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Polarization Directed Surface Plasmon Polariton Launching Y Gong, Alan G. Joly, Patrick Z. El-Khoury, and Wayne P Hess J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02509 • Publication Date (Web): 02 Dec 2016 Downloaded from http://pubs.acs.org on December 2, 2016

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The Journal of Physical Chemistry Letters

Polarization Directed Surface Plasmon Polariton Launching 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, WA 99352, USA

Abstract The relative intensities of propagating surface plasmons (PSPs) simultaneously launched from opposing edges of a symmetric trench structure etched into a silver thin film may be controllably varied by tuning the linear polarization of the driving field. This is demonstrated through transient multiphoton photoemission electron microscopy measurements performed using a pair of spatially separated phase-locked femtosecond pulses. Our measurements are rationalized using finite-difference time domain simulations, which reveal that the coupling efficiency into the PSP modes is inversely proportional to the magnitude of the localized surface plasmon fields excited at the trench edges. Our combined experimental and computational results allude to the interplay between localized and propagating surface plasmon modes in the trench; strong coupling to the localized modes at the edges correlates to weak coupling to the PSP modes. Polarization directed PSP launching measurements reveal an optimal PSP contrast ratio of 4.2 using a 500 nm-wide trench.

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Propagating surface plasmons (PSPs) are electromagnetic waves coupled to charge density fluctuations at the surface of metals.1 PSPs may travel at metal-dielectric or metalvacuum interfaces at speeds comparable to the speed of light in vacuum.2-3 Understanding and ultimately controlling the launching of PSPs and their field intensities is prerequisite to advancing high-speed electro-optic devices4-6 and may greatly benefit emerging ultrasensitive molecular spectroscopy techniques.

7-8

In this regard, the ability to launch, focus, and guide PSP

waves to specific locations presents itself in the quest for designing elements of a plasmonic nanocircuit, including, waveguides,9 amplifiers10 and demultiplexers11. Nanoscale constructs that can control PSP direction are critical elements necessary to build these components. Currently, holes,12 ridges,13 gratings14-15, and slits,16 engineered in metal surfaces, are used as plasmonic couplers to launch PSPs.2, 17-18 Among these structures, studies of PSP directional launching have been mainly focused on slits and ridges.19-21 For example, asymmetric coupling structures22-24 when combined with tunable laser incidence angle20-21 can result in directional PSP launching and propagation. The underlying mechanism is primarily constructive and destructive interference of PSPs launched from designed plasmonic metamaterials or complex asymmetric coupling structures.25 Theory previously identified polarization control as a key method for directing localized surface plasmons propagating in nanoparticle array structures26-27 while recent experiments demonstrated polarization control of localized surface plasmon (LSP) routing in branched silver nanowires.28 Here, we study asymmetric PSP launching from polarized light coupled using a simple symmetric trench structure to create a plasmon propagating on a smooth metal surface. Thus, laser polarization control and a simple square trench structure can be considered as 3 ACS Paragon Plus Environment


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individual elements in the toolbox for directed PSP launching. In combination with other elements, such as ridges, holes and pillars, more complex control structures may be constructed with unique properties specifically tailored with an intended application in mind. The trench structure studied here reveals phenomena that are underrated in literature, in particular, the interplay between localized and propagating surface plasmons. Figure 1 shows a schematic representation of the experimental geometry and a PEEM image recorded following irradiation with phase-locked 780 nm fs pulses. Pump and probe pulses irradiate the trench longitudinally and a remote region to the right side of the trench, respectively. A clear beat pattern, parallel to the trench long axis, is apparent in the probeirradiated region. Ultrafast excitation of the trench, by pump pulses, launches surface plasmon wave packets with a wave vector kBeat that points in the ±y-direction perpendicular to the trench long axis. The probe is spatially and temporally offset from the pump, affording time-resolved imaging of launched PSPs at distances more than 100 µm away from the coupling trench structure. In typical time-resolved photoemission studies, pump and probe pulses are spatially overlapped and are essentially indistinguishable in single color experiments.3 The scheme employed here extends the spatial detection limit beyond the laser spot size, such that it can be used to probe PSPs in regions that are remote from the coupling structure and the pump irradiated region.


