**3. Propagated surface plasmon in silver nanowires**

Silver nanowires possess sub-wavelength mode confinement and low propagation loss, but the couple of light into such small nanowires are extremely challengeable. For this challenge, several methods to light coupling have been proposed, including: (1) total internal reflection method utilizing an optical prism to compensate for momentum mismatch between photon and surface plasmon [47]; (2) localized excitation method via focusing laser beam directly on a scattering center of silver nanowires [48, 49]; and (3) near-field coupling method using optical dipole with large momentum components to match that of surface plasmon, where quantum emitters usually located in the near-field of silver nanowire [50, 51]. However, above three coupling methods cannot easily interconnect silver nanowires with photonic waveguides for nanophotonic circuits. Herein, we describe a novel result of silver nanowire supported plasmonic waveguide by coupling photons from quantum-dot-doped nanowire.

with distinct lengths and diameters would be interconnected with the same quantum-dotdoped photonic nanowire at distinct angles and intervals, which could provide versatile

**Figure 2.** SEM characterization of silver nanowires. (a) One silver nanowire was interconnected with a quantum-dotdoped nanowire at an angle of 60°. (b) Enlarged view of the crossing angle in (a). (c) Silver nanowires 1 and 2 were interconnected with the same quantum-dot-doped nanowire at crossing angles of 60 and 65°, respectively. Note: NW

Localized and Propagated Surface Plasmons in Metal Nanoparticles and Nanowires

http://dx.doi.org/10.5772/intechopen.78284

25

Optical characterization of the silver nanowires was performed under an optical microscopy using a micro-spectrophotometer (CRAIC, 20/20 PV, USA). **Figure 3a** shows a reflection-type optical microscope image of silver nanowires 1 and 2 with the same diameters of 300 nm was interconnected with an 800-nm diameter quantum-dot-doped nanowire at angles of 57 and 80°, respectively. The lengths of silver nanowires 1 and 2 are about 5 and 4 μm, respectively, and the mutual distance between them is of 10 μm. **Figure 3b** gives a scattering-type micrograph under dark-field for the crossing structure in **Figure 3a** illuminated by a beam of 532-nm laser at an optical power of 10 mW. The scattering position J0 comes from individual defects in the quantum-dot-doped nanowire. The larger scattering positions J1 and J2 come from the photonic-plasmonic junctions formed with the silver and quantum-dot-doped nanowires [55]. In the photonic-plasmonic junctions, when the surface plasmon of silver nanowires were launched by photoluminescence from quantum-dot-doped nanowire. Among the launched plasmonic modes, the dominant mode is longitudinal mode and the produced resonance could have Fabry-Perot characteristics [56]. The smaller scattering positions S1 and S2 come from the end tips of the silver nanowires, where the propagated surface plasmons were converted into

multi-functionalities in nanoscale circuits [52, 53].

means nanowire, and QD means quantum dot. Reprinted with permission [54].

free space photons.

A scanning electron microscope (SEM) micrograph of one silver nanowire with a diameter of 300 nm was interconnected with a quantum-dot-doped nanowire with a diameter of 800 nm at an angle of 60° is presented in **Figure 2a**. To inspect this crossing angle in better detail, an enlarged view is given in **Figure 2b**. For more practical device applications, the interconnection of more than one silver nanowire with a same quantum-dot-doped polymer nanowire is extremely desirable. For example, **Figure 2c** presents an SEM micrograph of silver nanowires 1 and 2 with the same diameters of 300 nm were interconnected with a quantum-dot-doped nanowire (800-nm in diameter) at angles of 60 and 65°, respectively. The mutual distance between silver nanowires 1 and 2 is about 6.4 μm. To realize hybrid interconnection with highdensity of plasmonic and photonic nanowires, more than three silver plasmonic nanowires Localized and Propagated Surface Plasmons in Metal Nanoparticles and Nanowires http://dx.doi.org/10.5772/intechopen.78284 25

In fact, emission quenching could happen because free electrons can transfer from quantum dots to gold nanorods once they are closely contacted. For instance, emission quenching of quantum dots by gold nanoparticles has been previously reported [44, 45]. As shown in **Figure 1a**, the locally enhanced inhomogenously distributed emission spots can be explained by the unevenly-increased in the local density of plasmonic sates around gold nanorods. This inhomogeneous can be improved by linking quantum dots and gold nanorods via chemical/ biological coupling to ensure their efficient interactions. In any case, the total emission efficiency depends on the quantum yield, self-absorption of quantum dots, resonance properties of localized surface plasmon in gold nanorods, and propagation loss of the plasmonic waveguide. For the measurements of emission intensity, the dark-field optical microscope images with RGB modes were transformed into gray levels employing software (Adobe Photoshop), and the gray values were summed up to acquire the normalized intensity, this method was previously reported [46]. The yellow rectangular in **Figure 1a** and **b** is taken as the sum-up region

the plasmonic waveguide (red line) and photonic waveguide (green line) as a function of propagation distance. It indicates that the existence of gold nanorods seriously affects the propagation distance of plasmonic waveguide, which is due to the larger scattering loss. In addition, some local regions with non-uniform particle density, refractive index, and crystal-

