Abstract
Plasmonics is a rapidly expanding field, founded in physics but now with a growing number of applications in biology (biosensing), nanophotonics, photovoltaics, optical engineering and advanced information technology. Appearing as charge density oscillations along a metal surface, excited by electromagnetic radiation (e.g., light), plasmons can propagate as surface plasmon polaritons, or can be confined as standing waves along an appropriately-prepared surface. Here, we review the latter manifestation, both their origins and the manners in which they are detected, the latter dominated by near field scanning optical microscopy (NSOM/SNOM). We include discussion of the “plasmonic halo” effect recently observed by the authors, wherein cavity-confined plasmons are able to modulate optical transmission through step-gap nanostructures, yielding a novel form of color (wavelength) selection.
Reviewed Publication:
Lewis Aaron
1 Introduction
Surface plasmon polaritons (SPPs) are two-dimensional (2D) electron density oscillations along a metal/dielectric interface resulting from an interaction between free photons in the dielectric and free electrons in the metal [1]. SPPs decay exponentially with distance away from the interface, but propagate along the interface, being subject to ohmic and radiation losses [2]. Since their discovery in the 1950s [3, 4], SPPs have found a broad range of applications, such as waveguiding [2], light trapping for photovoltaic solar cells [5–9], surface-enhanced Raman scattering (SERS) [10, 11], plasmon rulers [11], localized surface plasmon resonance bio/chemical sensing [11, 12], and plasmonic circuits [13], among many others [14]. Accompanying the still-growing number of applications of SPPs is an increased understanding of their underlying physics. Spatial mapping techniques of SPPs are playing a major role in in-depth investigations of SPP properties [15, 16]. Due to the hybrid optical-electronic nature of SPPs, they can be excited by either photons or electrons. This fact leads to two branches of spatial mapping techniques, namely, optical mapping and electrical mapping. Due to the fast speed of SPPs and the relatively slow speed of existing spatial mapping techniques, standing wave patterns (e.g., 2D cavity modes) of SPPs need to be established in order to capture or image their wave nature. Thus, optical and electrical mappings of SPP cavity modes have become the standard techniques for SPP investigations [17].
We review the state-of-the-art in optical and electrical mapping techniques for SPP cavity modes. We start with a brief summary of SPP excitation schemes in Section 2, given their close relation to detection techniques. We then review in Section 3 near-field scanning optical microscopy (NSOM) as a major spatial mapping technique of SPPs. Several recently published works, including our own contribution of the plasmonic halo effect, are described in some detail to illustrate the NSOM mapping technique. In Section 4, another near-field mapping technique, fluorescence-based SPP mapping, is introduced, along with several engaging examples. Finally, we discuss in Section 5 electrical mapping techniques for SPPs, including electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) microscopy. Other SPP mapping techniques, including leakage radiation microscopy (LRM) [18–22] and dark field imaging [23] for local SPP field intensity mapping, are not described in this review.
2 Excitation of surface plasmon polaritons
Before delving directly into the spatial mapping techniques of surface plasmon cavity modes, it will be useful to briefly review SPP excitation methods. The hybrid optical-electronic nature of SPPs leads to two branches of excitation schemes, namely, via photon coupling and via electron coupling. We organize this section according to these two schemes in a chronological manner. In general, a particular plasmon mapping technique works well with a specific SPP generation scheme. Typically, a mapping technique is the downstream development of the corresponding SPP excitation technique. In the following, we will give a brief summary of various methods of SPP excitation, including critiques on them and their interfaces with the plasmon mapping techniques, especially the NSOM technique.
2.1 Photon coupling
SPPs cannot be excited directly by injecting photons on a perfectly flat metal/dielectric interface, due to a momentum mismatch between SPPs and freely-propagating photons [24]. Since the early 20th century, various methods have been developed to compensate for this mismatch in order to, in the language of optics, achieve phase-matching.
2.1.1 Prism coupling
Prism coupling provides phase-matching by sending light into a medium with higher refractive index (relative to air or free space, e.g., a glass prism) before engaging the metal/air interface, as shown in Figure 1(A). This scheme is also called attenuated total internal reflection. In 1968, Kretschmann [25] and Otto [26] demonstrated their own prism coupling schemes, which are now widely used in optical SPP generation, and SPP-based biosensing [12]. Figure 1(A) shows the Kretschmann scheme on the left, and the Otto scheme on the right. The advantages of the prism coupling technique are that it can achieve very high coupling efficiency between photons and SPPs, and that it can be easily integrated with the NSOM technique to serve as an SPP source for NSOM probes. As a result, it is one of the most widely used SPP generation schemes for NSOM measurements. The drawback is that it is not easy to achieve local SPP excitation by this technique.
Schematic cartoons illustrating optical excitation schemes for SPPs. (A) Prism coupling schemes, with Kretschmann configuration on the left and Otto configuration on the right. (B) Grating coupling scheme. (C) Highly focused beam coupling scheme. (D) Near-field coupling scheme. (E) End-fire coupling scheme. (F) Step-gap leakage coupling scheme.
2.1.2 Grating coupling
Another route to achieve phase-matching is by gratings on metal surfaces. For a 1D grating with periodicity a, the reciprocal vector (G=2π/a) provides the needed momentum for SPP launching, as sketched in Figure 1(B). The pioneering work of Ebbesen et al. of extraordinary optical transmission through subwavelength hole arrays [27] can actually be explained by the excitation of SPPs on the periodic arrays [28]. Devaux et al. used 2D subwavelength holes on a metal film for SPP generation and detection [29]. Gratings can also be used as out-couplers for SPPs to freely-propagating photons. Park et al. have shown out-coupling of SPPs to photons using a dielectric grating with few-nanometer depth with an efficiency of about 50% [30]. Radko et al. systematically studied the efficiency of local SPP excitation on 1D periodic ridges [31]. Like the prism coupling scheme, the grating coupling scheme is a mature technique now widely adopted as an SPP source in NSOM experiments. Compared to prism coupling, the grating coupling scheme allows one to define a much better confined region as the SPP source. However, it is still not well focused enough for many nanoplasmonic applications. In the following, we show several more localized optical coupling schemes.
2.1.3 Highly focused beam coupling
A microscope objective of high numerical aperture (NA) together with immersion oil can be used to generate SPPs at its focal point [32]. As shown in Figure 1(C), incident photons are focused by the lens with NA=1.4, through immersion oil index-matched to the metal surface, where SPPs are generated and propagate out. A lens with NA>1 is used to ensure that the lateral component of the wave vector of the incident photon matches that of the excited SPPs, i.e., to achieve phase-matching. This works in the reverse way of leakage radiation microscopy. The advantage of this coupling scheme is that it conveniently works for white (broadband) incident light [32], although usually a single frequency laser is used as light source. The measurement techniques for SPPs generated by a highly focused beam are usually fluorescence mapping and LRM [22], due to the possible issue of an NSOM probe blocking the incoming beam.
2.1.4 Near-field coupling
While the above coupling schemes are, in principle, subject to the diffraction limit of light, the spatial extent of near-field coupling of SPPs with a sharpened metallic tip or tapering metal aperture is defined by the dimensions of the excitation tip itself, which can be made to be much smaller than the excitation wavelength [24]. By squeezing photons through an aperture smaller than the SPP wavelength, they acquire enough lateral momentum to couple to SPPs. Figure 1(D) shows a typical setup for near-field coupling of SPPs, where photons are first coupled to the metallic aperture via an optical fiber, then converted to SPPs on the sample surface. As will be discussed in Section 3, the transmission-mode NSOM works in the reverse process of near-field coupling [15].
2.1.5 End-fire coupling
Instead of providing a phase-matching mechanism, end-fire coupling couples photons into SPPs by matching their spatial field profiles. In doing so, it maximizes the probability of photons coupling to SPPs. Figure 1(E) shows a schematic of the end-fire coupling method. This scheme can reach high coupling efficiency [33], and is also highly compatible with the conventional fiber optics technique [34]. Chang’s theoretical work shows single photon coupling to SPP by this scheme [35–37]. Maier also demonstrated wave guiding with a tapered optical fiber and metal nanoparticle chains with 75% efficiency [34].
2.1.6 Step-gap leakage coupling
As a special case of a leakage coupling scheme, Ye et al. (present authors) recently developed a step-gap leakage coupling method [38]. As shown in Figure 1(F), the step-gap structure consists of a dielectric support layer with refractive higher than 1 (here, PMMA), an optically-thick top metal layer (Ag) and a base metal layer connected by an optically-thin metal film, forming a step-gap. Photons are injected from the bottom side of the structure. Optically-thick top and base metal layers preclude the direct transmission of photons through the device, such that the only path is through the step-gap. When photons from the dielectric side leak through the step gap, they acquire enough momentum to excite and couple to SPPs along the top surface of the base metal layer. Thus, the step-gap leakage scheme defines a localized SPP source, with minimum requirements for collimation of the input light source. It also naturally separates input photons from the out-coupled SPPs, optimizing the signal-to-noise ratio in subsequent SPP measurements [38]. As will be discussed in detail later, the step gap leakage scheme can be easily designed to excite SPP cavity modes. As a result, the step-gap leakage scheme of SPP generation works seamlessly with a conventional NSOM under collection mode.
An advantageous aspect of the step-gap leakage structure is ease of fabrication. By using the photoresist PMMA as the supporting dielectric step layer, a lengthy PMMA lift-off process was avoided, and by utilizing the quasi-directional nature of sputter deposition to form the thin metal layer along the step-gap, multiple metal depositions were avoided. The fabrication procedure is thus succinct, with ~1/2 h of sputtering, and electron beam lithography (EBL) dominating the total time consumption (several hours for large area writing patterns). For testing purposes, with relatively small EBL patterns (~1 mm2 writing area), the fabrication process takes <2 h.
