The development of nanotechnology in photonics offers significant scientific and technological potentials for miniaturized photonic integrated circuits (PICs) [1,2]. It fosters the substantial efforts for exploring novel materials, developing easy fabrication techniques, reducing the size of photonic components, improving device integration density, and fabricating low-cost nanodevices. One-dimensional (1D) nanomaterials are promising candidates for photonics integration due to their intriguing optical, electronic, and mechanical properties [3-9]. In 2003, L. Tong et al. experimentally demonstrated low-loss optical waveguiding in silica micro/nanofibers (MNFs) with diameters far below the wavelength of the guided light , which renewed research interests in optical MNFs as potential building blocks for applications in high density and miniaturized PICs [11-22]. In particular, polymers have been widely regarded as useful materials for manipulating light in optical waveguide applications due to their good processability, biocompatibility, tunable properties, flexibility, and low cost for integration [15-22]. On the other hand, polymeric materials offer a unique opportunity to carry other chemical composition and to readily produce hybrid nanocomposites. 1D nanostructures fabricated from polymers have been the hot subject of recent research with regard to their special physical, chemical, electronic, and photonic properties [19,23-38]. Demonstration of viable polymer MNF based elements and architectures will require the development of reliable methods for the production of such structures with good control over critical parameters such as diameter, length, morphology, and chemical composition. Up to now, a bunch of new materials and new techniques have been developed to fabricate subwavelength and nanometer diameter polymer fibers, including electrospinning, phase separation, self assembling and one-step drawing, etc. . Polymer MNFs with diameter down to several nanometers and length up to hundreds of millimeters have been reported. Besides its low cost and tiny size, polymer MNFs are easy to couple with other general optical devices, enabling them to fulfill their potentials in optical functions. Due to their good flexibility and large tunability, many shapes and structures can be achieved from polymer MNFs, including optical splitters [18,30], resonator , couplers , Mach–Zehnder Interferometers (MZI) , light-emitting polymer nanofibers , photodetector , organic nanofiber laser , sensors [37,38], polymeric nanofiber architecture .
In this chapter, we will introduce the fabrication techniques of polymer MNFs. Then we will focus on polymer MNF-based elements and assess their potential used as passive and active components in miniaturized photonic devices. Final is a perspective.
2. Fabrication methods of polymer MNFs
A number of different techniques including physical, chemical, thermal, and electrostatic method have been used to fabricate polymer MNFs. Ten of these fabrication methods are briefly described in the following section .
Electrospinning is an electrostatically driven method of fabricating polymer nanoﬁbers. Nanofibers are formed from a liquid polymer solution or melt that is feed through a capillary tube into a region of high electric field . The electric field is most commonly generated by connecting a high voltage power source in the kilovolt range to the capillary tip (Fig. 1). As electrostatic forces overcome the surface tension of the liquid, a Taylor cone is formed and a thin jet is rapidly accelerated to a grounded or oppositely charged collecting target. Instabilities in this jet cause violent whipping motions that elongate and thin the jet allowing the evaporation of some of the solvent or cooling of melts to form solid nanoﬁbers on the target site. Nanoﬁber size and microstructure can be controlled by several processing parameters including: solution viscosity, voltage, feed rate, solution conductivity, capillary to collector distance, and oriﬁce size . The electrospinning technique is very versatile and a wide range of polymer and copolymermaterials with a wide range of fiber diameters (several nanometers to several microns) can be fabricated using this technique. Many different types of molecules can be easily incorporated during the electrospinning fabrication process to produce functionalized nanofibers. Electrospun nanoﬁbers are usually collected from an electrospinning jet as non-woven randomly or uniaxially aligned sheets or arrays.
2.2. Phase separation
Nanofibrous foam materials have been fabricated by a technique called thermally induced liquid-liquid phase separation . This fabrication procedure involves (a) the dissolution of polymer in solvent (b) phase separation and polymer gelatination in low temperature (c) solvent exchange by immersion in water and (d) freezing and freeze-drying (Fig. 2). The morphology of these structures can be controlled by fabrication parameters such as gelatination temperature and polymer concentration. Interconnected porous nanofiber networks have been formed from polymers such as, poly-L-lactide acid (PLLA), poly-lactic-co-glycolic acid (PLGA), and poly-DL-lactic acid (PDLLA) with fiber diameters from 50–500 nm, and porosities up to 98.5%.
2.3. Self assembly
Self assembly is a process by which molecules organize and arrange themselves into patterns or structures through non-covalent forces such as hydrogen bonding, hydrophobic forces, and electrostatic reactions. Dialkyl chain amphiphiles containing peptides were developed to mimic the ECM. These peptide amphiphiles (PA), derived from a collagen ligand, allow for a self assembling system that consists of a hydrophobic tail group and a hydrophilic head group . Figure 3 shows the hierarchy of structure found in the self-assembled PA nanofiber networks . The specific composition of amino acid chains in peptide amphiphile systems determines the assembly, chemical, and biological properties of the system, and therefore PA systems can be tailored to specific applications [45,46]. Nanofibers with diameters around 5–25 nm can be formed by the self assembly process. Cells can be encapsulated in a nanofibrous PA structureif they are added during the self assembly process and PA can also be injected in vivo where they subsequently self assemble into a nanofibrous network. It has been demonstrated that self assembled peptide nanofibers can spontaneously undergo reassembly back to a nanofibers scaffold after destruction by sonication, and after multiple cycles of destruction and reassembly, the peptide nanofibers scaffolds were still indistiquishable from their original structures .
