Article pubs.acs.org/JPCC
Nonlinear Photoemission Electron Micrographs of Plasmonic Nanoholes in Gold Thin Films Yu Gong, Alan G. Joly, Patrick Z. El-Khoury, and Wayne P. Hess* Physical Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States S Supporting Information *
ABSTRACT: Nonlinear photoemission electron microscopy of isolated nanoholes in gold thin films maps propagating surface plasmon polaritons (SPPs) launched from the lithographically patterned plasmonic structures. A damped elongated ringlike photoemission beat pattern is observed from the nanoholes, following low angle of incidence irradiation of these structures with sub-15 fs 780 nm laser pulses. A notable agreement between finite difference time domain simulations and experiment corroborates our assignment of the observed photoemission patterns to SPPs launched from isolated nanoholes and probed through nonlinear photoemission. We also demonstrate how the efficiency of coupling light waves into isolated plasmonic holes can be tuned by varying hole diameter. In this regard, a simple intuitive geometrical model, which accounts for the observed and simulated diameter dependent plasmonic response, is proposed. Overall, this study paves the way for designing nanohole assemblies where optical coupling and subsequent plasmon propagation can be rationally controlled through 2D SPP interferometry.
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we report on the generation and manipulation of SPP fields through isolated nanoholes etched in thin films of gold. We take the first step toward attaining a more detailed understanding of SPPs launched from nanoholes using a combination of nonlinear photoemission electron microscopy (PEEM) and finite-difference time-domain (FDTD) simulations. A damped sinusoidal elongated ringlike photoemission beating pattern is observed in nonlinear PEEM maps of the isolated holes, recorded following low angle of incidence irradiation of the nanostructures with femtosecond laser pulses. We first establish the connection between the generated SPP waves and the measured beat pattern, guided by finitedifference time-domain simulations. We then analyze the hole-diameter-dependent plasmonic response using a combination of experiment and FDTD simulations. We find that the measured and simulated hole-diameter-dependent response can be understood on the basis of an intuitive geometrical model which accounts for the efficiency of light coupling into the lithographically etched plasmonic structures. Our results reveal the coupling efficiency and spatial pattern of propagating surface plasmons and serve as a precursor to understanding larger array structures capable of guiding and interfering light confined to nanoscale metal structures. Overall, our analysis paves the way for controlling and manipulating SPPs in 2D using isolated holes and their assemblies.
INTRODUCTION Trapping light waves at a metal−dielectric interface1,2 in the form of surface plasmon polaritons (SPPs) is the concept behind several emerging nanotechnologies. These include the fabrication of optical devices3 and optoelectronic circuits,4 as well as the development of ultrasensitive plasmonic detectors for chemical and biological sensing applications,5,6 all operating well beyond the diffraction limit. Over the past decade, several techniques have been devoted to imaging SPPs, including dark field scattering microscopy,7 fluorescence microspectroscopy,8−10 coherent anti-Stokes Raman scattering microspectroscopy, 11 near-field scanning optical microscopy (NSOM),12 and nonlinear photoemission electron microscopy.13,14 Among the various tools of trade, PEEM has the distinct advantage of directly mapping SPPs at nanometer resolution through photoemission, without the need for molecular reporters or a scanning probe tip. Herein, we employ multiphoton PEEM to probe propagating SPPs launched from isolated plasmonic holes lithographically etched in a gold thin film. That light incident on an otherwise nominally flat metal surface can be coupled to a SPP mode through subwavelength structures is not a new concept. The most commonly used plasmonic couplers are ridges15 and slits,16,17 engineered to reflect and refract SPPs at metal/dielectric interfaces.18−20 Nanohole arrays have also been previously exploited to achieve extraordinary optical transmission,21,22 high brightness photoemission,23 and enhanced energy conversion when coupled to photovoltaic devices.24,25 Nonetheless, few works12,26 have been devoted to understanding the operative physics that governs light coupling into and SPP generation from individual nanoholes, fundamental geometric coupling structures. Here, © XXXX American Chemical Society
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METHODS The isolated nanohole structures (e.g., see Figure 1b) were milled into a 100 nm thick gold thin film using focused ion Received: October 7, 2014