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Figure 1. Schematic of experimental configuration (a,b) and PEEM image of trench and propagating surface plasmon (c). The relative orientations of wave vectors of the pump laser, PSP and interference beat, KL, KPSP, KBeat respectively, are displayed in (a). The trench dimensions are 1 µm × 300 µm × 0.1 µm. Laser pulses propagate from the lower left to the upper right in all panels. Separated pump and probe laser pulses are incident at 75° to the surface normal (b). The PSP generated by the pump pulse coupled at the trench is detected at the probe-illuminated region (c). Beat pattern shown in the PEEM image results from PSP wave packet and probe beam interference using P-polarized pump and probe pulses. Typical laser exposure times range between 0.25 and 1.0 seconds to obtain the image displayed in (c). Background subtracted photoemission line profile recorded at the position of the red line is shown in the inset (c).

The wave vector relation between incident laser and PSP wave packet satisfies ∙ ∙ sin

in which η1 and η2 are the in plane angles of the laser and PSP propagation with respect to the inplane normal of the trench, as indicated in Fig. 1(b), and is the in-plane component of the laser wave vector.29 5 ACS Paragon Plus Environment


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∙ where k0 is the incident laser wave vector, and θ = 75° is the incident angle with respect to the surface normal. The PSP wave vector is given by /1 +

in which εm is frequency-dependent dielectric of silver and ω denotes the frequency of the laser light. Given λ0 = 780 nm, εm = −29.4 + 0.8i,30 and 2"⁄# , we obtain λPSP = 766.6 nm. Since the in-plane component of the laser field is parallel to the long axis of the trench, η1 = 90°. We therefore obtain arcsin# ∙ ⁄# 71.6° Figure 1(c) shows a PEEM image recorded following ultrafast laser irradiation using Ppolarized excitation pulses impinging on a 1 µm x 300 µm trench milled into a 100 nm thick silver film. Here, the spatially offset probe pulse is temporally delayed by 276 fs from the pump pulse to simultaneously arrive with the PSP wave at the probe position. Towards the right of the trench, photoemission from the coherent polarization state obtained by interaction of the probe pulse and the PSP wave is clearly observed as interference fringes atop a smooth Gaussian background. The background is due to three-photon photoemission as a result of the interaction between the probe laser and the nominally smooth silver surface. The inset in Fig. 1(c) shows a line profile obtained by subtracting the Gaussian background extracted from the probe irradiated region. The interference fringes beat regularly at a spacing of approximately 2.5 microns. The interference fringe spacing, λi, can be calculated using 31 :


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#,

-. ∙-/0/

1-2. 3-2/0/ 4∙-. ∙-/0/ ∙567 89 482

= 2.4 µm

which is in good agreement with the experimental result. The amplitude extracted from the sineGaussian fitting to the interference fringe intensity reports on the relative intensity of the PSP wave packet.

Figure 2. PEEM image of trench and plasmonic interference fringes following photoemission induced by a probe beam at the (a) left and (b) right side of the trench. The pump polarization for both images is rotated 45° clockwise compared to P polarization, which is defined as +45°. (c) Interference fringe intensity as a function of pump polarization of the left-side launched PSP. (d) Interference fringe intensity as a function of pump polarization of the right-side launched PSP. The probe beam is always P-polarized.

By tuning the polarization of the pump laser, an asymmetry in the intensity of left-side and right-side detected PSPs is observed indicating asymmetric light coupling and plasmon launching from the trench structure. Figure 2(a) and (b) show PEEM images of PSP fringes at the left and right sides of the trench. The probe positions at the left and right sides of the trench are axially symmetric which ensures that the PSP wave packets that propagate in either direction 7 ACS Paragon Plus Environment