Silver nanowires possess sub-wavelength mode confinement and low propagation loss, but the couple of light into such small nanowires are extremely challengeable. For this challenge, several methods to light coupling have been proposed, including: (1) total internal reflection method utilizing an optical prism to compensate for momentum mismatch between photon and surface plasmon [47]; (2) localized excitation method via focusing laser beam directly on a scattering center of silver nanowires [48, 49]; and (3) near-field coupling method using optical dipole with large momentum components to match that of surface plasmon, where quantum emitters usually located in the near-field of silver nanowire [50, 51]. However, above three coupling methods cannot easily interconnect silver nanowires with photonic waveguides for nanophotonic circuits. Herein, we describe a novel result of silver nanowire supported plas-

A scanning electron microscope (SEM) micrograph of one silver nanowire with a diameter of 300 nm was interconnected with a quantum-dot-doped nanowire with a diameter of 800 nm at an angle of 60° is presented in **Figure 2a**. To inspect this crossing angle in better detail, an enlarged view is given in **Figure 2b**. For more practical device applications, the interconnection of more than one silver nanowire with a same quantum-dot-doped polymer nanowire is extremely desirable. For example, **Figure 2c** presents an SEM micrograph of silver nanowires 1 and 2 with the same diameters of 300 nm were interconnected with a quantum-dot-doped nanowire (800-nm in diameter) at angles of 60 and 65°, respectively. The mutual distance between silver nanowires 1 and 2 is about 6.4 μm. To realize hybrid interconnection with highdensity of plasmonic and photonic nanowires, more than three silver plasmonic nanowires

line would give rise to bulk scattering in the plasmonic waveguide.

monic waveguide by coupling photons from quantum-dot-doped nanowire.

**3. Propagated surface plasmon in silver nanowires**

. **Figure 1e** shows the normalized intensity of red emission along

with an area of 21 × 18 μm2

24 Plasmonics

**Figure 2.** SEM characterization of silver nanowires. (a) One silver nanowire was interconnected with a quantum-dotdoped nanowire at an angle of 60°. (b) Enlarged view of the crossing angle in (a). (c) Silver nanowires 1 and 2 were interconnected with the same quantum-dot-doped nanowire at crossing angles of 60 and 65°, respectively. Note: NW means nanowire, and QD means quantum dot. Reprinted with permission [54].

with distinct lengths and diameters would be interconnected with the same quantum-dotdoped photonic nanowire at distinct angles and intervals, which could provide versatile multi-functionalities in nanoscale circuits [52, 53].

Optical characterization of the silver nanowires was performed under an optical microscopy using a micro-spectrophotometer (CRAIC, 20/20 PV, USA). **Figure 3a** shows a reflection-type optical microscope image of silver nanowires 1 and 2 with the same diameters of 300 nm was interconnected with an 800-nm diameter quantum-dot-doped nanowire at angles of 57 and 80°, respectively. The lengths of silver nanowires 1 and 2 are about 5 and 4 μm, respectively, and the mutual distance between them is of 10 μm. **Figure 3b** gives a scattering-type micrograph under dark-field for the crossing structure in **Figure 3a** illuminated by a beam of 532-nm laser at an optical power of 10 mW. The scattering position J0 comes from individual defects in the quantum-dot-doped nanowire. The larger scattering positions J1 and J2 come from the photonic-plasmonic junctions formed with the silver and quantum-dot-doped nanowires [55]. In the photonic-plasmonic junctions, when the surface plasmon of silver nanowires were launched by photoluminescence from quantum-dot-doped nanowire. Among the launched plasmonic modes, the dominant mode is longitudinal mode and the produced resonance could have Fabry-Perot characteristics [56]. The smaller scattering positions S1 and S2 come from the end tips of the silver nanowires, where the propagated surface plasmons were converted into free space photons.

The two silver nanowires with same diameters of 300 nm can strongly confine the 580-nm photoluminescence into sub-wavelength scale dimensions, giving rise to their relatively compact interconnection with the quantum-dot-doped nanowire. For briefness, the photoluminescence spectra collected at positions J0, J1, J2, S1, and S2 were plotted in a single graph as **Figure 3c**. The peak wavelengths for positions J0, J1, J2, S1, and S2 are 580, 585, 590, 595, and 605 nm, respectively. The inset image of **Figure 3c** shows surface plasmon propagation in the silver nanowires and emitted photon propagation in the quantum-dot-doped polymer nanowire. The quantum-dot-doped nanowire produced distinct colors of yellow, yellow-orange, and orange from positions J0, J1, and J2, respectively. This phenomenon is called "wavelength-converted waveguiding", which also previously observed in dye-doped polymer nanofiber [57]. The color difference between yellow-orange (J1) and red-orange (S1) in silver nanowire 1 comes from the metallic loss induced energy dissipation, and the resulted peak wavelength changed from 585 to 595 nm. Similarly, the colors are different between orange (J2) and red (S2) in silver nanowire 2, and the peak wavelength changed from 590 to 605 nm. This is a frequency-dependent dispersion effect in these two silver nanowires [58].