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irradiation are shown in Figure 1c. In this picture, electromagnetic waves are coupled through the nanohole structure, and a SPP is launched on the metal surface. As noted in prior reports,13,14,30 the same laser pulse also illuminates a larger area of the flat metal surface and interferes with the launched SPP wave, which accounts for the observed photoemission patterns discussed next. Figure 2a shows a PEEM image recorded following 780 nm laser irradiation of a single 1200 nm hole in gold. The
Figure 1. (a) Sample setup and experimental geometry. The thickness of the gold layer is 100 nm, while the diameter (D) is varied between 0.1 and 14 μm. The laser wave vector is at 75° from sample surface normal. (b) 52° tilted SEM image of a single 1200 nm hole in a 100 nm thick gold film. (c) 1D schematic of SPPs launched from a single nanohole under femtosecond laser illumination. Figure 2. (a) PEEM image of a 1200 nm single hole illuminated with a 780 nm 15 fs laser pulses incident at 75° with respect to the surface normal. The laser propagation direction and electric field polarization are indicated in the inset. The image intensity scale is set to reveal the photoemission fringes on the metal thin film surface, which renders the front side signal (photoemission from the nanohole area) saturated. The exposure time is 10 s. (b) Sketch of the origin of the photoemission beat pattern. The wave vector of the beat pattern (kB, blue arrows) is the difference between the wave vectors of the incident light source (kL, red arrows) and the SPP wave (kSPP, black arrows).
beam lithography and imaged using a scanning electron microscope (FEI QUANTA 3D dual beam SEM/GaFIB). The roughness of the sputtered gold film is ∼3 nm (rms), consistent with previously reported values for thin Au films prepared using physical vapor deposition.27 Our photoemission electron microscopy (PEEM III, Elmitec, GmbH) has been previously described elsewhere.28 Femtosecond laser pulses (90 MHz repetition rate) were generated using a commercial titanium−sapphire femtosecond oscillator (Griffin-10, KM Labs), which produces sub-15 fs pulses centered at 780 nm. The p-polarized laser beam is focused onto the sample surface using a 20 cm focal length lens, at an angle of incidence of 75° with respect to the surface normal. At this incidence angle, the elliptically shaped laser spot has major/minor axes of 120/20 μm on the sample surface. The laser power used throughout is ∼80 mW. Power-dependent PEEM measurements were performed, and the results are summarized in the Supporting Information. Numerical simulations were performed using a commercial FDTD package (Lumerical FDTD Solutions). The interaction of electromagnetic plane waves with an individual nanohole is calculated by iteratively solving finite-difference analogues of the time-dependent Maxwell equations. The iterative process is repeated until the desired transient or steady-state electromagnetic field behavior is well-resolved. The sample and experimental geometries are identical to their experimental analogues. The dielectric permittivity of gold is taken from Johnson and Christy.29
femtosecond laser pulse induces nonlinear photoemission from the sample that exhibits a three-photon power dependence (see Supporting Information). As noted above, the pulse effectively creates interference between a SPP and the same light source impinging on the surface surrounding the hole. At least three photons are required to exceed the work function of the metal and hence to probe the polarization state prepared by the coupling structure (nanohole) through photoemission.13 The observed three-photon photoemission can be sequential or coherent. In analogy to a prior analysis,14 the photoemission intensity, IP, is proportional to the sixth power of the total polarization field integrated over time (roughly the convolution of the femtosecond pulse duration and the plasmon decay time), −∞
IP = α
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∫+∞
(Plaser + Pspp)6 dt
(1)
in which Plaser is the polarization induced by the laser field and Pspp is polarization due to the surface plasmon polariton. The proportionality constant α accounts for the overall detection efficiency in PEEM, including the efficiencies of the microchannel plate, phosphor screen, and CCD camera. Constructive interference leads to regions of strong near-field intensity and subsequent electron emission from the gold surface. Our recorded nonlinear PEEM images are contrasted with their analogues for a linear trench structure.13 The wave vectors of light (kL), the SPP (kSPP), and the beat pattern (kB) → ⎯ ⎯⎯⎯→ → ⎯ again fulfill the phase matching condition kB = k SPP − kL (see Figure 2b).31−33 However, as light is incident at the curved boundary of the nanohole and vacuum, wave vectors of the beat