2.4. One-step drawing technique
Nanofibers can be mechanically drawn from viscous polymer liquids directly . In one example, nanofibers were drawn directly when a rod was placed in a polymer melt and moved up forming a thin filament that cooled to form a nanofiber . Figure 4 shows the schematic illustration of the drawing process. Fig. 4a shows a vertical direction tip-drawing process. Poly(trimethylene terephthalate) (PTT) pellets (melt temperature Tm = 225°C) was melt by a heating plate and thetemperature was kept at around 250°C during the wire drawing. First, an iron or silicarod/tip with radius of about 125 μm is being approached and its tip is immerged into the molten PTT. Then the rod tip is retracted from the molten PTT with a speed of 0.1−1 m/s, leaving a PTT wire extending between the molten PTT and the tip. The extended PTT wire is quickly quenched in air and finally, a naked amorphous PTT nanowire is formed. PTT nanofibers with diameters as low as 60 nm, and lengths up to 500 mm have been achieved. A SEM image (Fig. 4b) shows part of the coiled nanofiber with a length of about 200 mm and an average diameter of 280 nm. The diameter variation ratio is about 8.4x10-8. Figure 4c demonstrates flexible and elastic connection by pulling the polymer nanofibers with diameters of 140 and 170 nm. To examine surface roughness of the polymer nanofibers, high-magnification transmission electron microscope (TEM) was done. Figure 4d shows a TEM image of a 190-nm-diameter nanowire, indicating no visible defect and irregularity on the surface of the polymer nanofiber. Typical average sidewall root-mean-square roughness of the polymer nanofiber is 0.28 nm. The electron diffraction pattern (inset of Fig. 4d) demonstrates that the obtained PTT MNF is amorphous. The results demonstrate that the obtained polymer nanofibers exhibit high surface smoothness, length uniformity, high mechanical properties, and excellent flexibility.
Polymer nanofibers can be fabricated using templates such as self-ordered porous alumina. Alumina network templates with pore diameters from 25 to 400 nm, and pore depths from around 100 nm to several 100 µm have been be fabricated. Polymer nanofiber arrays can be released from these molds by destruction of the molds or mechanical detachment (Fig. 5) [49,50]. The length of polycaprolactone (PCL) nanofibers fabricated from alumina templates can be controlled as a function of parameters such as melt time and temperature .
2.6. Vapor-phase polymerization
Polymer MNFs have also been fabricated from vapor-phase polymerization. Plasma-induced polymerization of vapor phase vinyltrichlorosilane produced organosiloxane fibers with diameters around 25 nm and typical lengths of 400–600 nm and cyanoacrylate fibers with diameters from 100 to 400 nm and lengths of hundreds of microns (Fig. 6) [52,53].
Nanofibers can be extracted from natural materials using chemical and mechanical treatments. Cellulose fibrils can be disintegrated from plant cell walls. In one example, cellulose nanofibers were extracted from wheat straw and soy hull with diameters ranging from 10 to 120 nm and lengths up to a few thousand nanometers (Fig. 7) . Invertebrates have also been used as a source for the extraction of nanofibers. Chitin nanofibers 3−4 nm in diameter and a few micrometers in length were extracted from squid pen and Poly-N-acetyl glucosamine nanofibers isolated from a marine diatom demonstrated prothrombotic interactions with red blood cells [55,56].
2.8. Conventional chemical oxidative polymerization of aniline
Chemical oxidative polymerization of aniline is a traditional method for synthesizing polyaniline and during the early stages of this synthesis process polyaniline nanofibers are formed (Fig. 8). Optimization of polymerization conditions such as temperature, mixing speed, and mechanical agitation allows the end stage formation of polyaniline nanofibers with diameters in the range of 30–120 nm [58,59].
2.9. Bacterial cellulose
Cellulose nanofibers produced by bacteria have been long used in a variety of applications, including biomedical applications . Cellulose synthesis by Acetobacter involves the polymerization of glucose residues into chains, followed by the extracellullar secretion, assembly and crystallization of the chains into hierarchically composed ribbons (Fig. 9). Networks of cellulose nanofibers with diameters less than 100nmare readily produced, and fibers with different characteristics may be produced by different strains of bacteria . Copolymers have been produced by adding polymers to the growth media of the cellulose producing bacteria [50,51].
