In this article, we reviewed new designs for plasmonic nanostructure and its nanofocusing, coupling, resonance, and waveguide characterizations. With in-plane Fresnel zone plates, a 15 times field enhancement was achieved for the plasmonic nanofocusing. Plasmonic antennas placed at both ends of a single nanowire, consisting of a nanooptical circuit, was successfully realized in an enhanced plasmon coupling and emission. Plasmonic Fano resonance was also observed in a single sliced Ag nanodisk with the structure symmetry breaking. A hybrid plasmonic waveguide with a CdS nanoribbon placed on Ag surface showed an excellent mode confinement and energy low dissipation. More potential applications based on these plasmonic configurations are also discussed in this article.
Surface plasmon polaritons (SPPs) are collective electromagnetic excitations that propagate at an interface between dielectric and metallic layers, evanescently confined in the normal surface of the interface . With the field enhancement, subwavelength propagation, and near-field coupling effects, the application of SPP nanostructure has induced great attention, such as plasmon nanofocusing [2, 3, 4], subwavelength waveguide [5, 6, 7], molecules detection , surface Raman scattering enhancement [9, 10, 11], and quantum dots photoluminescence emission [12, 13]. In this article, we reviewed the latest designs of plasmonic nanostructure and the characterizations of corresponding plasmon enhanced focusing, coupling, resonance, and waveguide.
The in-plane Fresnel zone plates (FZPs) nanostructures are designed to realize plasmonic focusing, which are realized by phase modulation to achieve constructive interference [2, 14]. With regard to SPP coupling, the SPP modes focused at the FZP focus area used as a source of SPP waveguide was coupled to a CdS nanoribbon, and realized the separation of the SPP modes and the CdS photoluminescence background. A new Ag nanowire-nanoantenna optical circuit with a single Ag nanowire placed at both feed-gaps of receiving and transmitting bowtie-antennas pairs has been designed [15, 16], which shows a significant plasmon coupling and emission enhancement over previous designs. For the plasmon resonance, a Fano transmission window was observed in a single symmetry breaking Ag nanodisk at a normal incidence, which is induced by the overlap between the broad hybridized dipole and a narrow quadrupole mode . For the plasmonic waveguide, a CdS nanoribbon placed on the Ag film was served as the dielectric medium on the top of metal surface to generate a hybrid plasmonic waveguide [18, 19, 20]. The dielectric-loaded SPP (DLSPP) waveguide achieved a less energy dissipation, and great mode confinement than the traditional metallic stripe waveguides. The DLSPP induced a spectroscopic redshift of 30 meV for the CdS nanoribbon photoluminescence, which presents color-tuning and switching optical transport characters.
2 Plasmonic nanostructures
New plasmonic nanostructures and their characterization of plasmon focusing, coupling, resonance, and enhancement waveguide are reviewed as following. In Sect. 2.1, an in-plane Fresnel zone plates (FZPs) nanostructure was designed to realize plasmonic focusing, which can be used as a source for the SPP waveguide, where the focused SPP wave was coupled into a CdS nanoribbon. A new Ag nanowire-nanoantenna optical circuit was investigated as a SPP coupling device in Sect. 2.2. For the plasmonic resonance, a Fano resonance was observed in a single symmetry broken Ag nanodisk as it shown in Sect. 2.3. For the hybrid SPP waveguide, a CdS nanoribbon placed on the Ag surface was investigated with details in Sect. 2.4 containing particular analysis of experimental results and FDTD simulations.
2.1 SPP focusing: a subwavelength source
With a shorter wavelength, SPP can be focused into a subwavelength spot size, which enable applications for nanooptics , super-resolution imaging [22, 23], nanolithography , high harmonic generation , near-field imaging and sensing [26, 27], etc. SPP have been focused by coupling the light into SPP modes with an array of concentric circular metallic slits , a planar circular grating milled into an Ag film , or V-shaped channels etched in metal film with tapered ends . However, all these focusing techniques are based on fabricated nanostructures, that is, SPP modes propagate along the track predefined by the structure and generate a huge field enhancement at the tapered end. This kind of SPP focusing induces a field disturbance caused by the predefined nanostructures, thus it cannot be used to investigate the origin of the SPP device’s properties. In this section, we demonstrate the in-plane Fresnel zone plates (FZPs) designed to realize plasmonic focusing by controlling the field transmission through the components [2, 15]. A focused SPP source at the FZP focus area was generated, which separated from the SPP extracting region to eliminate the field disturbance.
