Letter pubs.acs.org/NanoLett
Near Field of Strongly Coupled Plasmons: Uncovering Dark Modes Florian Schertz,*,† Marcus Schmelzeisen,‡,§ Reza Mohammadi,‡ Maximilian Kreiter,‡ Hans-Joachim Elmers,† and Gerd Schönhense† †
Institut für Physik, Johannes Gutenberg-Universität, Staudinger Weg 7, D-55128 Mainz, Germany Max-Planck-Institut für Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany § Center of Smart Interfaces, Technische Universität Darmstadt, Petersenstraße 32, D-64287 Darmstadt, Germany ‡
S Supporting Information *
ABSTRACT: Strongly coupled plasmons in a system of individual gold nanoparticles placed at subnanometer distance to a gold film (nanoparticle-onplane, NPOP) are investigated using two complementary single particle spectroscopy techniques. Optical scattering spectroscopy exclusively detects plasmon modes that couple to the far field via their dipole moment (bright modes). By using photoemission electron microscopy (PEEM), we detect in the identical NPOPs near-field modes that do not couple to the scattered far field (dark modes) and are characterized by a strongly enhanced nonlinear electron emission process. To our knowledge, this is the first time that both farand near-field spectroscopy are carried out for identical individual nanostructures interacting via a subnanometer gap. Strongly resonant electron emission occurs at excitation wavelengths far off-resonant in the scattering spectra. KEYWORDS: Plasmon coupling, subnanometer gap, gap resonance, dark mode, near field, sphere-on-plane
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separated by a small distance. An alternative system consists of a metal nanoparticle separated from a metallic film by a small gap. In general, the mutual interaction of plasmon supporting structures can lead to quite complex modes, where the resonances are shifted either to the blue or to the red when compared to the noninteracting systems. To describe the coupling of such complex plasmonic nanostructures of arbitrary shape, the group of Nordlander presented an intuitive model, the plasmon hybridization (PH) concept.12,13 The system nanoparticle-on-plane (NPOP) has the advantage that it can be easily fabricated with well-defined spacing and in a way that it resembles theoretical models used for its description.14,15 Theoretical13−17 and experimental17−21 studies have led to a good understanding of their radiating modes. The coupling between particle and plane leads to new electromagnetic modes, which are named gap modes, since the strongly enhanced near field is mainly localized in the gap between particle and film. The plasmonic interaction depends on the particle size, the gap size, and the dielectric functions of both the metals and the environment. In the quasi-static approximation, the electromagnetic response is scale invariant, governed by the ratio d/R of the particle film distance d and radius R of the nanoparticle. Both the redshift and the intensity of the gap mode increase with decreasing d/R. Thus, the resonance wavelength can be easily tuned, e.g., by changing the
he interest in the field of nano-optics has recently tremendously increased. Due to the rapid progress in nanoengineering, the understanding and controlling of light− matter interaction on a nanometer and subnanometer scale may finally be within reach. Particular attention has been given to surface plasmons in metal structures, resulting in interesting electrical effects.1−3 Within a few optical cycles, the collective oscillation of electrons leads either to a pile-up of charge at the surface of a film (surface plasmon polariton, SPP) or to a localized plasmon in a particle (localized surface plasmon, LSP). Both excitations establish a strong concentration of electromagnetic energy in a volume far below the diffraction limit of light (i.e., in the near field) and may cause a high scattering efficiency in the far field. Thus, nanoparticles can be seen as optical antennas which efficiently convert freepropagating light into strongly confined electromagnetic energy and vice versa.4 Current fundamental research focuses on the electrodynamical properties in the optical regime and on the femtosecond time scale in order to achieve coherent control of the nano-optical fields.5,6 In addition, the emerging enhancement of optical fields provides valuable opportunities for, e.g., single molecule spectroscopy,7 optical data storage,8 cancer therapy,9 or improvement of solar cells.10,11 In these applications, mostly the enhanced optical near field is the important property, instead of the more frequently measured far-field response. Recently, the interaction of individual plasmon supporting components has come into the focus of interest. The coupling is realized by exciting, e.g., two or more metallic nanostructures © 2012 American Chemical Society
