Spatiotemporal Characterization of SPP Pulse Propagation in Two

Letter pubs.acs.org/NanoLett

Spatiotemporal Characterization of SPP Pulse Propagation in TwoDimensional Plasmonic Focusing Devices Christoph Lemke,*,† Christian Schneider,‡ Till Leißner,† Daniela Bayer,‡ Jörn W. Radke,† Alexander Fischer,‡ Pascal Melchior,‡ Andrey B. Evlyukhin,§ Boris N. Chichkov,§ Carsten Reinhardt,§ Michael Bauer,† and Martin Aeschlimann‡ †

Institute for Experimental and Applied Physics, University of Kiel, Leibnizstr. 19, D-24118 Kiel, Germany Department of Physics and Research Center OPTIMAS, University of Kaiserslautern, Erwin Schroedinger Str. 46, D-67663 Kaiserslautern, Germany § Laser Zentrum Hannover e.V., Hollerithallee 8, D-30419 Hannover, Germany ‡

S Supporting Information *

ABSTRACT: The spatiotemporal evolution of a SPP wave packet with femtosecond duration is experimentally investigated in two different plasmonic focusing structures. A two-dimensional reconstruction of the plasmonic field in space and time is possible by the numerical analysis of interferometric time-resolved photoemission electron microscopy data. We show that the timeintegrated and time-resolved view onto the wave packet dynamics allow one to characterize and compare the capabilities of twodimensional components for use in plasmonic devices operating with ultrafast pulses. KEYWORDS: Plasmonic devices, Femtosecond phenomena, Nanooptics, Surface photoemission lectromagnetic fields can propagate along a metal− dielectric interface in the form of low-dimensional modes coupled to coherent charge oscillations at the metallic surface known as surface plasmon polaritons (SPP). This is the key property that makes this type of coupled electron-light excitations, whose intensity is concentrated at the interface, attractive for applications in nanoscale photonics with prospects in data transport and processing.1 Recently, several optical functional units for steering and manipulating guided SPP modes have been developed.2−6 For the characterization of passive and active plasmonic elements, especially to assess their reliability, appropriate microscopy techniques, such as leakage radiation microscopy (LRM)7−10 or scanning near field optical microscopy (SNOM),11−13 are usually employed. Furthermore, the preparation of femtosecond-SPP wave packets has an important impact on the realization of highspeed SPP devices for broadband signal processing14 or ultrafast recording.15 In this regard it is especially important to have experimental means to track and visualize the twodimensional SPP propagation in space and time at submicrometer lateral and femtosecond temporal resolution. Optical pump−probe schemes in combination with near-field microscopy16,17 and photoemission electron microscopy (PEEM)18 techniques have successfully been applied for tracking SPP wave packet propagation in plasmonic systems in the past.19−24 In these studies, however, the focus was on the investigation of one-dimensional or quasi one-dimensional propagation scenarios in linear waveguides or at planar surfaces.

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© 2013 American Chemical Society

In this article, we present an experimental approach for the spatiotemporal characterization of ultrashort SPP wave packets that propagate in a real two-dimensional structured metallic film. More specifically, we analyze the propagation in two different plasmon-optical devices that have been designed for spatiotemporal focusing of SPP wave packets. Femtosecondtemporal and sub-100 nm spatial resolution is experimentally provided by interferometric time-resolved two-photon photoemission electron microscopy (ITR-PEEM). The experimental data are interpreted by comparison with FDTD-based simulations and simulations that are performed using Huygens’s principle. The combination of experiment and modeling yields a comprehensive view of the complex spatiotemporal evolution of an ultrashort (sub-15 fs) SPP wave packet in the defined boundary conditions of the structured gold film. As a proof of principle for our new approach for timeresolved recording of SPP propagation in complex twodimensional plasmon-optical assemblies, we show in Figure 1 optical microcopy images of two plasmonic focusing devices. Both designs consist of structured gold films fabricated by electron beam lithography. For sample 1 (Figure 1a), the gold film is supported by a chromium adhesion layer of less than 5 nm thickness on top of a SiO2 substrate. Sample 2 (Figure 1b) uses an indium titanium oxide (ITO) substrate. Because of the thickness of the gold film of 100 nm (sample 1) and 200 nm Published: February 22, 2013 1053