2.2 Electron coupling
2.2.1 Fast electron coupling
The fact that fast electrons (with energy usually above 1 keV) bombarding metal thin films generate bulk and surface plasmons played an important role in the early stages of understanding SPPs. A typical fast electron SPP generation process is sketched in Figure 2(A). Powell and Swan’s observation using EELS of fast electrons bombarding on aluminum thin films [4] served as the first experimental proof of Ritchie’s theory of surface plasmons [3]. Later, Teng and Stern coupled fast electrons to a periodically-corrugated metallic surface for more efficient SPP launching, with the metallic gratings also serve as photon couplers for SPPs [39]. Heitmann also demonstrated similar effects in 1977 [40]. In recent years, as a highly localized SPP generation scheme, fast electron coupling has attracted increasing attention. Bashevoy et al. demonstrated the excitation of SPPs by electron beam bombardment in 2006, coupling the SPPs out as photons by a grating some distance apart [41]. García de Abajo et al. have demonstrated SPP generation by energetic electron incidence [42–44]. A more recent work by Liu et al. in 2012 demonstrated SPP generation by coupling fast electrons to metal surfaces with a grazing incident angle [45], as sketched in Figure 2(B).
Schematic cartoons illustrating electrical excitation schemes for SPPs. (A) Excitation of SPPs by energetic electron bombardment. (B) Excitation of SPPs by grazing incidence of energetic electrons. (C) Excitation of SPPs by electron tunneling. (D) Excitation of SPPs by hot electron relaxation.
2.2.2 Electron tunneling
In contrast to the high-energy bombardment method for SPP launching, a low-energy route based on inelastic electron tunneling was first reported by Lambe and McCarthy, with roughened metal-insulator-metal (MIM) junctions [46]. The reason tunneling electrons with very low energy electrons (on the order of 1 eV) can still excite SPPs is that the highly localized tunneling process is inelastic [47]. The inelastic tunneling process generates localized surface plasmon gap modes, which then couple into propagating SPPs [48], as sketched in Figure 2(C). Scanning tunneling microscopy (STM) turned out to be the ideal tool to explore this effect. A major difference between STM and optical excitation of SPPs is that STM generates a wide range of frequencies ν (with hν smaller than tunneled electron energy) at once, while optical excitation usually works at a single frequency [49]. Gimzewski [50] and Coombs [51] were the first to observe light emission from tunneled electrons in STM. Recently, Palash et al. demonstrated propagating SPPs along a gold nanowire excited on one end by tunneling electrons from the STM tip [48]. Generally speaking, metallic NSOM probes can be used to generate SPPs anywhere on conducting surfaces, through the electron tunneling scheme. The electron tunneling scheme, together with the near-field optical coupling scheme, makes the NSOM technique the preferred generator of SPPs on all sorts of surfaces.
2.2.3 Hot electron coupling
Recent theoretical work by Kempa shows that hot electrons generated in a semiconductor [52] can couple their excess free energy into an adjacent metal surface in the form of SPPs [53]. By comparing electron-phonon and electron-plasmon coupling probability amplitudes, he concluded that hot electrons are more probable to couple to surface plasmons. Thus, hot electrons can also serve as potential SPP sources. The schematic drawing of the hot electron-SPP coupling process is shown in Figure 2(D).
3 Mapping surface plasmon cavity modes by near-field scanning optical microscopy
3.1 Background and theory
Near-field scanning optical microscopy (NSOM) is a powerful technique to investigate SPPs along a metal-air interface, due to its sub-diffraction limited spatial resolution [24]. The acronym NSOM was proposed by Lewis [54], but the technique is also called scanning near-field optical microscopy (SNOM) [55]. When working in so-called collection mode, with SPPs excited by the Kreschtmann configuration, NSOM is also referred to as photon scanning tunneling microscopy (PSTM) [56]. The latter term emphasizes the similarity with the electron scanning tunneling microscope (STM) [57]. A comprehensive review of the different existing near field configurations can be found in books by Novotny [15] or Courjon [58].
In a typical NSOM measurement, a sharp optical probe (with or without an aperture) is first brought into close proximity with a metal sample surface, then raster scanned across the surface while keeping a constant distance (usually below 100 nm) to map out the local electric field intensity over the scanned area. Typical NSOM scans therefore map out the local electric field intensity distribution. Recent work by Denkova et al. also demonstrated the mapping of magnetic near field distributions, via plasmonic nanoantennas by an aperture-less probe [59] (details of this work will be discussed in Section 3.2).
The probe-sample distance is a key factor in near field microscopy [60]. Probe-sample distance is controlled by a feedback system based on interaction forces normal and/or lateral to the sample surface, borrowing from now standard atomic force microscopy (AFM) techniques [15]. Unlike most commercial AFMs, NSOM generally cannot introduce an optical auxiliary feedback mechanism, due to its sensitivity to environmental illumination. Mechanical feedback methods for NSOM include the shear-force methods [61] and the piezoelectric force detection by quartz tuning fork techniques [62]. In both cases, the probe is excited at its mechanical resonance frequency and caused to approach the sample surface. When the lateral/normal force between the probe and the sample surface is large enough to be sensed, an abrupt change in the amplitude or phase of oscillation of the probe serves as a good reference for subsequent control of the probe-sample distance [63]. A more detailed summary of the feedback methods is given in the book by Novotny [15].
The most common NSOM probes are uncoated fiber probes [64], aperture probes [58], sharp-tipped metal probes [65] and metallic nanoparticle probes [66]. The optical fiber probes are usually fabricated by heating and pulling [67] or chemical etching [68] of optical fibers. Metallic probes are usually chemically etched using techniques taken from the STM field [69]. The optical resolution of NSOM can beat the diffraction limit of light being, in principle, restricted only by the physical dimensions of the probe end [60]. Uncoated optical fiber probes can be made to 80 nm in diameter [70], and aperture probes typically have 40 nm-diameter aperture sizes [60]. Some reports showed that the resolution can be as high as 15 nm in the case of metallic probes [71].
Conventional SPP excitation schemes for NSOM measurements include prism coupling (Kreschtmann configuration) [56], grating coupling [72], NSOM probe coupling [73], and interactions between a metallic probe and a sample [71]. As introduced in Section 2, the step-gap leakage SPP generation scheme provides a controllable localized SPP source for NSOM probes, without strict requirements on the incident light source. Due to the slow scanning speed of NSOM, to capture the wave nature of SPPs along a metal surface, one needs to find ways to create SPP cavity modes [74–76], also called SPP standing waves. Otherwise, the detected signal will be smoothed out by the time averaging of the fast-propagating SPPs. In many cases, obtaining an NSOM scan of the SPP cavity modes can serve as direct proof of the existence of SPPs in the studied system.
3.2 Experimental realization
In recent years, the rapid development of near-field scanning techniques has facilitated the emergence of high-level research activities on NSOM mapping of SPP cavity modes. When choosing the application examples among thousands, we focus on the recent progress of NSOM mappings of SPP cavity modes, with clear measured NSOM data, and/or novelty and importance in the experiments carried out. We also try to cover a wide variety of problems studied by NSOM, presenting one example in each aspect. We provide here only a brief summary of the many selected works, leaving a more detailed discussion of the authors’ recent work on NSOM mapping of SPP cavity modes within a circular step-gap [38] to the next section.
Kwak et al. imaged SPP standing waves between periodic nano-holes on a metal surface [77]. Shortly after that, Gao et al. showed that by varying the metal film thickness, the SPP standing wave pattern changes from SPP-SPP interference to SPP-photon interference [28], thus giving direct evidence for the SPP as the origin for the observation of extraordinary optical transmission through subwavelength holes discovered by Ebbssen et al. [27]. Figure 3 shows the NSOM mapping of SPP standing wave patterns. Figure 3(B) shows the SPP-SPP interference pattern that arises when the gold film is 125-nm thick, with negligible direct tunneling of photons through the film. Figure 3(C) shows the SPP-photon interference pattern when the gold film is thin enough (75 nm) for direct photon tunneling through the film.
![Figure 3 SEM (A) and NSOM (B) and (C) images of nanohole arrays in gold films, with (B) showing SPP-SPP interference pattern on a 125 nm thick gold film, and (C) showing SPP-photon interference pattern on a 75-nm thick gold film. Reprinted with permission from Ref. [28]. Copyright 2006 American Chemical Society.](/document/doi/10.1515/nanoph-2013-0038/asset/graphic/nanoph-2013-0038_fig3.jpg)
SEM (A) and NSOM (B) and (C) images of nanohole arrays in gold films, with (B) showing SPP-SPP interference pattern on a 125 nm thick gold film, and (C) showing SPP-photon interference pattern on a 75-nm thick gold film. Reprinted with permission from Ref. [28]. Copyright 2006 American Chemical Society.
Liu et al. studied SPP cavity modes inside an area defined by symmetric circular and elliptical slits [78], demonstrating that such structures can behave as plasmonic lenses for SPP focusing, as shown in Figure 4. They also found that the measured electric field by NSOM is primarily the parallel component of the SPP, and confirmed their results by numerical simulations. Figure 4(C) shows the NSOM mapping of an SPP cavity mode inside a circular slit 14 μm in diameter fabricated on a Ag layer of 150 nm thickness. Figure 4(D) is an AFM scan of the plasmonic lithography pattern (recorded in photoresist under 365 nm UV exposure) of an elliptical slit with a 4 μm long axis and a 2.5 μm short axis, milled in an Al layer with 75 nm thickness.
![Figure 4 Experimental setup for (A) NSOM and (B) plasmonic lithography measurements for recording the near-field pattern for plasmonic lenses. (C) Near-field pattern for a 14 µm diameter circle cut into a 150 nm thick silver film recorded with NSOM. Polarization of incident light is indicated with an arrow. (D) Near-field pattern for an ellipse with a long axis of approximately 4 µm and a short axis of 2.5 µm cut into a 70 nm thick aluminum film recorded with plasmonic lithography. Reprinted with permission from Ref. [78]. Copyright 2005 American Chemical Society.](/document/doi/10.1515/nanoph-2013-0038/asset/graphic/nanoph-2013-0038_fig4.jpg)
Experimental setup for (A) NSOM and (B) plasmonic lithography measurements for recording the near-field pattern for plasmonic lenses. (C) Near-field pattern for a 14 µm diameter circle cut into a 150 nm thick silver film recorded with NSOM. Polarization of incident light is indicated with an arrow. (D) Near-field pattern for an ellipse with a long axis of approximately 4 µm and a short axis of 2.5 µm cut into a 70 nm thick aluminum film recorded with plasmonic lithography. Reprinted with permission from Ref. [78]. Copyright 2005 American Chemical Society.
