Dielectric effect on localized surface plasmons

Furthermore, the extinction scattering due to the excitation of localized surface plasmons is also strongly dependent on the dielectric property of surrounding environment. Figure 2.3 presents that the scattered particular light band can be observed be immersing gold nanoparicle with a diameter ~16 nm in the solution of refractive index (from left to right, 1.336, 1.407, 1.481, 1.525, and 1.583). It is noted that the scattering light shifted to longer wavelength when the index is increasing.

Figure 2.3: Extinction scattering of gold nanoparticles was sensitive to the dielectric constant of solution environment. [4]

2.1.3 Localized Surface Plasmon of Metal Nanorod

At present, so many factors, such as size, shape, and dielectric effect, affected localized surface plasmon resonance were rapidly reviewed. However, the other factor of aspect ratio of metal nanorod also leads to an interesting optical property of metal nanopart. In S. Link’s work, he presents that the aspect ratio of gold nanorod significantly affects the optical property of localized surface plasmon. Two different excited plasmons modes, transverse and longitudinal mode, are found on the metal nanorod. In Figure 2.4 (a), the absorption spectrum shows two absorption maxima at 525 and 740 nm corresponding to two different modes. This is because the absorption of visible light both along the direction of the nanorod length (the longitudinal plasmon band) and along the direction of the nanorod width (the transverse plasmon band). Figure 2.4 (b) is the transmission electron micrograph (TEM) image of nanorods with average aspect ratio of ~3.3. A well control aspect ratio of nanorod has be done in this work.

Figure 2.4: (a) absorption spectrum of a gold nanorod sample with an average aspect ratio of 3.3. The band at 525 nm is referred to as the transverse plasmon resonance, while the one centered at 740 nm is the longitudinal plasmon absorption. (b) TEM image [6]

Although two different surface plasmon modes are excited due to the geometry of nanorods, the absorbance property due to dielectric variation seems to be the same

with that of metal spherical nanoparticles. The resonance wavelength shifts to red when increasing the dielectric constant of surrounding environment. In Figure 2.5 (a) and (b), the theoretical dielectric and aspect-ratio dependent absorbance spectra of metal nanorod are shown. By increasing the environment dielectric constant and the aspect ratio of metal nanorod, the maximum absorbance in the spectra shifts to red band. This characteristic determined the macroscopic optical property and that provides a potential to externally modulate the localized surface plasmon effect.

Figure 2.5: Theoretical absorbance spectra of metal nanorod with various aspect ratios and surrounded in environment with various dielectric constants. [6]

Besides the above theoretical analysis, Catherin J. Murphy and Nikhil R. Jana proposed an experimental observation of aspect-ratio dependent optical absorbance spectra. In this experiment, a seed-mediated growth approach is used to make metallic nanorods and nanowires in homogeneous solution. Figure 2.6 (a) shows the evolution begins with the synthesis of metallic nanospheres by chemical reduction of a metal salt with a strong reducing agent such as sodium borohydride. Citrate is employed as a capping agent to prevent particle growth. The gold or silver spheres with diameter of 3-5 nm are generated and become as seeds to grow more anisotropic nanostructures.

Figure 2.6 (b) is the transmission electron micrograph (TEM) of the gold nanorods with an aspect ratio of 18.

Figure 2.6: (a) A seed-mediated growth for gold and silver nanorods and nanowires. (b) TEM of gold nanorods, aspect ratio 18, made by the seed-mediated growth approach. [7]

This seed-method provides a very good control of the aspect ratio of metal nanorod. As shown in figure 2.7 (a), the maximum absorbance shifted to longer wavelength as increasing the aspect ratio of metal nanorods. This variation of absorbance spectra lead to various scattering light due to aspect ratio. The result confirms the theory prediction.

Figure 2.7: Aqueous solutions of silver nanorod show (A) absorbance spectra and (B) a beautiful variation in visible color depending on the aspect ratio of the suspended nanoparticles. : (a)-(f) silver nanorods of aspect ratio 1-10 [7].

In summary, there are so many factors, such as geometry and dielectric property of surrounding environment, significantly affect the excitation of localized surface plasmons on the metallic nanoparticle and leading to the particular optical property.

On the base of these advantages, high potential of sensor and classifying applications, can be accomplished simply.

2.2 Preparation and Arraying of metal material

There are two general ways available to produce nano-structure materials. As shown in Figure 2.8, the first way is to start with a bulk material and then piece it into smaller fragments via using mechanical, chemical or other form of energy (top-down).

The opposite approach is to synthesise the material from atomic or molecular species by chemical reactions, allowing for the precursor particles to grow and become bigger size (bottom-up). Most of the researchers are interested to well control of particle size, shape, size distribution, particle composition, and the degree of particle agglomeration.