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h. The experimental setup is described in detail in ref 26. Both excitation of plasmonic wave packets and their tracking via twophoton photoemission is achieved with the 800 nm output of the laser incident onto the sample at 65° with respect to the surface normal (Figure 1c). Note that the dispersion mismatch guarantees that SPP excitation is only possible by light scattering at the edges of the structure, even though the laser beam illuminates the whole focusing device. Figure 2a,b shows PEEM images of the elliptically shaped focusing device (compare Figure 1a) and the Fresnel-type structure (compare Figure 1b), respectively. Both images were recorded in the single-beam excitation mode (static PEEM, one interferometer arm blocked). In order to facilitate two-photonphotoemission at λ = 800 nm, the work function of the gold film was decreased by covering it with a small amount of cesium (coverage ≪1 atomic layer). SPP dispersion measurements show that the cesium does not influence the propagation of the SPP (see Supporting Information). The periodically modulated photoemission intensity pattern at the gold film is a characteristic PEEM signature that is indicative of the presence of SPP propagation at the gold−vacuum interface. It is the result of alternating constructive and destructive superposition of the illuminating laser field and the phase-coupled SPP, which both propagate along the gold surface.21 The PEEM data provides, therefore, an indirect view of the SPP, which has to be analyzed by suitable theoretical models. This is particularly necessary for a real two-dimensional scenario, being the case for the focusing structures studied here. Here, the (near-) planar phase front of the laser beam interferes with the temporally and spatially varying phase front of the SPP, resulting in a complex intensity pattern as shown in Figure 2. For a quantitative analysis, the static PEEM results are compared with simulated PEEM intensity maps using two different numerical approaches. A first, numerically less demanding approach uses Huygens’s principle to describe the light-induced SPP excitation at the edges of the gold structure. The coupling edge of each structure is indicated by the red dashed line in Figure 2c,d. In this model, each point of a structural edge illuminated by the exciting light field acts as a source of a SPP wavelet emitted along the gold film surface. The resulting SPP polarization field and the (phase-coupled)

Figure 1. Optical microscopy images of the samples investigated in this work; the gold covered areas are yellow; (a) elliptically shaped focusing structure; the red rectangle marks the area analyzed in the PEEM experiments; (b) Fresnel-type focusing structure; (c) laser excitation geometry applied in the PEEM experiments.

(sample 2), respectively, an influence of the substrate on the SPP propagation can be neglected. The focusing structure of sample 1 is the elliptically shaped edge, which efficiently couples femtosecond light pulses to SPP modes supported by the gold vacuum interface. Sample 2 is a two-dimensional Fresnel-type structure consisting of extended shadowing bars connected to the coupling edge of a homogeneous gold area. For the characterization of SPP propagation in these structures we used a photoemission electron microscope operated in the (ITR-)PEEM mode25 using a sub-15 fs femtosecond laser system equipped with a stabilized optical interferometer with a timing accuracy of less than 30 as over 10

Figure 2. Static PEEM images recorded in two-photon photoemission mode at 800 nm laser excitation of the elliptically shaped focusing device (a) and the Fresnel-type focusing structure (b); the modulated intensity pattern is the experimental signature for SPP excitation and propagation along the gold-vacuum interface. (c,d) Simulation of the experimental PEEM intensity maps; an approach based on Huygens’s principle has been used to account for SPP excitation at the edges of the structures. (e,f) FDTD simulations of the experimental PEEM intensity maps. 1054