2.10. Kinetically controlled solution synthesis
Nanofibers and nanowires have been fabricated in solution using linear aligned substrates as templating agents such as iron-cation absorbed reverse cylindrical micelles and silver nanoparticles . Poly(vinyl alchohol)-poly(methyl methacrylate) nanofibers were fabricated using silver nanoparticle that were linearly aligned in solution by vigorous magnetic stirring (Fig. 10) . These nanoparticle chain assemblies acted as a template for further polymerization of nanofibers with diameters from 10 to 30 nm and lengths up to 60 µm.
3. Polymer MNF based photonic components and devices
Optical fiber based components and devices have been very successful in the past 30 years and will surely continue to thrive in a variety of applications including optical communications, optical sensing, power delivery and nonlinear optics [63-65]. With increasing requirements for higher performance, wider applicability and lower energy consumption, there is a strong demand for the miniaturization of fiber-optic components or devices. When operated on a smaller spatial scale, a photonic circuit can circulate, process and respond to optical signals on a smaller time scale. Only at wavelength or subwavelength size does the photonic structure manifest evident near-field features that can be utilized for interlinking and processing optical signals highly efficiently. For example, it was estimated that to reach an optical data transmission rate as high as 10 Tb/s, the size of photonic matrix switching devices should be reduced to 100-nm scale . At the same time, to perform a given function that relies on a certain kind of light-matter interaction, usually less energy is required when smaller quantities of matter are involved. MNFs featured at subwavelength scale, provide a number of interesting properties such as mechanical flexibility, high optical cross-sections, large and ultrafast nonlinear responses, and broad spectral tunability that are highly desirable for functionalizing high density and miniaturized PICs . Besides its low cost and tiny size, MNFs are also easy to couple with other general optical devices, providing excellent compatibility with standard optical fiber systems.This section gives an up-to-date review of polymer MNF based photonic components/devices that have been investigated very recently.
3.1. Polymer MNF-based passive components/devices
3.1.1. MxN coupling photonic splitters and sensors
X. Xing et al. have demonstrated PTT nanofibers with high surface smoothness, diameter uniformity, as well as high mechanical strength and excellent flexibility, make them promising candidates for building blocks to construct ultracompact photonic devices and device arrays [17,18]. Fig.11a shows a 4x4 photonic coupling splitter assembled by twisting four PTT nanofibers with diameters of 450, 450, 510, and 570 nm for branches A to D . The inset of Fig. 11a shows that the coupling section is composed of a 3x4 and a 1x4 couplers, where the total width of the coupling section is 1.98 μm. The maximum length of the coupling regionis about 16.1 μm, and that for the 1x4 splitter is about 8.5 μm. When two red lights (650 nm) are simultaneously launched into the branches A and B, the measured splitting ratio is 29182924. As shown in Fig. 11b, blue light (532 nm) are coupled into the branchG of an 8x8 coupling splitters and divided into branches 1 to 8.The coupling section of the splitter (Fig. 11b, inset) is 38-μm-long and 2.5-μm-wide. We also launched other visible lights into the devices to observe the splitting phenomenon. Experimental demonstration shows that the properties of the splitters are dependent on the operation wavelength and the input branch which the optical signal launched into. For a fixed operation wavelength and the input branch, desirable splitting ratio can be tuned by controlling input/output branching angle.
As shown in Fig. 11c-e, a tunable refractive index sensor with ultracompact structure in a 2x2 PTT nanofiber coupling splitter assembled by twisting two flexible PTT nanowires with diameter of 400 nm . The sensor consists of two input branches, a twisted coupling region, and two output branches. The changes of optical power caused due to variations in the surrounding medium around the twisted coupling region were measured in the output branches.The highest measured sensitivity of the sensor is 26.96 mW/RIU (refractive index unit) and the maximum detection limit on refractive index change is 1.85x10-7.
3.1.2. Cascaded MZI
Figure12 shows an optical microscope image of the assembled two-cascaded MZI (wire diameter, 900 nm) . The inset (a) showsa scanning electron microscope image of MZI 1 and the inset (b) shows guided red light (650 nm) in the cascaded MZI. Themeasured insertion loss is about 0.94 dB for the red light. The total length of the cascaded MZI is 327 μm. The width and length of each bow-shaped MZI are 32 μm and 121 μm, respectively. According to the analysis, to get coupling ratios of 0.147, 0.501, and 0.147, the lengths of the couplers C1, C2, and C3 are 27, 31, and 27 μm, respectively. The estimated totalpath-length difference is 40 μm. The bright spot in the inset (b) of Fig. 12 is the scattering spot of the input light at the end of the tapered fiber I. The average bandwidth of 3-dB pass-band is measured to 33 nm for the cascaded MZI over the wavelengthsof 1.3 to 1.6 μm. The measured extinction ratio for the cascaded MZI is 16 to 19 dB, and optical insertion loss is 1.1 to 1.8 dB at wavelengths of 1.3 to 1.6 μm.This is good for band-pass filter applications.
