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.
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 , 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.
A variety of techniques can be used to fabricate optical nanofibers. Particularly low-loss nanofibers  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 .
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) , which can act as notch filters , and can be used in fundamental research .
- 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 .
- 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.
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