2.2 SPP coupling: nanoantenna
How to enhance the efficiency of source light coupling into a nanoscale plasmonic component is a challenge for the plasmonic devices. The plasmonic focusing, as discussed above, gives a way to avoid this problem. But sometimes, it still needs the coupling of a light wave to a nanoscale optical component directly. Ag nanoparticles , nanorods , nanogaps  were reported to be efficient antennas for coupling of visible light into the localized SPP field. Further considering of the nanoscale optical nanocircuits designing, it is also important to coupling of the localized SPP filed in nanoantenna into another optical component, such as the SPP waveguids .
In the theoretical analysis, the configuration can be seen as an analog to an equivalent of two-wire transmission line (OTL) electrical circuit. The principle of impedance matching was used to optimize the OTL at optical frequencies. With an optimized geometry of bow tie antennas and the incident angle, an Ag nanowire with the 10 μm length, 250 nm for the arm length of the bow tie antennas, and 28∘ for the incident angle of the excitation laser were obtained by both theoretical calculation and experiment; an enhancement factor of 45 was recorded for the maximum plasmon emission was measured. The optical nanocircuit consists of a metallic nanowire and receiving/transmitting antenna pairs, which can realize a great plasmon emission enhancement, providing a practical way to build future plasmonic devices by using metallic nanowires that suffer huge Ohmic dissipation and low plasmon coupling and emission efficiencies.
2.3 Fano resonance
The SPP waveguide, with a cross section at the order of hundreds of nanometers in diameter and a propagation length of ten to hundreds of microns, has been served as a prospective type of optical information carrier in an integrated photonic device. The groove etched on a metal surface and the metal stripe have been investigated for the plasmonic coupling and propagation [35, 36], but due to the Ohmic losses, the SPP propagation length is limited to the order of 10 μm in the case of noble metals at visible wavelengths. In order to achieve less energy dissipation and excellent mode confinement, dielectric SPP waveguides on a metal surface [dielectric-loaded SPP (DLSPP)] have been implemented. The refractive index for the SPP wave on the metal-dielectric interface is significantly higher than on the outer air-metal interface. Due to the effect that the optical field tends to be confined in regions with higher refractive indexes, the SPP confinement is primarily achieved. By choosing the appropriate DLSPP waveguide geometry parameter, the low propagating loss can be achieved simultaneously. Using SiO2 and polymer stripes with an Au surface to generate the DLSPP waveguide has been proposed, and analyzed both theoretically and experimentally [37, 38]. However, this kind of DLSPP mode can only be excited at a near-infrared range, and its architecture is highly dependent on the nanofabrication technique. CdS nanoribbons, with photoluminescence and optical transport properties at visible range, can serve as a dielectric supporter on the top of the metal surface to generate a DLSPP waveguide. More recently, X. Zhang et al. reported a nanometer-scale plasmonic laser, by using a hybrid plasmonic waveguide consisting of a CdS nanowire separated from a silver surface by a 5 nm-thick insulating gap, generated optical modes a hundred times smaller than the diffraction limit .
A series of experimental works and theory analysis on this kind of hybrid plasmonic waveguide, from excitation to propagation, and to coupling [18, 19, 20] have been investigated. We experimentally confirmed the excitation of this kind hybrid plasmon mode by using the CdS nanoribbon deposited at the Ag film. The DLSPP induced a spectroscopic redshift of 30 meV of CdS nanoribbon photoluminescence at different positions of the stripe by SNOM. The CdS hybrid plasmonic waveguide shows color-tuning and switching optical transport characters. The guided PL spectra under various waveguide lengths demonstrate a spectroscopic red-shift caused by the semiconductor self-absorption effect (Urbach tail), and an energy compensation at the Ag film induced by the SPP resonance. A similar color-changeable properties of was observed on the plasmonic waveguide based on the Se-doped CdS nanoribbon. By using the FZP focusing structure, the focused SPP modes coupling into CdS nanoribbon were separated from the CdS photoluminescence background. The result demonstrates a novel approach for planar plasmonic focusing and the extraction of SPP modes from the dielectric PL background. The investigated properties suggest that this plasmonic structure has potential applications for the future photonics, and integrated circuits, especially for the color-changeable plasmonic device.