Received: December 5, 2011 Revised: March 12, 2012 Published: March 19, 2012 1885
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diameter of the particle.20 As a consequence, they are simple and highly efficient structures. Plasmonic excitations decay within a few femtoseconds, either radiatively by emitting photons into the far field or nonradiatively by electron−hole excitations.22 Modes with a strong dipole moment couple very efficiently to the far field and thus can decay radiatively (bright modes). If a certain mode exhibits a weak dipole moment, its radiative decay channel is suppressed. Therefore, they have been named dark modes. The strong coupling of nanostructures can give rise to these dark modes when excited optically.23 While bright modes enhance both the near- and the far-field responses, dark modes are pure near-field modes. Due to the absence of the radiative decay channel, dark modes have a longer lifetime,24 representing resonators of a high quality factor and therefore providing an extremely large near-field enhancement. For applications long lived, excited states of a high quality factor are of great importance. The experimental investigation of dark modes remains an ambitious task. As its name implies, the decay of a dark mode does not emit photons into the far field, and thus standard scattering methods are precluded for measuring these modes. In some cases, dark modes can be detected in the far-field indirectly, e.g., in terms of the involvement of evanescent fields23 or the creation of Fano resonances,25 where dark modes interfere with dipolar modes, provided that a spectral overlap exists. In the case of single nanoparticles, differences of nearand far-field resonances were previously obtained in calculations.26 For example, the near field peaked at larger excitation wavelengths than the far field. An increase of this redshift with increasing particle size was observed27 and could be explained by a damped oscillator model.28 Thus, for the interaction of plasmons the question arises: Does the coupling of plasmons affect the near and the far field differently? In this paper, we present a study of both the far and the near field of identical individual NPOPs serving as a model system for strongly coupled plasmons. The gap sizes of the NPOPs were smaller than a nanometer, implying a near-field concentration on a length scale of a thousandth part of the excitation wavelength. Recently it was shown, that photoemission electron microscopy (PEEM) is one of the few methods offering experimental access to the near field.29−31 In this study we demonstrate that this holds true even for subnanometer gaps. We show that valuable information on both dipolar and dark modes can be gained by exploiting the nonradiative decay of an excited plasmon. A single electron may capture the energy of the oscillating ensemble of conduction electrons.32 However, it can only be detected if the captured energy is large enough to overcome the work function of the metallic surface. As the energy of the plasmon modes is usually much lower than the work function, this condition requires highly nonlinear electron emission processes. PEEM provides high lateral resolution and therefore an unambiguous assignment of the emission to individual nanostructures.29−33 We compare spectroscopic results from PEEM, revealing the near field of both bright and dark modes, with optical scattering showing exclusively bright modes both for the same individual NPOPs. To correlate the precise shape and size of the individual NPOPs with their optical response, they are further characterized by high-resolution scanning electron microscopy (SEM). Our experimental results prove the existence of narrow nonradiative near-field modes (dark modes) for excitation near 800 nm with TM polarization. In contrast, the bright gap
modes occur at shorter excitation wavelengths, as detected in the optical scattering spectra for the identical individual NPOP. The structure of the sample is sketched out in Figure 1a. The relevant components of the sample are Au nanoparticles20,34,35
Figure 1. Experimental setups. (a) Sample cross section; (b) experimental setup of the dark-field confocal microscope to record the far-field scattering spectra; and (c) setup of the near-field induced electron emission experiment using a PEEM.