In this section, we will review three previous works included the fabrication of metal nanoparticles and metal nano-array.

Figure 2.8: Two basic approaches to nanomaterials fabrication: top-down (from left to the right) and bottom-up (from right to the left).

2.2.1 Photoinduced conversion of silver naonspheres to nanoprisms

In general, spherical silver particles are prepared by injecting NaBH4 solution to an aqueous solution of AgNO3, with a presence of trisodium citrate.

Bis(p-sulfonatophenyl) phenylphosphine dihydrate dipotassium salt solution (BSPP) is subsequently added by drop-wise addition to the solution as a particle stabilizing agent. After the radiation of the conventional fluorescent light, the metal nanoparticle can be produced in the solution.

Rongchao Jin presented a time-dependent spectroscopic observation correlated with TEMs of silver nanoparticles. In Fig. 2.9 (a), we can find that the initial silver particles (~8 nm) are converted from spherical to triangle structure. During the initial stages of growth, both spheres and prisms exist at the same time. After 70 hours, most of the initial spheres (.99%) are converted to the prismatic structures.

In this growth reaction, they also observed a set of color changes of the sample.

Initially, the solution is yellow and then turned green and finally blue in whole growth of 70 hours. From the analysis time dependent extinction spectrum in Fig. 2.10 (B), a decrease in intensity of the surface plasmon band at λmax = 400 nm (for the spherical particles) was turned to a growth of three new bands of λmax = 335 (weak), 470 (medium), and 670 nm (strong), respectively. After 70 hours, the band at 400 nm completely disappeared. This optical response also implied the growth evolution of silver nanoparticle from spherical to triangle shape.

Figure 2.9: (A): Three distinctive stages are identified in nanoprism formation: induction, growth, and termination. (B) Time-dependent UV-Vis spectra showing the conversion of silver nanospheres to nanoprisms (a) before irradiation and after (b) 40, (c) 55, and (d) 70 hours of irradiation. [2]

In Figure 2.10 (a) and (d), TEM pictures shows the whole growth of nanoparticles. Size and population of the Ag prisms increase with time and a concomitant decrease in the number of spherical particles can be found. Figure 2.10 (D) shows the most of nanoparticles turned to triangle shape. These data clearly show

that the silver nanoprisms evolve from the initial spherical nanoparticles. This work proposes a detail time dependent observation on the evolution of photoinduced chemical synthesis and that leads to a advance understanding of chemical synthesis mechanism.

Figure 2.10: TEM images (reverse print) mapping the morphology changes (A) before irradiation and after (B) 40, (C) 55, and (D) 70 hours of irradiation. Except for the inset in (A), the scale bar is 200 nm for all four images. [2]

2.2.2 Electron beam lithography (nano-array)

Electron beam lithography is employed a beam of electrons to scan in a pattern a resistant surface and the pattern can be transferred on the sample surface by the selectively removing either exposed or non-exposed regions of the resist. The purpose is to create very small structures and to create very nano-scale electronic devices.

The primary advantage of electron beam lithography is to beat the diffraction limit of light and make features in the nano-scale regime. This form of maskless lithography has found wide usage in mask-making for photolithography, low-volume production of semiconductor components, and research and development.

Figure 2.11 shows a two-dimensional photoresist dot array covered by a thin silver film. The pattern was prepared by an e-beam lithography system. This approach provides a precise control of array structure in and a good band gap property can be obtained as this photonic device. By altering the factors, such as spacing and size of the metal dots, of this nano-array, the band gap can be modulated for certain purpose in optical photonic application.

Figure 2.11: (a) A SEM of hexagonal array of dots. (b) The energies of the upper and lower branches of the SPP energy gap as a function of the propagation direction. [9]

2.2.3 Focused ion beam (FIB)

Focused ion beam (FIB) is inherently destructive to the specimen. When a high-energy gallium ion-beam strikes the sample, the atoms are sputtered from the surface. FIB, like e-beam lithography, is often used in the semiconductor industry to patch or modify an existing semiconductor device. For example, in an integrated circuit, the gallium beam could be used to cut unwanted electrical connections, or to deposit conductive material for a connection.

In the surface plasmon study area, FIB is a good tool to fabricate a metal nano-structure or metal array. V. Vlasko-Vlasov uses FIB system to fabricate Ag circular nanoslits with width of 140 nm and diameter of 7.55 um, as shown in Fig.

12(a). The circular nanoslits excite strong surface plasmon polaritons (SPPs), resulting a very interesting interference patterns due to plasmons emitted from opposite sides of the circle. The sample is illuminated from the bottom by a tunable gas laser generating a series of sharp lines between 476 and 676 nm. The resulting SPP interference patterns were imaged by a near-field scanning optical microscope (NSOM) with a 50 nm fiber aperture (Figure 2.12 (b)).