RESULTS AND DISCUSSION Figure 1 schematically illustrates our experimental setup and geometry. In our PEEM setup, a sub-15 fs laser source centered at 780 nm is incident onto the sample surface at an angle of 75° with respect to the surface normal (see Figure 1a). The samples probed consist of isolated nanoholes featuring diameters that were systematically varied in the 100−1400 nm range. Figure 1b shows the morphology of a lithographically etched 1200 nm nanohole, visualized using a scanning electron microscope (image recorded at a viewpoint titled by 52° from the axis of the electron beam). The configuration of the single hole and a schematic of the SPP launched following femtosecond laser B
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pattern, following symmetry considerations, are always perpendicular to the line tangent to the curved surface.34 As such, instead of the grating fringes of a trench,13 we observe elongated ringlike beating patterns. The simple vectorial diagram shown in Figure 2b, while rigorously justified for linear coupling structures,34 is only approximate in describing curved coupling structures, including holes and arc-shaped trenches.35 This is because under short pulse excitation, at offnormal incidence, various positions of the curved hole edge are excited at slightly different time delays. The SPP excitation time is therefore position dependent, resulting in a phase lag between each SPP generation site. It is the time-dependent interference of the infinite set of phase-delayed propagating SPP waves in combination with the laser pulse impinging on the surrounding surface that ultimately governs the experimentally observed images. There are secondary contributions to the generated SPP waves in addition to the initial direct excitation of the hole edge. Scattering from the rear edge as well as reflection from the forward edge can also excite additional forward and rear sites within the hole structure. This is analogous to reflection and scattering in off-normal excitation of linear trench coupling structures.34 Square trench structures are also known to support Fabry−Perot modes30 that contribute to the total propagating SPP field. In principle, cylindrical hole structures may support concentric Fabry−Perot modes for appropriate combinations of hole diameter and laser wavelength. The effects of position dependent excitation time, scattering, and reflection are comparable in the PEEM results and FDTD calculations to the extent that our experimental hole structure matches a geometrical cylinder. With this caveat, the aforementioned effects are explicitly included in both measurement and modeling. Although these secondary sources of SPP generation complicate the analysis of the experimental images, the simple wave vector analysis illustrated in Figure 2b constitutes a useful and intuitive model for qualitatively rationalizing the recorded images. We find that the complex 2D structure of the SPP eigenmodes of nanoholes not only needs to be taken into account in designing hole arrays on otherwise flat metal surfaces but also needs to be invoked to rationalize the nonlinear PEEM images of micron sized nanoholes, vide infra. In Figure 3, nonlinear PEEM images are compared with the near-field intensity maps (E/E0) 6 predicted by FDTD simulations. We show representative experimental and FDTD simulated images and corresponding intensity line profiles for isolated nanoholes with diameters of 1200 (Figure 3a,b) and 600 nm (Figure 3c,d). The intensity scale, of paired experimental and corresponding calculated images, is normalized to the maximum of the first photoemission beat maxima, although the relative intensity scale between the two different hole diameters is maintained. In all plots, the laser light is incident onto the sample from the left side of the image, as schematically illustrated in Figure 3a. The FDTD simulations qualitatively reproduce the measured emission patterns, namely, the spatial distribution of strong near-field intensities and the elongated ring patterns on the far (right) side of the nanohole. Moreover, the simulations predict weaker optical near-field intensities for the 600 nm nanohole as compared to its 1200 nm analogue, which is also consistent with our PEEM observables. The wavelength of the beating photoemission pattern follows the form32
Figure 3. Comparison between the experimental (a, c) and FDTD near-field intensity maps (E/E0)6 (b, d) for isolated 1200 and 600 nm nanoholes, respectively. The 780 nm laser pulses are incident at 75° with respect to the surface normal and are p-polarized, with an electric field vector pointing out of the sample plane, as denoted in (a). The intensity scale is normalized to the maximum of the first photoemission beat to the right side of the hole. The beat pattern and field intensities predicted by the numerical simulations agree well with the experimental observables. Line intensity profile cuts along the laser propagation direction (e−h) are taken as indicated by the red line in (a).