Another novel result is optical routing in single silver nanowires [59]. Semiconductor microribbon and silver nanowires were assembled to realize routing structure, as shown in SEM micrograph of **Figure 4a**. One silver nanowire was sitting on top of a semiconductor SnO<sup>2</sup> ribbon, and a second silver nanowire just cling to it with a 12 μm overlapping. The three color arrows show the end tips of the silver nanowires, and the Inset shows the enlarged view of the overlapping region. The surfactants on the silver nanowires have already been cleared so that the two silver nanowires were contact directly. **Figure 4b** and **c** show the intensity mapping and real-color micrograph of the optical routing from semiconductor SnO2 ribbon

photoluminescence. Surface plasmons were launched into the first silver nanowire in the

**Figure 4.** Optical routing of silver nanowires. (a) SEM micrograph of two overlapped silver nanowires coupling to

into silver nanowires with three different emission spots at distal ends. (c) the real-color micrograph of the same view as in (b). (d−f) individual red (R), green (G), and blue (B) color channels from (c), giving a gradual decrease in propagation loss from short (f) to long (d) wavelengths. The colors are obtained with the Bayer filter from the CoolSnap. Reprinted

Intensity mapping of the routing structure when the photoluminescence of the SnO2

ribbon. The length of overlapped region is 12 μm. (inset) overlapped region of the two silver nanowires. (b)

Localized and Propagated Surface Plasmons in Metal Nanoparticles and Nanowires

http://dx.doi.org/10.5772/intechopen.78284

27

ribbon was launched and coupled

low scattering loss in the silver-silver overlapped region. This efficient coupling in the silversilver region is benefited from the large surface plasmon mode overlaps due to the closely contact between the nanowires. Interestingly, the relationship of the end tip emission color with propagation distance on the case of single silver nanowire was also investigated. **Figure 4d**–**4f** individually display the red, green, and blue color panel of the real-color micrograph in **Figure 4c**. The closer the propagation distance, the emission composes more short-wavelength components. The blue-wavelength components just exist at the first silver nanowire tip with a propagation distance of 8 μm, while the green-wavelength components survived 20 μm of propagation and the red-wavelength components were the only visible one with propagation

Different from dielectric micro-cavities, plasmonic nano-cavities could confine light into the nanometer scale mode volume [60, 61]. Optical investigations based on circular nano-cavities are extremely interesting due to the whispering-gallery-modes, which are attributed to the total internal reflection of light at the plasmonic interface along the equator. Herein, we describe

overlapped region, and evanescently coupled into the second silver nanowire with

silver-SnO2

with permission [59].

a SnO2

distance up to 40 μm.

**4. Plasmonic nano-cavity in gold nanospheres**

**Figure 3.** Optical characterization of silver nanowires. (a) Reflection-type optical microscope image of silver nanowires 1 and 2 were interconnected with the same quantum-dot-doped nanowire at crossing angles of 57 and 80°, respectively. Note that the black box (area: 1 μm2 ) near silver nanowire 1 is the sampling spot to spectrum measuring. (b) Scatteringtype optical microscope image of the crossing structure in (a) was illuminated by a 532-nm excitation laser with a spot diameter of 15 μm at an optical power of 10 mW. The yellow arrow gives the propagation direction of excited 580-nm photoluminescence. Inset was captured by focalizing to the two silver nanowires and measures the photoluminescence spectra on positions J0, J1, J2, S1, and S2. The scale bar in (a) is applicable to (b). (c) Photoluminescence spectra measured at positions J0, J1, J2, S1, and S2. Inset shows the propagation of photons and plasmons. NW means nanowire, and QD means quantum dot. PL means photoluminescence. Reprinted with permission [54].

Localized and Propagated Surface Plasmons in Metal Nanoparticles and Nanowires http://dx.doi.org/10.5772/intechopen.78284 27

The two silver nanowires with same diameters of 300 nm can strongly confine the 580-nm photoluminescence into sub-wavelength scale dimensions, giving rise to their relatively compact interconnection with the quantum-dot-doped nanowire. For briefness, the photoluminescence spectra collected at positions J0, J1, J2, S1, and S2 were plotted in a single graph as **Figure 3c**. The peak wavelengths for positions J0, J1, J2, S1, and S2 are 580, 585, 590, 595, and 605 nm, respectively. The inset image of **Figure 3c** shows surface plasmon propagation in the silver nanowires and emitted photon propagation in the quantum-dot-doped polymer nanowire. The quantum-dot-doped nanowire produced distinct colors of yellow, yellow-orange, and orange from positions J0, J1, and J2, respectively. This phenomenon is called "wavelength-converted waveguiding", which also previously observed in dye-doped polymer nanofiber [57]. The color difference between yellow-orange (J1) and red-orange (S1) in silver nanowire 1 comes from the metallic loss induced energy dissipation, and the resulted peak wavelength changed from 585 to 595 nm. Similarly, the colors are different between orange (J2) and red (S2) in silver nanowire 2, and the peak wavelength changed from 590 to 605 nm. This is a frequency-dependent dispersion effect in these two silver nanowires [58].

