**3.1. Polymer MNF-based passive components/devices**

#### *3.1.1. MN coupling photonic splitters and sensors*

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**Figure 9.** (a) Schematic of Acetobacter cells depositing cellulose nanofibers, (b) an SEM image of a

**Figure 10.** (a) Schematic of silver nanoparticle embedded polymer nanofibers fabrication (b) SEM and

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 [66]. 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 [67]. 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

TEM images of silver nanoparticle embedded Polymer nanofibers [62].

components/devices that have been investigated very recently.

**3. Polymer MNF based photonic components and devices** 

cellulose nanofiber mesh produced by bacteria [60].

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 44 photonic coupling splitter assembled by twisting four PTT nanofibers with diameters of 450, 450, 510, and 570 nm for branches A to D [18]. The inset of Fig. 11a shows that the coupling section is composed of a 34 and a 14 couplers, where the total width of the coupling section is 1.98 m. The maximum length of the coupling regionis about 16.1 m, and that for the 14 splitter is about 8.5 m. 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 88 coupling splitters and divided into branches 1 to 8.The coupling section of the splitter (Fig. 11b, inset) is 38-m-long and 2.5-m-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 22 PTT nanofiber coupling splitter assembled by twisting two flexible PTT nanowires with diameter of 400 nm [19]. 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 1.8510-7.

**Figure 11.** Optical microscope images [18,19]: (a) A 44 photonic coupling splitterwith diameters of 450, 450, 510, and 570 nm for branches A to D when red light (650 nm) was launched into branches of A and B. (b) An 88 photonic coupling splitter of guided blue light (532 nm) with diameters of 400, 400, 400, 400, 400, 750, 750, and 600 nm for branches A to H. The inset of Fig. a and b shows the coupling section of devices. (ce) the sensor (400-nm-diameter) taken with (c) red light, (d) green light (532 nm), and (e) blue light (473 nm) (without sample solution).The white arrows show the propagation directions of the launched lights.

## *3.1.2. Cascaded MZI[21]*

Figure12 shows an optical microscope image of the assembled two-cascaded MZI (wire diameter, 900 nm) [21]. 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.

**Figure 12.** Optical microscope image of the assembled cascaded MZI (wire diameter, 900 nm) [21]. Inset (a) shows the scanning electron microscope image of bow-shaped MZI 1 and inset (b) shows the optical microscope image of guided red light (650 nm) in the cascaded MZI. Estimated path-length difference is 40 m. The white arrow in the inset (b) indicates propagation direction of the light.

#### *3.1.3. Ring resonator[31]*

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.

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*3.1.2. Cascaded MZI[21]* 

*3.1.3. Ring resonator[31]* 

with adiameter of 3.5 μm.

Figure12 shows an optical microscope image of the assembled two-cascaded MZI (wire diameter, 900 nm) [21]. 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.

**Figure 12.** Optical microscope image of the assembled cascaded MZI (wire diameter, 900 nm) [21]. Inset (a) shows the scanning electron microscope image of bow-shaped MZI 1 and inset (b) shows the optical microscope image of guided red light (650 nm) in the cascaded MZI. Estimated path-length difference is

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

40 m. The white arrow in the inset (b) indicates propagation direction of the light.

**Figure 13.** (a) Schematic illustration of the experimental setup. The red arrows show propagation directions of the optical signals whereas the yellow one indicates moving direction of the tunable stage. (b) SEM image of a microring assembled by the PTT wire. The wire diameter is 3.5 μm and the ring radius is 70 μm [31].

### *3.1.4. Arbitrary and vertical optical couplers using flexible polymer MNFs[32]*

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 m (wire 1) and 1.00 m (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 =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 m (wire 1) and 1.40 m (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 (=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 m (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 ( = 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 m (wire 1), 1.37 m (wire 2), and 1.20 m (wire 3). The cross angle between channels C1 and C3 is 87° and that between channels C4 and C5 is 39°. When the light ( = 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.

**Figure 14.** Optical microscope images of different arbitrary and vertical optical couplers [32]. The insets schematically represent respective structures,and the white arrows indicate the transmission direction of the guided light. (a) Optical microscope image of a crossed interconnect structure assembled by wires 1 and 2 with diameters of 1.05 and 1.00 m, respectively. (b) Optical microscope image of a crossed interconnect structure assembled by wires 1 and 2 with diameters of 1.18 and 1.40 m,respectively. (c) Optical microscope image of a star topology network assembled by three wires with diameters of 1.08 m (wire 1), 917 nm (wire 2), and 970 nm(wire 3). (d) Optical microscope image of a tree topology network assembled by three wires with diameters of 1.17 m (wire 1), 1.37 m (wire2), and 1.20 m (wire 3).

