Applied Physics A

, Volume 112, Issue 1, pp 15–22 | Cite as

Plasmonic properties and device in nanostructures



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.

1 Introduction

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 [1]. 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 [8], 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 [17]. 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 [21], super-resolution imaging [22, 23], nanolithography [24], high harmonic generation [25], 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 [28], a planar circular grating milled into an Ag film [29], or V-shaped channels etched in metal film with tapered ends [30]. 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.

Ag-column arrays and the in-plane Fresnel zone plate (FZP) structure with a Cu grating layer were fabricated by an electron beam lithography (EBL) and lift-off technique on the smooth Ag surface. The SPP field excitation and wave propagation with energy compensation were realized by using Ag-column arrays fabricated on the Ag surface with a Cu grating underneath, as shown in Fig. 1. With a p-polarized laser illuminating the Ag film, the SPP modes resonance can be excited when the phase matching condition is fulfilled. The Ag-column arrays were used to compensate the mismatch in the wave vector of SPP modes between the in-plane momentums of impinging photons. The Cu grating underneath was used as a mirror to collect the laser transmissions and couple them back to the SPP modes, functioning as a reflection layer. The in-plane Fresnel zone plate at the right side, consisting of a series of zones alternating in transmittance between transparent and opaque, was used to focus the SPP waves. Considering the diffraction of the SPP wave striking this structure from the left, the constructive interference of the SPP fields can be obtained at the focal point. Taking into account the effective detection point located at about 100 nm above the FZP focus, we obtained about 15 times of the field intensity enhancement at the focal length of 60 μm.
Fig. 1

(a) SEM image of Ag-column arrays and schematic of Cu grating for SPP energy compensation (inset); (b) nearfield optical image of SPP scattering during propagation in Ag-column arrays; (c) SEM image of Ag nanostructures with a 12-zone FZP for SPP focusing; (d) near-field optical image of SPP focusing with a 1 μm diameter focus spot. Figure reproduced with permission from [2]

This plasmonic focusing spot can be used as a source to stimulate CdS waveguide as shown in Fig. 2. Because the excitation source was the focused plasmons instead of incident laser, that is, no CdS PL modes would be excited. The SPP excitation and detection structures are located in different places (the left and right side of the FZP structure). Thus, the SPP modes can be successfully extracted/separated from the PL background.
Fig. 2

(a) Sketch of focused SPP coupled into CdS nanoribbon; (b) near-field optical image of SPP scattering with the ribbon end at the FZP focus. Inset is a schematic of nanoribbon deposited perpendicular to the SPP wave vector with one end close to the FZP focus area; (c) SPP scattering at the FZP focus and emission at the other end of the ribbon captured by a color CCD in far-field. Figure reproduced with permission from [2]

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 [31], nanorods [32], nanogaps [33] 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 [31].

An 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], as shown in Fig. 3. The receiving and transmitting bow tie antenna pairs placed at the two ends of the Ag nanowire respectively enhanced the plasmon coupling and emission efficiencies of the Ag nanowire. This design offers significant plasmon coupling and emission enhancement over previous designs.
Fig. 3

(a) Schematic of the antenna-wire optical nanocircuit with an Ag nanowire placed at the feed gaps of both receiving and transmitting antenna pairs. (b) The FDTD simulated electric field distribution of the antenna-wire nanocircuit. (c) Scanning near-field optical image for the plasmon propagation and end emission. (d) The equivalent electrical circuit for the model configuration. The generator represents the receiving antennas illuminated by the incident laser, ZS is the characteristic impedance of the Ag nanowire and corresponding SiO2 substrate, and ZL is the load impedance of the transmitting antenna pair. Figure reproduced with permission from [15]

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

Fano resonance arises from the coherent interference of “bright” and “dark” hybridized plasmon modes. Interest in coherent plasmonic phenomena in nanosystems is motivated by the usefulness of these effects in applications, such as high-sensitivity localized surface plasmon resonance sensing [34]. To investigate this resonance feature, it shows that a Fano resonance can be achieved in a remarkably simple, educed-symmetry planar plasmonic nanostructure consisting of a single nanodisk with a missing wedgeshaped slice [18]. As shown in Fig. 4, using the hydridization concept, the dipolar plasmon modes of the symmetry broken nanodisk can be understood as bonding (DB) and antibonding (DAB) combinations of the dipolar nanodisk plasmon DD and the slice plasmon DH. The charge density distributions associated with the spectral features of this structure provide a clear picture regarding the origin of the Fano resonance, as shown in Fig. 5. Feature 2, the spectral dip between features 1 and 3, corresponding to the Fano resonance, exhibits a distinctive and quite different charge distribution, surrounded by a larger dipole mode supported by the disk. The Fano resonance is due to interference between the DAB and the quadrupolar slice plasmon QH.
Fig. 4