3.1.3. Ring resonator
Owing to relatively high quality (Q) factor and ease off abrication, fiber-based microrings were widely used for resonators. Y. Wang et al. have assembled microring resonators by knotting a uniform PTT wire with micromanipulator assistance under an optical microscope. Subsequently, two ends of the microring resonator were fixed on two branches of a tunable microstage as schematically shown in Fig. 13a. Then an input microtaper and an output microtaper were used to launch and collect optical signals, respectively, by evanescent wave coupling. In Fig. 13a, when we adjust the tunable microstage to the right, the wire will be pulled tightly and finally, the ring radius will become smaller. The red arrows in Fig. 13a show the propagation directions of the optical signals while the yellow one indicates the moving direction of the tunable stage. As an example, Fig. 13b shows a SEM image of the microring with aradius of 70 μm, which was assembled by the PTT wire with adiameter of 3.5 μm.
Three tunable microrings were assembled by PTT wire withdiameters of 3.5, 2.5, and 2.0 μm, respectively. Ring radius and wire diameter dependent optical properties were demonstrated by measuring the FSRs and the Q factors and showed that, the maximum Q factors are 28090, 28071, and 23528 at the ring radii of 417 (wire diameter, 3.5 μm), 258 (wire diameter, 2.5 μm), and 204 (wirediameter, 2.0 μm) μm, respectively. Their corresponding FSRs are 0.55, 0.92, and 1.19 nm. The FSR is mainly dependent on the ringradius and little affected by the wire diameter. By using anappropriate wire diameter with an appropriate ring radius, amaximum Q can be obtained with a desirable FSR. The tunable microring resonators would be useful for optical filters and sensors. Also, the assembly method used in this work could become acandidate for fabricating highly-integrated photonic devices basedon micro/nano-wire rings.
3.1.4. Arbitrary and vertical optical couplers using flexible polymer MNFs
The polymer MNFs were directly drawn from a polymethyl methacrylate (PMMA) solution in which acetone was used as a solvent. To investigate the possibility of arbitrary and vertical optical couplers, different topologies were assembled. Figures 14a, b show the optical microscope images of two crossed interconnect structures with different cross angles. The structure of Fig. 14a as assembled by two polymer fibers with diameters of 1.05 μm (wire 1) and 1.00 μm (wire 2). Cross angle between channels C1 and C2 is 57° while the cross angle between channels C3 and C4 is 42°. Figure 14a shows that a light (wavelength λ=650 nm) was launched into channel C1 and divided into channels C2, C3, and C4. The output powers were collected using the tapered fibers. For this structure, 4% optical power was divided into channel C2. The powers divided into channels C3 and C4 are 41% and 42%, respectively. The total insertion loss is 0.6 dB. Figure 14b further shows a crossed interconnect structure, which was assembled by two wires with diametersof 1.18 μm (wire 1) and 1.40 μm (wire 2). The cross angle between channels C1 and C2 is 55° and that between channels C3 and C4 is 89°. For this structure, when the light (λ=650 nm) was launched into channel C1, the measured powers at channels C2, C3, and C4 are 1%, 39%, and 42%, respectively. The total insertion loss is 0.86 dB. Figure 14c shows the optical microscope image of the star topology network with five channels. It was assembled by twisting three wires with diameters of 1.08 μm (wire 1), 917 nm (wire 2), and 970 nm (wire 3). The cross angle between channels C1 and C2 is 29°, that between channels C2 and C3 is 57°, between channels C3 and C4 is 110°, and between channels C4 and C5 is 35°. When the light (λ = 650 nm) was launched into channel C2, the measured powers in the channels C3, C4, and C5 are 1%, 40%, and 36%, respectively. The total insertion loss is 1.14 dB. Figure 14d shows a tree topology network, which was assembled by three wires with diameters of 1.17 μm (wire 1), 1.37 μm (wire 2), and 1.20 μm (wire 3). The cross angle between channels C1 and C3 is 87° and that between channels C4 and C5 is 39°. When the light (λ = 650 nm) was launched into channel C2, it was dividedinto channels C1 and C3 by the first node A with a splitting power ratio of 0.41:0.40. At the second node B, the light inchannel C3 was divided into channels C4 and C5 with a splitting power ratio of 0.16:0.17. The total insertion loss for this structure is 1.31 dB. By using the structures shown in Figs. 14a–c, optical channels can be further increased while keeping one node. This is very useful for ultracompact arbitrary and vertical optical interconnect structures. We hope that the arbitrary and vertical coupling structures could find applications in high-density PICs and miniaturized optical networks.