Here, we present the latest work on the CdS/Ag hybrid plasmonic waveguide and the photoluminescence spectra of the CdS nanoribbon with two luminescence emission bands (intrinsic band edge emission and defect-related emission). The peak place and the intensity of the emission spectrum modulated by the SPP resonance was observed by the near-field spectroscopy detection. These optical phenomena can be well understood by the Franz–Keldysh effect (FKE), which is induced by the electric field enhancement due to the SPP modes. The suggested plasmonic structure provides valuable information for the implementation of photonic integrated nanocircuits, especially for the color-changeable plasmonic device.
2.4.1 Preparation of nanoribbon
2.4.2 Characterization and analysis
On the other hand, due to the influence of SP’s electric field, the excited electrons in the conduction band and holes in the valence band tend to separate spatially, as shown in Fig. 7. This results in that the spatial overlap between electrons and holes wavefunctions is reduced, and thereby the oscillator strength is decreased. Eventually, the efficiency of the irradiative recombination between bands must be reduced. Such phenomenon is also reflected from the PL spectra in Fig. 8(b), where the intensity of the band I ceases to increase as the laser power increases to 35 mW from 80 mW, while the intensity of the band II increases remarkably at the laser power of 80 mW. This indicates that the amount of the electron-hole pairs (excitons) formed by laser illumination reduces, and the electrons in the conduction band tend to recombine irradiatively to the defect-related levels, as marked by red arrows in Fig. 8.
2.4.3 FDTD simulation
The inset in Fig. 10(b) shows the simulation model of the silver film with the randomly distributed hemispherical silver particles in the radius range of 50–100 nm mimicking silver islands. The relative dielectric constant of the silver is given by the modified Debye model, as ε=−7.6321+0.7306i. The CdS nanoribbon (refractive index n=2.64) is placed on a Ag surface. The size of the spatial grid cell is set as 5 nm. The wavelength of incident plane is 442 nm. When the laser beam is incident on this film, localized SPs are excited at the interface between air/silver, with the extraordinary enhancement of the electric field near the silver islands. Figure 10(b) shows the simulated electric field intensity distributions at the z=100 nm plane over the silver film, as the excited light is used. The electric field intensity is concentrated, and typically 30-fold enhanced, around the silver islands due to the localized surface plasmon resonance. Figure 10(c) shows the electric field intensity distributions in a rectangular section of the CdS nanoribbon placed on the rough silver film. The enhanced field induced by LSP extends into the CdS nanoribbon, which confirmed the contribution of LSP to the different stimulated luminescence behavior of CdS nanoribbons on the silver film.
In this article, we reviewed plasmonic nanostructures that were designed for plasmonic nanofocusing, coupling, field enhancement, and a subwavelength waveguide. The in-plane Fresnel zone plates (FZPs) nanostructures were fabricated to realize a 15 times plasmon enhanced nanofocusing. An Ag nanowire nanoantenna optical circuit with a single Ag nanowire placed at both feed-gaps of bowtie-antennas pairs shows significant plasmon coupling and emission enhancement. Plasmonic Fano resonance also was realized in a single sliced Ag nanodisk at a normal incidence, which is the result of the spectroscopic superposition between the bright dipole and a dark quadrupole mode. CdS hybrid plasmonic waveguide with a CdS nanoribbon placed on the Ag surface represents an excellent performance of mode confinement and low energy dissipation. The PL of the CdS hybrid plasmonic waveguide was proved that can be modulated by the plasmon resonance, which shows a spectroscopic redshift. These designs provide potential applications for the future optoelectronic devices.
This work is supported by the National Basic Research Program of China (Grant No. 2007CB936800), and the National Natural Science Foundation of China (Grants Nos. 60977015, 61176120).
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