#### **3.2. Polymer MNFs as active waveguides**

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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 m (wire 1) and 1.40 m (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 (=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 m (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 ( = 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 m (wire 1), 1.37 m (wire 2), and 1.20 m (wire 3). The cross angle between channels C1 and C3 is 87° and that between channels C4 and C5 is 39°. When the light ( = 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.

**Figure 14.** Optical microscope images of different arbitrary and vertical optical couplers [32]. The insets schematically represent respective structures,and the white arrows indicate the transmission direction of the guided light. (a) Optical microscope image of a crossed interconnect structure assembled by wires 1 and 2 with diameters of 1.05 and 1.00 m, respectively. (b) Optical microscope image of a crossed interconnect structure assembled by wires 1 and 2 with diameters of 1.18 and 1.40 m,respectively. (c) Optical microscope image of a star topology network assembled by three wires with diameters of 1.08 m (wire 1), 917 nm (wire 2), and 970 nm(wire 3). (d) Optical microscope image of a tree topology network assembled by three wires with diameters of 1.17 m (wire 1), 1.37 m (wire2), and 1.20 m (wire 3).

The development of miniaturized PICs are challenging directions in next generation alloptical 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 [72] 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 [34]. 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 [34]. Upon 473-nm laser (λex) launched from the left side with excitation power (*P*ex) 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.

**Figure 15.** PL microscope images of a 380-nm-diameter 520-μm-length RhB-PS nanofiber excited by 473-nm light from the left side at *P*ex = 70 nW, taken with (a) and without (b) the long-pass emission filter. (c) PL spectra of the RhB-PS nanofiber under 70-nW waveguiding (orange line) and 3-μW irradiation (blue line) schemes. The absorption spectrum of the NF is also provided (black line). (d) PL intensity of the RhB-PS nanofiber at 579 nm with *P*ex = 30 nW as a function of time [34].

#### *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 [16]. 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. Nanometersized QDs were clearly recognizable, and were observed to be concentrated in a constrained region as a strand parallel to the fiber axis (Fig. 16b).

**Figure 16.** QD distribution in the SU8 nanofiber [16]: a) Fluorescence image of ensembles of QDs embedded in an SU8 nanofiber, showing the distribution of luminescence along the fiber length. b) TEM image of QD distribution inside the SU8 nanofiber.

## *3.2.3. QD-Decorated Polymer MNFs*

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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

**Figure 15.** PL microscope images of a 380-nm-diameter 520-μm-length RhB-PS nanofiber excited by 473-nm light from the left side at *P*ex = 70 nW, taken with (a) and without (b) the long-pass emission filter. (c) PL spectra of the RhB-PS nanofiber under 70-nW waveguiding (orange line) and 3-μW irradiation (blue line) schemes. The absorption spectrum of the NF is also provided (black line). (d) PL

H. Liu et al. incorporated semiconductor QDs as integrated light sources into the polymer fiber to produce subwavelength-diameter optical waveguides [16]. 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. Nanometersized QDs were clearly recognizable, and were observed to be concentrated in a constrained

intensity of the RhB-PS nanofiber at 579 nm with *P*ex = 30 nW as a function of time [34].

*3.2.2. Quantum-dot-doped Polymer MNFs* 

region as a strand parallel to the fiber axis (Fig. 16b).

measured absorption coefficient α of the RhB-PS NFs at 473 nm is ~50 cm-1.

H. Yu et al.developed a one-step process to decorate PTT nanofiber with the CdSe/ZnS core/shell QDs [73]. 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 *P*ex = 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 *P*ex = 1.2 μW.

**Figure 17.** Optical microscope images of nanofibers with a rugby-ball-like QDs-decorated cross structure and a cascaded rugby-ball-like QDs-decorated structure [73]. (a) Crossed structurewith a guided 473 nm blue light in nanofiber 2 and excited 630 nm red light at the cross junction. The inset schematically representsthe cross structures. (b) Cascaded rugby-ball-like QDs-decorated structure. The lengths of the QDs B1 to B7are 2.3, 2.2, 2.3, 2.2, 1.8, 2.0, and 2.0 μm, respectively. The corresponding maximum widths are 1.6, 1.5, 1.6, 1.6, 1.0, 1.1, and 1.1 μm. (c) Rugby-ball-like QDs-decorated fiber excited by 473 nm blue light at *P*ex = 1.2 μW. The blue arrow shows the direction of propagation of the input blue light.
