.001300
[14] D. Yeom et al., “Low-threshold supercontinuum generation in highly nonlinear chalcogenide nanowires”, Opt. Lett. 33 (7), 660 (2008); doi:10.1364/OL.33.000660
[15] R. Yan et al., “Nanowire photonics”, Nature Photon. 3 (10), 569 (2009); doi:10.1038/nphoton.2009.184
[16] Shahraam Afshar V. et al., “Small core optical waveguides are more nonlinear than expected: experimental confirmation”, Opt. Lett. 34 (22), 3577 (2009); doi:10.1364/OL.34.003577
[17] R. Yan, D. Gargas and P. Yang, “Nanowire photonics”, Nature Photonics 3, 569 (2009); doi:10.1038/NPHOTON.2009.184
[18] G. Brambilla et al., “Optical fiber nanowires and microwires: fabrication and applications”, Advances in Optics and Photonics 1 (1), 107 (2009); doi:10.1364/AOP.1.000107
[19] J. J. Morrissey et al., “Spectroscopy, manipulation and trapping of neutral atoms, molecules, and other particles using optical nanofibers: a review”, Sensors 13 (8), 10449 (2013); doi:10.1117/12.942540
[20] H. Sun et al., “Giant optical gain in a single-crystal erbium chloride silicate nanowire”, Nature Photonics 11, 589 (2017); doi:10.1038/nphoton.2017.115

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