Later, Babayan et al. [79] proposed an optical version of the quantum corral [80, 81]. By measuring the fields inside circular and elliptical corrals using an NSOM system, they found that the fields depend on the geometry and local dielectric environment of such corrals. In the circular corrals, only certain wavelengths produced constructive interference, creating a bright spot in the center, while in the elliptical corrals, the eccentricity was said to have suppressed or enhanced the focal points. Figure 5(A) shows an SEM image of the elliptical corral with different eccentricities, e=0.60 and 0.75, from top to bottom, respectively. Figure 5(B) shows the measured fields by NSOM inside the elliptical corrals for different wavelengths, 457 nm circularly polarized light incidence. Figure 5(C) is the calculated fields inside the corrals using these same eccentricities and wavelength.
![Figure 5 Effect of elliptical shape on patterned waves. (A) SEM images of the elliptical corrals with eccentricities of 0.60 (top) and 0.75 (bottom). (B) NSOM images obtained by using 457 nm circularly polarized incidence. (C) Calculated fields using FDTD with same geometries and wavelength used in (B). All scale bars are 1 μm. Reprinted with permission from Ref. [79]. Copyright 2009, American Chemical Society.](/document/doi/10.1515/nanoph-2013-0038/asset/graphic/nanoph-2013-0038_fig5.jpg)
Effect of elliptical shape on patterned waves. (A) SEM images of the elliptical corrals with eccentricities of 0.60 (top) and 0.75 (bottom). (B) NSOM images obtained by using 457 nm circularly polarized incidence. (C) Calculated fields using FDTD with same geometries and wavelength used in (B). All scale bars are 1 μm. Reprinted with permission from Ref. [79]. Copyright 2009, American Chemical Society.
In 2010, Fang et al. demonstrated plasmonic focusing in symmetry-broken circular corrals with NSOM observation [82]. They demonstrated that it is possible to focus the SPP into a spot smaller than the diffraction limit using a symmetry-broken corral under linearly polarized incidence because of SPP interference. The interference pattern can be controlled by changing the polarization and the degree of symmetry-breaking. Figure 6 shows an SEM image of the symmetry-broken corral (left) and the near field image acquired by NSOM in the same structure (right), when linearly polarized light is interacting with the structure. The direction of the electric field is indicated by the arrow.
![Figure 6 SEM image of symmetry broken cavity (left) and NSOM image using linearly polarizaed light in the direction of the arrow (right). Reprinted with permission from Ref. [82]. Copyright 2010 American Chemical Society.](/document/doi/10.1515/nanoph-2013-0038/asset/graphic/nanoph-2013-0038_fig6.jpg)
SEM image of symmetry broken cavity (left) and NSOM image using linearly polarizaed light in the direction of the arrow (right). Reprinted with permission from Ref. [82]. Copyright 2010 American Chemical Society.
Recently, Lin et al. demonstrated polarization-controlled tunable directional coupling of SPPs [83]. Using a special set of apertures arranged in a specific geometry, they showed that it is possible to control the propagation direction of the generated SPPs by controlling the polarization of the incident light. Photons with circular polarization couple to unidirectional SPPs, launching the SPPs to the left for clockwise polarization, and to the right for counter-clockwise polarization [83]. Their specific geometry also allows recovering the common bidirectional SPP coupling for linearly polarized light. Figure 7(A) shows an SEM image of the proposed coupler winding in a circular shape. Figures 7(B) and (C) show the measured field by NSOM for clockwise and counter-clockwise circular polarizations, respectively. Figure 7(D) is the case for linear polarization with direction indicated by the arrow.
![Figure 7 (A) SEM image of the tunable directional coupler. (B) NSOM image of the field using clockwise polarization. (C) NSOM image of the field using counter clockwise polarization. (D) NSOM image of the field using linear polarization (arrow). Color bars in (B)–(D) represent field intensities with arbitrary units. Adapted from Ref. [83]. Reprinted with permission from AAAS.](/document/doi/10.1515/nanoph-2013-0038/asset/graphic/nanoph-2013-0038_fig7.jpg)
(A) SEM image of the tunable directional coupler. (B) NSOM image of the field using clockwise polarization. (C) NSOM image of the field using counter clockwise polarization. (D) NSOM image of the field using linear polarization (arrow). Color bars in (B)–(D) represent field intensities with arbitrary units. Adapted from Ref. [83]. Reprinted with permission from AAAS.
Finally, Denkova et al. demonstrated the mapping of lateral magnetic near field distributions of plasmonic nanoantennas [59]. Using aperture probes in NSOM, they used the strong coupling of the probe-sample system to induce an effective magnetic dipole that generates surface plasmon resonances only at lateral magnetic field maxima. In the experimental results appears a spectral mismatch that can be explained by the difference between real conditions of the experiment, i.e., geometry, dielectric constants and roughness in both the sample and probe, compared with the ideal case used in the numerical simulations. Figure 8 shows a comparison between experimental and numerical results, top and bottom, respectively, for a 1120 nm long nanoantenna using the wavelengths indicated by the number above each image. The labels l=2, l=3, l=4 and l=5 are the predicted modes in each wavelength.
![Figure 8 NSOM transmission maps of a 1120 nm long nanobar at different wavelengths (top) and simulated |Hy|2 profiles (bottom) at different wavelengths showing the mode numbers l=2, 3, 4, 5, respectively. Reprinted with permission from Ref. [59]. Copyright 2013 American Chemical Society.](/document/doi/10.1515/nanoph-2013-0038/asset/graphic/nanoph-2013-0038_fig8.jpg)
NSOM transmission maps of a 1120 nm long nanobar at different wavelengths (top) and simulated |Hy|2 profiles (bottom) at different wavelengths showing the mode numbers l=2, 3, 4, 5, respectively. Reprinted with permission from Ref. [59]. Copyright 2013 American Chemical Society.
3.3 Plasmonic halos: mapping of the SPP drumhead modes via NSOM
As introduced in Section 2, the step-gap leakage SPP coupling scheme provides a convenient way to locally excite SPPs with a macroscopic light source. As a specific example, let us consider curving the step-gap into a circular shape. The metal surface surrounded by the step-gap then forms a circular drumhead for 2D SPP waves. By scanning the NSOM tip on the surface of the metal drumhead, one can map out the SPP drumhead modes [38].
The drumhead mode profiles can be calculated from the 2D Helmholtz equation governing the dynamics of SPPs: (∇2+kSPP2)Ez=0, with Ez the vertical electric field, kSPP=k0(ε0εAg/(ε0+εAg))1/2 the SPP propagation constant in the x−y plane, and k0 the free space wavenumber. By separation of variables, we obtain the resonant condition for the SPP drumhead modes: Ro=ρm, nλSPP/2π, with ρ≡rkSPP, ρm, n being the mth zero-crossing of the nth Bessel function of the first kind: Jn(ρ). The resonating dimensionless normalized radius ρm, n can be calculated for various m, n combinations, e.g., for m=1, n=0, ρ10=2.40.
We first showed the excitation of SPP drumhead modes on the SPP drumheads implicitly via the effect of SPP modulated transmission. As shown in Figure 9(A) and (B), the transmission minima of a circular step-gap array form branches as one increases the radius of the circular cavity. The fact that these branches of transmission minima coincide with the SPP drumhead mode branches indicates that the transmission spectra are modulated by SPP drumhead modes. This transmission modulation by SPP drumhead modes produces color tuning of the circular step-gaps, yielding attractive “plasmonic halos” [38], some of which are shown in Figure 9(C).
![Figure 9 Optical properties of step-gap circular cavities. (A) Optical transmission images for groupings of circular cavities of different radii under bottom white light illumination (corresponding radii indicated by arrows pointing to the exact values along the radius axis in (B). The field of view of each image is 60×30 μm2. (B) Normalized transmission spectrum contour map of plasmonic circular cavity arrays vs. SPP wavelength at the Ag/air interface, when tuning circular cavity radius from 0.98 to 1.90 μm, in 20 nm increments. A linear color scale (from 0 to 6%) is employed for the transmission spectra, with red (blue) indicating high (low) transmission. Groups of black and red lines indicate SPP drumhead modes within the plasmonic circular cavity, with corresponding mode indices labeled. (C) Optical images of transmission through plasmonic circular cavities with different radii and electron-beam writing dosages, showing color tunability. Scale bar in (C), 4 μm. (D) NSOM image of a plasmonic circular cavity with Ro=1.68 μm under linearly polarized 660 nm laser light (corresponding to λSPP=644 nm along the Ag/air interface) incident from below. This NSOM scan corresponds to the point in (B) as labeled by a yellow star. The calculated drumhead mode profile with (n, m)=(1, 5) is shown to its right, showing a good match to the measured data. (E) NSOM image of a plasmonic circular cavity with Ro=1.35 μm under the same condition as (D) (at yellow diamond), with the corresponding calculated drumhead mode profile (n, m)=(1, 4) on the right. The dashed circles in c and d represent 3.35 and 2.70 μm diameters, respectively. Reprinted with permission from Ref. [38]. Copyright 2013 American Chemical Society.](/document/doi/10.1515/nanoph-2013-0038/asset/graphic/nanoph-2013-0038_fig9.jpg)
Optical properties of step-gap circular cavities. (A) Optical transmission images for groupings of circular cavities of different radii under bottom white light illumination (corresponding radii indicated by arrows pointing to the exact values along the radius axis in (B). The field of view of each image is 60×30 μm2. (B) Normalized transmission spectrum contour map of plasmonic circular cavity arrays vs. SPP wavelength at the Ag/air interface, when tuning circular cavity radius from 0.98 to 1.90 μm, in 20 nm increments. A linear color scale (from 0 to 6%) is employed for the transmission spectra, with red (blue) indicating high (low) transmission. Groups of black and red lines indicate SPP drumhead modes within the plasmonic circular cavity, with corresponding mode indices labeled. (C) Optical images of transmission through plasmonic circular cavities with different radii and electron-beam writing dosages, showing color tunability. Scale bar in (C), 4 μm. (D) NSOM image of a plasmonic circular cavity with Ro=1.68 μm under linearly polarized 660 nm laser light (corresponding to λSPP=644 nm along the Ag/air interface) incident from below. This NSOM scan corresponds to the point in (B) as labeled by a yellow star. The calculated drumhead mode profile with (n, m)=(1, 5) is shown to its right, showing a good match to the measured data. (E) NSOM image of a plasmonic circular cavity with Ro=1.35 μm under the same condition as (D) (at yellow diamond), with the corresponding calculated drumhead mode profile (n, m)=(1, 4) on the right. The dashed circles in c and d represent 3.35 and 2.70 μm diameters, respectively. Reprinted with permission from Ref. [38]. Copyright 2013 American Chemical Society.