λB =
λLλSPP 2
2
λL + λSPP − 2λLλSPP cos(θ1 − θ2)
(2)
in which the laser wavelength λ0 = 780 nm is projected onto the surface plane as λL = λ0/sin(75°) = 808 nm, and the wavelength of the SPP wave is λSPP = 2π/kSPP. The SPP wave vector is given by kSPP = Re{(ω/c)[εm/(1 + εm)]1/2}, where εm is frequencydependent dielectric function of gold and ω denotes the frequency of light. At 780 nm, εm = −24.2 + 1.6i 29 and λSPP = 763.7 nm. Plugging the values of λL and λSPP back into Eq 2 yields a beat wavelength λB ≈ 14.3 μm, consistent with the values extracted from intensity line profiles of (14.4 μm) in Figure 3 and corresponding FDTD results (14.0 μm) in Figure 3f,h. We note that there are two significant differences between the PEEM measured and FDTD simulated images. The first has to do with the backscattered SPP waves (visible toward the left side of the nanoholes) which are observed in the simulations but not noticeable in the experimentally recorded images. This observation parallels previous observations and analyses of nonlinear PEEM maps of single trenches.13 The second discrepancy between experiment and theory has to do with the high brightness regions around the individual nanoholes, observed in the experimental but not in the simulated images. In practice, this results in intense localized photoemission from the hole structures, signals that saturate the PEEM images in the regions of the lithographically milled hole structures. Both discrepancies can be attributed to imperfections in the hole structures (e.g., edge curvature) when compared to the idealized cylindrical models used in the FDTD simulations and/or experimental limitations, i.e., the accessible dynamic range of our detector. With the two aforementioned differences in mind, a good general agreement between the experimental and simulated images validates the use of the FDTD method as an interpretive and even predictive tool to analyze plasmon coupling nanostructures in otherwise flat metal surfaces. C
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Figure 4 shows the simulated and measured near-field intensity maps and corresponding intensity line profiles for
Figure 5. Integrated photoemission intensity as a function of hole diameter. A simple geometrical model that accounts for the laseraccessible inner surface area of the nanohole gives a good fit (solid blue line) to the experimental and simulated results. The inset of this panel shows the geometry of a cylindrical aperture with a depth Z and a diameter D illuminated by a laser source incident at an angle θ. See text for more details.
Figure 4. Comparison between experimental (a, c) and computed FDTD (E/E0)6 near-field intensity maps (b, d) for isolated 5000 and 6000 nm holes in gold, respectively. The intensity scale is normalized to the maximum of the first photoemission beat to the right of the hole. The simulated beat node/antinode patterns agree well with the experimental observables. (e) to (h) are intensity lines profile cuts taken perpendicular to the laser propagation direction as indicated by the red line in (a).
