We perform two-photon excitation confocal experiments on coupled gold nanoantennas and observe time-integrated luminescence spectra that match plasmonic mode emission in the far-field. We show that the transversal particle plasmon mode can be excited, using excitation light that is cross-polarized with respect to the gold luminescence signal and therefore oriented along the long axis of the dipole gold antenna. We provide evidence for losses in polarization information from the excitation channel to the luminescence response due to the nature of the energy and momentum transfer. Finally, we map out the two-photon induced luminescence intensity profile for a fixed excitation wavelength λ and varying antenna arm length L.
© 2011 Optical Society of America
The properties of optical nanoantennas [1–5] are currently under extensive investigation from an experimental as well as a theoretical point-of-view. With electron beam lithography [6–10], possibly combined with focused ion beam milling [11, 12], or chemical synthesis in addition to functionalization approaches , such prototype antennas are accessible with high precision and reproducibility. The scattering spectrum of optical gold nanoantennas can be interpreted as an effect that is basically plasmonic in nature, with a tunable spectral response due to size, shape, material and the dielectric environment. Two-arm nanoantennas, two closely spaced single gold rods coupled in the near-field, include an additional feature. The nano-sized gap in between the arms enhances highly localized optical near-fields at wish. Two-arm optical antennas thus bridge the world of plasmonic optics and electrical engineering at the nanoscale with an account for ultra high frequencies .
Bulk gold under single- or two-photon excitation conditions yields broadband luminescence from the visible to the infrared (IR) [15, 16]. For single nanoparticles, this broadband spectrum is limited to sharper resonances, which have been shown to match their scattering peaks [17,18]. This observation is currently discussed as a mechanism where d-band electrons are promoted to the sp-band. The resulting holes are partly scattered and receive additional momentum, allowing them to emit a particle plasmon, which can then decay radiatively . Nano-gold luminescence can therefore be enhanced and spectrally reshaped according to the nanoparticle plasmon resonances. We have recently shown that this is also true for two-arm antennas under two-photon excitation conditions . One important issue, however, has not yet been discussed in detail - if the gold nanoantenna two-photon induced luminescence is really mediated by electronic states, then the luminescence is not necessarily polarization conserving.
As is commonly known, nanoantennas can exhibit both longitudinal and transversal modes. For gold antennas with small widths, such transversal modes are typically challenging to observe using scattering observations due to interband transitions which promote significant losses. If, however, the plasmonic gold luminescence is indeed not polarization conserving, then it should contain observable features cross-polarized to the excitation light, with an intensity corresponding to the density of states of the transversal modes. In the following, we present such luminescence spectra obtained from two-arm nanoantennas and uncoupled gold nanorods (termed single-arm antennas) under two-photon excitation conditions, where the excitation laser light is polarized along the long antenna axis and cross-polarized photoluminescence intensity data is observed. The claim that this cross-polarized light is indeed the transversal antenna mode is supported by numerical simulations. This observation is a powerful tool to map out the gold antenna resonant arm length L for a fixed laser excitation wavelength λ using the cross-polarized transversal mode emission intensity.
Our gold nanoantenna fabrication protocol has been outlined previously . In short, indium-tin-oxide (ITO, 30 nm thickness) was deposited onto a glass substrate. An electron-beam resist (PMMA, poly(methylmethacrylate)) was spun onto it, thereafter, electron-beam lithography was applied. After resist developement, gold with a final layer thickness of 30 nm was thermally evaporated. Following a lift-off process, optical two-arm antennas had been fabricated. They are 20 nm in width, have arms of varying length, and are coupled by a 20 nm gap. As reference structures, we also fabricated single-arm antennas of similar dimensions.