Figure 2.12: (a) SEM image (aken at an angle of 52°) of circular nanoslits of 7.55 um diameter and 140 nm width. (b) Schematic of the NSOM setup. (c) NSOM image of the SPP interference pattern at λlight=568 nm. [10]

Figure 2.12 (b) illustrates a typical NSOM picture of the plasmon intensity. The obvious interference pattern presents the existence of SPP. NSOM images are largely a representation of the in-plane electric field intensity on the surface of the film. The interference pattern has a period equal to half the SPP wavelength, λ^SPP/2, the usual period of the standing wave intensity. With decreasing frequency the period increases, however, remaining always smaller than λ^light /2. The FIB approach to precisely fabricate metal nano-structure leads to an advance study for localized surface plasmon effect.

A precise hole-array can also be fabricated by FIB. Figure 2.13 shows a gold hole-array with a diameter and spacing of 300 nm and 400 nm, respectively.

Figure 2.13: A silver nano-hole array fabricated by FIB system

In Figure 2.14, the transmission spectra of hole-arrays display peaks that can be tuned by adjusting the period and the symmetry. For the blue, green and red arrays, the periods were 300, 450 and 550 nm, respectively, the hole diameters were 155, 180 and 225 nm and the peak transmission wavelengths 436, 538 and 627 nm. The arrays were made in a free standing 300 nm thick silver film.

Figure 2.14: (a) Normal incidence transmission image and (b) spectra for the subwavelength hole-arrays. [11]

In summary, the development of nano-technology promotes the advance of metal nano-structure system. Since the nano-structure can be designed and fabricated in demanded nano-optics device, the optical phenomena of reflection, transmission, propagation, and diffraction, are duplicated in marco-vision optics. The preparations of metal nanoparticle system mentioned in above system provide a easy way to fabricate nano-device for certain purpose. A well control of metal nanoparticle shape can be achieved by chemical synthesis. However, for a complicate metal nanoparticles system, E-beam lithography and FIB is proper to the fabrication of device. Actually, no matter which way to manufactory of optical metal nano-device, a creative idea is essential to advance study.

2.3 Application and development of localized surface plasmon resonance (LSPR) In this section, I will introduce the previous works which present the potential application and development of metal nano-device, such as sensor, light channel, condenser, bio-detector, and so on. The realization of these devices not only exhibits interesting physical and chemical phenomena but also leads to a potential for nano-techonology.

2.3.1 Surface plasmon enhanced Raman scattering [12, 13]

Due to the weak signal from the molecular structure, it’s difficult to detect the information from the molecular bound. The surface plasmon effect excited on the metallic nano-particles can enhance the signal, known as the surface plasmon enhanced Raman scattering. For this purpose, metallic nanoparticles were widely applied to the observation of weak signal exhibited from molecules.

Shuming Nie proposed the Raman spectra of a single rhodamine 6G molecules adsorbed on the selected nanoparticles, the intrinsic Raman enhancement factors were on the order of 10¹⁴ to 10¹⁵, much larger than the ensemble-averaged values of conventional measurements. This strong enhancement leads to a vibrational Raman signals with more intense and more stable than single-molecule fluorescence. The enhancement of localized electromagnetic field of molecule due to the exciting surface plasmons improves the sensitivity for detection of individual molecule or individual metal nano-particle.

Figure 2.14 show the surface-enhanced Raman spectra of R6G obtained with a linearly polarized confocal laser beam from two Ag nanoparticles. The R6G concentration was 2x10^-11 M, corresponding to an average of 0.1 analyte molecule per particle. By changing the polarization direction of laser corresponding to the expected particle orientation, the enhanced spectra can be obtained when the polarization parallel to the long axis of particle. All spectra were plotted on the same intensity

scale in arbitrary units of the CCD detector readout signal.

Figure 2.15: Surface-enhanced Raman spectra of R6G obtained with a linearly polarized confocal laser beam. When polarization direction is parallel to the long axis of particle, the weak signal can be enhanced via the excitation of surface plasmons on Ag nanopartilces. [12]

2.3.2 Interference of locally excited surface plasmons

Surface plasmon interaction in a metal nano-slit has mentioned in section 2.2.3.

In the following, the SPPs emerged metal nanoparticles also inferred each others and the interference pattern due to the presence of surface plasmons wave can be observed SNOM system. In L. Novotny’s work, a system, shown in Figure 2.15 (a), consists of several protruding particles located on the surface of the layer and the protrusions on the silver layer are considered as small, dipolar silver particles of variable size. The probing particle is locally excited by the monochromatic field Eo. An optical scan image of the surface of the silver layer is obtained by recording the far-field radiation for every position (x,y) of the optical probe.

