**2. Localized surface plasmon in gold nanorods**

Metal-enhanced fluorescence can be realized via the resonant coupling with localized surface plasmon in metallic nanoparticles, nanorods, and nanostructures [35–40]. Different form two-dimensional films and/or three-dimensional solutions, metal-enhanced fluorescence in one-dimensional waveguides could provide a lower power consumption and higher density integration for plasmonic circuits. Herein, we describe a novel result of gold nanorod enhanced light emission, which is realized by embedding gold nanorods into polymer photonic waveguide doped with quantum dots at low concentration.

To clearly examine the distributions of gold nanorods and quantum dots embedded in the plasmonic waveguide, both a bright-field transmission electron microscope (TEM) and an energy dispersive X-ray spectroscopy (EDS) were simultaneously performed. **Figure 1c** shows EDS spectrum of a plasmonic waveguide while the inset shows the corresponding TEM image. The EDS spectrum verifies the existence of Au (9.43 wt%), Cd (0.02 wt%), Se (0.40 wt%), Zn (0.69 wt%), and S (1.00 wt%) elements. They came from gold nanorods and CdSe-ZnS core-shell quantum dots. The embedded concentrations for gold nanorods and

**Figure 1.** Plasmonic waveguides in polymer embedded with gold nanorods. Dark-field optical microscope images are corresponding to (a) plasmonic waveguide with gold nanorods and (b) photonic waveguide without gold nanorods. (c) TEM and EDS analysis of a plasmonic waveguide. (d) The relationship between emission efficiency and propagation distance. (e) Dependence of normalized intensity on the propagation distance. Red curve is for plasmonic waveguide, while blue curve is for photonic waveguide. GNRs mean gold nanorods, and QDs mean quantum dots. Reprinted with

Localized and Propagated Surface Plasmons in Metal Nanoparticles and Nanowires

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

23

**Figure 1d** shows the light emission efficiency at positions A to E (with gold nanorods) and positions A1 to E1 (without gold nanorods). At position B, the emission enhancement is maximized, where plasmonic wavelength overlapped with emission wavelength of quantum dots [42]. The mechanism for emission enhancement can be interpreted as follows: (1) by coupling a 473-nm blue light into plasmonic waveguide, the embedded quantum dots are photoexcited and then emit 600-nm red light. (2) The emitted 600-nm red light excites the localized surface plasmon of gold nanorods, which leads to an enhancement of local near-field. (3) The enhanced local near-field increases the stimulated radiative decay rate of quantum dots, thus leading to the increasing of quantum yields and the decreasing of fluorescence lifetime [43].

quantum dots are estimated to be of 4 μm−3 and 3.2 × 103 μm−3, respectively.

permission [41].

To study plasmonic properties, a single plasmonic waveguide was placed on MgF2 substrate with refractive index (*n*) of 1.39. A blue light at 473-nm wavelength was coupled into the plasmonic waveguide via evanescent field. **Figure 1a** shows the dark-field optical microscope image with red emission excited by the incident light at an optical power (*P*in) of 0.1 μW, where positions A to E manifesting gold nanorod enhanced light emission from embedded quantum dots for representative measurements. To make a comparison, a single photonic waveguide has a similar diameter as that of the plasmonic waveguide, was also excited by the incident light at an optical power (*P*in) of 0.1 μW (**Figure 1b**).

Localized and Propagated Surface Plasmons in Metal Nanoparticles and Nanowires http://dx.doi.org/10.5772/intechopen.78284 23

to guide light in various geometries such as 90° bending [1]. This promises the scaling of optical devices down to the diffraction limit for miniaturized photonic circuits. The localized surface plasmons have the functionality to scatter, absorb and squeeze light into nanometer scale, providing large enhancements of local near-fields [7]. It holds the potential applications in data storage, light energy generation, sub-wavelength optics, nano-optical tweezers, biophotonics and nanoscopy [8–11]. Gold nanoparticles with sphere/rod shapes and size below 100 nm are highly investigated for many useful applications, such as non/radiative enhancement of nano-crystals [12], inter-particle coupling effect [13], single particle plasmon spectroscopy [14–16], plasmonic sensing [17–19], plasmonic photocatalysis [20–22], and so on. Different from the localized surface plasmons of individual nanoparticles, the propagated surface plasmons existing at flat/curved surfaces in metallic planes, films, and wires also exhibit intriguing plasmonic phenomena [23]. The propagation length of surface plasmon modes is inevitably limited by metallic absorption, could also be strongly confined in the lateral section normal to propagation direction. This implies that plasmonic waveguides could transport larger bandwidth of information than that of conventional photonic waveguides. However, there is a trade-off between propagation loss and mode confinement in plasmonic waveguides [24]. To balance this trade off, an alternative method by seamlessly integrating photonic waveguides into plasmonic waveguides can be used [25]. Silver nanowires usually act as plasmonic waveguides with lateral size of 200−300 nm and axial length up to 10 μm, exhibiting many interesting applications. For example, plasmonic interference [26], waveparticle duality [27], remote excitation of Raman scattering [28–30], long-distance plasmonic gain [31–33], broadside nano-antennas [34], etc. It is impossible to introduce every result on plasmonics in this short chapter. In the following sections, we will specifically describe some novel results with gold nanorods/nanospheres and silver nanowires for achieving optical