To obtain optical information, we used an epi-fluorescent microscope in the confocal mode with a sample piezo stage movable in (x,y,z)-dimensions (Fig. 1). The excitation laser (Coherent Mira Ti:Sa, 120 fs pulses, 76 MHz repetition rate, with center peak wavelength tuned to 810 nm) is first limited to wavelengths greater than 800 nm by a long-pass filter (Semrock Edge Basic BLP01-785R-25). The laser beam intensity can be adjusted by neutral density filters. Polarizers effectively set the state of the excitation polarization. The expanded laser beam is then sent into the microscope and directed via a 50:50 beam-splitter to the oil immersion objective (100x, NA 1.46). The diffraction limited laser spot interacts with the nanoantenna sample. The emitted light from the gold antenna is subsequently collected by the same objective. Only light with wavelength below 785 nm then passes through a short-pass filter (Semrock RazorEdge 785RU-25) in the detection path. A polarizer is introduced to only detect the transversal spectral response of the antennas. For detection, either an avalanche photo-diode detector (APD) (for intensity investigations) or a spectrometer (Acton 2500i) equipped with an Andor EMCCD camera, can be used, both accessible via a turnable mirror. The typical non-destructive laser power applied (as measured in front of the objective lens) was 10 μW for resonant two-arm antenna structures. For each pulse of approximately 500 fs width (corrected for broadening, e.g. due to beam-expanding lenses, at 76 MHz repetition rate), we thus have a laser fluence I of about , assuming a focal area of π × 2002 nm2 and an objective lens transmission efficiency of 0.7  at 810 nm wavelength. The laser excitation power required to excite single-arm antennas to obtain a similar SNR was around 10 times the power required for the corresponding two-arm structures.
3. Results and discussion
We present gold luminescence spectra of individual two-arm gold nanoantennas and additionally of single-arm antennas where the excitation laser light is polarized along the long axis of the nanostructure, but the cross-polarized gold luminescence is observed. Besides the fact that the transversal mode is observed, the measurements follow the scheme our group has presented earlier . The total integration time per spectrum was 30 s. The laser power was adjusted for each antenna length to yield a response with a favorable SNR avoiding the destruction of the nanostructures. Transversal luminescence spectra obtained from the two-arm and single-arm antennas can be found in Fig. 2. All response spectra have been normalized to unity to allow for an easy comparison of the spectral characteristics. If a two-arm antenna were measured with the same excitation power as a single-arm structure, the former would be about 10 to 100 times brighter. The two-photon nature of the process is confirmed by double-logarithmic excitation intensity - emission intensity plots which yield a slope of 2 (data not shown).
Looking at the TPL spectra depicted in Figs. 2(a) and 2(b), we observe a clear peak of the transversally polarized luminescence around 2.3 eV. The spectral shape appears to be fairly independent of the long axis length (although a small aspect-ratio dependency may be present) and also of any two-arm coupling. In addition, the signal is considerably less intense compared to the longitudinal signal which we have published previously  (about two orders of magnitude). The peak position, which is significantly blue-shifted to the well-known longitudinal scattering resonances of such antenna structures , suggests that we in fact observe the transversal plasmon mode of the structures under investigation.
To support this claim, we performed numerical simulations using the FDTD-based software Lumerical . Those calculations are intended to simulate the scattering response, rather than the actual TPL experiment and hence to confirm that the measured TPL signal can be interpreted as the plasmonic scattering resonance. In these simulations, a plane wave with polarization perpendicular to the antenna length and parallel to the substrate surface was incident on gold nanoantennas. In agreement with experimental conditions, the antenna arm width was chosen to be 20 nm and the height 30 nm. The antenna is placed on a glass substrate (n=1.5) covered with a 30 nm ITO layer (complex, frequency dependent dielectric function ). The dielectric function of gold is taken from measurements by Johnson and Christy . All material parameters were fitted to a generalized multi-coefficient model supported in the FDTD simulation. The corners and edges of the antenna arms were rounded with a radius of 3 nm.
This circumvents singularities in the simulation, which would lead to unrealistically high field enhancements at theses corners. A simulation area of 8003 nm3 surrounded by 12 perfectly matched layers (PMLs) proved to be sufficient. The mesh cell size varied between 0.15 nm and 8 nm throughout the simulation area, and the plane wave was incident from inside the glass substrate. The scattered power was detected with a power monitor situated in the glass substrate 100 nm above the antenna. The former was then normalized by the incident intensity and is proportional to the power scattered backwards into the glass substrate. Figure 3 shows the relative scattering intensity of single-arm and two-arm antennas with arm lengths of 45 nm, 50 nm and 65 nm and a further single-arm antenna with arm length 150 nm. As can be seen, the peak position at 2.25 eV is in very good agreement with the peak position from the measured TPL emission spectra. Also, in accordance with measured results, there is hardly any shift in the peak position for different arm length and also not for coupled versus uncoupled antennas. With increased antenna volume, the scattering response simply increases. We can thus in fact assume that the transversal plasmon mode is observed.