Another novel result is optical routing in single silver nanowires [59]. Semiconductor microribbon and silver nanowires were assembled to realize routing structure, as shown in SEM micrograph of **Figure 4a**. One silver nanowire was sitting on top of a semiconductor SnO<sup>2</sup> ribbon, and a second silver nanowire just cling to it with a 12 μm overlapping. The three color arrows show the end tips of the silver nanowires, and the Inset shows the enlarged view of the overlapping region. The surfactants on the silver nanowires have already been cleared so that the two silver nanowires were contact directly. **Figure 4b** and **c** show the intensity

mapping and real-color micrograph of the optical routing from semiconductor SnO2

**Figure 3.** Optical characterization of silver nanowires. (a) Reflection-type optical microscope image of silver nanowires 1 and 2 were interconnected with the same quantum-dot-doped nanowire at crossing angles of 57 and 80°, respectively.

type optical microscope image of the crossing structure in (a) was illuminated by a 532-nm excitation laser with a spot diameter of 15 μm at an optical power of 10 mW. The yellow arrow gives the propagation direction of excited 580-nm photoluminescence. Inset was captured by focalizing to the two silver nanowires and measures the photoluminescence spectra on positions J0, J1, J2, S1, and S2. The scale bar in (a) is applicable to (b). (c) Photoluminescence spectra measured at positions J0, J1, J2, S1, and S2. Inset shows the propagation of photons and plasmons. NW means nanowire, and QD

means quantum dot. PL means photoluminescence. Reprinted with permission [54].

) near silver nanowire 1 is the sampling spot to spectrum measuring. (b) Scattering-

Note that the black box (area: 1 μm2

26 Plasmonics

ribbon

**Figure 4.** Optical routing of silver nanowires. (a) SEM micrograph of two overlapped silver nanowires coupling to a SnO2 ribbon. The length of overlapped region is 12 μm. (inset) overlapped region of the two silver nanowires. (b) Intensity mapping of the routing structure when the photoluminescence of the SnO2 ribbon was launched and coupled into silver nanowires with three different emission spots at distal ends. (c) the real-color micrograph of the same view as in (b). (d−f) individual red (R), green (G), and blue (B) color channels from (c), giving a gradual decrease in propagation loss from short (f) to long (d) wavelengths. The colors are obtained with the Bayer filter from the CoolSnap. Reprinted with permission [59].

photoluminescence. Surface plasmons were launched into the first silver nanowire in the silver-SnO2 overlapped region, and evanescently coupled into the second silver nanowire with low scattering loss in the silver-silver overlapped region. This efficient coupling in the silversilver region is benefited from the large surface plasmon mode overlaps due to the closely contact between the nanowires. Interestingly, the relationship of the end tip emission color with propagation distance on the case of single silver nanowire was also investigated. **Figure 4d**–**4f** individually display the red, green, and blue color panel of the real-color micrograph in **Figure 4c**. The closer the propagation distance, the emission composes more short-wavelength components. The blue-wavelength components just exist at the first silver nanowire tip with a propagation distance of 8 μm, while the green-wavelength components survived 20 μm of propagation and the red-wavelength components were the only visible one with propagation distance up to 40 μm.

### **4. Plasmonic nano-cavity in gold nanospheres**

Different from dielectric micro-cavities, plasmonic nano-cavities could confine light into the nanometer scale mode volume [60, 61]. Optical investigations based on circular nano-cavities are extremely interesting due to the whispering-gallery-modes, which are attributed to the total internal reflection of light at the plasmonic interface along the equator. Herein, we describe a novel result of whispering-gallery-mode emission in plasmonic nano-cavity, which combines gold nanosphere and quantum dots by using electrostatic attraction. **Figure 5a** shows polarization-dependent photoluminescence intensity of the emitted light. The dependence can be described by a sinusoidal function. The minimum (−90°) and the maximum (10°) photoluminescence intensities are corresponding to perpendicular and parallel to the extinction direction of the gold nanosphere, respectively. The period of 180° is similar to that reported in quantum dots on gold nanodiscs [62].

states density and the near-field coupling strengths. **Figure 5d** shows the photoluminescence decay spectra of quantum-dot-coated gold nanosphere (blue) and the quantum dots solution (red). They are multiple-exponential process, including intrinsic decay, plasmonic quenching, and plasmonic enhancement. The lifetimes were calculated from 1/*e* values to be of 9 and 13 ns, respectively. The decrease in lifetime verifies the near-field coupling between the localized surface plasmon of gold nanosphere and the radiative decay of quantum dots. As the light waves were confined along the equator of gold nanosphere by localized surface plasmon,

