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May 31, 2017 - various PSP coupling and control structures, waveguides,18 amplifiers,19 and ... couple/launch PSPs.27−29 These coupling structures a...
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Surface Plasmon Coupling and Control Using Spherical Cap Structures Yu Gong, Alan G. Joly, Xin Zhang, Patrick Z. El-Khoury, and Wayne P Hess J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 31 May 2017 Downloaded from http://pubs.acs.org on June 4, 2017

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Surface Plasmon Coupling and Control using Spherical Cap Structures Yu Gong, Alan G. Joly, Xin Zhang, Patrick Z. El-Khoury and Wayne P. Hess* Physical Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, USA Abstract: Propagating surface plasmons (PSPs) launched from a protruded silver spherical cap structure are investigated using photoemission electron microscopy (PEEM) and finite difference time domain (FDTD) calculations. Our combined experimental and theoretical findings reveal that PSP coupling efficiency is comparable to conventional etched-in plasmonic coupling structures. Additionally, plasmon propagation direction can be varied by a linear rotation of the driving laser polarization. A simple geometric model is proposed in which the plasmon direction selectivity is proportional to the projection of the linear laser polarization on the surface normal. An application for the spherical cap coupler as a gate device is proposed. Overall, our results indicate that protruded cap structures hold great promise as elements in emerging surface plasmon applications.

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The interplay between light waves and propagating surface plasmon (PSP) modes has attracted significant attention.1-4 Remarkable properties of both localized plasmons and PSPs allow for their manipulation on the nanometer length scale at metal/dielectric boundaries, which has facilitated a broad range of applications in photovoltaics5-6, catalysis7-9 plasmonic sensing10-11, cancer treatment12-13, ultrasensitive chemical detection14-15 and plasmonic circuitry.16-17 Among various PSP coupling and control structures, waveguides,18 amplifiers19 and demultiplexers20 are particularly valuable, as traditional semiconductor-based electronics are rapidly approaching fundamental limitations on interconnect and information transmission speeds and heat generation. In this regard, plasmonic devices could potentially overcome these limitations as optical frequency PSPs travel near light speed21 and generate little heat16-17. To realize these various promising applications requires development of specialized structures for efficient coupling of light into PSP modes. Typically, coupling structures such as holes,22 ridges23 slits,24 and gratings25-26 are milled into metal surfaces to compensate for momentum mismatch and to couple/launch PSPs.27-29 These coupling structures also exhibit capabilities to focus and guide plasmon waves to specific locations, a useful property for applications ranging from ultrasensitive chemical analysis to functional elements of plasmonic nanocircuits. For example, greater understanding of coupling mechanisms, launching, and directing PSPs is prerequisite to advancing particular high-speed electro-optic devices.1, 4, 30 We characterize light coupling and PSP launching from a silver spherical cap structure using photoemission electron microscopy (PEEM), which maps out the surface field distribution.22, 31 Photoemission micrographs in combination with numerical simulations indicate that the coupling and launching efficiency of PSPs by cap structures is at least as effective as that of a standard etched-in plasmonic coupling structure such as a hole of similar size. We propose a 3 ACS Paragon Plus Environment

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simple geometrical model to understand coupling efficiency and PSP polarization anisotropy. Furthermore, we propose that the coupling and polarization-dependent PEEM results indicate that spherical cap structures can be employed as controllable plasmonic gates. Figure 1a displays an SEM image of a self-assembled hollow cap structure. The structure is produced upon deposition of approximately 100 nm of silver on a silicon substrate cleaned in methanol. The spherical cap has a smooth surface (roughness < 2 nm RMS) and geometrically precise boundary connected with the surrounding flat thin film. The SEM image indicates a cap with a diameter of 9.2 µm and AFM indicates the cap height is 0.94 µm (Figure 1b). Hemispherical caps between 1 and 10 micron diameter form during this process; however, the height to diameter ratio was found to be ~0.1 for all structures encountered in this study. The formation of the spherical cap structures likely proceeds through solvent diffusion and bubble formation forced by the non-wetting interaction of the solvent with the substrate. Residual solvent on the substrate forms small bubbles underneath the silver coating, resulting in an upward force that deforms the silver coating without compromising its integrity. The resulting bubble produces a protrusion but leaves the film otherwise intact. We have used focused ionbeam (FIB) milling to demonstrate that the resulting bubble is filled with gas (see supporting information figure S1) as the FIB cut causes the cap to partially deflate. The silver film thickness is approximately the same as that of the surrounding film. It is likely that other volatile solvents such as acetone will produce similar structures. The formation process is reproducible, although controlling the exact number and size density will require further experimentation.