The energy transfer via excited electron states described by Dulkeith et al.  would specifically allow for the non-polarization-conserving effects we measure. Our experiment in conjunction with our simulation results thus gives further evidence that the model proposed there for single-photon excitation also holds true for TPL effects. It is interesting to see that the two-photon excitation technique in fact allows for a higher signal-to-noise-ratio (SNR) compared to scattering experiments, as now the transversal mode can easily be accessed due to efficient filtering of the excitation light.
It could be argued that the observed cross-polarized signal is in fact material-intrinsic, as a similar spectral response was observed for chemically synthesized gold nanorods under TPL exictation by e.g. Imura et al.  and Scolari et al. . We deem this unlikely for our case, however, as our structures are polycrystalline. Furthermore, we have not observed any L symmetry point luminescence contribution (which would appear in a similar spectral range around 2.3 eV) from non-resonant TPL on thin gold nanorods . It would be interesting to conduct comparative electron energy loss spectroscopy (EELS) measurements to possibly shed light on these apparent differences between chemically fabricated structures and antennas made using electron beam lithography.
The selective observation of the transversal plasmon mode allows for a complete resonance intensity mapping of the antenna response, which is impossible using the longitudinal mode emission intensity as it is maximal at the two-photon excitation wavelength, which needs to be filtered out in the detection channel . As the spectral shape of the transversal resonance is fairly constant, however, and within the detection window limited by the filtering of the excitation wavelength, the transversal intensity observed can be used as a direct (albeit nonlinear) measure for the strength of the antenna resonance. We can thus compare the spectrally integrated intensities measured from antennas of different lengths. A normalized curve of such a mapping with a constant illumination intensity and constant illumination wavelength can be seen both for two-arm and single-arm antennas in Fig. 4. Please note that while the position of the maximum for both curves is independent of the excitation intensity, the curve shape is not, as a nonlinear process is observed. Also, it is still the absorption mediated by the excitation and thus the longitudinal excitation efficiency that is mapped out (similar to experiments e.g. conducted by Wang et al. ), albeit the energy is transferred to the transversal mode for the luminescence response. A Gaussian fit is given in addition to the actual measurement data points as a guide to the eye. We find the main antenna excitation resonance for a two-arm antenna to be for an arm length of about 70 nm, while the corresponding single-arm antennas yield a resonance maximum for an arm length of about 85 nm. Both those values are in very good agreement with extrapolation from the known peak wavelengths for shorter structures obtained from linear scattering experiments . Cross-polarized TPL measurements thus provide a powerful tool for quick characterization of antenna resonance properties and proper antenna selection for the chosen excitation conditions. On a side note, this measurement also proves that we do not observe a signal generated by possibly transversally polarized components of the excitation light (which are well-known to be present for high numerical aperture objective lenses under linear polarization, see for example reference ) - if that were the case, the emission signal observed would need to become more intense for larger structures. Additionally, we had already shown in a previous publication  that even completely transversally polarized excitation light was unable to generate an optical response from the structures.
We have verified with this detailed study that resonant gold dipole antennas both with and without gap under two-photon laser excitation along their long axis yield a cross-polarized luminescence response with a spectral peak that can be attributed to the transversal particle plasmon. We thus provide further evidence for the involvement of electronic states in the plasmon generation and subsequent photon emission process. In fact, this method is a largely tunable non-linear frequency conversion process - a matching antenna can be found for any desired excitation and emission wavelength regime, as long as one operates within the suitable wavelengths outside of absorption maxima for the material chosen. The resonant structure for a fixed excitation wavelength can be easily found using this TPL technique, exploiting the fact that transversal mode plasmon emission can be triggered by longitudinal excitation, and that the transversal modes fit the spectral detection window needed. This has extremely useful applications in setups where only laser excitation is available for the characterization of the nanostructures and can be used easily hand-in-hand with atomic force microscopy or tip-enhanced near-field techniques.
The authors thank H. Wermund for help with the gold evaporation process. M. W. and C. M. acknowledge generous support by the Karlsruhe School of Optics and Photonics (KSOP). K. I. and M. S. are partly supported by the Center for Functional Nanostructures (CFN). The corresponding author likes to acknowledge support through Deutsche Forschungsgemeinschaft (DFG) under the project DFG EI 442/3-1 and the DFG Heisenberg Excellence Fellowship DFG EI 442/2-2.
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