Surface Plasmon Waveguides

Interest in exploiting SPs for opto-electronic device applications is happening at a time when continuous miniaturization of integrated electronic circuits is approaching fundamental limitations. For example, ever since the semiconductor industry entered the sub-100 nm technology nodes, there has been growing concerns over the issues associated with electronic interconnects [3] [5], such as signal delay and heat generation. While conventional fibre- optic interconnects offer speed-of-light propagation, large band-width and low power consumption, the dimensions of these dielectric components are generally at the microscale. This has severely limited their applications in nanoscale integrated electronics circuits.

In contrast to the free propagating light, SPs can be sustained and guided by nanoscale metallic structures without the constraint of the diffraction limits. In 1997 Takahara and co-workers first proposed the use of metallic nanowires to guide 1-D optical beams with nanometer diameter [88]. They suggested that in visible and near-infrared ranges the negative dielectric constants of metal removes the constraints of conventional dielectric waveguides:

0 < εclad < εcore

β, kx, ky, κx, κy ≤ ω√εcoreµo (3.1)

where εclad and εcore are the dielectric constants of the core and cladding; β

kx and ky are the transverse components of the wavevector inside the core

and κx and κy are those in the cladding. As kx and ky become imaginary

in metallic nanowires, the beam diameter is free from the limitation of the wavelength. Simulation [88] reveals that for the lowest order TM mode β increases as the core radius decreases and no transmission cut off in the core size.

The concept of nanoscale 1-D metallic waveguides has motivated many new researches in the field of plasmonics. However, currently realizing such structures still presents an engineering challenge. On the other hand, various 2-D SPP waveguides (waveguides of 2-D SPP waves) have been experimentally demonstrated in recent years, including: metallic stripes [55] [57] [89] [16] and wedges [90] [91] (insulator-metal-insulator configuration), metallic gaps [92] [93] and grooves [56] (metal-insulator-metal configuration), etc. Moreover, some complex SPP waveguiding elements based on the principle of straight waveguides, such as S-bent waveguides, four-port coupler, Y-splitter and coupler have also been reported [94].

Figure 3.3 illustrates light transport through SPP waveguides [16]. The device used for SPP waveguiding is shown in Fig. 3.3a; it consists of an array of 50-nm-thick varying width Au stripes attached to a large Au area, from which optical waves are converted to SPPs and launched onto Au stripes (2-D SPP waveguides) via a prism coupler. Photon scanning tunnelling microscope(PSTM) images of SPPs excited at λ = 780 nm and propagating along 3.0 µm, 1.5 µm, and 0.5 µm wide Au stripes are shown in Fig. 3.3b, 3.3c and 3.3d, respectively. It is clear that the propagation distance of SPPs decreases with decreasing stripe width. The losses are mainly due to the intrinsic resistance of the metal leading to resistive heating and wave decay. The losses of stripe waveguides embedded in symmetric environments are

§ 3. Plasmonic Applications and Recent Research Activities 42

considerably lower than that in asymmetric environments because the former support long range SPP excitation. It has been reported that the attenuation lengths in such arrangement can be as long as 13.6 mm when operated with light at λ = 1550 nm [89].

Figure 3.3: (a) A SPP waveguide consisting of an arrays of Au stripes attached to a large Au launchpad. (b)–(d) PSTM images of SPPs excited at λ = 780 nm and propagating along 3.0 µm, 1.5 µm, and 0.5 µm wide Au stripes, respectively [16]

.

An alternative approach for SP guiding is to use metallic nanoparticle arrays. It was initially proposed by Quinten et al. [95] that a linear chain of spherical metal nanoparticles can be used as a subwavelength-sized light guide in which light is transmitted by the coupling of interparticle plasmon oscillations. The subwavelength light confinement in metallic nanoparticle arrays was first verified by Krenn et al. [60] through collective excitation of LSPs on the arrays of nanoparticles, as shown in Fig. 3.4; and the EM energy transport along the nanoparticle waveguides was later demonstrated by Maier et al. through local spot excitation of Ag nanoparticle arrays interspersed with dye molecules via near-field scanning optical microscope (NSOM) tip pumping [96]. The trade-off between the field confinement lever and the

propagation loss also exists in nanoparticle waveguides. However, as LSP- fields are highly concentrated in the space between metallic nanoparticles rather than on the metallic structures in nanoparticle waveguides, they generally produce the tighter field confinement and lower Ohmic losses when compared to SPP waveguides [60] [15].

Figure 3.4: Constant height PSTM image of optical near-field intensity above a chain of Au particles on an ITO substrate, illuminated at λ = 633 nm. The individual particle size is 100 × 100× 40 nm3_{) and the separation between}

particles is 100 nm. The PSTM tip to Au particles distance is less than 45 nm [60].

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Recently, a 2-D fibre-accessible metallic nanoparticle waveguide, with measured light coupling efficiency to be as high as 75%, has been reported [14], which could offer the solution to the coupling problem between 3-D optical signals and low-dimensional SP modes. The structure consists of Au nanoparticles whose sizes are gradually decreased from the centre of the waveguide to the edge of the waveguide, as illustrated in Fig. 3.5, which

§ 3. Plasmonic Applications and Recent Research Activities 44

allows EM energy to be highly confined to the middle of the guide.

Figure 3.5: Finite-difference time-domain (FDTD) simulation of electric field in a 2-D fibre-accessible metallic nanoparticle waveguide. To enhance lateral field confinement, the sizes of the Au particles vary across the waveguide [14].

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SP waveguides offer opportunities for future integrated optical or photonic devices, although currently propagation losses and coupling problems are still major challenges for their implementation. In the parallel development of SP waveguides, a variety of 2-D optical elements and devices for SP manipulation have also been demonstrated recently, which are discussed in the next section.