Nanofibers

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

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

DOI: 10.61835/5su Cite the article: BibTex plain text HTML Link to this page! LinkedIn

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

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 which 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.

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). For keeping 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:

Supercontinuum generation in nanowires [7, 11] is possible for low peak powers, as the light propagates in a highly concentrated manner.
Strongly bent nanowires can form very tiny ring resonators (micro cavities, microloop interferometers) [6], which can act as notch filters [9], and can be used in fundamental research [3].
The strong evanescent field suggests applications in the area of fiber-optic sensors for chemical or biological species.
The small dimensions allow probing of fluorescent light from atoms or similar particles [12].
It is conceivable that lasers with very low threshold pump power could be built by incorporating some laser-active dopants in a small nanofiber resonator.

More to Learn

fibers tapered fibers waveguides cavities wavelength

Bibliography

[1]	R. J. Black et al., “Tapered fibers: an overview”, Proc. SPIE 0839 (1988)
[2]	K. J. Vahala, “Optical microcavities”, Nature 424, 8394346 (2003); https://doi.org/10.1038/nature01939
[3]	S. M. Spillane et al., “Ideality in a fiber-taper-coupled microresonator system for application to cavity quantum electrodynamics”, Phys. Rev. Lett. 91 (4), 043902 (2003); https://doi.org/10.1103/PhysRevLett.91.043902
[4]	L. Tong et al., “Subwavelength-diameter silica wires for low-loss optical wave guiding”, Nature 426, 816 (2003); https://doi.org/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); https://doi.org/10.1364/OPEX.12.001025
[6]	M. Sumetsky et al., “Fabrication and study of bent and coiled free silica nanowires: Self-coupling microloop optical interferometer”, Opt. Express 12 (15), 3521 (2004); https://doi.org/10.1364/OPEX.12.003521
[7]	S. G. Leon-Saval et al., “Supercontinuum generation in submicron fibre waveguides”, Opt. Express 12 (13), 2864 (2004); https://doi.org/10.1364/OPEX.12.002864
[8]	G. Brambilla et al., “Ultra-low-loss optical fiber nanotapers”, Opt. Express 12 (10), 2258 (2004); https://doi.org/10.1364/OPEX.12.002258
[9]	M. Sumetsky et al., “The microfiber loop resonator: theory, experiment, and application”, IEEE J. Lightwave Technol. 24 (1), 242 (2006); https://doi.org/10.1109/JLT.2005.861127
[10]	M. Sumetsky, “How thin can a microfiber be and still guide light?”, Opt. Lett. 31 (7), 870 (2006); https://doi.org/10.1364/OL.31.000870
[11]	R. R. Gattass et al., “Supercontinuum generation in submicrometer diameter silica fibers”, Opt. Express 14 (20), 9408 (2006); https://doi.org/10.1364/OE.14.009408
[12]	K. P. Nayak et al., “Optical nanofiber as an efficient tool for manipulating and probing atomic fluorescence”, Opt. Express 15 (9), 5431 (2007); https://doi.org/10.1364/OE.15.005431
[13]	M. A. Foster et al., “Nonlinear optics in photonic nanowires”, Opt. Express 16 (2), 1300 (2008); https://doi.org/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); https://doi.org/10.1364/OL.33.000660
[15]	R. Yan et al., “Nanowire photonics”, Nature Photon. 3 (10), 569 (2009); https://doi.org/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); https://doi.org/10.1364/OL.34.003577
[17]	R. Yan, D. Gargas and P. Yang, “Nanowire photonics”, Nature Photonics 3, 569 (2009); https://doi.org/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); https://doi.org/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); https://doi.org/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); https://doi.org/10.1038/nphoton.2017.115

(Suggest additional literature!)

Suppliers

The RP Photonics Buyer's Guide contains 20 suppliers for waveguides. Among them:

Shalom EO

Periodically poled lithium niobate (PPLN) crystal is a type of nonlinear optical crystal with distinguished high conversion efficiencies and a wide transparent spectrum of 0.4–5 μm. The 5% MgO doping in LiNbO₃ improves the damage threshold of the PPLN to a significant extent and broadens its phase-matching bandwidth. MgO: PPLN crystals exhibit a unique quasi-phase-matching (QPM) phenomenon, which allows the utilization of a higher nonlinear coefficient than in the case of birefringent phase matching.

Shalom EO offers standard and off-the-shelf MgO:PPLN crystal and waveguides for SHG of 976 nm to 2100 nm and for various upconversion and downconversion applications including DFG, SFG, OPO, and OPA. Different dimensions, MgO concentrations and AR coatings are available.

Teem Photonics

Teem Photonics offers photonic integrated circuits based on its versatile and cost effective IoNext platform. Specifics include high quality waveguides with high or variable confinement for mode diameters from 3 to 20 μm, propagation losses below 0.1 dB/cm, low bending radius (<1 mm), efficient coupling to single-mode fibers (<0.2 dB loss), low polarization-dependent loss and a large optical bandwidth range (400–2100 nm).

Covesion

Covesion’s MgO:PPLN waveguide solutions provide high efficiency for SHG and SFG wavelength conversion processes. Ideal for a wide range of pump sources, these solutions offer robust performance from femtosecond to continuous wave applications, ensuring reliable and versatile frequency conversion for both scientific research and industrial applications.

We offer a range of free space and fiber coupled solutions for one-off orders to large-volume manufacture with customizable options including:

free space or fiber coupled solutions
tailored AR coatings
2×1, 2×0, 1×1 or 1×0 fiber input/output configurations
resistive or Peltier temperature control
integrated or external temperature control
custom wavelength coverage
power monitoring, control and output filtering
compatibility with both CW and pulsed lasers

HC Photonics

HC Photonics (HCP) provides high conversion efficiency PPLN waveguides (periodically poled lithium niobate waveguides) to enable full-spectrum applications (i.e. single photon or Solc filter modulator) available for up-conversion (SHG/SFG) and down-conversion (DFG/OPA/OPG) frequency mixing configurations.

Two types of the waveguide are available to fulfill different applications. One is a proton-exchanged waveguide (up to 50 mm long) and the other one is ridge waveguide for high power handling (i.e. >2 W at 780 nm output). Tailored waveguides are available based on specific demands.

Octave Photonics

Octave Photonics provides supercontinuum generation waveguides that are packaged into easy-to-use devices. These devices feature standard fiber connectors and enable femtosecond lasers to be broadened to octave-spanning supercontinuum with low pulse energies (<150 pJ). The devices can be customized depending on the desired output spectrum.

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