We then carried out NSOM scans to selected SPP drumheads along the SPP drumhead mode branches, with a linearly polarized 660 nm laser as the excitation source. Due to the polarization property of our laser, the n=1 drumhead modes are excited. As shown in Figure 9(D) and (E), the theoretical calculations match well with the measured electric field intensities. These NSOM mappings give direct proof of the excitation of SPP drumhead modes.
The transmission modulation effect is perhaps better illustrated by the full wave electromagnetic simulations, where one sees that when SPP drumhead modes are excited, far field radiation is minimized, as demanded by energy conservation. In contrast, when SPP drumhead modes are not excited, far field radiation is maximized. For more detailed discussions on transmission modulations of the circular step-gap structures, the reader is referred to the paper by Ye et al. [38].
3.4 Summary and outlook
NSOM is a scanning probe-based technique for spatial mapping of the near electric field profile of relatively planar samples. NSOM has been proven to be a powerful technique for direct mapping of the near field profiles of SPP cavity modes. SPPs excited by different schemes, confined in various cavity shapes, such as circular, elliptical, asymmetrical, and rectangular, have been measured by NSOM. Our proposed step-gap coupling scheme for SPPs is shown to be a convenient way to generate SPP cavity modes with different cavity geometries, and highly compatible with the NSOM measurement technique. The step-gap design combined with the traditional bottom illumination scheme gives a controllable local SPP source, enhancing the signal-to-noise ratio of detected SPPs.
It is important to note that although the NSOM technique is a unique and powerful tool, it can be very time consuming, and the measurements are rather complicated using research or commercial systems. In this framework, it would be advantageous to develop techniques that allow access to the information contained in the near field by indirect methods in an inexpensive and real-time way. Our group is working in this direction, by integrating leakage radiation microscopy with the step-gap SPP generation scheme. Progress on that front will be published elsewhere.
4 Mapping surface plasmon cavity modes by fluorescence techniques
Unlike the previously described techniques (NSOM), nano-sized emitters such as semiconductor quantum dots or fluorescent molecules can be directly placed in the vicinity of the enhanced electric field of SPPs, to serve as real time local indicators of the SPP field intensity. A prominent advantage of the fluorescence techniques is a short image acquisition time (as compared to scanning-based techniques such as NSOM), enabling one the ability of capturing real time dynamics of SPPs [84, 85]. Two counteracting processes occur when fluorescent molecules are in the vicinity of metal structures: (1) enhanced fluorescence emission by the enhanced local electric field and (2) quenched fluorescence by non-radiative decay in the metal. Kramer et al. observed the first process by showing fluorescence enhancement for molecules placed near metal nanotips [86]. Dulkeith et al. showed the quenching effect of gold nanoparticles [87]. Anger et al. provided a comprehensive description of these two processes, by investigating the fluorescence rate of a single molecule as a function of its separation from a spherical gold nanoparticle [88]. They concluded that when the separation is too small, the quenching effect dominates, killing the fluorescence; when the separation is too large, the near-field enhancement is too weak to enhance fluorescence. There is thus an optimum separation between fluorescent molecules and metal surfaces for maximized fluorescence enhancement. This separation is usually achieved by coating a thin dielectric layer on the metal surface before fluorescent molecule engagement.
By using the above dielectric spacer technique, Ditlbacher et al. demonstrated the fluorescence mapping of propagating SPPs generated by focusing a laser beam on metal nanowires and nanoparticles [84]. As shown in Figure 10(A) and (B), Ag nanoparticles and nanowires were fabricated by EBL on a 70 nm thick Ag film; a 10 nm SiO2 layer was coated on top to serve as the dielectric spacer to prevent quenching; a sub-monolayer of Rhodamine 6G fluorescent molecules was then coated on top of the SiO2 layer to map the SPP field profiles in real time. Figure 10(C) and (D) show the fluorescence mapping of the interference between SPPs generated by two Ag nanoparticles, aligned parallel and perpendicular, respectively, to the polarization direction of incident light.
![Figure 10 Fluorescence images of the field intensity of SPs excited via (A) A silver particle and (B) A silver wire. (C) Two silver particles along the horizontal axis, and (D) Two silver particles along the vertical axis. Reprinted with permission from Ref. [84]. Copyright 2002, AIP Publishing LLC.](/document/doi/10.1515/nanoph-2013-0038/asset/graphic/nanoph-2013-0038_fig10.jpg)
Fluorescence images of the field intensity of SPs excited via (A) A silver particle and (B) A silver wire. (C) Two silver particles along the horizontal axis, and (D) Two silver particles along the vertical axis. Reprinted with permission from Ref. [84]. Copyright 2002, AIP Publishing LLC.
Recently, Xu’s group has shown beautiful applications of fluorescence mapping of SPP modes along single-crystal metal nanowires [85, 89, 90]. By using CdSe@ZnS quantum dot emitters (QDs) with 30 to 50 nm Al2O3 spacer layer, Wei et al. demonstrated interferometric optical logic gates based on intersecting Ag nanowires (NWs) [89]. By tuning the relative phases between two highly focused incident laser beams on two joining Ag NWs, controllable ON-OFF states on the far end of one of the NWs are achieved. Subsequent work by Wei et al. showed good control of the SPP mode profiles along Ag NWs by tuning the radius of the Ag NW, the thickness of the Al2O3 layer, and the embedding environment, as shown in Figure 11 [90]. Based on the same technique, Zhang et al. have demonstrated the excitation of SPP chiral modes along Ag NWs [85]. Fluorescence images of various SPP chiral modes were obtained along a Ag NW with tuned input polarization. They concluded that the SPP chiral modes are a result of interference between different orders of plasmonic modes along the Ag NW. Since for Ag NWs with small enough radii, only the first order SPP mode (TM0) is supported, with higher order modes cutting off, the excitation of SPP chiral modes can only be seen on relatively thick Ag NWs (about 150 nm in radius).
![Figure 11 Near-field distribution images for NWs with different dielectric environments. (A) Schematic cross-section of the samples. (B) Schematic illustration of the excitation/collection configuration and the plasmons propagating along the NW. (C) SEM images of a typical bare Ag NW (Upper Left) and NWs coated with T=30 nm (Lower Left), 50 nm (Upper Right), and 80 nm (Lower Right) Al2O3 layers. (Scale bar, 500 nm.) (D) QD emission images under excitation at the left ends of the NWs. From Top to Bottom, the radius of the NWs is 160, 155, and 157 nm, and the corresponding Al2O3 thicknesses are 30, 50, and 80 nm. (E) QD emission images for a 162-nm radius NW with a 50-nm Al2O3 coating measured in air (Top), and then after depositing 5 nm of Al2O3 (Middle), and finally with an additional 5 nm of Al2O3 (Bottom). The white dashed lines are visual guides to show the shift of the plasmon near-field pattern. (F) QD emission images for a 155-nm radius NW initially coated with 10 nm of Al2O3 and QDs, and then capped with another 5 nm of Al2O3 to protect the water-soluble QDs from being removed, measured in air (Top), water (Middle), and oil (Bottom). (Scale bar in D for D–F, 5 μm.) Reprinted with permission from Ref. []. Copyright 2013 National Academy of Sciences, USA.](/document/doi/10.1515/nanoph-2013-0038/asset/graphic/nanoph-2013-0038_fig11.jpg)
Near-field distribution images for NWs with different dielectric environments. (A) Schematic cross-section of the samples. (B) Schematic illustration of the excitation/collection configuration and the plasmons propagating along the NW. (C) SEM images of a typical bare Ag NW (Upper Left) and NWs coated with T=30 nm (Lower Left), 50 nm (Upper Right), and 80 nm (Lower Right) Al2O3 layers. (Scale bar, 500 nm.) (D) QD emission images under excitation at the left ends of the NWs. From Top to Bottom, the radius of the NWs is 160, 155, and 157 nm, and the corresponding Al2O3 thicknesses are 30, 50, and 80 nm. (E) QD emission images for a 162-nm radius NW with a 50-nm Al2O3 coating measured in air (Top), and then after depositing 5 nm of Al2O3 (Middle), and finally with an additional 5 nm of Al2O3 (Bottom). The white dashed lines are visual guides to show the shift of the plasmon near-field pattern. (F) QD emission images for a 155-nm radius NW initially coated with 10 nm of Al2O3 and QDs, and then capped with another 5 nm of Al2O3 to protect the water-soluble QDs from being removed, measured in air (Top), water (Middle), and oil (Bottom). (Scale bar in D for D–F, 5 μm.) Reprinted with permission from Ref. []. Copyright 2013 National Academy of Sciences, USA.
5 Mapping surface plasmon cavity modes by electron beams
As introduced in Section 1, electron impinging is another major method (besides optical excitation) for SPP generation. As a result, scanning-mode electron microscopes (both SEM and STEM), combined with electron energy loss spectroscopy (EELS) or cathodoluminescence (CL) analysis, are becoming more popular for SPP cavity mode mapping. Due to rapid development in the precise control of electron beam emission, electron microscopes now reach unprecedented spatial resolution (sub-Angstrom) and energy span (from ~1 eV to 300 keV) [91]. Thus, no other existing technique can match EELS or CL in combined space-energy resolution [16]. A recent comprehensive review on this subject by Garcia de Abajo is available [16]. Here we give a brief summary focusing on the spatial mapping aspect of EELS and CL.