Schematic of the plasmonic hybridization between the dipolar modes of the disk and hole structures, and the energy diagram of the degenerated plasmon bright and dark modes [17]

Fig. 5

Normalized extinction spectra (left) of a symmetric Ag (red) and symmetry broken (blue) nanodisks normal incidence.; (1–4, right column) edge charge distributions for the modes 1–4 denoted on the extinction curve, calculated as the divergence of the simulated electric field. The red and blue color represents the positive and negative charge, respectively. The figure was reproduced with permission from [17]

2.4 Waveguide

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 [39].

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

The CdS nanoribbons were synthesized by a chemical vapor deposition (CVD) method [13]. In the synthesis process, CdS (99.995%) powders were used as the source, and pieces of Si wafers covered with a 10 nm thick thermally evaporated Au catalysts were used as the substrates [40]. Figure 6(a) presents a representative scanning electron microscopy (SEM) image of the as-prepared CdS nanoribbons on a silicon substrate. We can see that the CdS nanoribbons have smooth surfaces and uniform widths along the growth directions. Usually, the CdS nanoribbons are about several hundred microns in length, 0.3–10 μm in width, and 50–300 nm in thickness, which can be partially controlled by the synthesis temperature. The silver film with thickness of 80 nm was deposited on a glass substrate by thermal evaporation, and then it was lithographically fabricated into a 100×100 μm2 quadrate grid. The nanoribbons were dispersed in alcohol by ultrasonication and dropped onto the silver grid. The investigated individual nanoribbon was shown in the optical image in Fig. 6(b). This nanoribbon with rough estimation of the width of 1 μm and thickness of 50 nm lies above both the dark region (bare glass) and bright region (silver film). During the photoluminescence (PL) measurement, the excitation laser beam of 442 nm (He–Cd laser) was first focused onto the position A of the nanoribbon located above the bare glass, and secondly the position B located above the silver film. The luminescence signal of the microarea of A and B was detected by an Al-coated, tapered scanning near-field optical microscope (SNOM) fiber tip with a 100 nm apex diameter, which transmits the optical signal to a spectrometer.
Fig. 6

(a) SEM image of the as-prepared CdS nanoribbons and TEM image of a representative individual CdS nanoribbon (inset). (b) Microscope image of the investigated individual CdS nanoribbon placed on the silver film and the bare glass. The position A (over the bare glass) and B (over the silver film) are the locations where the laser was focused

2.4.2 Characterization and analysis

Figure 7 shows the PL spectra obtained separately from the microarea position A and B of the individual CdS nanoribbon at the different laser power, where the luminescence band I peaked at 510 nm is ascribed to the intrinsic band edge emission (C–V transition) and the band II peaked at 605 nm is ascribed to the defect-related emission (C–A transition) [41]. Figure 7(a) presents the PL of the segment nanoribbon located on the bare glass (position A), where the intensity of band I and II both increases monotonously, and the peak positions of the two bands stay unchanged with an increasing excitation laser power from 35 mW to 80 mW. While for the PL from the segment of the nanoribbon located on the silver film (Fig. 7(b), position B), the peak position of the band I occurs red-shift; the red-shift magnitude becomes larger as the laser power increasing. The maximum of the measured red-shift magnitude is about 37 meV (see the inset of Fig. 7(b)). For band II, there is no red-shift in the peak position, only the intensity increases monotonously.
Fig. 7

The PL spectra of the segment nanoribbon (a) located on the bare glass, (b) on the silver film, with incident laser power from 35 mW to 80 mW. Inset shows red-shift value of band I (on the silver) vs. incident laser power