Localized and Propagated Surface Plasmons in Metal Nanoparticles and Nanowires

http://dx.doi.org/10.5772/intechopen.78284

29

Molecular emitters located in a plasmonic nano-cavity experience an intriguing process to be coupled into the surrounding optical field [64]. Cucurbit(7)uril is hydro-soluble and can encapsulate only one molecule of methylene-blue inside it. Encapsulation of methylene-blue inside cucurbit(7)uril is verified by absorption spectroscopy analysis (**Figure 6a**): methyleneblue dimmers (with characteristic 'shoulder' of small peak at 625 nm on the red solid curve) disappear when mixing low concentrations of methylene-blue into cucurbit(7)uril (**Figure 6a**, blue solid curve). Contrast experiments use the smaller molecule of cucurbit(5)uril also have this shoulder peak (red dashed curve), eliminating the possibility of parasitic binding. Encapsulating single molecules of methylene-blue into cucurbit(7)uril can be used to avoid molecular aggregation. Carboxide portals at either end of the 0.9-nm in height cucurbit(*n*)uril molecules with rims were binding flatly onto the gold surface (**Figure 6b**). The formed nano-

**Figure 6.** Plasmonic nano-cavity with a single dye molecule. (a) Absorption spectra for methylene-blue in water, with (blue solid line) and without (red solid line) encapsulation in cucurbit(*n*)urils. Blue and red icons show individual

molecule in cucurbit(*n*)uril, located within the gold nanosphere-on-mirror geometry configuration. Reprinted with

) and paired molecular dimmers, respectively. (b) Schematic illustration of a methylene-blue

. This decorating of molecules via light opens

the propagation medium of the light waves are the coated layers with quantum dots.

cavity volume can be down to less than 40 nm3

molecules (centred at *ω*<sup>0</sup>

permission [64].

up the venue to manipulate chemical bonds [65].

**Figure 5b** shows the emission spectrum containing five narrow bands, which indicates the characteristic of the whispering-gallery-modes. The maximum peak of whispering-gallerymode is well coincidence with that of solution photoluminescence. Meanwhile, the optical feedback is strong enough to resist the plasmonic loss. **Figure 5c** shows the photoluminescence spectra of 800 and 550 nm diameter quantum-dot-coated gold nanosphere producing the several narrow bands, while the maximum peaks towards to short wavelength range as diameter increasing. This phenomenon is governed by changing in whispering-gallery-mode

**Figure 5.** Whispering-gallery-mode emission. (a) Photoluminescence intensity as a function of polarization angle. Insets show optical micrographs of quantum-dot-coated gold nanosphere with polarization angles of 90 and 10°. (b) Whisperinggallery-mode emission spectrum from a quantum-dot-coated gold nanosphere (solid blue line) and the quantum dots solution emission spectrum (dashed red line). (c) Whispering-gallery-mode emission spectra of quantum-dot-coated gold nanosphere with diameters of 490 nm (blue), 550 nm (green), and 800 nm (red). Insets: Corresponding TEM micrographs. (d) Photoluminescence decay spectra of quantum-dot-coated gold nanosphere (blue line) and the quantum dots solution (red line) with lifetime at 9 and 13 ns, respectively. Reprinted with permission [63].

states density and the near-field coupling strengths. **Figure 5d** shows the photoluminescence decay spectra of quantum-dot-coated gold nanosphere (blue) and the quantum dots solution (red). They are multiple-exponential process, including intrinsic decay, plasmonic quenching, and plasmonic enhancement. The lifetimes were calculated from 1/*e* values to be of 9 and 13 ns, respectively. The decrease in lifetime verifies the near-field coupling between the localized surface plasmon of gold nanosphere and the radiative decay of quantum dots. As the light waves were confined along the equator of gold nanosphere by localized surface plasmon, the propagation medium of the light waves are the coated layers with quantum dots.

a novel result of whispering-gallery-mode emission in plasmonic nano-cavity, which combines gold nanosphere and quantum dots by using electrostatic attraction. **Figure 5a** shows polarization-dependent photoluminescence intensity of the emitted light. The dependence can be described by a sinusoidal function. The minimum (−90°) and the maximum (10°) photoluminescence intensities are corresponding to perpendicular and parallel to the extinction direction of the gold nanosphere, respectively. The period of 180° is similar to that reported in

**Figure 5b** shows the emission spectrum containing five narrow bands, which indicates the characteristic of the whispering-gallery-modes. The maximum peak of whispering-gallerymode is well coincidence with that of solution photoluminescence. Meanwhile, the optical feedback is strong enough to resist the plasmonic loss. **Figure 5c** shows the photoluminescence spectra of 800 and 550 nm diameter quantum-dot-coated gold nanosphere producing the several narrow bands, while the maximum peaks towards to short wavelength range as diameter increasing. This phenomenon is governed by changing in whispering-gallery-mode

**Figure 5.** Whispering-gallery-mode emission. (a) Photoluminescence intensity as a function of polarization angle. Insets show optical micrographs of quantum-dot-coated gold nanosphere with polarization angles of 90 and 10°. (b) Whisperinggallery-mode emission spectrum from a quantum-dot-coated gold nanosphere (solid blue line) and the quantum dots solution emission spectrum (dashed red line). (c) Whispering-gallery-mode emission spectra of quantum-dot-coated gold nanosphere with diameters of 490 nm (blue), 550 nm (green), and 800 nm (red). Insets: Corresponding TEM micrographs. (d) Photoluminescence decay spectra of quantum-dot-coated gold nanosphere (blue line) and the quantum dots solution

(red line) with lifetime at 9 and 13 ns, respectively. Reprinted with permission [63].

quantum dots on gold nanodiscs [62].