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light field are then superimposed. The PEEM intensity map is finally calculated from the fourth power of the total field to account for the second order nonlinearity of the two-photon photoemission process. To check the validity of the Huygens model, the static PEEM intensity maps were additionally calculated within the framework of the finite-difference timedomain (FDTD) method (for details, see Supporting Information).27−30 The FDTD results are shown in Figure 2e,f. On the basis of these simulations, the PEEM signal was derived from the superposition of the light field with the calculated z-component of the SPP electric field (the component parallel to the surface normal). Conventional photoemission experiments show that for the relevant wavelengths the measured signal is predominantly governed by this field component.31−33 The results that will be presented below justify this choice, even though the situation can be more complex in the case of a SPP-mediated photoemission process. Although it provides no information of the vector character of the SPP field, the approach based on Huygens’s principle reduces CPU times for the simulation significantly in comparison to FDTD. This makes the Huygens approach particularly attractive for the analysis of the time-resolved interferometric data discussed below. For both simulation methods, the dielectric response of the gold−vacuum interface was described using experimental optical data reported by Johnson and Christy.34 While the static PEEM measurements yield no information of the propagation of ultrashort SPP pulses, the aspect of temporal SPP wavepacket evolution clearly emerges in time-resolved experiments, as will be shown below. The PEEM intensity maps calculated within the Huygensbased approach and the FDTD simulations for the two structures are shown in Figure 2c,d and Figure 2e,f, respectively. The very good agreement between the Huygensbased simulation and the experimental PEEM data confirms that illumination of the elliptic focusing device results in the generation of an SPP field as expected from a pure, or at least predominant light coupling to the structural edges. Furthermore, the good agreement between the two simulations indicates the validity of the Huygens approach for modeling the SPP field as probed in the PEEM experiment. We would like to add that the general PEEM pattern obtained from the Huygens-simulations depends on the excitation wavelength and the dielectric response of the interface. However, for the intensity distribution, the agreement with experiment can be optimized significantly by adjustment of the relative coupling efficiency at the edges, the phase shift between excitation light pulse and SPP field, and the effective SPP damping length. The experimental data of the elliptically shaped focusing structure is best reproduced by a sin ϑ dependence of the coupling efficiency, where ϑ is the angle between the edge tangent and the incidence direction of the laser light. For the phase shift, the Huygens-simulations provide a consistent value of 120° for both structures, which is in agreement with results reported before.27 The time-integrated SPP polarization field distribution probed in the PEEM experiment can be derived from the Huygens-simulations that have been adjusted to the experimental data. Results for the two focusing structures are compared in Figure 3. In both structures, the formation of a distinct focus at a distance of 20.6 μm (Fresnel structure) and 36.2 μm (elliptic structure) from the coupling edge is observed. Overall, the Fresnel structure provides a significantly tighter focus with a focal width of 0.85 μm (fwhm) in comparison to

Figure 3. Time-integrated SPP polarization field of the elliptic shaped focusing device (a) and the Fresnel-type focusing device (b), calculated from the simulated PEEM intensity maps using Huygens’s principle (see Figure 2c,e); the focusing effect of both structures is clearly visible.

1.2 μm (fwhm) found for the elliptic focusing device. Noteworthy, these values are very well reproduced by the position and extension of the localized superposition field maxima in the experimental and simulated PEEM intensity maps of Figure 2. The positions and focal widths for the Fresnel-type and elliptically shaped structure measured from the static PEEM images are 20.2 and 0.78 μm (Fresnel structure) and 36.7 and 1.1 μm (elliptically shaped focusing structure), respectively. Even without a detailed analysis of the data it is evident that the bare PEEM images already provide relevant quantitative information on the capabilities of the different focusing devices. In the actual PEEM experiment, we do not prepare a stationary SPP field in the focusing devices. Instead, because of the pulsed excitation by the laser, an ultrashort SPP wave packet is generated, whose propagation is influenced by the structural boundary conditions as well as the dielectric response of the gold vacuum interface. Interferometric time-resolved PEEM experiments combined with appropriate simulations are capable of monitoring and analyzing the resulting complex twodimensional evolution of this wave packet within the focusing structure. In these two-pulse correlation experiments, a second laser pulse with a time delay with respect to the excitation laser pulse that is adjusted in a highly stable manner probes the wave packet motion in the focusing structure. Note that the delaydependent two-pulse correlation signal is always superimposed by a static PEEM signal (as shown in Figure 2) caused by the 2PPE emission from each pulse separately. Figure 4 shows ITR-PEEM images of the elliptically shaped focusing device recorded at four different time delays ΔT = 0 fs, ΔT = 25.25 fs, ΔT = 26.52 fs, and ΔT = 49.98 fs between excitation and probe laser pulse. The upper parts of the images are the experimental data. The lower parts are results of corresponding time-dependent Huygens-based calculations (a complete movie of the experimental and simulated ITR-PEEM scans of the elliptic shaped focusing structure can be found in the Supporting Information). The most notable differences in the experimental data observed for different delay times ΔT are signatures of the propagation of the SPP wave packet as probed by the second laser pulse. Comparison of Figure 4a,b shows that on the intermediate time scale a complex, additional 1055