placed on a ∼50 nm thick Au film which is covered by a selfassembled monolayer (SAM) of the organic molecule cysteamine. The nanoparticles have a mean diameter of ∼90 nm. The cysteamine layer acts as a spacer between the Au film and the nanoparticles, establishing a well-defined gap of 0.8 nm size that determines the interaction of the LSP with the SPP. The Au nanoparticles are strongly immobilized by this process. In contrast to lithography methods, this process allows for a welldefined assembly of samples with subnanometer gaps. In addition, it has the advantage of leaving no residual metallic atoms in the gap which would affect the optical response significantly.36 In order to retrieve the identical sample position in the different microscopes, the samples are marked with a grid structure and scratches. The optical scattering spectra of individual NPOPs are measured by using a customized scanning confocal optical 1886
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microscope in a plasmon mediated dark-field mode, see Figure 1b. Details of the setup are described in detail elsewhere.21,34 The sample is illuminated with linearly polarized light of a Xenon lamp, allowing the recording of spectra in the wavelength range of 450−850 nm. Electron emission is induced by illuminating the sample with light of a femtosecond laser tunable from 750 to 850 nm. The angle of incidence is 65° with respect to the surface normal. A commercial PEEM is used to detect the emitted electrons as shown in Figure 1c. The detection process itself is background free. In fact, no electrons are emitted from the Au/cysteamine film without nanoparticles, since the energy of a single photon is smaller than the work function of Au/cysteamine. If no resonant excitation is involved, the occurrence of electron emission is extremely unlikely. In this sense, the number of detected electrons per time is assumed to be a nonlinear measure for the plasmon induced near field (see discussion below). Finally, the NPOPs are imaged by SEM. This sequence of methods is necessary because the intense electron beam of the SEM induces carbon contamination, thus changing the optical properties of the sample. The individual NPOPs measured in both the PEEM and the confocal optical microscope are identified by their position with respect to the markers and to each other. More details of the sample preparation and the experimental setups can be found in the Supporting Information. In Figure 2, a 12 × 12 μm2 large section of the characterized sample is shown, recorded with the PEEM (a), the confocal
Figure 3. Experimental results: Excitation spectroscopy of four representative nanoparticles on plane. The black curves show the scattering efficiency, measured by means of dark-field confocal microscopy. The dip at ∼720 nm occurred in all spectra and is attributed to an artifact in the normalization spectrum of the Xenon illumination. The red curves show the wavelength dependence of the electron emission yield for a constant intensity of the excitation light. SEM images of the corresponding nanoparticles are represented in the insets. Peak A marks the particle resonance, peak B the far-field gap resonance, and peak C the near-field gap resonance. The blue arrows drawn in the plot of object 23 indicate the fwhm of both the far- and the near-field gap resonance.
interaction of the SPP film plasmon and the LSP of the nanoparticle. This is in agreement with previous observations17−20 and theoretical studies.13−17 However, the theoretical investigations predict narrower linewidths of the scattering resonances, which is also confirmed by single particle spectroscopy of comparable silver systems.21 The higher energy mode at wavelengths of about 500−600 nm is assigned to a mode preferentially incorporating the entire particle or a mode parallel to the film surface, while the lower energy mode at wavelengths around 700 nm is mainly localized in the gap.16 No scattering resonance occurs above 730 nm. The electron emission spectra show peaks with very narrow bandwidths in the wavelength regime of 750−850 nm for all investigated NPOPs. The corresponding scattering spectra of the identical NPOP reveal a featureless tail of the gap resonance in this energy interval. This occurrence of near-field modes observed with the PEEM at wavelengths being off-resonant in the far-field spectra for the identical NPOP is a clear evidence for the existence of dark modes uncovered by the PEEM. The electron emission spectra show particularly narrow resonances. However, part of the observed sharpness of the spectral peaks for electron emission is due to the nonlinear electron emission process. Considering the nonlinearity, we determine a full width at half-maximum (fwhm) for peak C (electron emission peak) of object 23 in Figure 3 of ΔEfwhm (C) = 45 meV. The corresponding fwhm of Peak B (far-field gap resonance) is ΔEfwhm (B) = 350 meV. This implies a lifetime τDM = h/ (2π·ΔEfwhm) = 14 fs of the electron emission (near field) mode compared to τBM = 2 fs for the scattering resonances (far-field mode). The narrow linewidths indicating longer lifetimes of the electron emission modes support the assumption of the detection of a dark mode. The spectra shown in Figure 3 were obtained by illumination with TM-polarized light. When irradiating the sample with TE-
Figure 2. Particle identification of the different detection techniques. Images recorded by means of (a) PEEM, (b) dark-field confocal microscopy, and (c) SEM of a small sample section. The field of view is 12 × 12 μm2. The NPOPs identified as isolated particles on the Au surface are marked by circles. The dark rectangles in the SEM image indicate carbonated areas already scanned with higher magnification.