spatially integrated intensity Iintegrated = ∫ I(x,y) dx dy around the first SPP recursion in our combined experimental and theoretical analysis. Assuming that the efficiency of coupling the laser field to the holes (assumed cylindrical; see the inset of Figure 5) is proportional to the irradiated inner surface area of the structures, the observed hole size-dependent photoemission intensity can be fitted to
isolated nanoholes in a gold thin film with diameters of 5000 (Figure 4a,b) and 6000 nm (Figure 4c,d). The scale is normalized to the maximum intensity of first photoemission beat right of the hole. The experimental and simulated images and corresponding intensity profiles reveal that holes with diameters much larger than the excitation wavelength, on the order of several micrometers, support SPP modes featuring noticeable nodal and antinodal intensity patterns in their first recursion. Note that the number of antinodes increases with increasing hole diameters (see Supporting Information). This behavior is reminiscent of a classical single slit diffraction of a point light source experiment, in which increasing the single silt width renders intensity antinodes (arising from higher order interference of the diffracted light source) visible in the diffracted pattern. Herein, the isolated holes act as the sources of SPPs. When hole diameters are larger (>4×) than the excitation wavelength, SPPs launched from multiple sites of the circular aperture interfere. As such, interferometric intensity nodes and antinodes are visible in the recorded as well as the simulated images. This suggests that the spatial distribution of SPP fields generated from single holes can be manipulated in 2D simply by tuning the hole size. How this effect can be exploited to tailor well-defined high brightness photoemission regions of the substrate is addressed next. The PEEM images in concert with FDTD simulations reveal that systematically increasing the nanohole diameter enhances the efficiency of near-field emission. PEEM images of isolated holes featuring diameters in the 100−14000 nm range reveal that the largest structures probed give rise to the largest photoemission signals (see Figure 5). Note that we use the
Iintegrated
⎡ ∝E ∝⎢ ⎢⎣ 2
∫0
arcsin( z tan θ ) D
D2 (tan 15)(sin φ) dφ 2
2 ⎛π ⎛ z tan θ ⎞⎞ D ⎤ ⎥ ⎜ ⎟ ⎟ z + ⎜ − arcsin ⎝ D ⎠⎠ 2 ⎥⎦ ⎝2
(3)
where Z is the depth, D is the diameter of the cylinder, and θ is the incidence angle of the laser source, as schematically illustrated in the inset of Figure 5. Using the experimental values for nanohole depth, Z = 100 nm, and laser incident angle, θ = 75°, we obtain a reasonable fit (solid blue line in Figure 5) to the simulated and measured diameter dependent profiles. This suggests that the laser-accessible area of the inner surface of the hole is a major factor that governs the overall response. Note that hole-depth-dependent results follow the same intensity trend and provide further support for this premise (see Supporting Information).
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CONCLUSION We probe the generation and manipulation of SPPs from individual holes lithographically etched in thin gold films through a combination of nonlinear PEEM and FDTD simulations. Individual holes constitute an efficient and D
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fundamental light coupling structure in metal substrates that is easily produced and combined into more complex structures. For hole diameters ranging from 200 nm to several micrometers, we found that SPPs generated by low angle 780 nm femtosecond laser irradiation exhibit elongated ringlike interference patterns surrounding the hole. Systematically increasing hole diameter enhanced the efficiency of near-field emission as described by a simple and intuitive geometrical model. PEEM images of propagating SPPs were modeled by the FDTD method and compared as the sixth order of the calculated electric field commensurate with the observed third order dependence of the electron yield on laser intensity. The derived rules for coupling ultrafast radiation into nanometric and micrometer sized holes in thin films of gold, for generating SPPs at the gold-vacuum interface, and for interfering SPPs nascent from the largest patterns probed all provide a basis for constructing tunable devices (such as plasmonic lenses) based on assemblies of plasmonic holes such as lines, arcs, or arrays. Studies of hole arrays for guiding and focusing SPPs at the metal−vacuum interface are currently underway in our laboratory.
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ASSOCIATED CONTENT
S Supporting Information *
A plot of photoemission intensity as a function of laser irradiance, a three-photon PEEM image of a single hole, and a plot of FDTD simulation results. 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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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. P.Z.E.-K. acknowledges support from the Laboratory Directed Research and Development Program through a Linus Pauling Fellowship at Pacific Northwest National Laboratory (PNNL), a multiprogram national laboratory operated for DOE by Battelle. The research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at PNNL.
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