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reconstruct SPP wave packet snapshots at different propagation times t after arrival of the excitation laser pulse maximum at the apex of the ellipse. Note that the actual SPP propagation time t and the time delay ΔT between excitation pulse and probe pulse are not identical. This is due to the non-normal geometry of the light excitation in combination with the difference between the excitation and probing positions. One finds that the two quantities are connected via the relation vg,SPP ⎞−1 ⎛ ⎟ t = ΔT ⎜1 − sin(δ) ⎝ c ⎠

Here, δ is the angle of incidence of the laser beam (δ = 65° in this work), vg,SPP the SPP group velocity and c the vacuum speed of light. For vg,SPP = 0.939c, as experimentally determined in other publications,24,36 our experimental configuration quantitatively yields t = 6.71ΔT. Figure 5 shows three-dimensional plots of reconstructed snapshots of the SPP wave packet at four different propagation

Figure 4. (a−d) ITR-PEEM data of the parabolic focusing structure (upper part of each image) in comparison to Huygens-based simulations (lower part of each image) for four different time delays ΔT between excitation and probing laser pulse.

superposition signal appears at large distances from the apex of the elliptic structure, visible behind the focal region of the structure. This contribution to the signal disappears again as the time delay ΔT is further increased, as shown in Figure 4d. Here, one observes essentially the motion of the SPP wave packet as it first enters this distant area and finally leaves the region probed by the microscope. The quantitative comparison of the ITR-PEEM data with time-dependent Huygens-based calculations allows one then to reconstruct the complex twodimensional propagation of the SPP wave packet across the structure. In contrast to the static PEEM simulations, the simulations of the ITR-PEEM data are more involved as a complete set of time-dependent snapshots of the SPP wave packet propagation needs to be calculated at sufficiently small time steps. The PEEM pattern for a given temporal delay ΔT is then generated by numerical integration of the calculated snapshots (at time steps of 1 fs) after superposition with timedelayed Gaussian pulses mimicking the temporally and spectrally well characterized excitation and probe laser pulses.35 Simulations and experimental PEEM data compared in Figure 4 show a good agreement and prove that the simulations yield the relevant propagation dynamics probed by our experiment. Note, for instance, the shift of the pattern signature marked by the arrows in Figure 4b,c, which is very well reproduced by the Huygens-based calculation. These distinct variations in the interference pattern within 1.28 fs variation in the pulse delay ΔT (corresponding to a phase Δφ = 0.96π) is the result of a local modulation of the interference conditions between the SPP and the probe laser pulse induced by the applied phase delay Δφ. Such a behavior has been reported for quasi one-dimensional SPP propagation, before.21,24 It is an indicator of the phase propagation of the SPP and the magnitude of the shift is a measure of the actual phase velocity of the wave packet. The numerical support of our experimental results by the Huygens-based simulations enables us to

Figure 5. (a−d) Huygens-based, reconstructed SPP wave packet snapshots at four selected propagation times t after arrival of the excitation laser pulse maximum at the apex of the ellipse; shown is the polarization field of the SPP; note that the propagation time t and time delay ΔT, shown in Figure 4, are not identical.