5.1 Electron energy loss spectroscopy
Since the first demonstration by Hillier and Baker [92], EELS has become standard in electron microscopy, in both TEM and SEM. EELS detects material properties by engaging electron beams with the sample, and recording the energy loss spectra of the incident beams. When an energetic electron beam passes through a thin metal film, parts of its energy and momentum couple to the free electron gas, forming bulk and surface plasmon modes [3, 4]; parts of those couple to core electrons, exciting electron-hole pairs, while parts couple to very-low-energy excitations (e.g., phonon modes), resulting in the major zero-loss peak (ZLP) of unscattered electrons [16]. Excitations of bulk plasmons and SPPs belong to the low energy loss region of EELS. In the early stage of the development of SPP theory, EELS of aluminum thin film [4] served as the first experimental proof of Ritchie’s surface plasmon theory [3].
EELS has been used to detect SPPs since the mid-20th century. However, the application of EELS to spatial mapping of SPP cavity modes has only emerged in recent years [93], thanks to maturing techniques in precise control of emission and detection of electron beams. As shown in Figure 12(A), conventional EELS works in transmission mode, by sending energetic electron beams through thin-film samples. EELS can also work in reflection mode (when integrated with a SEM and detecting thick samples), called REELS [94–96], as shown in Figure 12(B).
![Figure 12 Schematic representation of various types of electron microscope equipped to perform spectroscopy. (A) Scanning transmission electron microscopes (STEMs) allow analysis of the energy distribution of electrons transmitted through a thin specimen (<100 nm thick), on which the incident beam is focused down to a <0.1 nm spot. Large momentum transfers are collected through annular dark-field detectors (ADFs). Cathodoluminescence (CL) light detection is also possible on a STEM. (B) Scanning electron microscopes (SEMs) collect secondary electrons (SEs) to form images or CL emission to perform spectroscopy, with spatial resolution down to ~2 nm. Adapted with permission from Ref. [16].](/document/doi/10.1515/nanoph-2013-0038/asset/graphic/nanoph-2013-0038_fig12.jpg)
Schematic representation of various types of electron microscope equipped to perform spectroscopy. (A) Scanning transmission electron microscopes (STEMs) allow analysis of the energy distribution of electrons transmitted through a thin specimen (<100 nm thick), on which the incident beam is focused down to a <0.1 nm spot. Large momentum transfers are collected through annular dark-field detectors (ADFs). Cathodoluminescence (CL) light detection is also possible on a STEM. (B) Scanning electron microscopes (SEMs) collect secondary electrons (SEs) to form images or CL emission to perform spectroscopy, with spatial resolution down to ~2 nm. Adapted with permission from Ref. [16].
Nelayah et al. demonstrated the EELS mapping of SPP cavity modes on a Ag nanoprism for frequencies ranging from infrared to ultraviolet in 2007 [93]. The EELS experiment was done in an STEM, on a 10 nm-thick single-crystal Ag triangular nanoprism with side length of 78 nm, supported by a mica layer. Figure 13(A) and (B) show the measured and simulated SPP cavity modes on the Ag nanoprism, at three different frequencies. Each SPP spatial map consists of 32×32 resolutions, with each resolution point corresponding to an EEL spectrum subtracted by the ZLP. For a mapping at a selected frequency, the EELS signal is translated to a color at each resolution point. Figure 13(C) shows spatial mapping with the EELS fitted by Gaussian parameters, showing a better localized spatial distribution.
![Figure 13 Mapping SPP cavity modes in a Ag triangular nanoprism supported on mica through EELS. (A) Spatial mapping of SPP cavity modes with spectra centered at 1.75, 2.70 and 3.20 eV respectively. The color scale, common to the three maps, is linear and in arbitrary units. The outer contour of the particle, deduced from its HAADF image, is shown as a white line. (B) Simulated amplitude maps of the three main resonances resolved in the simulated EEL spectra of the nanoprism with 78-nm-long sides. (C) EELS amplitude distributions obtained after Gaussian fitting of the three modes mapped in (A). Reprinted by permission from Macmillan Publishers Ltd: Nature Physics (Ref. [93]), copyright 2007.](/document/doi/10.1515/nanoph-2013-0038/asset/graphic/nanoph-2013-0038_fig13.jpg)
Mapping SPP cavity modes in a Ag triangular nanoprism supported on mica through EELS. (A) Spatial mapping of SPP cavity modes with spectra centered at 1.75, 2.70 and 3.20 eV respectively. The color scale, common to the three maps, is linear and in arbitrary units. The outer contour of the particle, deduced from its HAADF image, is shown as a white line. (B) Simulated amplitude maps of the three main resonances resolved in the simulated EEL spectra of the nanoprism with 78-nm-long sides. (C) EELS amplitude distributions obtained after Gaussian fitting of the three modes mapped in (A). Reprinted by permission from Macmillan Publishers Ltd: Nature Physics (Ref. [93]), copyright 2007.
EELS is also shown to be an ideal tool for measuring dark (nonradiative) SPP modes [97]. Figure 14(A) schematically shows the dark and bright modes within a symmetric metallic dimer as a result of plasmon hybridization. While the lower energy bright modes can be directly excited by incident photons, the higher energy dark modes can only be excited electrically due to their anti-symmetry properties. By measuring EELS on a Ag nanoparticle dimer, Koh et al. showed the excitation of both bright and dark modes. As shown in Figure 14(B), when electron beam is incident on position A, both bright and dark modes are excited, resulting in two peaks at 2.2 and 3.3 eV. When electron beam is incident on positions B1 and B2, bulk plasmon modes are excited, with energy of 3.8 eV. When electron beam is at position C, only dark modes are excited, with energy 3.4 eV, as the bright mode is symmetry-forbidden.
![Figure 14 (A) Hybridization model of a symmetric dimer. (B) Experimental EELS data set of a symmetrical silver nanoparticle dimer, showing plasmon energies as a function of electron probe position. The spectra are obtained at regular intervals of 4 nm. Reprinted with permission from Ref. [97]. Copyright 2009 American Chemical Society.](/document/doi/10.1515/nanoph-2013-0038/asset/graphic/nanoph-2013-0038_fig14.jpg)
(A) Hybridization model of a symmetric dimer. (B) Experimental EELS data set of a symmetrical silver nanoparticle dimer, showing plasmon energies as a function of electron probe position. The spectra are obtained at regular intervals of 4 nm. Reprinted with permission from Ref. [97]. Copyright 2009 American Chemical Society.
5.2 Cathodoluminescence
First discovered in the mid-19th century as light emission stemming from cathode (electron) rays impacting a glass substrate, cathodoluminescence (CL) is now widely used as a material characterization technique in mineralogy, semiconductor physics, and many other fields [98]. The most extensive industrial application of CL was as the emission source in CRT monitors, before the LCD, LED, etc. display eras [99].
CL includes direct and indirect emission processes. The main direct emission processes involved in CL are Cherenkov radiation and transition radiation. Cherenkov radiation happens when a charged particle (electron) moves in a transparent medium, with speed v faster than the phase velocity of light in that medium (v>c/n), and is thus mainly observed in dielectrics and not in metals [100]. Transition radiation is emitted if a charged particle passes through a boundary between two media with different dielectric constants [101]. It is generated by the time-dependent interaction between the incident charge and its image charge in the other dielectric. This effect was first predicted by Ginzburg and Frank [101] and observed by Goldsmith and Jelly [102] for metals.
The observation of bulk and/or surface plasmons via electron bombardment is an indirect process of CL. Bulk plasmons and/or SPPs are excited when traveling electrons couple to the free electron gas of a metal. SPPs then couple out as free-propagating photons through properly designed gratings on metal surfaces. Thus, this electron-SPP-photon process is an indirect CL process. The first observations of light emission from SPPs were done by Teng and Stern [39].
A schematic of a typical electron microscope coupled with a CL collection system is shown in Figure 12(B). The parabolic mirror has a through hole on the top for electron beam passage. It is designed to be confocal with the electron beam, so as to maximize the collected optical signal. Out-coupled light is usually detected by a spectrometer [16]. For the purpose of spatial mapping of SPP cavity modes, an optical signal at a selected wavelength suffices. Spatial mapping is realized by scanning the electron beam across the measured sample. CL usually has a poorer signal-to-noise ratio than EELS, but it has the unique property of probing local optical properties of a sample [16]. Several examples of CL SPP cavity mode mapping have been presented over the last few years. The following includes some representative examples.
Barnard et al. imaged the hidden modes of ultrathin plasmonic strip antennas by CL [103] in 2011. A 30 keV electron beam is injected on a 20 nm thick Ag wedge with tuning width from 200 to 700 nm. The Ag wedge is deposited on a silicon-on-insulator substrate. CL image corresponding to a free-space wavelength of 700 nm is shown in Figure 15. The formation of the measured SPP cavity mode pattern is attributed to the interference between the short-range SPPs and the long-range SPPs, with simulation details shown in Figure 15(C) and (D).
![Figure 15 (A) Cathodoluminescence image for a free-space emission wavelength of 700 nm. The scan has been stretched 4 times in the horizontal direction to more clearly show the fine structure across the width of the antenna. Horizontal lines denote widths at which resonances occur. (B) Line cuts of the CL signal taken at resonant widths from the image in (A), Showing SPP standing wave patterns. (C) Calculated intensity of SPP standing waves according to the Fabry-Perot model for short-range SPP resonant modes. (D) Calculated intensity of SPP standing Fabry-Perot model for a superposition of resonant short-range and long-range SPP modes. Reprinted with permission from Ref. [103]. Copyright 2011, American Chemical Society.](/document/doi/10.1515/nanoph-2013-0038/asset/graphic/nanoph-2013-0038_fig15.jpg)
(A) Cathodoluminescence image for a free-space emission wavelength of 700 nm. The scan has been stretched 4 times in the horizontal direction to more clearly show the fine structure across the width of the antenna. Horizontal lines denote widths at which resonances occur. (B) Line cuts of the CL signal taken at resonant widths from the image in (A), Showing SPP standing wave patterns. (C) Calculated intensity of SPP standing waves according to the Fabry-Perot model for short-range SPP resonant modes. (D) Calculated intensity of SPP standing Fabry-Perot model for a superposition of resonant short-range and long-range SPP modes. Reprinted with permission from Ref. [103]. Copyright 2011, American Chemical Society.