optical microscope (b), and the SEM (c). There is a one-to-one correspondence between the white objects in the SEM image, the false color spots in PEEM, and the bright spots in the scattering image of the confocal microscope. The objects are identified as surface-immobilized gold particles by increasing the magnification in the SEM. The tagged objects were characterized by the two spectroscopic methods. The complete investigated sample area with all characterized particles is shown in Figure S2, Supporting Information. Scattering spectra in the wavelength range between 450 and 850 nm and electron emission spectra between 750 and 850 nm are displayed for a selection of four representative NPOPs in Figure 3, the spectra of the remaining particles are presented in Figure S3, Supporting Information. The scattering spectra shown in Figure 3 reveal the common characteristics of the two hybridized modes, resulting from the 1887
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NPOP, the critical gap size for the onset of the CTP regime might be smaller due to the fact that the local electric field in the gap for a given gap size is spatially less concentrated in comparison to the case of two convex surfaces in a particle dimer. Nevertheless, CTP might still occur in our NPOPs, as the gap is only 0.8 nm. Tunneling is a nonohmic process, to the effect that the conductivity increases with increasing electric field strength or the (square root of) laser power density, respectively. The conductivity determines the damping of a CTP mode and thus its spectral width and resonance frequency. Hence, in case of a CTP, changes in the spectral characteristics are expected to occur in the electron emission experiment for different laser power densities. In contrast, our experimental results exhibit spectral characteristics independent of the excitation power, as shown in Figure S6, Supporting Information. Therefore, we conclude that CTPs may exist under the given experimental conditions, considering the ultrasmall gap, but do not contribute directly to the measured total electron yield. The alternative nonradiating modes are dark modes originating from field-coupled plasmons. We tentatively ascribe the observed resonances to these modes. The strongly increased near field of the dark mode may induce ionization of the organic spacer molecule cysteamine. Alternatively, a previously emitted electron may ionize cysteamine via impact ionization.42 The ionization may not be detectable at sample locations without nanoparticles, since the electric field strength provided by the laser radiation without plasmonic enhancement is too low. However, due to the high ionization potential of cysteamine of >9 eV,43 it is more likely to extract electrons from the Au surfaces involved in the plasmonic excitation (work function ∼5 eV)44 than from the spacer molecule. Consequently, we assume that the emitted electrons originate from the Au surfaces. Possible electron emission mechanisms are either multiphoton photoemission45 or field emission, induced by the highly enhanced electric field in the gap. Our studies favor the latter process, in which conduction electrons are directly emitted by optical field emission and afterward accelerated by the time-dependent electric field (ponderomotive force). This model is supported by our observation of a wide distribution of kinetic energies (up to 7 eV), depending on the laser intensity. In both possible emission mechanisms, the enhanced near-field intensity caused by excited plasmon modes provokes the electron emission. By definition, we designate the observed electron emission modes as dark modes, since they are not observable in the far-field spectra. For dark modes a narrower line width is expected as their lifetime increases with the suppression of radiative decay, in agreement with the observed spectra. Previous theoretical investigations concerning NPOP systems13−17 were restricted to spherical nanoparticles. Semianalytical approaches require at least cylindrical symmetry of the structure. Therefore, most nonspherical nanoparticle shapes, as in our experiment, necessitate numerical calculations. We used the software package “CST Microwave Studio” based on the finite integration technique (FIT)46,47 for our electrodynamical simulations to consider the influence of the particle shape on the near field in an NPOP system. The dielectric function of Au was taken from Johnson and Christy.48 A comparison of the calculated near fields of a spherical NPOP and a prolate deformed NPOP is shown in Figure 4. The field was evaluated in the center of the gap. The deformation of the
polarized light, no photoemission intensity is observed in the accessible excitation range, indicating that the excited modes are normal modes (perpendicular to the Au film surface), as expected for gap resonances. A comparison of PEEM images of NPOPs illuminated by TM- and TE-polarized light, respectively, is shown in Figure S4, Supporting Information. The electron emission signal stayed constant over hours and was reproducible after months. The scattering spectra shown in Figure 3 were measured after the PEEM experiment in order to avoid artifacts from changes of the NPOP structure induced by the ultrashort laser pulses. By following this sequence, influences of the laser pulses on the sample composition would affect both the electron emission and the scattering results. However, no influence of the illumination with the laser pulses on the NPOP structure could be observed. In order to estimate the effect of the excitation with ultrashort laser pulses on the NPOPs, we measured the scattering spectra prior and after the illumination with fs-laser light of ∼5 times higher intensities than used for electron emission. No relevant changes of the scattering spectra were observed (see Figure S5, Supporting