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arrive at the probe illuminated region at the same relative probe delay. The pump polarization is oriented 45° clockwise from P polarization (defined as +45°) as indicated by the inset of Figure 2(a). The interference fringe intensity detected at the right side of the trench is higher than that detected at the left side, indicating the PSP wave packet launched from the right side of the trench is more intense than the PSP launched from the left. Moving the relative phase between the pump and probe pulse moves the positions of the fringe peaks and valleys but has no effect on the relative fringe intensity difference between the left- and right-hand launched wave packets. Figure 2(c) and (d) display the fringe intensity as a function of pump polarization angle for both the right- and left-side detected PSPs. The angular distribution as a function of pump polarization shows a tilted uniaxial anisotropy. When the probe is positioned at the left side of the trench, the pump polarization dependence shows that the PSP wave packet intensity is larger than when the pump polarization is rotated counter-clockwise (< 90°) from P polarization. The maximum appears close to -45° while the minimum is near +45°. When the probe is placed at the right side of the trench (Fig. 2(d)), the interference fringe intensity reaches a maximum at about +45° while the minimum appears at about -45°, consistent with observations in Figure 2(a) and (b). The results rule out the possibility that the differences in interference fringe intensity from left- and right-side launched PSPs are due to asymmetric imperfections in the sample. Most importantly, it shows the feasibility of directing the right-side launched/left-side launched PSP intensity ratio by rotating the excitation laser polarization.


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Figure 3. FDTD simulations of a 1 µm ×15 µm trench showing field enhancement for pump polarizations of (a) -45° and (b) +45°. The laser propagates aligned with the long axis of the trench as indicated by the red arrow. Surface plasmon field enhancement line profiles (c) and (d) calculated at the positions indicated by blue solid lines in (a) and (b), respectively. The trench position is marked by two dashed arrows at the top center of both (c) and (d).

Figure 3 displays FDTD simulations of a trench illuminated by a laser field with -45° and +45° polarization showing the magnitude of the surface field distribution. The Asymmetric surface field distributions can be clearly observed for the two pump polarizations. Surface field intensity line profiles are displayed in Figures 3(c) and (d)). When the field polarization is rotated to -45°, the intensity line profile shows that the PSP field on the flat silver surface is stronger at the left side of the trench when compared to the right side (Fig. 3 (c)). When the field polarization is rotated +45°, the left/right distribution is reversed (Fig. 3 (d)). In addition, there are field intensity spikes at both edges of the trench, attributed to field enhancement at the sharp apex of the trench edge. The FDTD simulations exhibit remarkable agreement with our experimental observations. The left-edge versus right edge intensity peak are clearly evident in the pump pulse only PEEM images (see Figure S1 of supporting information).



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Figure 4. (a) Schematic of spatially offset pump-probe experiment on a single plasmonic step edge of a large 40 µm x 40 µm trench. (b) PEEM image displaying photoemission interference patterns at the right side of a 40 µm ×40 µm trench due to the PSP wave packet excited by the pump beam. Laser pulses propagate from the lower right to the upper left. (c) Interference fringe intensity as a function pump polarization when the probe illuminates the right side of the trench. (d) Schematic of PSP asymmetric launching mechanism.

Insights into the origin of the asymmetric PSP launching with respect to input polarization can be obtained by considering a single step edge, essentially removing one side of the trench. This can be done by FIB milling a square structure with dimensions much larger than the pump beam 1/e width. Figure 4 shows analogous pump-probe experiments on a 40 µm × 40 µm trench. The two trench edges in figure 4 are displaced by a distance larger than the pump spot lateral size (~ 20 µm). As a result, the time difference between the fields produced at the two edges is greater than the pulse duration in addition to the fact that there is negligible field overlap.