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Figure 1 (a) 52° tilted SEM image of a silver spherical cap. (b) AFM image of a silver spherical cap structure. The diameter of the base of the cap is 9.2 µm and the height of the cap is 0.94 µm. (c) Photoemission image of a spherical cap structure under the illumination of a 780 nm fs laser. The laser is incident at 75° relative to the sample surface normal. (d) Numerical simulation of the spherical cap structure that maps out the surface field distribution under the illumination of the laser. The cap position is marked by a red dashed circle.

Much previous plasmonics work has focused on etched-in structures. Work on protruding structures includes solid structures such as nanowires or nanoparticles grown or deposited upon various substrates or protusions sculpted through removal of peripheral material. The spherical cap produced by our process represents a unique, hollow shell structure of remarkable homogeneity seamlessly connected to a smooth, continuous silver film coating. Figure 1c display a PEEM image of a silver spherical cap structure, obtained following fs laser excitation. A comparison of the experimental image to a Finite-Difference Time-Domain (FDTD) calculated electric field enhancement map is illustrated in Fig.1d. The dashed red circles in Figure 1c and d highlight the position of the cap. The FDTD calculation monitors the surface electric field with

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an additional monitor on the peak of the cap to clarify the cap position and field intensity. The PEEM image was recorded following irradiation with p-polarized transform-limited (~15 fs pulse duration) laser pulses centered at 780 nm. The pulses are incident onto the surface at an angle of 75° with respect to the surface normal. As noted in prior analyses,32-34 the photoemission pattern observed on the flat silver surface beyond the structure is created by interference between the launched PSP and the fs light pulse impinging directly on the surface surrounding the plasmonic coupling structure (silver spherical cap). In addition, the photoemission pattern also has contributions from ordinary diffraction from the protruding cap structure.35-36 Both the experiment and FDTD calculations capture the entire field which is composed of the incident light pulse, plasmon field, and diffracted field. Experimental and simulation results both display a weaker bifurcated pattern close to the cap and very strong elliptical pattern farther beyond the cap structure but within the 30×30 µm2 region comprising the PEEM field-of-view (FOV). Under 780 nm fs laser excitation, absorption of at least three photons is required to induce photoemission; therefore the photoemission intensity is proportional to the sixth order of the combined polarization fields.21 The coupled plasmon field dramatically increases the probability of multiphoton absorption and therefore the photoemission intensity directly maps the PSP field. Empirically, we note that the protruded cap structure produces enhanced photoemission at least as large as typical etched-in structures we have investigated previously. This suggests that a protruded structure can launch PSPs with at least comparable efficiency. In addition, we propose that coupling efficiency is proportional to the projection of the laser polarization on the spherical cap surface normal integrated over the cap surface. These

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assignments are supported by results displayed in Figure 2 obtained from a 9.0 µm (base diameter) spherical cap.

Figure 2 PEEM images of a 9.0 µm spherical cap following laser excitation using (a) p-polarization and (b) linear polarization clockwise rotated 70° from p-polarization respectively. Note that the relative photoemission intensity at position 1 and position 2 is altered by tuning the laser polarization. (c) Proposed model of light coupling efficiency under linear laser polarization. The laser polarization is rotated from p- to s-polarization. The projection of the laser polarization onto the surface normal of the cap structure determines the relative coupling efficiency. The laser propagates into the page, tilted at a 15 degree angle relative to the substrate plane. (d) The intensity ratio of position 1 over position 2 obtained from experiment (black curve) and the model (red curve) showing a similar trend and peak position.