3.2. Polymer MNFs as active waveguides
The development of miniaturized PICs are challenging directions in next generation all-optical signal processing, in which light-emitting sources are important elements for integration [68,69]. In the past decades, doped glass optical fibers have been widely used as solid hosts for photonic applications such as tunable lasers and amplifiers for optical communication [70,71]. For most widely used rare-earth-doped glass fibers, the concentrations of rare-earth dopants are limited by the concentration quenching, for example, typically lower than 1% for Er3+ in silica fibers, resulting in relatively low energy conversion efficiency within a limited length. Therefore, it is difficult to obtain efficient energy conversion in doped glass fibers within a scale comparable to the compactness of a nanophotonic integrated system. Compared to those of glass fibers, the most attractive prospects of polymer MNFs are as follows: First, the polymer matrix can host functional dopants ranging from metal oxides, quantum dots (QDs), and fluorescent dyes to enzymes that can be used to tailor the properties of the MNFs with greater versatility. Second, the dopants can be easily doped into the solvated polymer and drawn into polymer MNFs at room temperature with higher doping concentration than that in glass fibers. Also, the mechanical flexibility, perm-selective nature to gas molecules, biocompatibility, easy processing, and low cost of the polymer materials  offers more opportunities for doped polymer MNFs over doped glass fibers in nanophotonic systems. Especially, high efficient light emission dopants (e.g., fluorescent dyes and QDs), which are much more compatible with polymer MNFs, show great potential to realize compact light-emitting sources with feature sizes acceptable in nanophotonic integrated systems.
3.2.1. Fluorescent dye doped polymer MNFs
In 2010, F. Gu et al. reported light-emitting polymer nanofibers based on waveguiding excitation . By waveguiding excitation light along the polymer nanofiber, the interaction of light with polymer nanofiber is enhanced over 3 orders of magnitude compared with the currently used irradiating excitation. Intriguing advantages such as enhanced excitation efficiency, low excitation power operation down to nW levels, tightly confined excitation with low cross talk, and high photostability of the light-emitting polymer nanofibers are obtained. The waveguiding excitation allows incorporation of various fluorescent dyes into polymer nanofibers to generate multicolor emitting sources covering the entire visible spectrum.
Figure15a shows a photoluminescence (PL) microscope image of a 380-nm-diameter 520-µm-length RhB-PS nanofiber taken with a long-pass emission filter . Upon 473-nm laser (λex) launched from the left side with excitation power (Pex) of 70 nW, bright fluorescent emission with a peak (λem) around 578 nm is generated and guided along the nanofiber. For reference, Fig. 15b shows the PL image taken without the emission filter, in which a small light spot at the output end of the nanofiber and no obvious scattering along the nanofiber are observed, suggesting that the excitation light was efficiently absorbed during its waveguiding excitation. The absorption spectrum of the RhB-PS nanofiber in Fig. 15c (black line) exhibits a peak at 560 nm due to the absorption of the doped RhB molecules. The measured absorption coefficient α of the RhB-PS NFs at 473 nm is ~50 cm-1.
3.2.2. Quantum-dot-doped Polymer MNFs
H. Liu et al. incorporated semiconductor QDs as integrated light sources into the polymer fiber to produce subwavelength-diameter optical waveguides . Fibers of polymeric photoresist were fabricated with diameters down to 50 nm and lengths up to tens of millimeters using an electrospinning method, and QDs consisting of CdSe/ZnS (core/shell) were incorporated into the polymer fibers. A fluorescence optical image (Fig. 16a), taken by uniformly illuminating the fiber by a mercury lamp light source and imaging the 605 nm wavelength fluorescence, shows the distribution of luminescence along the fiber length. The image suggests that the QDs were not uniformly distributed along the fiber. Nanometer-sized QDs were clearly recognizable, and were observed to be concentrated in a constrained region as a strand parallel to the fiber axis (Fig. 16b).
3.2.3. QD-Decorated Polymer MNFs
H. Yu et al.developed a one-step process to decorate PTT nanofiber with the CdSe/ZnS core/shell QDs . The QDs-decorated nanofibers with diameters of 400800 nm can be used as active subwavelength waveguides with some advantages such as good photostability, low excitation power operation (less than 0.2 μW), low propagation loss (11.3 dB/cm), low absorption coefficient (down to 2.6 cm1), and 200 times enhancement in excitation efficiency excited by the evanescent waveguiding excitation than that of by the irradiation excitation.
Figure 17a shows an optical microscope image of adrop of a rugby-ball-like QDs-decorated cross structure, composing of a PTT nanofiber 1 (495 nm) and a PTT nanofiber 2 (590 nm), excited by the EW excitation from laser B at Pex = 0.1 µW. It can be seen that red light was excited in the rugby-ball-like QDs. Figure 17b shows the optical microscope image of the cascaded rugby-ball-like QDs-decorated nanofiber with diameter of 597 nm, in which there are seven rugby-ball-like QDs. The lengths of the QDs B1 to B7 are 2.3, 2.2, 2.3, 2.2, 1.8, 2.0, and 2.0 µm, respectively. Their corresponding maximum widths are 1.6, 1.5, 1.6, 1.6, 1.0, 1.1, and 1.1 µm. The average length of the rugby-ball-like QDs is 2.1 µm and the average value of maximum width is 1.4 µm. Figure 17c shows the corresponding optical microscope image with seven red spots excited by the 473 nm blue light fromlaser B at Pex = 1.2 μW.