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Therefore, the 40 µm ×40 µm trench excludes the possibility of interference from fields produced simultaneously at the two edges and represents a single plasmonic edge structure. Figure 4(b) shows a PEEM image recorded following ultrafast laser irradiation of the right hand edge of the 40 µm ×40 µm trench. Once again, interference fringes generated from the interaction of the PSP and probe field are observable. Figure 4(c) displays the fringe intensity as a function of pump laser polarization angle. The fringe intensity is again maximal using pump pulses polarized at +45° and minimized using pump pulses polarized at -45°. The angular distribution is similar to the result displayed in Figure 2(d). FDTD calculations modeling a single-edge structure confirm these results. Therefore, the asymmetric PSP launching observed with a symmetric trench structure occurs for a single step edge alone, and does not require interaction between PSPs generated from left and right trench edges. One possible explanation for the PSP launching asymmetry lies in the relationship of the electric field vector relative to the apex (corner) of the trench edge (see Figure 4(d)). Field enhancement can be very large at sharp edges or corners of metal structures with a pronounced polarization dependence.32 When the excitation polarization is parallel to the angular bisector of the corner, a strong localized field is observed (see Figure 3 and supporting information Figure S1). If the excitation polarization is perpendicular to the angular bisector of the trench corner, the localized field is weaker. We note that the photoelectron emission intensity displays a sharp peak at both trench edges that also shows strong dependence on the pump polarization. Therefore, field intensities at the right side edge are high and sharply peaked when the trench is excited using -45° polarized pump pulses. Lower intensity, although still sharply peaked field distributions, are calculated using equivalent but polarization rotated +45°, pump pulses. The electron emission and field intensities on the left side edge show an opposite relationship. We 11 ACS Paragon Plus Environment


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attribute this intensity peak to the localized plasmon field generated at the trench apex. More importantly, when the localized intensity is high, the PSP field on the flat silver surface at the same side of the trench is weak and vice versa. This relationship suggests that the localized and propagating surface plasmon fields compete; the accessible plasmon field intensities are effectively partitioned between localized and propagating eigenmodes. As illustrated in Figure 4(d), at the right side of the trench edge when the pump is +45° polarized, the pump laser field does not generate as strong a localized field because its polarization is orthogonal to the angular bisector of the corner. Thus more field is available to couple into the PSP on the silver surface. However, when the pump is -45° polarized, more field can couple to the localized plasmon field and the generated PSP field is comparatively weak. Polarization dependent PSP coupling is a general feature of step edges. For trenches whose edges are separated by a few microns or less, coupling between opposite edges can lead to an increase in PSP launching asymmetry. Previous studies of trenches etched into silver films have observed periodic Fabry-Perot resonances and effects of cylindrical waves in the PSP intensity,33 demonstrating that interaction between edges is an important consideration. Figure 5 displays experimental data and FDTD-simulated results of the photoemission intensity ratio detected at the right side of the trench following +45° polarized and -45° polarized laser excitation, I+45°/ I-45°, plotted as function of trench width. The intensity ratio quantifies the degree of PSP launching asymmetry. The PSP launched following +45° polarized laser excitation is always stronger than that of -45° polarized laser excitation when detected at the right side of the trench such that the right side to left side intensity ratio is always greater than one for the rightside detection geometry. In both experiment and simulation, the intensity ratio first increases and 12 ACS Paragon Plus Environment

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then decreases with trench width (Figure 5). The simulation shows a greater variation in the field intensity ratio, compared to experiment, although both curves display similar behavior as a function of trench width. The intensity ratio maximum occurs at a trench width of about 500 nm and a minimum is evident near 2000 nm. While this is suggestive of a Fabry-Perot mode, there is a lack of secondary recurrences as would be expected in this case. Still, the total polarization has contributions from the PSP, Fabry-Perot, and cylindrical waves and their relative contributions to the PSP asymmetry cannot be easily untangled. The intensity discrepancy between the simulated and experimental results may be due to the quality (sharpness) of the FIB milled trench edge. FIB lithographic precision is impacted by grain structure in the polycrystalline silver thin films, as milling rates depend on crystal orientation. The physical vapor deposited silver film is polycrystalline and has a roughness of approximately 2 nm (root mean square as measured by atomic force microscopy). Different crystal orientations inherent in the silver film are etched at different rates to produce nanometric imperfections such as rounded corners and structured edges. We have therefore conducted simulations of trenches with rounded corners (see supporting information figures S2 and S3). The results demonstrate that rounding decreases the left/right asymmetry for both the PSP and localized plasmon. Nonetheless, both experimental and simulated results suggest that the best trench width for asymmetrical PSP launching is approximately 500 nm for 780 nm excitation and that increased launching asymmetry can be achieved by fabricating more precise (sharper) trench edges.