28 Plasmonics

Molecular emitters located in a plasmonic nano-cavity experience an intriguing process to be coupled into the surrounding optical field [64]. Cucurbit(7)uril is hydro-soluble and can encapsulate only one molecule of methylene-blue inside it. Encapsulation of methylene-blue inside cucurbit(7)uril is verified by absorption spectroscopy analysis (**Figure 6a**): methyleneblue dimmers (with characteristic 'shoulder' of small peak at 625 nm on the red solid curve) disappear when mixing low concentrations of methylene-blue into cucurbit(7)uril (**Figure 6a**, blue solid curve). Contrast experiments use the smaller molecule of cucurbit(5)uril also have this shoulder peak (red dashed curve), eliminating the possibility of parasitic binding. Encapsulating single molecules of methylene-blue into cucurbit(7)uril can be used to avoid molecular aggregation. Carboxide portals at either end of the 0.9-nm in height cucurbit(*n*)uril molecules with rims were binding flatly onto the gold surface (**Figure 6b**). The formed nanocavity volume can be down to less than 40 nm3 . This decorating of molecules via light opens up the venue to manipulate chemical bonds [65].

**Figure 6.** Plasmonic nano-cavity with a single dye molecule. (a) Absorption spectra for methylene-blue in water, with (blue solid line) and without (red solid line) encapsulation in cucurbit(*n*)urils. Blue and red icons show individual molecules (centred at *ω*<sup>0</sup> ) and paired molecular dimmers, respectively. (b) Schematic illustration of a methylene-blue molecule in cucurbit(*n*)uril, located within the gold nanosphere-on-mirror geometry configuration. Reprinted with permission [64].

**Figure 7.** Plasmonic nano-cavity design. (a) Schematic of the hybrid nanosphere, indicating silica shell doped with dye molecules. (b) TEM micrograph of gold core. (c) SEM micrograph of gold-silica-dye core-shell nanospheres. (d) Localized surface plasmon laser mode with *λ* of 525 nm and *Q* factor of 14.8. The outer and inner circles stand for the 44-nm shell and 14-nm core, respectively. Strength color bar is on the right. Reprinted with permission [66].

Another novel result shows that 44-nm in diameter nanospheres with a gold core and dyedoped silica shell can be used to overcome the plasmonic loss with optical gain and realize a stimulated emission of surface plasmons [66]. **Figure 7a** shows a nanosphere with a gold core, supporting for localized surface plasmon modes, and coated by a silica shell doped with organic dye OG-488 (Oregon Green 488), supporting for optical gain. TEM and SEM measurements show the diameter of gold core and the thickness of silica shell are about 14 and 15 nm, respectively (**Figure 7b** and **c**). The molecule number of dye per nanosphere was estimated to be about 2.7 × 103 , and the nanosphere concentration in suspension was about 3 × 1011 cm−3. The calculation of plasmonic laser mode (**Figure 7d**) obtains the stimulated emission wavelength (*λ*) to be of 525 nm and the quality (*Q*)-factor to be of 14.8. Although this *Q*-factor is governed by optical absorption, the produced gain is large enough to overcome the plasmonic loss.

> (**Figure 8b**–**8e**). Importantly, the full colors were outputted with the sub-wavelength silver nanowire at end tips (white circles). This demonstrates the typical feature of surface plasmon

> **Figure 8.** Sub-wavelength output of micro-lasers. (a) Diagram of the strategy to dope π-conjugated dye into microdisk for silver-polymer nanostructures. (b−e) Dark-field photoluminescence micrographs of the silver-polymer microdisks doped with organic dyes of (b) trans-DPDSB, (c) C153, (d) CNDPASDB, and (e) HMDMAC, respectively, under ultraviolet excitation. Photoluminescence spots enlarged in the white circles come from surface plasmon scattering in

Localized and Propagated Surface Plasmons in Metal Nanoparticles and Nanowires

http://dx.doi.org/10.5772/intechopen.78284

31

Another novel result of energy conversion between photonic nanowire and plasmonic nanowire will be described [72]. The photonic nanowire is CdSe-ZnS core-shell quantum dots doped polymer nanowire, and the silver nanowire served as plasmonic nanowire. The conversion efficiency is about 32%, which is realized by the Förster resonance energy transfer. **Figure 9a** shows SEM micrograph of quantum-dot-doped photonic nanowire were interconnected with silver plasmonic nanowire at a crossed angle of 45°. Once the plasmonic and photonic nanowires are directly contact, the surface plasmon field and evanescent field around photonic nanowire would overlap, leading to efficient conversion from photon to plasmon at the coupling area [73]. **Figure 9b** gives the dark-field optical micrograph of the silver-polymer crossed structure with red emission. The silver nanowire is about 55 μm in total length, and the lengths of the long (white arrow) and short (yellow arrow) regions on the two side of the photonic nanowire (refer as 'limbs') are 40 and 10 μm, respectively. The plasmonic conversion can be understood by the energy transition: photon-exciton-plasmon, and not need to match momentum, because the dipolar near-field of quantum dots has momentum components matching that of surface plasmon. Hence, the crossed angle cannot influence the plasmonic conversion efficiency [74]. The exponential decay curves of **Figure 9c** and **d** indicate surface

waveguide, and the plasmonic conversion at the silver-polymer connections.

silver nanowire tips. Scale bar, 10 μm. Reprinted with permission [71].