During the experiment, except for distinguish between the substrate with and without Ag film, all the other experimental conditions are unique for the two positions. The excitation condition of a DLSPP mode in the structure can be described by the phase matching condition:
$$ k_0n_1\sin \theta\sin\delta =k_0R(n_{\mathrm{eff}}) $$
where k0 is the wave vector in free space, n1 is the refractive index of the dielectric, and neff is the associated complex effective index. The laser beam illuminates the sample with an angle of incidence θ and an azimuth δ; the scattering on the Ag surface generates a continuum of wave vectors, some of which satisfy the condition given in Eq. (1) and excite the supported mode. Moreover, the LSPs are excited by the incident laser at the Ag surface without the matching condition of the wave vector. We consider that the DLSPP modes and LSP modes contribute to different stimulated luminescence behavior. The PL from the nanoribbon’s segment located on the Ag film (on position B) must be influenced by the enhanced electric field of SPP modes that is excited at the same time with nanoribbon’s fluorescence.
This phenomenon can be explained as the typical optical phenomenon of semiconductors called the Franz–Keldysh effect (FKE), which states that the electric field can cause a red-shift of the adsorption edge, giving rise to the presence of an absorption tail for band-to-band transitions [42, 43]. For the CdS nanoribbon, Fig. 8 presents the band structure with (a) and without (b) an SP’s electric field perpendicular to the direction of nanoribbon surface. Under the influence of electric field, the electrons and holes shift into their respective corners, reducing their energetic separation [16]. Such narrowing of the energy gap renders the red-shift of the interband emission, as shown in Fig. 8(b). The magnitude of red-shift ΔW of the FKE is expressed as [44]
$$ \Delta W =\biggl[\frac{\hbar^2e^2E^2}{2m_r}\biggr]^{\frac{1}{3}} $$
where e and mr are the electron charge and the reduced mass for the electron and hole. From Fig. 7(c), the range in the magnitude of red-shift is about 10–30 meV in our case, according to Eq. (1); the SP’s electric field is at the order of magnitude of 104 V/cm.
Fig. 8

Sketch of the band structure of a CdS nanoribbon without (a) and with (b) an applied static electric field. With an external electric field, the electrons and holes shift into their respective nanoribbon’s boundary, reducing their energetic separation. Here, CB refers to the conduction band, VB the valence band, and IEL the defect-related level

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.

Generally, the Franz–Keldysh absorption coefficient α can be approximated by an exponential function [45]:
$$ \alpha (\hbar\omega, E)\sim \exp\biggl[-\biggl(\frac{E_g-\hbar\omega}{\Delta W} \biggr)^{\frac{3}{2}}\biggr] $$
Here, we consider that the absorption occurs mainly at the wavelength of 605 nm, and calculate α as a function of the electric field E that is in the range of about 104 V/cm obtained from Eq. (1). Figure 9 shows that the intensity of band II from the experimental PL spectra agree well with the calculated results based on Eq. (2), which is consistent with the analysis that the recombination of the electron and hole is mainly completed by the C–A transition instead of the C–V transition under the influence of the SP’s electric field.
Fig. 9

The integral intensity of emission band II of the nanoribbon’s segment over the silver film, and the corresponding calculated result

2.4.3 FDTD simulation

Finite-difference time-domain (FDTD) simulations were preformed to confirm the electric field enhancement induced by the SPP resonance. The simulation of the excitation of DLSPP can reference the previous work of our group. Here, we calculated the field enhancement of LSP at the rough surface by loading mimicking silver islands. The experimental prepared 80-nm thick silver film deposited by thermal evaporation has a rough surface and full of nanometer-sized silver islands, as shown in Fig. 10(a).
Fig. 10

(a) Topographic image of the 80-nm-thick rough silver film surface measured by atomic force microscopy with the area of 1×1 μm2. (b) Calculated electric field distribution on the rough silver film surface, with the wavelength of the incident light 442 nm, the inset of (b) is the calculation model for the rough silver film. (c) Calculated electric field distribution in the cross section of CdS nanoribbon, which is placed on the silver film. The black dotted rectangle indicates the cross section of the CdS nanoribbon

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.

3 Conclusions

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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Copyright information

© Springer-Verlag 2012

Authors and Affiliations

  • Shan Huang
    • 1
  • Zheyu Fang
    • 1
  • Jie Li
    • 1
  • Feng Lin
    • 1
  • Xing Zhu
    • 1
    • 2
  1. 1.School of Physics, State Key Laboratory for Mesoscopic PhysicsPeking UniversityBeijingChina
  2. 2.National Center for Nanoscience and TechnologyBeijingChina

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