Yu’s group has been using CL to map out SPP cavity modes within ultra-smooth metal cavities with various shapes [104–106]. As shown in Figure 16, plasmonic triangular nanocavities with ultra-smooth surfaces were fabricated by a template stripping method [104]. High quality CL mappings were obtained due to the smoothness of the SPP cavities. Using the same technique, Zhu et al. mapped SPP cavity modes within open circular cylinder cavities for SPPs via CL [105]. They also investigated the control of SPP cavity modes by tuning of the height of the cylindrical cavity. Later, they also measured the vertical SPP cavity modes within a vertical nanocavity [106].
![Figure 16 Panchromatic CL spectrum and mode patterns of the equilateral triangle cavity with a 935 nm side length. (A) CL spectrum of the cavity. The inset image is the SEM picture of the cavity. The spectrum from 400 to 650 nm was collected near the vertex, and the spectrum from 650 to 900 nm was collected at the center of the cavity (the arrow indicates the break). (B–D) Typical monochromatic CL images at wavelengths 818, 580, and 452 nm in vacuum, respectively. The brightness corresponds to the emitted photon intensity. (E–G) Simulated mode patterns. Scale bars are 500 nm. Adapted with permission from Ref. [104].](/document/doi/10.1515/nanoph-2013-0038/asset/graphic/nanoph-2013-0038_fig16.jpg)
Panchromatic CL spectrum and mode patterns of the equilateral triangle cavity with a 935 nm side length. (A) CL spectrum of the cavity. The inset image is the SEM picture of the cavity. The spectrum from 400 to 650 nm was collected near the vertex, and the spectrum from 650 to 900 nm was collected at the center of the cavity (the arrow indicates the break). (B–D) Typical monochromatic CL images at wavelengths 818, 580, and 452 nm in vacuum, respectively. The brightness corresponds to the emitted photon intensity. (E–G) Simulated mode patterns. Scale bars are 500 nm. Adapted with permission from Ref. [104].
Vesseur et al. measured the SPP whispering gallery modes via CL within circular grooves in a gold surface [107, 108]. Such resonators with various geometries were fabricated by focused-ion-beam milling. A 30 keV electron beam was used to generate CL spectra. SPP whispering gallery mode mappings with angular quantum numbers m=0, 1, and 2 are obtained. More recently, by measuring the asymmetric SPP cavity modes within elliptical metal cavities via CL, Schoen et al. have demonstrated that surface plasmon physics can be employed to realize a planar broadband unidirectional parabolic antenna at optical frequencies [109]. The CL SPP cavity mode mapping is also shown to be representing the local density of optical states (LDOS) across the Ag wedge. Sapienza et al. have recently shown the determination of the LDOS of dielectric (Si3N4) photonic crystals by CL [110].
6 Conclusion
Beginning with various excitation schemes for SPPs, we have reviewed current mapping techniques for SPP standing wave cavity modes. We have also shown the connection between the excitation schemes and mapping techniques for SPPs. SPPs can be excited by both photon and electron injection, and thus can be spatially-mapped by both optical and electrical methods. Optical schemes include NSOM mapping, fluorescence mapping, LRM mapping, dark-field mapping; electrical schemes include EELS mapping and CL mapping. SPP spatial mapping techniques provide direct evidence and fundamental information toward a deepening understanding of surface plasmon physics. With the continued development of nanofabrication and nanocharacterization techniques, SPP spatial mapping will continue to play an important role in the plasmonics community.
Acknowledgment
This work was supported by the W.M. Keck Foundation.
References
[1] Barnes WL, Dereux A, Ebbesen TW. Surface plasmon subwavelength optics. Nature 2003;424:824–30.10.1038/nature01937Search in Google Scholar PubMed
[2] Han Z, Bozhevolnyi SI. Radiation guiding with surface plasmon polariton. Rep Prog Phys 2013;76:016402.10.1088/0034-4885/76/1/016402Search in Google Scholar PubMed
[3] Ritchie RH. Plasma losses by fast electrons in thin films. Phys Rev 1957;106:874–81.10.1103/PhysRev.106.874Search in Google Scholar
[4] Powell CJ, Swan JB. Origin of the characteristic electron energy losses in aluminum. Phys Rev 1959;115:869–75.10.1103/PhysRev.115.869Search in Google Scholar
[5] Pillai S, Catchpole KR, Trupke T, Green MA. Surface plasmon enhanced silicon solar cells. J Appl Phys 2007;101:093105.10.1063/1.2734885Search in Google Scholar
[6] Atwater HA, Polman A. Plasmonics for improved photovoltaic devices. Nature Mat 2010;9:205–13.10.1038/nmat2629Search in Google Scholar PubMed
[7] Ye F, Burns MJ, Naughton MJ. Embedded metallic nanopatterns for enhanced optical absorption. Proc of SPIE 2011;8111:811103.10.1117/12.892618Search in Google Scholar
[8] Ye F, Burns MJ, Naughton MJ. Embedded metal nanopatterns for near-field scattering enhanced optical absorption. Phys Status Solidi A 2012;209:1829–34.10.1002/pssa.201228459Search in Google Scholar
[9] Gu M, Ouyang Z, Jia B, Stokes N, Chen X, Fahim N, Li X, Ventura MJ, Shi Z. Nanoplasmonics: a frontier of photovoltaic solar cells. Nanophotonics 2012;1:235–48.10.1515/nanoph-2012-0180Search in Google Scholar
[10] Kneipp K, Wang Y, Kneipp H, Perelman LT, Itzkan I, Dasari RR, Feld MS. Single molecule detection using surface-enhanced Raman scattering (SERS). Phys Rev Lett 1997;78:1667–70.10.1103/PhysRevLett.78.1667Search in Google Scholar
[11] Halas NJ, Lal S, Chang W-S, Link S, Nordlander P. Plasmons in strongly coupled metallic nanostructures. Chem Rev 2011;111:3913–61.10.1021/cr200061kSearch in Google Scholar PubMed
[12] Phillips KS, Cheng Q. Recent advances in surface plasmon resonance based techniques for bioanalysis. Anal Bioanal Chem 2007;387:1831–40.10.1007/s00216-006-1052-7Search in Google Scholar PubMed
[13] Wei H, Xu H. Nanowire-based plasmonic waveguides and devices for integrated nanophotonic circuits. Nanophotonics 2012;1:155–69.10.1515/nanoph-2012-0012Search in Google Scholar
[14] Wang Y, Plummer E W, Kempa K. Foundations of plasmonics. Adv Phys 2011;60:799–898.10.1080/00018732.2011.621320Search in Google Scholar
[15] Novotny L, Hecht B. Principles of nano-optics. UK: Cambridge University Press, 2006.10.1017/CBO9780511813535Search in Google Scholar
[16] Garcia de Abajo FJ. Optical excitations in electron microscopy. Rev Mod Phys 2010;82:209–75.10.1103/RevModPhys.82.209Search in Google Scholar
[17] Kociak M, Garcia de Abajo J. Nanoscale mapping of plasmons, photons, and excitons. MRS Bull 2012;37:39–46.10.1557/mrs.2011.335Search in Google Scholar
[18] Bouhelier A, Huser Th, Tamaru H, Guntherodt H-J, Pohl DW. Plasmon optics of structured silver films. Phys Rev B 2001;63:155404.10.1103/PhysRevB.63.155404Search in Google Scholar
[19] Grandidier J, Massenot S, Colas des Francs G, Bouhelier A, Weeber J-C, Markey L, Dereux A, Renger J, González MU, Quidant R. Dielectric-loaded surface plasmon polariton waveguides: Figures of merit and mode characterization by image and Fourier plane leakage microscopy. Phys Rev B 2008;78:245419.10.1103/PhysRevB.78.245419Search in Google Scholar
[20] Drezet A, Hohenau A, Koller D, Stepanov A, Ditlbacher H, Steinberger B, Aussenegg FR, Leitner A, Krenn JR. Leakage radiation microscopy of surface plasmon polaritons. Mat Sci Eng B 2008;149:220–9.10.1016/j.mseb.2007.10.010Search in Google Scholar
[21] Zhang D, Yuan X, Bouhelier A. Direct image of surface-plasmon-coupled emission by leakage radiation microscopy. Appl Opt 2010;49:875–9.10.1364/AO.49.000875Search in Google Scholar PubMed
[22] Liu Y, Palomba S, Park Y, Zentgraf T, Yin X, Zhang X. Compact magnetic antennas for directional excitation of surface plasmons. Nano Lett 2012;12:4853–8.10.1021/nl302339zSearch in Google Scholar PubMed
[23] Kang T, Choi W, Yoon I, Lee H, Seo M-K, Park Q-H, Kim B. Rainbow radiating single-crystal Ag nanowire antenna. Nano Lett 2012;12:2331–6.10.1021/nl3002414Search in Google Scholar PubMed
[24] Maier SA. Plasmonic: Fundamentals and applications. New York: Springer, 2007.Search in Google Scholar
[25] Kretschmann E, Raether H. Radiative decay of non radiative surface plasmons excited by light. Z Naturforsch A 1968;23:2135–6.10.1515/zna-1968-1247Search in Google Scholar