Information). Therefore, neither the illumination with ultrashort pulses nor the electron emission process cause any measurable structural defects on the NPOPs as, e.g., observed by Grady et al.38 The electron emission is induced by monochromatic laser radiation. In contrast, the scattering is observed upon whitelight excitation, affecting the formation of the broadband SPP. Nevertheless, the dark-field confocal microscope setup excites and detects a wide wavelength range, including the excitation wavelength range used for the PEEM experiment. If far-field modes were excited near 800 nm, they would have been observed in the optical scattering spectra. Another aspect worth mentioning is the difference regarding the angles of incidence in both setups. For the PEEM experiment, the angle of incidence is fixed to 65° with respect to the sample surface normal. In contrast, in the optical microscopy setup a range of angles is focused onto the sample by means of the high NA objective lens. The angle of incidence changes the peak heights but not the fwhm, since the gap resonance is solely sensitive to the electric field component normal to the Au film surface (z-direction). Hence, the excitation under a smaller angle of incidence implies a smaller z-component of the electric field, thus contributing a smaller fraction to the scattering signal of the gap mode. Since we do not compare absolute values of both detection techniques, the different angles of incidence do not have any crucial impact. In the following, we discuss the possible microscopic mechanisms of the electron emission process. Two nonradiative plasmon modes are currently discussed occurring in strongly coupled plasmonic structures: charge transfer plasmons (CTP)39−41 and dark modes.23−25 In a CTP, the electrons tunnel through the gap from the particle to the film and vice versa. Zuloaga et al. predict a crossover from radiative to nonradiative behavior for nanoparticle dimers when the distance between the nanoparticles decreases to 0.5 − 1 nm.40 In this case, a classical description does not suffice, and quantum mechanical effects have to be taken into account. They analyzed particles with diameters of a few nanometers and predict an onset of the CTP regime with decreasing distance. The critical distance increases with increasing particle size. This fact was explained by an enhanced impact of the image potential and an increasing density of states, resulting in more conductivity channels. In the case of a 1888
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NPOPs; (S3) optical and PEEM spectra of the residual NPOPs; (S4) polarization dependence of the electron emission intensity; (S5) influence of femtosecond-laser irradiation on the optical properties of the NPOPs; (S6) excitation spectroscopy of electron emission for different laser power densities. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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Figure 4. Numerical calculations of the near field of a spherical (black curve) and a nonspherical (red curve) NPOP. The deformation of the particle gives rise to a red shift and a splitting of both resonances in comparison with the spherical NPOP.
ACKNOWLEDGMENTS Financial support from Deutsche Forschungsgemeinschaft through SPP 1391 and SFB 625 is gratefully acknowledged. We thank Gunnar Glasser for his support in scanning electron microscopy and H.-J. Butt and D. Panzer for the fruitful discussions.
particle causes a slight red shift and a splitting of both the particle-like resonance at 600−680 nm and the gap resonance at 750−870 nm. These multiple gap resonances qualitatively agree with NPOP resonances observed in the PEEM experiment (see Figure 3 and S3, Supporting Information), whereas multiple peaks were not observed for the gap resonance of the scattering spectra. The scattering cross-section of a NPOP could not be calculated with this simulation method because the numerically limited size of the plane causes interference patterns that mask the far-field response of the real system. In this paper we showed that plasmon induced electron emission provides experimental access to the near field of strongly coupled plasmonic structures solely separated by subnanometer gaps. Single particle spectroscopy of the same individual NPOPs was carried out by means of two complementary detection techniques, optical scattering spectroscopy to detect far field and PEEM to obtain near-field characteristics. The direct comparison of the near-field spectra with optical scattering spectra for each NPOP showed significant differences: Strong resonances occurred in the near field at wavelengths, being clearly off-resonant in the far-field spectra. The results suggest the requirement of a near-field detection technique, provided, e.g., by photoelectron emission microscopy, besides the common far-field spectroscopy for a complete understanding of strongly coupled plasmon systems. A deeper insight into the near field of strongly coupled plasmons is of utmost importance for the optimization of devices exploiting near-field phenomena, such as plasmonenhanced solar cells. The dependency of the near-field resonances of a NPOP on the particle shape allows for the tailoring of plasmonic structures. For prospective experiments, PEEM offers a variety of photoelectron detection techniques, for instance, kinetic energy or emission angle distribution experiments, to allow for a better understanding of the underlying physics of strongly coupled plasmons. Moreover, time-resolved PEEM experiments30,31,37,49,50 give an insight into the temporal evolution of the near-field modes.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
(S1) Detailed description of sample preparation and experimental setup; (S2) particle identification of all investigated 1889
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