Metal-enhanced fluorescence can be realized via the resonant coupling with localized surface plasmon in metallic nanoparticles, nanorods, and nanostructures [35–40]. Different form two-dimensional films and/or three-dimensional solutions, metal-enhanced fluorescence in one-dimensional waveguides could provide a lower power consumption and higher density integration for plasmonic circuits. Herein, we describe a novel result of gold nanorod enhanced light emission, which is realized by embedding gold nanorods into polymer pho-

with refractive index (*n*) of 1.39. A blue light at 473-nm wavelength was coupled into the plasmonic waveguide via evanescent field. **Figure 1a** shows the dark-field optical microscope image with red emission excited by the incident light at an optical power (*P*in) of 0.1 μW, where positions A to E manifesting gold nanorod enhanced light emission from embedded quantum dots for representative measurements. To make a comparison, a single photonic waveguide has a similar diameter as that of the plasmonic waveguide, was also excited by the

substrate

To study plasmonic properties, a single plasmonic waveguide was placed on MgF2

energy generation, propagation, and conversion.

22 Plasmonics

**2. Localized surface plasmon in gold nanorods**

tonic waveguide doped with quantum dots at low concentration.

incident light at an optical power (*P*in) of 0.1 μW (**Figure 1b**).

**Figure 1.** Plasmonic waveguides in polymer embedded with gold nanorods. Dark-field optical microscope images are corresponding to (a) plasmonic waveguide with gold nanorods and (b) photonic waveguide without gold nanorods. (c) TEM and EDS analysis of a plasmonic waveguide. (d) The relationship between emission efficiency and propagation distance. (e) Dependence of normalized intensity on the propagation distance. Red curve is for plasmonic waveguide, while blue curve is for photonic waveguide. GNRs mean gold nanorods, and QDs mean quantum dots. Reprinted with permission [41].

To clearly examine the distributions of gold nanorods and quantum dots embedded in the plasmonic waveguide, both a bright-field transmission electron microscope (TEM) and an energy dispersive X-ray spectroscopy (EDS) were simultaneously performed. **Figure 1c** shows EDS spectrum of a plasmonic waveguide while the inset shows the corresponding TEM image. The EDS spectrum verifies the existence of Au (9.43 wt%), Cd (0.02 wt%), Se (0.40 wt%), Zn (0.69 wt%), and S (1.00 wt%) elements. They came from gold nanorods and CdSe-ZnS core-shell quantum dots. The embedded concentrations for gold nanorods and quantum dots are estimated to be of 4 μm−3 and 3.2 × 103 μm−3, respectively.

**Figure 1d** shows the light emission efficiency at positions A to E (with gold nanorods) and positions A1 to E1 (without gold nanorods). At position B, the emission enhancement is maximized, where plasmonic wavelength overlapped with emission wavelength of quantum dots [42]. The mechanism for emission enhancement can be interpreted as follows: (1) by coupling a 473-nm blue light into plasmonic waveguide, the embedded quantum dots are photoexcited and then emit 600-nm red light. (2) The emitted 600-nm red light excites the localized surface plasmon of gold nanorods, which leads to an enhancement of local near-field. (3) The enhanced local near-field increases the stimulated radiative decay rate of quantum dots, thus leading to the increasing of quantum yields and the decreasing of fluorescence lifetime [43].

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 with an area of 21 × 18 μm2 . **Figure 1e** shows the normalized intensity of red emission along 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 crystalline would give rise to bulk scattering in the plasmonic waveguide.