Under p-polarized laser illumination, the intensities of the two near-edge photoemission interference patterns (marked 1 and 2) in Figure 2a are equivalent. However, when the polarization is linearly rotated clockwise towards s-polarization, the photoemission intensity at position 1 becomes larger than its analogue at position 2. Figure 2b displays the PEEM image obtained when the polarization is rotated 70o from pure p-polarization. The overall photoemission intensity decreases because p-polarization results in more efficient photoelectron ejection and detection compared with s-polarization. The intensity ratio increases until 80 degrees then decreases as s-polarization is reached at 90 degrees (Fig. 2d). We model the polarization asymmetry by assuming that the relative PSP field intensity at 7 ACS Paragon Plus Environment

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points 1 and 2 can be determined by integrating the projection of the polarization vector to the surface normal for each half of the spherical cap ( or  ) over the cap surface area. Figure 2c displays a cross-sectional schematic of a spherical cap under laser illumination with clockwiserotated laser polarization. The symbols  and  represent the vector component normal to the surface at two axisymmetric positions on the left and right sides of the cap. From Figure 2c, the polarization projection onto the surface normal is asymmetric with respect to left versus right hemisphere for polarization angles rotated from pure p- or s-polarization. In order to calculate the overall coupling efficiency ratio we sum the laser polarization projection normal to the surface at each surface position for both right and left hemispheres and take their ratio. Using our model, the projection is given by P*cosβ where β is the angle between the polarization vector and surface normal. The values of Cosβ for both left- and right-sided projections are given by: cos  ( )   ⋅   15 +    −    ∙ ( −  ⋅ 15 ⋅  ∙ ) + ( ∙ 15 +  ∙ 15 ∙  ∙ )  =   ⋅   15

(1) cos  ( )   ⋅   15 +    −    ∙ ( +  ⋅ 15 ⋅  ∙ ) + ( ∙ 15 +  ∙ 15 ∙  ∙ )  =   ⋅   15

(2) Where α is laser polarization angle relative to p-polarization (α=0 for pure p-polarization) and cos15° is the projection of the laser incident angle (15° relative to the sample plane). Angles θ and φ are the Euler angles of the surface normal, where θ represents the polar angle in the sample surface plane and ϕ is the azimuthal angle. The intensity ratio can be expressed as:  ( ) = ∑",$ cos  ( ) / ∑",$ cos  ( )

(3)

The calculated result is displayed as the red curve in Figure 2d. Both calculations and experiments show ratios increasing from 0 to 80 degrees, followed by a rapid decrease in intensity in the 80-90 degrees range. 8 ACS Paragon Plus Environment

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The protruded cap structure provides an intense focused PSP field with a polarization sensitive response. For comparison, we use FDTD calculations to determine the protruded cap enhanced PSP field for an inverted (etched-in) cap of the same dimensions and a cylindrical hole of diameter 9 microns and 940 nm depth (Figure 3). Of the three structures, the inverted spherical cap produces the most intense focused fields at both the near edge and region extending over 10 microns from the edge. The protruding spherical cap supports slightly less intense PSP fields, particularly near the edge of the structure. The spherical hole has a noticeably de-focused field pattern although with comparable field intensity. The similarity between the protruded and inverted cap structures is not surprising as both structures have roughly identical illuminated regions. The main difference is that in the protruded structure, the front area (towards the propagation direction) is effectively cast in shadow (with respect to the incident beam) while in the inverted structure, the posterior area is in shadow.

Figure 3. FDTD calculations displaying enhanced field (E/E0) for the protruded cap structure (top), inverted cap structure (middle), and cylindrical hole (bottom). All three calculations shared similar dimensions; a 940 nm height or depth, and a 9.0 micron diameter.

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The protruded spherical cap result displays two regions of high field enhancement that make it useful as a plasmonic device element. The strongest enhancement region is the highly concentrated, focused elliptical field profile located approximately 14 microns from the cap center in Figure 1. This large field enhancement can be exploited to provide dramatic signal detection enhancement by taking advantage of non-linear mixing between the PSP signal wave and the PSP field generated (coupled) at the protruding cap structure.37 Figure 4a shows a schematic of the setup in which the p-polarized laser beam is split into a ‘pump’ and ‘probe’ pair. The pump beam launches a signal PSP at a remote coupling structure (hole) located approximately 100 microns before the spherical cap. The pump-generated PSP wave packet (signal) travels at nearly the speed of light 21, therefore it is slightly delayed relative to the probe pulse at position B in Figure 4b. The probe beam then launches a reference PSP field from the spherical cap structure at a time delay commensurate with the arrival of the pump-launched PSP. The timing between the two pulses can be adjusted to maximize the constructive interference between the PSP launched at the hole (signal wave), the PSP launched at the spherical cap, and the probe field. The fields interfere nonlinearly resulting in a homodyne detection scheme, whereby the photoelectron signal is significantly enhanced.37 Experimentally, we monitor the time-resolved photoemission intensity at positions A and B in Figure 4b. Photoemission interferograms are obtained as the relative delay between the pump and probe pulses is scanned resulting in peaks and nulls from constructive and destructive interference between the oscillatory fields (Figure 4c, d). The interferograms show two distinct peaks as the delay between pump and probe is changed (Figure 4c, d). The first peak centered at 0 time delay is photoemission resulting from the overlap between pump and probe fields. Due to a lower group velocity, the PSP waves are delayed relative to the light pulses leading to a second 10 ACS Paragon Plus Environment