3.3. Polymer nanofiber laser
3.3.1. Dye Doped Polymer MNF Knot Resonator Laser
Recently, many kinds of microsized polymer fiber and dye doped polymer fiber were also successfully demonstrated. Q. Song et al. reported the lasing action in a dye doped PNF knot ring resonator . The dye doped polymer nanofiber was fabricated by general fiber drawing technique. Then it was bent to a knot ring resonator and fixed onto glass tubes (the inset of Fig. 18a), which can increase the stability of the knot resonator and fix its diameter . Laser emission was observed. The emission spectrum of knot resonator is shown in Fig. 18b. Here the pump intensity is 4.2 mJ/cm2. Periodic peaks can be observed in the laser spectrum. This group of peaks is considered to come from the whispering galley mode resonance inside the ring resonator. The linewidth of laser peaks is only 0.07 nm and the mode spacing in Fig. 18 is about 0.206 nm.
3.3.2. Random lasering in a singleorganic Nanofiber
One-dimensional light random lasing in individual p-sexiphenyl nanofibers (p-6P) is investigated by F. Quochi et al . The laser action happens in single p-6P nanofibers grown on (001)-oriented muscovite mica. Isolated nanofibers are shown to yield low-threshold random laser emission in the deep blue. Random lasing from isolated nanofibers starts at pump fluences on the order of 10 µJ/cm2 per pulse. Lasing nanofibers are demonstrated. Figure 19a shows a lasing micrograph taken in imaging mode slightly above threshold. It displays both lasing from vertically aligned nanofibers and spontaneous emission from a set of neighboring nanofibers oriented approximately at 60° with respect to the vertical axis of the detection system. The latter faintly appear in the lower part of the graph. Scattering of the lasing emission into out-of-plane directions does not take place homogeneously along the nanofibers’ axis; conversely, scattering is highly spotted, indicating that wave-guiding is interleaved with light scattering and outcoupling at special sites along the fibers. Emission spectra relating to the ~100 µm long nanofiber placed at the center of the imaging field of view in Fig. 19a are reported in Fig. 19b. They refer to the emission spatially integrated over the whole nanofiber length. Below threshold, spontaneous emission exhibits a broad vibronic progression with 0–1 and 0–2 emission bands peaked near 425 and 450 nm, respectively.
3.3.3. Optically pumped lasing in single conjugated polymer nanowires
Conjugated polymers have chemically tuneable opto-electronic properties and are easily processed, making them attractive materials for photonics applications [75,76]. Conjugated polymer lasers, in a variety of resonator geometries such as microcavity, micro-ring, distributed feedback and photonic bandgap structures, have been fabricated using a range of coating and imprinting techniques [77-80]. D. O’Carroll et al. reported the first observation of optically pumped lasing in single conjugated polymer nanowires . The waveguide and resonator properties of each wire are characterized in the far optical field at room temperature. The end faces of the nanowire are optically flat and the nanowire acts as a cylindrical optical cavity, exhibiting axial Fabry–Pérot mode structure in the emission spectrum. Above a threshold incident pump energy, the emission spectrum collapses to a single, sharp peak with an instrument-limited line width that is characteristic of single-mode excitonic laser action. Fluorene-based conjugated polymers are attractive.
Fluorene-based conjugated polymers are attractive photonic materials because they exhibit high photoluminescence quantum efficiencies, large stimulated emission crosssections and chemically tuneable emission wavelengths . Poly(9,9-dioctylfluorene) (PFO) is a prototypical main-chain liquid-crystalline homopolymer that emits in the blue and exhibits polymorphic behaviour, with striking implications for its photophysical properties . Isolated PFO nanowires were uniformly excited by the 355-nm output of a 0.7 ns, 1.25 kHz pulsed Nd/YVO laser and spatially resolved photoluminescence spectra were acquired from the bodies and tips (Fig. 20a). To confirm that the wires operated as axial Fabry–Pérot microcavities, the mode spacing at 460 nm was plotted versus the inverse nanowire length for 14 wires, and was shown to exhibit a linear dependence (Fig. 20b). The number of Fabry–Pérot modes per guided mode that could propagate in a nanowire microcavity was estimated from Δλspont/Δλm, where Δλspont is the full-width-at-half-maximum, FWHM, of the spontaneous emission and is ~19 nm.
Single isolated PFO nanowire microcavities were then uniformly excited (355 nm, 0.7 ns, 1.25 kHz) and tip emission spectra were collected as a function of pump energy (Fig. 21a). At lower pump energies, tip spectra exhibited nanowire microcavity emission, with pronounced Fabry–Pérot modes apparent at the 0–1 peak. Above an energy threshold of ~100 nJ (2.8 mJ cm–2), a single spectrally narrowed emission peak developed by preferential gain in a single Fabry–Pérot mode and the onset of lasing. A slight blue shift in peak position with increasing pump energy suggested that stimulated emission occurred on timescales comparable to or faster than exciton energy migration. Concerning the dependence of tip emission intensity on pumpenergy, below the energy threshold, emission increased linearly with excitation energy (Fig. 21b). Above threshold, a kink in emission output was followed by a super-linear increase due to optical gain. Also, the emission peak width narrowed from 19.6 nm to 1.4 nm (instrument-limited) at threshold, indicating the high quality factor of the nanowire cavity.