[26] Otto A. Excitation of nonradiative surface plasma waves in silver by the method of frustrated total reflection. Z Phys 1968;216:398–410.10.1007/BF01391532Search in Google Scholar
[27] Ebbesen TW, Lezec HJ, Ghaemi HF, Thio T, Wolff PA. Extraordinary optical transmission through sub-wavelength hole arrays. Nature 1998;391:667–9.10.1038/35570Search in Google Scholar
[28] Gao H, Henzie J, Odom TW. Direct evidence for surface plasmon-mediated enhanced light transmission through metallic nanohole arrays. Nano Lett 2006;6:2104–8.10.1021/nl061670rSearch in Google Scholar PubMed
[29] Devaux E, Ebbesen TW, Weeber J-C, Dereux A. Launching and decoupling surface plasmons via micro-gratings. Appl Phys Lett 2003;83:4936–8.10.1063/1.1634379Search in Google Scholar
[30] Park S, Lee G, Song SH, Oh CH, Kim PS. Resonant coupling of surface plasmons to radiation modes by use of dielectric gratings. Opt Lett 2003;28:1870–2.10.1364/OL.28.001870Search in Google Scholar
[31] Radko IP, Bozhevolnyi SI, Brucoli G, Martín-Moreno L, García-Vidal FJ, Boltasseva A. Efficiency of local surface plasmon polariton excitation on ridges. Phys Rev B 2008;78:115115.10.1103/PhysRevB.78.115115Search in Google Scholar
[32] Bouhelier A, Wiederrecht GP. Surface plasmon rainbow jets. Opt Lett 2005;30:884–6.10.1364/OL.30.000884Search in Google Scholar PubMed
[33] Stegeman GI, Wallis RF, Maradudin AA. Excitation of surface polaritons by end-fire coupling. Opt Lett 1983;8:386–8.10.1364/OL.8.000386Search in Google Scholar PubMed
[34] Maier SA, Friedman MD, Barclay PE, Painter O. Experimental demonstration of fiber-accessible metal nanoparticle plasmon waveguides for planar energy guiding and sensing. Appl Phys Lett 2005;86:071103.10.1063/1.1862340Search in Google Scholar
[35] Chang DE, Sorensen AS, Hemmer PR, Lukin MD. Quantum optics with surface plasmons. Phys Rev Lett 2006;97:053002.10.1103/PhysRevLett.97.053002Search in Google Scholar PubMed
[36] Chang DE, Sorensen AS, Hemmer PR, Lukin MD. Strong coupling of single emitters to surface plasmons. Phys Rev B 2007;76:035420.10.1103/PhysRevB.76.035420Search in Google Scholar
[37] Chang DE, Sorensen AS, Demler EA, Lukin MD. A single-photon transistor using nanoscale surface plasmons. Nat Phys 2007;3:807–12.10.1038/nphys708Search in Google Scholar
[38] Ye F, Burns MJ, Naughton MJ. Plasmonic Halos-optical surface plasmon drumhead modes. Nano Lett 2013;13:519–2310.1021/nl303955xSearch in Google Scholar PubMed
[39] Teng Y-Y, Stern EA. Plasma radiation from metal grating surfaces. Phys Rev Lett 1967;19:511–4.10.1103/PhysRevLett.19.511Search in Google Scholar
[40] Heitmann D. Radiative decay of surface plasmons excited by fast electrons on periodically modulated silver surfaces. J Phys C: Solid State Phys 1977;10:397–405.10.1088/0022-3719/10/3/010Search in Google Scholar
[41] Bashevoy MV, Jonsson F, Krasavin AV, Zheludev NI, Chen Y, Stockman MI. Generation of traveling surface plasmon waves by free-electron impact. Nano Lett 2006;6:1113–5.10.1021/nl060941vSearch in Google Scholar PubMed
[42] Cai W, Sainidou R, Xu J, Polman A, Garcia de Abajo FJ. Efficient generation of propagating plasmons by electron beams. Nano let 2009;9:1176–81.10.1021/nl803825nSearch in Google Scholar PubMed
[43] Cai W, Wang L, Zhang X, Xu J, García de Abajo FJ. Controllable excitation of gap plasmons by electron beams in metallic nanowire pairs. Phys Rev B 2010;82:125454.10.1103/PhysRevB.82.125454Search in Google Scholar
[44] Adamo G, MacDonald KF, Fu YH, Wang C-M, Tsai DP, Garcia de Abajo FJ, Zheludev NI. Light well: a tunable free-electron light source on a chip. Phys Rev Lett 2009;103:113901.10.1103/PhysRevLett.103.113901Search in Google Scholar PubMed
[45] Liu S, Zhang P, Liu W, Gong S, Zhong R, Zhang Y, Hu M. Surface polariton Cherenkov light radiation source. Phys Rev Lett 2012;109:153902.10.1103/PhysRevLett.109.153902Search in Google Scholar PubMed
[46] Lambe J, McCarthy SL. Light emission from inelastic electron tunneling. Phys Rev Lett 1976;37:923–5.10.1103/PhysRevLett.37.923Search in Google Scholar
[47] Berndt R, Gimzewski JK, Johansson P. Inelastic tunneling excitation of tip-induced plasmon modes on noble-metal surfaces. Phys Rev Lett 1991;67:3796–99.10.1103/PhysRevLett.67.3796Search in Google Scholar PubMed
[48] Bharadwaj P, Bouhelier A, Novotny L. Electrical excitation of surface plasmons. Phys Rev Lett 2011;106:226802.10.1103/PhysRevLett.106.226802Search in Google Scholar PubMed
[49] Wang T, Boer-Duchemin E, Zhang Y, Comtet G, Dujardin G. Excitation of propagating surface plasmons with a scanning tunneling microscope. Nanotech 2011;22:175201.10.1088/0957-4484/22/17/175201Search in Google Scholar PubMed
[50] Gimzewski JK, Reihl B, Coombs JH, Schlittler RR. Photon emission with the scanning tunneling microscope. Z Phys B 1988;72:497–501.10.1007/BF01314531Search in Google Scholar
[51] Coombs JH, Gimzewski JK, Reihl B, Sass JK. Photon emission experiments with the scanning tunneling microscope. J Microsc 1988;152:325–36.10.1111/j.1365-2818.1988.tb01393.xSearch in Google Scholar
[52] Kempa K, Naughton MJ, Ren ZF, Herczynski A, Kirkpatrick T, Rybczynski J, Gao Y. Hot electron effect in nanoscopically thin photovoltaic junctions. Appl Phys Lett 2009;95:233121.10.1063/1.3267144Search in Google Scholar
[53] Kempa K. Plasmonic protection of the hot-electron energy. Phys Status Solidi (RRL) 2013;7:465–8.10.1002/pssr.201307101Search in Google Scholar
[54] Lewis A, Isaacson M, Harootunian A, Muray A. Development of a 500 Å spatial resolution light microscope: I. light is efficiently transmitted through λ/16 diameter apertures. Ultramicroscopy 1984;13:227–32.10.1016/0304-3991(84)90201-8Search in Google Scholar
[55] Pohl DW, Denk W, Lanz M. Optical stethoscopy: image recording with resolution λ/20. Appl Phys Lett 1984;44:651–3.10.1063/1.94865Search in Google Scholar
[56] Bozhevolnyi SI, Coello V. Elastic scattering of surface plasmon polaritons: modeling and experiment. Phys Rev B 1998;58:10899–910.10.1103/PhysRevB.58.10899Search in Google Scholar
[57] Binnig G, Rohrer H, Gerber Ch, Weibel E. Surface studies by scanning tunneling microscopy. Phys Rev Lett 1982;49:57–61.10.1103/PhysRevLett.49.57Search in Google Scholar
[58] Courjon D. Near-field microscopy and near field optics. London: Imperial College Press, 2003.10.1142/p220Search in Google Scholar
[59] Denkova D, Verellen N, Silhanek AV, Valev VK, Van Dorpe P, Moshchalkov VV. Mapping magnetic near-field distributions of plasmonic nanoantennas. ACS Nano 2013;7:3168–76.10.1021/nn305589tSearch in Google Scholar PubMed
[60] Ohtsu M, Kobayashi K. Optical near fields: Introduction to classical and quantum theories of electromagnetic phenomena at the nanoscale. New York: Springer, 2004.10.1007/978-3-662-09104-3Search in Google Scholar
[61] Betzig E, Finn PL, Weiner JS. Combined shear force and near-field scanning optical microscopy. Appl Phys Lett 1992;60:2484–6.10.1063/1.106940Search in Google Scholar
[62] Karrai K, Grober RD. Piezoelectric tip-sample distance control for near field optical microscopes. Appl Phys Lett 1995;66:1842–4.10.1063/1.113340Search in Google Scholar
[63] Merlo JM, Aguilar JF, Martí-Panameño E, Cortés R, Coello V. Angle dependence of the interaction distance in the shear force technique. Rev Sci Instrum 2011;82:083704.10.1063/1.3624691Search in Google Scholar PubMed
[64] Vohnsen B, Bozhevolnyi SI. Characterization of near-field optical probes. Appl Opt 1999;38:1792–7.10.1364/AO.38.001792Search in Google Scholar PubMed
[65] Inouye Y, Kawata S. Near-field scanning optical microscope with a metallic probe tip. Opt Lett 1994;19:159–61.10.1364/OL.19.000159Search in Google Scholar PubMed
[66] Sugiura T, Okada T, Inouye Y, Nakamura O, Kawata S. Gold-bead scanning near-field optical microscope with laser-force position control. Opt Lett 1997;22:1663–5.10.1364/OL.22.001663Search in Google Scholar
[67] Lazarev A, Fang N, Luo Q, Zhang X. Formation of fine near-field scanning optical microscopy tips. Part II. By laser-heated pulling and bending. Rev Sci Instrum 2003;74:3684–8.10.1063/1.1589584Search in Google Scholar
[68] Lazarev A, Fang N, Luo Q, Zhang X. Formation of fine near-field scanning optical microscopy tips. Part I. By static and dynamic chemical etching. Rev Sci Instrum 2003;74:3679–83.10.1063/1.1589583Search in Google Scholar
[69] Ekvall I, Wahlstrom E, Claesson D, Olin H, Olsson E. Preparation and characterization of electrochemically etched W tips for STM. Meas Sci Technol 1999;10:11–8.10.1088/0957-0233/10/1/006Search in Google Scholar
