定义：
横向维度小于1微米的光纤。

光纤，通常也称为光子纳米线，是指直径在几十到几百纳米的光纤。而亚波长光纤则是强调这一特性。这种纳米线具有很特殊的机械和光学特性。 

目录

• 性质
• 制备过程
• 应用

性质 
由于光纤和空气之间很大的折射率差，数值孔径很高，并且有效模式面积非常小。为了更准确的计算模式特性，需要采用全矢量模型，因为不满足傍轴近似。 
石英纳米线具有非常特殊的机械强度，弯曲半径可以达到几微米。它的高数值孔径使其弯曲损耗即使在强烈弯曲时也较低。紧密缠绕在一起的光纤可以用来得到小型光纤谐振腔。 
由于纳米光纤很小的有效模式面积，光在其中传播会经历很强的非线性，并且在光纤表面还有很强的衰逝场。如果光纤直径小于约0.6微米时（石英光纤的情况），导波的模式半径随着光纤直径的减小而增大，主要是由于更细的光纤的导波占的功率变小。大多数光以衰逝场形式在光纤外传播。 

制备过程 
制备纳米光纤由很多方法。通过加热然后在火焰处拉锥光纤（石英光纤）可以得到非常低损耗的纳米光纤。为了保持较低的损耗，锥形区域需要很平缓（绝热拉锥）。但是，即使在固定光纤直径的情况下，当直径很小时损耗也会很高。 

应用 
尽管光学纳米线是一个比较新的研究领域，已经发现了很多可能的应用，而有的情况已经实现。例如： — 在低峰值功率时，纳米线中可以得到超连续谱产生。 — 强弯曲的纳米线可以形成很小的环形谐振腔（微腔，微环干涉仪），可以用作陷波滤波器，也可以用于基础研究。 — 强的衰逝场可以用于光纤传感器中用在化学或生物领域。 — 小维度可以探测原子或类似例子发出的荧光。 — 可以想象，具有非常小阈值泵浦功率的激光器可以利用小的纳米谐振腔结合一些激光活性掺杂来实现。

Definition: optical fibers with transverse dimensions below one micrometer

Alternative terms: photonic nanowires, optical nanowires

More general term: optical fibers

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.

Bibliography