times t (a complete movie is provided in the Supporting Information). The data are the result of the Huygens-based simulations that provide the best agreement between calculated and the experimental ITR-PEEM data (see Figure 4). For the time-resolved data, the adjustment parameters for the Huygensbased simulations are the coupling efficiency and the phase shift of the SPP. As expected, the SPP wave packet is initially generated at the edge of the focusing device (Figure 5a) and propagates into the focal region of the structure were it arrives approximately at time t = 130 fs after the pulsed illumination of the apex of the ellipse (Figure 5c). On longer time scales and beyond the focus, the SPP wave packet breaks up into two parts and finally leaves the probed region (Figure 5d). The experimental signature for this split of the wave packet is well resolved in the experimental ITR-PEEM data, where it shows up at time delay ΔT ≈ 26 fs, corresponding to a propagation 1056

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time t = 175 fs. This result is in very good agreement with the reconstructed dynamics shown in Figure 5d. In general, one expects that changes observed in the ITRPEEM scans depend critically on phase and group-velocity of the SPP and the details of the spatiotemporal evolution of the SPP wave packet pattern. This is confirmed by PEEM experiments (and corresponding simulations) for different excitation wavelengths. The sensitivity to SPP propagation parameters was furthermore investigated by calculations using slightly different values for group and phase velocity. The results of these simulations do not agree with the experimental data anymore.37 In recent years, ultrafast optical approaches have increasingly been applied to nano-optical problems.38−42 ‘Ultrafast nanooptics’, in particular, needs sophisticated methods for a comprehensive characterization of the complex dynamics of localized optical excitations simultaneously in space and time. Femtosecond laser-based PEEM provides such a capability, as has been shown in the past by a number of pioneering studies.18,25,43,44 With regard to the real-time observation and characterization of plasmonic wave packet motion, this technique, as well as other approaches, were mainly restricted to one-dimensional scenarios in the past. In the present work, we demonstrated how the interferometric time-resolved PEEM technique, together with proper numerical simulations, can be used to reconstruct and visualize the two-dimensional evolution of ultrashort SPP wave packets in space and time. The analysis presented here provided quantitative insights into details of the group propagation of plasmonic fields in complex structures. In particular, we experimentally demonstrated the two-dimensional propagation and spatiotemporal focusing of sub-15 fs SPP wave packets. The approach presented here is not only restricted to ITR-PEEM experiments but can also be applied to other experimental techniques where plasmon propagation is probed by the interference with a reference laser field.45 Prospects of this approach include the analysis of time-resolved data in terms of SPP phase modulation in highly dispersive plasmonic systems that should give rise to a wave packet broadening during propagating. The method demonstrated here for time-resolved recording of SPP propagation in complex two-dimensional plasmon-optical assemblies further provides the possibility to analyze the temporal evolution of SPP pulses prepared with defined amplitude and/or phasemodulation by laser pulse-shaping techniques.

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Nanolaboratory at the University of Kiel for support in the preparation of the nanostructures. We further acknowledge support by the Centre of Quantum Engineering and SpaceTime Research (QUEST) and the Laboratory of the Nano and Quantum Engineering (LNQE) of the Leibniz University Hannover. This work was funded by the Deutsche Forschungsgemeinschaft through Priority Program 1391 “Ultrafast Nanooptics”.

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ASSOCIATED CONTENT

S Supporting Information *

The complete ITR-PEEM measurement of the elliptical structure, shown for selected time delays in Figure 4, is available as an .avi movie. The reconstructed polarization field of the SPP wave packet, shown in Figure 5, is also added as an .avi movie. 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].

ACKNOWLEDGMENTS We acknowledge help with the setup of the interferometer for the ITR-PEEM experiments from Frank Meyer zu Heringdorf (University of Duisburg-Essen). We thank the Nano-Structuring Center (NSC) at the University of Kaiserslautern and the 1057

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