[70] Merlo JM, Aguilar JF, González-Hernández H, Caballero NA. Properties of the near field interactions produced by spherical nanoparticles. Proc of SPIE 2011;8011:801141–7.10.1117/12.903412Search in Google Scholar
[71] Ichimura T, Fujii S, Verma P, Yano T, Inouye Y, Kawata S. Subnanometric near-field Raman investigation in the vicinity of a metallic nanostructure. Phys Rev Lett 2009;102:186101.10.1103/PhysRevLett.102.186101Search in Google Scholar PubMed
[72] Koev ST, Agrawal A, Lezec HJ, Aksyuk VA. An efficient large-area grating coupler for surface plasmon polaritons. Plasmonics 2012;7:269–77.10.1007/s11468-011-9303-7Search in Google Scholar
[73] Hecht B, Bielefeldt H, Pohl DW, Novotny L, Heinzelmann H. Influence of detection conditions on near-field optical imaging. J Appl Phys 1998;84:5873–82.10.1063/1.368902Search in Google Scholar
[74] Cao L, Nome RA, Montgomery JM, Gray SK, Scherer NF. Controlling plasmonic wave packets in silver nanowires. Nano Lett 2010;10:3389–94.10.1021/nl101285tSearch in Google Scholar PubMed
[75] Costantini D, Greusard L, Bousseksou A, Rungsawang R, Zhang TP, Callard S, Decobert J, Lelarge F, Duan G-H, De Wilde Y, Colombelli R. In situ generation of surface plasmon polaritons using a near-infrared laser diode. Nano Lett 2012;12:4693–7.10.1021/nl302040eSearch in Google Scholar PubMed
[76] Gjonaj B, Aulbach J, Johnson PM, Mosk AP, Kuipers L, Lagendijk A. Optical control of plasmonic Bloch modes on periodic nanostructures. Nano Lett 2012;12:546–50.10.1021/nl204071eSearch in Google Scholar PubMed
[77] Kwak E-S, Henzie J, Chang S-H, Gray SK, Schatz GC, Odom TW. Surface plasmon standing waves in large-area subwavelength hole arrays. Nano Lett 2005;5:1963–7.10.1021/nl051339sSearch in Google Scholar PubMed
[78] Liu Z, Steele JM, Srituravanich W, Pikus Y, Sun C, Zhang X. Focusing surface plasmons with a plasmonic lens. Nano Lett 2005;5:1726–9.10.1021/nl051013jSearch in Google Scholar PubMed
[79] Babayan Y, McMahon JM, Li S, Gray SK, Schatz GC, Odom TW. Confining standing waves in optical corrals. ACS Nano 2009;3:615–20.10.1021/nn8008596Search in Google Scholar PubMed
[80] Crommie MF, Lutz CP, Eigler DM. Confinement of electrons to quantum corrals on a metal surface. Science 1993;262: 218–20.10.1126/science.262.5131.218Search in Google Scholar PubMed
[81] Fiete GA, Heller EJ. Colloquium: theory of quantum corrals and quantum mirages. Rev Mod Phys 2003;75:933–48.10.1103/RevModPhys.75.933Search in Google Scholar
[82] Fang Z, Peng Q, Song W, Hao F, Wang J, Nordlander P, Zhu X. Plasmonic focusing in symmetry broken nanocorrals. Nano Lett 2011;11:893–7.10.1021/nl104333nSearch in Google Scholar PubMed
[83] Lin J, Balthasar Mueller JP, Wang Q, Yuan G, Antoniou N, Yuan X-C, Capasso F. Polarization-controlled tunable directional coupling of surface plasmon polaritons. Science 2013;340:331–4.10.1126/science.1233746Search in Google Scholar PubMed
[84] Ditlbacher H, Krenn JR, Felidj N, Lamprecht B, Schider G, Salerno M, Leitner A, Aussenegg FR. Fluorescence imaging of surface plasmon fields. Appl Phys Lett 2002;80:404–6.10.1063/1.1435410Search in Google Scholar
[85] Zhang S, Wei H, Bao K, Hakanson U, Halas NJ,Nordlander P, Xu H. Chiral surface plasmon polaritons on metallic nanowires. Phys Rev Lett 2011;107:096801.10.1103/PhysRevLett.107.096801Search in Google Scholar PubMed
[86] Kramer A, Trabesinger W, Hecht B, Wild UP. Optical near-field enhancement at a metal tip probed by a single fluorophore. Appl Phys Lett 2002;80:1652–4.10.1063/1.1453479Search in Google Scholar
[87] Dulkeith E, Ringler M, Klar TA, Feldmann J. Gold nanoparticels quench fluorescence by phase induced radiative rate suppression. Nano Lett 2005;5:585–9.10.1021/nl0480969Search in Google Scholar PubMed
[88] Anger P, Bharadwaj P, Novotny L. Enhancement and quenching of single-molecule fluorescence. Phys Rev Lett 2006;96:113002.10.1103/PhysRevLett.96.113002Search in Google Scholar PubMed
[89] Wei H, Li Z, Tian X, Wang Z, Cong F, Liu N, Zhang S, Nordlander P, Halas NJ, Xu H. Quantum dot-based local field imaging reveals plasmon-based interferometric logic in silver nanowire networks. Nano Lett 2011;11:471–5.10.1021/nl103228bSearch in Google Scholar PubMed
[90] Wei H, Zhang S, Tian X, Xu H. Highly tunable propagating surface plasmons on supported silver nanowires. PNAS 2013;110:4494–9.10.1073/pnas.1217931110Search in Google Scholar PubMed PubMed Central
[91] Pennycook SJ, Nellist PD, editors. Scanning transmission electron microscopy: imaging and analysis. New York: Springer, 2011.10.1007/978-1-4419-7200-2Search in Google Scholar
[92] Hillier J, Baker RF. Microanalysis by means of electrons. J Appl Phys 1944;15:663–75.10.1063/1.1707491Search in Google Scholar
[93] Nelayah J, Kociak M, Stephan O, Garcia de Abajo FJ, Tence M, Henrard L, Taverna D, Pastoriza-Santos I, Liz-Marzan LM, Colliex C. Mapping surface plasmons on a single metallic nanoparticle. Nature Phys 2007;3:348–53.10.1038/nphys575Search in Google Scholar
[94] Werner WSM. Dielectric function of Cu, Ag, and Au obtained from reflection electron energy loss spectra, optical measurements, and density functional theory. Appl Phys Lett 2006;89:213106.10.1063/1.2397026Search in Google Scholar
[95] Werner WSM, Went MR, Vos M. Surface plasmon excitation at a Au surface by 150–40000 eV electrons. Surf Sci 2007;601:L109–13.10.1016/j.susc.2007.06.076Search in Google Scholar
[96] Went MR, Vos M, Werner WSM. Extracting the Ag surface and volume loss functions from reflection electron energy loss spectra. Surf Sci 2008;602:2069–77.10.1016/j.susc.2008.04.011Search in Google Scholar
[97] Koh AL, Bao K, Khan I, Smith WE, Kothleitner G, Nordlander P, Maier SA, McComb DW. Electron energy-loss spectroscopy (EELS) of surface plasmons in single silver nanoparticles and dimmers: influence of beam damage and mapping of dark modes. ACS nano 2009;3:3015–22.10.1021/nn900922zSearch in Google Scholar PubMed
[98] Kuttge M. Cathodoluminescence plasmon microscopy. The Netherlands: Utrecht University, 2009.Search in Google Scholar
[99] Kumar A. Cathodoluminescence spectroscopy. Urbana Champaign: University of Illinois, 2010.Search in Google Scholar
[100] Kuttge M, Vesseur EJR, Koenderink AF, Lezec HJ, Atwater HA, García de Abajo FJ, Polman A. Local density of states, spectrum and far-field interference of surface plasmon polaritons probed by cathodoluminescence. Phys Rev B 2009;79:113405.10.1103/PhysRevB.79.113405Search in Google Scholar
[101] Ginzburg VL, Frank IM. On the transition radiation theory. Sov Phys JETP 1946;16:15.Search in Google Scholar
[102] Goldsmith P, Jelley JV. Optical transition radiation from protons entering metal surfaces. Philos Mag 1959;4:836–44.10.1080/14786435908238241Search in Google Scholar
[103] Barnard ES, Coenen T, Vesseur EJR, Polman A, Brongersma ML. Imaging the hidden modes of ultrathin plasmonic strip antennas by cathodoluminescence. Nano Lett 2011;11:4265–9.10.1021/nl202256kSearch in Google Scholar PubMed
[104] Zhu X, Zhang Y, Zhang J, Xu J, Ma Y, Li Z, Yu D. Ultrafine and smooth full metal nanostructures for plasmonics. Adv Mat 2010;22:4345–9.10.1002/adma.201001313Search in Google Scholar PubMed
[105] Zhu X, Zhang J, Xu J, Li H, Wu X, Liao Z, Zhao Q, Yu D. Dispersion control in plasmonic open nanocavities. ACS Nano 2011;5:6546–52.10.1021/nn201916nSearch in Google Scholar PubMed
[106] Zhu X, Zhang J, Xu J, Yu D. Vertical plasmonic resonant nanocavities. Nano Lett 2011;11:1117–21.10.1021/nl104024jSearch in Google Scholar PubMed
[107] Vesseur EJR, Garcia de Abajo FJ, Polman A. Modal decomposition of surface-plasmon whispering gallery resonators. Nano Lett 2009;9:3147–50.10.1021/nl9012826Search in Google Scholar PubMed
[108] Vesseur EJR, Polman A. Plasmonic whispering gallery cavities as optical nanoantennas. Nano Lett 2011;11:5524–30.10.1021/nl203418cSearch in Google Scholar PubMed
[109] Schoen DT, Coenen T, García de Abajo FJ, Brongersma ML, Polman A. The planar parabolic optical antenna. Nano Lett 2013;13:188–93.10.1021/nl303850vSearch in Google Scholar PubMed
[110] Sapienza R, Coenen T, Renger J, Kuttge M, van Hulst NF, Polman A. Deep-subwavelength imaging of the modal dispersion of light. Nature Mat 2012;11:781–7.10.1038/nmat3402Search in Google Scholar PubMed
©2014 Science Wise Publishing & De Gruyter Berlin/Boston