3.4. Polymer nanofiber photodetector
Semiconducting polymers are attractive materials due to their chemically tunable optical and electronic properties, as well as their facility for solution processing [84,85]. Garret A. O’Brien et al. have demonstrated that solution-assisted template wetting may be successfully exploited for high-yield controlled synthesis of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(bithiophene)] (F8T2) nanofibes and employed single nanowire devices as ultraminiature photodetectors.  Discrete nanowires with average lengths of 15 µm and mean diameters of 200 nm has been fabricated. As expected, structural data point to a low degree of crystallinity within the wires. Individual nanowires can be electrically interfaced using either bottom- or top-contact geometries. Top-contacted single-nanowire devices with interelectrode gaps of approximately 5 μm were fabricated on glass substrates using shadow masking and gold evaporation, see Fig. 22a. The blue curve in Figure 22b shows the measured dark current (Idark) for a typical device. The data show quasilinear characteristics at low bias, similar to measured data for bottom-contacted nanowires, with some asymmetry at higher bias. The red curve in Fig. 22b shows the measured current (Iillum) under continuous 405 nm illumination. A marked increase in the measured current is observed across the entire bias range. The green curve in Fig. 22b shows the measured current under manually chopped 405 nm illumination, where the illumination was switched on or off at 10 V intervals during the bias sweep, corresponding to a duty cycle of 50 %. The present photoconductivity measurements of F8T2 nanowire devices, which yield single-nanowire responsivities of approximately 0.4 mAW–1 and external quantum efficiencies of approximately 0.1% under monochromatic illumination; these values are comparable with data reported for single-inorganic-nanowire devices. The results demonstrate the promise of these novel nanostructures as ultraminiature photodetectors with the potential for integration into future hybrid nanophotonic devices and systems.
3.5. Nanopatterning of single light-emitting polymer nanofibres
Polymer nanofibers are compatible with sub-micrometre patterning capability and electromagnetic confinement within subwavelength volumes [2,86], they can offer the benefits of organic light sources to nanoscale optics. F. D. Benedetto et al. reported on the optical properties of fully conjugated, electrospun polymer nanofibres, demonstrated the enhancement of the fibre forward emission through imprinting periodic nanostructures using room-temperature nanoimprint lithography, and investigate the angular dispersion of differently polarized emitted light . Fibers with diameters intentionally produced to be in the range 0.5–5 μm are used to collect reliable fluorescence micrographs and photoluminescence spectra. Confocal microscopy (Fig. 23a) on nanopatterned fibres shows uniformly bright imprinted gratings. The spectra of a typical poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) fibre are displayed in Fig. 23b. in the emission of the untextured fibre the two replicas exhibit almost the same relative intensity, that is, a relative decrease of the high-energy spectral component (λ < 600 nm). Different angular dispersion for light with s- and p-polarization, namely with the electrical field vector parallel or perpendicular to the grating grooves and corresponding to transversal electric (TE) or transversal magnetic (TM) guided modes, is demonstrated in Fig. 23c, respectively. The two modes are concomitantly present in the Bragg outcoupled emission, with TM peaks blueshifted by 18–46 meV with respect to the TE peaks, indicating a lower effective refractive index (neff) for light polarized perpendicularly to the nanoimprinted features. For an MEH-PPV nanopatterned fibre, neff for TM and TE light monotonously decreases from ~1.9 to 1.2 upon increasing the wavelength from 570 to 575 nm, whereas the difference, Δneff = neff,TE –neff,TM, slightly increases from 0.01 to 0.04 (Fig. 23d).
3.6. Optical sensors
Single-nanowire detection presents special advantages of high sensitivity and fast response and may offer potentials for highly localized sensing with small footprint and high spatial resolution, as have been recently demonstrated in single-polymer-nanowire electrical sensors [88,89]. And optical sensing offers potentials of high sensitivity, fast response, immunity to electromagnetic interference, and safe operation in explosive or combustive atmosphere, as well as more options for signal retrieval from optical intensity, spectrum, phase, polarization, and fluorescence lifetime . L. Tong group have demonstrated that polymer single-nanowire optical sensors with extraordinary fast response and high sensitivity for humidity and gas sensing [37,91].
When blended or doped with other functional materials,polymer nanowires can be used for optical sensing with high versatilities. For instance, here a 250-nm-diameter PANI/PS nanowire was employed for gas sensing. The nanowire is drawn from a polymer-blend solutionof 2 wt % PANI doped with 10-camphorsulfonic and 5 wt% PS in chloroform and is suspended by a 250-μm-width MgF2 microchannel and optically connected to fiber tapers at both ends. The sensor is operated by applying a nitrogen-diluted NO2 gas onto the nanowire witha probing light of 532-nm wavelength. When exposed to NO2, the increase of the oxidation degree of PANI resultsin spectral absorption at the wavelength of the probing light, in which the absorbance is proportional to the degree of the oxidation that increases with the concentration of NO2. Figure 24a shows a typical response of a 250-nm-diameter PANI/PS nanowire to 1 ppm NO2. A clear absorbance is observed,with a response time of about 7s, which is orders ofmagnitude faster than in other types of NO2 sensors . The time-dependent absorbance of the nanowire at roomtemperature to cyclic NO2/nitrogen exposure with NO2 concentration from 0.1 to 4 ppm is given in Figure 24b, indicating good reversibility of the nanowire response. The linear dependence of the absorbance over the NO2 concentration(see inset) suggests that the PANI/PS nanowire couldfunction as a NO2 optical sensor with a detection limit below 0.1 ppm.