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Figure 5. Experimental and simulated ratio of photoemission intensity (I+45°/ I-45°) for PSPs detected to the right side of the trench, plot as a function of trench width.

In summary, a simple symmetric coupling structure, such as the trench studied here, can produce asymmetric coupling and launching of PSPs from different trench sides under different light polarizations. By selecting polarization, the extent of PSP directional launching can be varied. FDTD calculations confirm experimental results and show that the PSP field intensity is inversely proportional to the strength of the localized field at the edges of the trench. When the laser polarization is parallel to the angular bisector of the corner, a strong localized field is observed and weaker PSP intensity results. If the excitation polarization is perpendicular to the angular bisector of the corner, the localized field is weaker and the PSP intensity is increased. The degree of PSP asymmetry increases with trench width up to 500 nm then decreases to a minimum near 2000 nm due to the field coupling between trench edges. Our results demonstrate a new and simple way to control the ratio of PSPs launched in different directions from a symmetric coupling structure. While the optimal experimental intensity asymmetry ratio is 2.5 for a simple individual symmetric trench structure, one can envision multiple structures arranged


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in appropriate geometric patterns to maximize PSP launching asymmetry to achieve greater PSP directional control.25 Additional control using time-delayed pulse pairs with unique polarizations could yield combined spatial and temporal control of PSPs for a variety of applications. Methods Linear square trenches were milled into 100 nm thick silver thin films vapor deposited on mica using focused ion beam (FIB) lithography and characterized using scanning electron microscopy (SEM). Plasmon polarization fields are imaged using a photoemission electron microscope (PEEM). The PEEM instrument has been previously described in detail;34 only specifics pertinent to the current experimental schemes are described here. Namely, we used a commercial titanium-sapphire oscillator (Griffin-10, KM Labs) producing sub-15 fs pulses centered at 780 nm at a 90 MHz repetition rate, to induce nonlinear photoemission from the silver surface. The pump and probe laser beams are focused onto the sample surface with a 20 cm focal length lens, at an incidence angle of 75 degrees with respect to the surface normal. Under our focusing conditions and using a incidence angle, the laser spot is elliptical with major/minor axes of 120/20 µm on the sample surface. The incident light impinges along the long axis of a symmetric trench resulting in PSP waves launched from the right- and left-side of the trench. The PSP fields are then imaged using nonlinear photoemission induced by timedelayed and spatially-separated probe pulses. PEEM, as implemented in our set up, allows spatial imaging down to 20 nm, all while also retaining femtosecond time resolution. The incident laser powers of the pump and probe pulses were maintained at ~45 mW. At least three 780 nm photons are required to induce photoemission from silver. For a single laser beam experiment (or two-beam excitation with indistinguishable beams), a coherent three-photon photoemission process is expected to be the dominant mechanism. For a three-photon process, the 15 ACS Paragon Plus Environment


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photoemission intensity, IP, is proportional to the sixth power of the total polarization field integrated over time. Laser power-dependence versus electron yield measurements were performed at the probe position and showed a linear dependence on the pump power and a quadratic dependence on the probe laser power, consistent with an overall three-photon emission process, as described previously for gold samples under 780 nm fs excitation. 12 Numerical simulations were performed using a commercial FDTD package (Lumerical FDTD Solutions). The interaction of electromagnetic plane waves with an individual surface – trench structure is calculated by iteratively solving finite-difference analogues of the timedependent Maxwell equations. The iterative process is repeated until the desired transient or steady-state electromagnetic field behavior is well-resolved. The calculations model a 100 nm deep trench etched in a silver film on a dielectric substrate. A plane wave source that illuminates the entire simulation region is used, injected at a 73 degree angle of incidence and polarized at either ±45°. Laser pulses of 80 fs are used to ensure that the angular dispersion in the injected wave vector is minimized. The dielectric permittivity of silver is taken from Johnson and Christy35 and is the only experimental data input to the calculations.

Acknowledgements The authors acknowledge support from the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. This work was performed in EMSL, a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at Pacific Northwest national Laboratory (PNNL). PNNL is operated by Battelle Memorial Institute for the DOE.


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