In Figure 2.16 (b), they found a strong iterference of the locally excited surface plasmons revealed by detecting the radiation emitted into the lower half-space at angles beyond the critical angle of total internal reflection. The interference was theoretically investigated via using coupled dipole formalism.

Figure 2.16: (a) Investigation of surface plasmon interactions with a four-particle, photo tunneling image model Forbidden light scan image for the configuration (b) Experimental images recorded with aperture SNOM. Forbidden light scan image (left) and shear-force scan image (right). The parameters of the layered reference system correspond to those of the theoretical model.[14]

Because of the long range interactions of surface plasmon waves, they assumed simply dividing the objects into a number of dipolar cells and analyze the interference pattern occurred between particle. The theoretical analysis is based on a coupled dipole formalism that uses Green’s functions of a layered reference system. Figure 2.15 (a) shown the theoretical calculation of interaction of surface plasmons emerged from particles. The theoretical prediction is in a good agreement with experimental result.

Coupling mechanisms between surface plasmon waves and their far-field radiation have been discussed in this work. The optical probe as well as the protruding particles is modeled as single dipolar. They simply the complicated physical system of larger objects with arbitrary shape into a dipolar assumption.

2.3.3 Plasmon wave between metal nano-particle [15-17]

On the base of the observation of interference of surface plasmon polariton emerged from metal nanoparticles, we can assume the SPP behaviors like wave. There are many studies show that the light can be propagated along a metallic nano-particles array and the electromagnetic wave is well confined between particles. Figure 2.17 (a) shows a ~15 um-length metal nano-particles chains consisted of Au and Ag rods, respectively.

Figure 2.17 (b-e) shows optical microscopy images of Au and Ag rods under prism-coupled total internal reflection (TIR) illumination conditions of wavevector components parallel to the long axes of the rods. When the particle chains are illuminated at 532 nm (b and d) and 820 nm (c and e), the rods scatter light and interacted the incident beam. Although light couples into the rod at all points along its long axis, the coupling efficiency is expected to be highest at the tip first interacting with the laser. In Figure 2.17 (b), only the input end of the Au rod is observed under illumination of 532 nm laser. Light is only observed at both ends of the Ag rod (Figure 2.17 (d)). Upon illumination at 820 nm, however, both the Au (c) and Ag (e) rods display intensity patterns that are similar to Ag at 532 nm, light emerges from both the input and output ends of the rods.

In this work, the plasmon can propagate several micron distance and the small dimensional limitations for plasmon excitation, plasmon waveguides from high aspect-ratio metal nanostructures can be created. When plasmon propagation is initiated parallel to the structure’s long axis, the incident optical energy passes down the length as a surface-bound plasmon mode and reemerges from the end as a photon.

Functionally, this process leads to creation of the two nanoscale optical tools described herein: (i) tens of micron-scale conduits for transporting optical information in nanometer-scale structures and (ii) optoelectronic components that control the

directional flow of optical information. If the light can propagate unidirectionally and the scattering loss can be controlled, this plasmon light channel provides a potential application.

Figure 2.17: (a) Photograph of 20 nm diameter Au and Ag rods. The two labeled rods are typical of the ones used for these experiments. (b) Optical microscope image of a 4.7mm long Au rod exposed to through-prism TIR illumination at 532 nm. Note the strong scattering at the rod input and the absence of scattering at distal tip. (c) Image of the same Au rod under TIR illumination at 820 nm. Under these conditions, both ends of the rod exhibit strong emission/scattering. (d)-(e) A 4.7mm long Ag rod illuminated at 532 and 820 nm, respectively. Emission is observed from both tips at both wavelengths in the case of Ag. [15]

In theoretical analysis of plasmon propagation between metal nano-particles, L.

A. Sweatlock shows a strong confine of electromagnetic wave between metal nanoparticles. Figure 2.18 illustrates the spatial images of the peak instantaneous electric field intensity at steady state for arrays of four 10-nm-diameter particles, excited on resonance. The distances between particle center are (a) 4 nm, (b) 2 nm, or (C) 1 nm. High confine electric energy is observed between particles. The maximum

local field occurs in the dielectric gap between the two metal particles at the midpoint of the array.

For comparison, the maximum field intensity enhancement near an isolated nanoparticle driven resonantly is typically 30. The giant 5000-fold intensity enhancement is consistent with previous reports of the enhancement of the effective Raman scattering cross section near metallic nanostructures. Since this high confine and high enhancement of electric field energy, the light can be propagate in a metal nano-particles chain.

Figure 2.18: Two-dimensional spatial images of the electric field intensity in a plane through the particle centers of four Ag nanospheres with interparticles spacing of (a) 4 nm, (b) 2 nm, and (c) 1 nm at resonant excitation. [16]

They also show that there are two kinds of longitudinal resonance appeared in

They also show that there are two kinds of longitudinal resonance appeared in