For optical sensing, we suspended a QD/PS nanofiber across a 245-μm-wide MgF2 microchannel with two ends of the NF coupled to fiber tapers for optical launching and signal collection, as schematically illustrated in Figure 25a. For robustness, the coupling region between the QD/PS NF and fiber tapers was bonded to the substrate using low-index UV-cured fluoropolymer (EFIRON PC-373; Luvantix Co. Ltd.), and the sensing element was sealed inside a glass chamber. To operate the sensor for humidity sensing, 532 nm excitation light was launched from the left-hand fiber taper, and measured the light output from the right-hand fiber taper, while changing the surrounding relative humidity (RH) from 7% to 81% by circulating moisture gases insidethe chamber. The optical response of the nanofiber sensor is shown in Fig. 25b, which works well with the excitation power ofabout 100 pW used here. The RH-dependent PL intensity can be attributed to the passivation of surface trap states of QDs by water molecules [93-95]. The monotonic increase of the PL output with increasing RH can be used for RH sensing, with estimated resolution (calculated from the response curve in Figure 25b) better than 1% RH. Excellent reversibility of the nanofiber sensor was obtained on alternately cycling 19% and 54% RH air, as shown in Figure 25c. The instant response of the sensor was investigated by introducing sudden changes of the humidity in the chamber, with measured response time less than 90 ms (Figure 25d), which is 1–2 orders of magnitude faster than those of RH sensors based on films ormonolayers [96–98]. In addition, the PL intensity of the CdSe/ZnS QD-doped PS matrix was also found to be sensitive to otherspecies, such as CN-ions ; the nanofiber sensor proposed here is promising for optical detection of many other types of samples.
4. Polymeric nanowire architecture as full-color micro/nanodisplays
Recently, a number of crossed nanofiber structures for full-color micro/nanodisplays has been reported by H. Yu et al.  The crossed array structures were formed by assembling flexible PTT nanofibers under an optical microscope with the assistance of micromanipulators while avoiding the use of color filters and complicated photolithography processing. The color pixels of the displays consist of micro/nanometer sized color spots in a radius of 3001500 nm. The colors and sizes of the spots were tuned by changing the total power and power ratios of the launched red, green, and blue (RGB) lights.
As shown in Fig. 26a, a 3x3 structure was assembled by using nanofibers 1, 2, and 3 with diameters of 558, 598, and 598 nm, respectively, and RGB lights were launched into the nanofibers 1, 2, and 3, respectively, with a total power of 63.4 μW and a power ratio of about 50:13: 10. Inset of Figure 25b shows that a white colour at the centre of the spot with a radius of about 818 nm was observed at the crossed junction. The colour coordinate was (0.41, 0.35). Its correlated colour temperature (CCT) is 3026 K.
5. Potential to construct integrated optical circuit
The requirement for any photonic device in optical computing, data communications or telecommunications is to be robust, lowcost, high speed and easy to integrate. Polymer MNFs are an ideal material for assembling structures that address both of these requirements, as they offer lowcost processing and are easy to integrate into existing and future networks made either entirely from polymeric or a combination of polymeric with semiconductive. Inaddition, polymer MNFs posses many other advantages such as versatility for biofunctionalization, and the promotion of speciﬁcdesired cell behaviors that are elicited by the nanofibers architecture. Polymer MNF shave can be easily produced by current technique, which exhibiting many advantages such as mechanical flexibility, perm-selective nature to gas molecules, biocompatibility, easy processing, and low cost. It is demonstrated that polymer nanofibers offer a unique materials platform for producing key photonic elements including lasing, light emitting diode, detectors, cavity resonator, and passive elements. All these indicate that polymer MNFs is an promising candidate for integrated optical circuit with high density and multifunction. As an excellent research platform merging ﬁber-optic technology and nanotechnology, there is no doubt that polymer MNFs will continue to open up new opportunities in broad areas including micro- and nanoscale photonics, nonlinear optics and quantum optics in the near future.
The work thanks for Prof. Baojun Li from State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-Sen University, for his guidance. This work was supported by China’s National Natural Science Foundation (6117707, 81071790, 11104162), China Postdoctoral Science Foundation Funded Project (201003359), Guangdong Excelent Doctoral Dissertation Funded Project (SYBZZM201126), the Key Project of Chinese Ministry of Education (211131), the Science and Technology Project of Guangzhou Nansha District (RG201001003).