### **5. Plasmonic conversion in silver-polymer nanostructures**

Organic polymers with extraordinary properties to incorporate with laser dyes for providing optical gain over broadband visible spectrum, which makes them potential for tunable fullcolor lasers [67, 68]. Meanwhile, the mechanical flexibility of polymers allows to integrate them with metallic nanostructures, confining light into a small mode volume by surface plasmon [69]. In addition, flexible polymers have an outstanding function to self-assemble into optical micro-cavities with high-quality, where propagated light beam will be continuously confined via total internal reflection. Consequently, the large evanescent field around the air-cavity interface results in efficient conversion between photon and surface plasmon [70]. Herein, we describe a plasmonic conversion to obtain sub-wavelength output of full-color lasers in a silver-polymer heterostructure [71]. **Figure 8a** shows the assemble process of PS and dye molecules via dropping a defined amount of water into the PS-dye blend solution. During the nucleus formation of PS, the powerful π-π interaction makes the dye molecules slowly diffuse into the PS matrix. Once the solvent was evaporated, the silver-polymer microdisks with dyes can be successfully fabricated. Four organic dyes, trans-DPDSB, C153, CNDPASDB, and HMDMAC, with emission colors across the full visible spectrum, were intentionally selected for optical gain media. Under ultraviolet (330–380 nm) excitation, the silver-polymer microdisks emitted homogenous blue (b), cyan (c), green-yellow (d), and red (e) colors, respectively

Another novel result shows that 44-nm in diameter nanospheres with a gold core and dyedoped silica shell can be used to overcome the plasmonic loss with optical gain and realize a stimulated emission of surface plasmons [66]. **Figure 7a** shows a nanosphere with a gold core, supporting for localized surface plasmon modes, and coated by a silica shell doped with organic dye OG-488 (Oregon Green 488), supporting for optical gain. TEM and SEM measurements show the diameter of gold core and the thickness of silica shell are about 14 and 15 nm, respectively (**Figure 7b** and **c**). The molecule number of dye per nanosphere was estimated to

**Figure 7.** Plasmonic nano-cavity design. (a) Schematic of the hybrid nanosphere, indicating silica shell doped with dye molecules. (b) TEM micrograph of gold core. (c) SEM micrograph of gold-silica-dye core-shell nanospheres. (d) Localized surface plasmon laser mode with *λ* of 525 nm and *Q* factor of 14.8. The outer and inner circles stand for the 44-nm shell and 14-nm core, respectively. Strength color bar is on the right. Reprinted with permission [66].

calculation of plasmonic laser mode (**Figure 7d**) obtains the stimulated emission wavelength (*λ*) to be of 525 nm and the quality (*Q*)-factor to be of 14.8. Although this *Q*-factor is governed by optical absorption, the produced gain is large enough to overcome the plasmonic loss.

Organic polymers with extraordinary properties to incorporate with laser dyes for providing optical gain over broadband visible spectrum, which makes them potential for tunable fullcolor lasers [67, 68]. Meanwhile, the mechanical flexibility of polymers allows to integrate them with metallic nanostructures, confining light into a small mode volume by surface plasmon [69]. In addition, flexible polymers have an outstanding function to self-assemble into optical micro-cavities with high-quality, where propagated light beam will be continuously confined via total internal reflection. Consequently, the large evanescent field around the air-cavity interface results in efficient conversion between photon and surface plasmon [70]. Herein, we describe a plasmonic conversion to obtain sub-wavelength output of full-color lasers in a silver-polymer heterostructure [71]. **Figure 8a** shows the assemble process of PS and dye molecules via dropping a defined amount of water into the PS-dye blend solution. During the nucleus formation of PS, the powerful π-π interaction makes the dye molecules slowly diffuse into the PS matrix. Once the solvent was evaporated, the silver-polymer microdisks with dyes can be successfully fabricated. Four organic dyes, trans-DPDSB, C153, CNDPASDB, and HMDMAC, with emission colors across the full visible spectrum, were intentionally selected for optical gain media. Under ultraviolet (330–380 nm) excitation, the silver-polymer microdisks emitted homogenous blue (b), cyan (c), green-yellow (d), and red (e) colors, respectively

**5. Plasmonic conversion in silver-polymer nanostructures**

, and the nanosphere concentration in suspension was about 3 × 1011 cm−3. The

be about 2.7 × 103

30 Plasmonics

**Figure 8.** Sub-wavelength output of micro-lasers. (a) Diagram of the strategy to dope π-conjugated dye into microdisk for silver-polymer nanostructures. (b−e) Dark-field photoluminescence micrographs of the silver-polymer microdisks doped with organic dyes of (b) trans-DPDSB, (c) C153, (d) CNDPASDB, and (e) HMDMAC, respectively, under ultraviolet excitation. Photoluminescence spots enlarged in the white circles come from surface plasmon scattering in silver nanowire tips. Scale bar, 10 μm. Reprinted with permission [71].