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peak in the interferogram at ~35 fs delay. The delayed interferogram peak is due to photoemission from the overlap of the pump-generated PSP signal field, probe-generated PSP field, and the probe field and represents the detection of the signal field. Using this scheme, a signal enhancement of ~30 is observed by comparing the signal amplitudes (~3000 in Figure 4c, ~100 in Figure 4d).

(c)

(d)

Figure 4 (a) Schematic illustration of the enhanced detection experimental setup. The pump laser impinges on the coupling structure (hole) producing a signal PSP. The signal PSP travels to the spherical cap where it interferes nonlinearly with the probe-induced PSP (reference) to generate a homodyne detection scheme. (b) PEEM image displaying the interference fringes between the pump-generated PSPs from the hole and the probe-generated PSP located at the spherical cap. Red dash circles depict laser pulse positions. (c) and (d) Time-resolved photoemission yield measured at positions marked in (b) as the relative delay between pump and probe is scanned. Due to a lower group velocity, the PSP waves are delayed relative to the time zero and are detected at a delay of ~35 fs. The relative intensity of the PSP waves before and after the spherical cap is indicated by the solid lines in c and d. A signal enhancement of about 30X is observed after the spherical cap at position A.

The same experimental construct and scheme can also be used to generate an optical gate using the spherical cap structure utilizing the PSP polarization dependence at positions 1 and 2 of Figure 2. Combining this with signal detection enhancement described above allows for the

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construction of an all-optical gate device. In this case, the enhanced detection field can selectively enhance either the PSP signal at position 1 or position 2, depending on the polarization of the probe detection field. Figure 5 displays time-resolved photoemission interferograms recorded as the probe pulse is delayed with respect to the pump pulse. The intensity of the photoemission interferences right after the spherical cap shows strong laser polarization dependence as the surface field intensity at position 1 is much stronger than that of position 2 when the probe laser polarization is rotated clockwise by 70 degrees. An incoming PSP (possibly encoded with information) transports through the spherical cap and interferes with the cap launched PSP at position 1 and position 2. By rotation of the linear polarization, PSP fields from the spherical cap enhance the signal at position 1 over that recorded at position 2. If position 1 and position 2 can be regarded as two separate channels, the detection field effectively directs selection of that channel. Since the PSP field from the spherical cap can be simply controlled by laser polarization, we can selectively choose to send the incoming PSP signal to either one of these two channels, which amounts to a polarization controlled plasmonic gate device. For further transport of incoming PSPs signal, the two channels can be linked to either electronic or plasmonic circuits.

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Figure 5 Time-resolved photoemission yield measured at position 1 and 2 marked in Figure 2(a) under the illumination of probe beam with 70° clockwise rotated laser polarization. The PSP signal at positon 1 is more than 5 times stronger than that at position 2. The relative intensity ratio between position 1 and position 2 can be controlled by probe polarization making it a tunable plasmonic control device in which position 1 and position 2 become two available signal channels.

In summary, experimental results and numerical simulations show that protruded silver cap structures couple and launch PSPs at least as efficiently as a conventional etched-in plasmonic structures of similar dimensions. We propose a simple model to disentangle the coupling mechanism and model the laser polarization dependence. The protruded spherical cap can be used as an efficient coupling element with aforementioned sensing applications or, as given in the example, a signal enhanced plasmonic control (gate) device incorporating a homodyne detection scheme.

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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 for the DOE by Battelle.

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