(**Figure 8b**–**8e**). Importantly, the full colors were outputted with the sub-wavelength silver nanowire at end tips (white circles). This demonstrates the typical feature of surface plasmon waveguide, and the plasmonic conversion at the silver-polymer connections.

Another novel result of energy conversion between photonic nanowire and plasmonic nanowire will be described [72]. The photonic nanowire is CdSe-ZnS core-shell quantum dots doped polymer nanowire, and the silver nanowire served as plasmonic nanowire. The conversion efficiency is about 32%, which is realized by the Förster resonance energy transfer. **Figure 9a** shows SEM micrograph of quantum-dot-doped photonic nanowire were interconnected with silver plasmonic nanowire at a crossed angle of 45°. Once the plasmonic and photonic nanowires are directly contact, the surface plasmon field and evanescent field around photonic nanowire would overlap, leading to efficient conversion from photon to plasmon at the coupling area [73]. **Figure 9b** gives the dark-field optical micrograph of the silver-polymer crossed structure with red emission. The silver nanowire is about 55 μm in total length, and the lengths of the long (white arrow) and short (yellow arrow) regions on the two side of the photonic nanowire (refer as 'limbs') are 40 and 10 μm, respectively. The plasmonic conversion can be understood by the energy transition: photon-exciton-plasmon, and not need to match momentum, because the dipolar near-field of quantum dots has momentum components matching that of surface plasmon. Hence, the crossed angle cannot influence the plasmonic conversion efficiency [74]. The exponential decay curves of **Figure 9c** and **d** indicate surface

**Acknowledgements**

**Conflict of interest**

**Author details**

**References**

Xianguang Yang and Baojun Li\*

The authors declare no conflict of interest.

Science. 2017;**356**(6341):908-909

nanoscale. Nano Letters. 2007;**7**(10):3145-3149

\*Address all correspondence to: baojunli@jnu.edu.cn

Institute of Nanophotonics, Jinan University, Guangzhou, China

[2] Polman A. Plasmonics applied. Science. 2008;**322**(5903):868-869

plasmonic nanosensors. Nature Materials. 2008;**7**(6):442-453

We thank the supports provided by National Natural Science Foundation of China (Grant 21703083), the Natural Science Foundation of Guangdong Province (Grants 2017A030313026 and 2017A030310463), the Science Research Project of Guangzhou (Grant 201804010468) and

Localized and Propagated Surface Plasmons in Metal Nanoparticles and Nanowires

http://dx.doi.org/10.5772/intechopen.78284

33

[1] Maier SA, Brongersma ML, Kik PG, Meltzer S, Requicha AA, Atwater HA. Plasmonics—A route to nanoscale optical devices. Advanced Materials. 2001;**13**(19):1501-1505

[3] Brongersma ML, Shalaev VM. The case for plasmonics. Science. 2010;**328**(5977):440-441

[5] Marinica DC, Zapata M, Nordlander P, Kazansky AK, Echenique PM, Aizpurua J, Borisov AG. Active quantum plasmonics. Science Advances. 2015;**1**(11):e1501095

[6] Naldoni A, Shalaev VM, Brongersma ML. Applying plasmonics to a sustainable future.

[7] Durach M, Rusina A, Stockman MI, Nelson K. Toward full spatiotemporal control on the

[8] Anker JN, Hall WP, Lyandres O, Shah NC, Zhao J, Van Duyne RP. Biosensing with

[4] Kauranen M, Zayats AV. Nonlinear plasmonics. Nature Photonics. 2012;**6**:737-748

the Fundamental Research Funds for the Central Universities (Grant 21617334).

**Figure 9.** Energy conversion from photonic to plasmonic nanowires. (a) SEM micrograph of a 300-nm-diameter silver nanowire and a 400-nm-diameter quantum-dot-doped nanowire interconnected at a crossed angle of 45°. (b) Dark-field optical micrograph of the crossed silver-polymer structure excited by a 532-nm-wavelength laser at an optical power of 5 mW. (c, d) intensities of red emission along the long (c) and short (d) limbs of the silver plasmonic nanowire. Solid lines are plotted with an exponential decay. Insets and green dashed arrows give a visual guide. (e) Emission spectra collected at coupling area, two distal ends at long and short limbs. Reprinted with permission [72].

plasmon propagation lengths of 10 and 5 μm for long and short limbs, respectively. In addition, **Figure 9e** gives the emission spectra collected at coupling area, two distal ends of long and short limbs with wavelength centered at 650, 660, and 654 nm, respectively.
