Spatiotemporal Imaging of Surface Plasmons Using Two-Color PEEM

Alan G. Joly, Patrick Z. El-Khoury, and Wayne P. Hess*. Physical Sciences Division, Pacific Northwest National ... Page 1 of 22. ACS Paragon Plus Envi...
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Spatiotemporal Imaging of Surface Plasmons Using Two-Color Photoemission Electron Microscopy 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

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S Supporting Information *

ABSTRACT: Time-resolved photoemission electron microscopy images recorded using a combination of ultrafast 800 nm (red) and 400 nm (blue) pulses track surface plasmon polaritons launched from lithographic patterns etched into silver thin films with joint femtosecond temporal and nanometer spatial resolution. The nondegenerate two-color scheme is found to significantly enhance photoelectron yields relative to single-color approaches. This enables both an enhanced visualization of surface plasmons and a more accurate determination of surface plasmon properties compared to single-color measurements. Power-dependent photoemission yield measurements reveal that the overall signal is linear with respect to blue excitation and slightly nonlinear for analogous red excitation. A numerical model based on wave packet propagation reproduces the experimental results and rigorously establishes that the polarization fields from both laser colors and their conjugate surface plasmons account for the observed photoelectron yield enhancement. Tuning the time delay between the red and blue laser pulses allows determination of the group velocity of the blue surface plasmon, in spite of its intrinsically rapid dissipation rate. Numerical analysis of the recorded interference patterns yields a surface plasmon group velocity of 0.70c ± 0.07c at 420 nm.

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INTRODUCTION Surface plasmon polaritons (SPPs) are electromagnetic waves coupled to charge density fluctuations at the surface of metals. Understanding and ultimately controlling the SPP fields and profiles on the nanoscale is prerequisite to advancing novel high-speed electro-optic devices1−3 and may greatly contribute to emerging ultrasensitive molecular spectroscopy techniques.4,5 To date, manipulation and control of electromagnetic radiation on the nanometer length scale at the metal/dielectric boundaries have facilitated a broad range of applications in photovoltaics,6,7 catalysis,8−10 ultrasensitive chemical detection,11,12 and plasmonic circuitry.13 In the same vein, traditional semiconductor-based electronics are approaching fundamental size and speed limitations, thereby necessitating novel plasmonic coupling and control structures such as waveguides,14 amplifiers,15 and demultiplexers.16 In this regard, existing nanoscale constructs that can be used to manipulate (e.g., couple and decouple) SPPs include holes,17,18 ridges,19,20 gratings,18,21,22 and slits,23,24 engineered in metal surfaces. Visualizing SPP propagation along metal/dielectric surfaces requires the use of experimental techniques featuring both high spatial and high temporal resolution. This has led to a number of experimental breakthroughs, including near-field scanning microscopy25−27 and time-resolved photoemission electron microscopy (tr-PEEM).28−32 The latter approach affords joint femtosecond time resolution and sub-50 nm spatial resolution. Dynamic tr-PEEM imaging thus allows direct measurements of SPP properties on the nanoscale, including temporal/spatial © XXXX American Chemical Society

dispersion and group velocity. Measurements of spatial and temporal dispersion are particularly important in the context of applied nanophotonics, as these properties determine transmission efficiencies, propagation speeds, and SPP propagation lengths. Most existing techniques used to visualize SPPs utilize degenerate photon sources. Although single-color experiments are technically convenient (particularly at high repetition rates), multicolor experiments yield both new information and unveil additional complexity. PEEM measurements utilizing multiwavelength approaches are particularly scarce, although multicolor PEEM measurements driven by 800 and 266 nm time-overlapped femtosecond pulses33 have been previously used to determine the SPP properties at the gold/ alkanethiolate self-assembled monolayer interface.34 Through the aforementioned measurements, it was found that the UV pulse enhances emission from the near-infrared-induced plasmon. We have previously used single-color spatially resolved femtosecond pulses to visualize the dynamics of SPP propagation on metal surfaces, leading to the determination of group velocities of plasmons propagating on gold.32 The drawback of such measurements is often that relatively high laser powers are needed to obtain signals from multiphoton photoemission processes, which can result in Received: June 19, 2018 Revised: July 26, 2018 Published: August 17, 2018 A

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scattered field plane wave source which allows monitoring of either the total field (laser plus SPP) or the scattered field (SPP only). The SPP field is monitored at different spatial points along the surface beyond the trench to determine the group velocity. As the wave velocities in FDTD calculations show considerable dispersion depending on the mesh size, the calculations were repeated with smaller and smaller mesh sizes until convergence was reached. The dielectric constants for silver were obtained from the work of Yang et al.35

melting and sample restructuring. Information about both spatial and temporal SPP dispersion is often obscured as a result of this requirement. Therefore, processes that enhance photoelectron yields allow for better imaging sensitivity and ultimately better signal transmission in plasmonic devices. Here, we report the tr-PEEM measurements utilizing a nearinfrared (∼800 nm) and near-UV (∼400 nm) femtosecond pulse pair to excite and probe the SPPs launched from trenches etched into silver thin films. In contrast to previous multicolor PEEM measurements,33,34 we utilize the fundamental (red) and second harmonic (blue) outputs from a femtosecond laser in a time-resolved scheme, resulting in an experiment where the photoemission yield is a nonlinear function of power in both beams. The resulting interference between the red pulseinitiated and blue pulse-initiated SPPs is monitored in real time and modeled using wave packet analysis. Our analysis allows determination of the blue pulse-initiated group velocity through a direct time-domain measurement.

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RESULTS AND DISCUSSION Figure 1 depicts a schematic representation of the experimental geometry along with a typical PEEM image. The red (∼800

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METHODS A mode-locked Ti:sapphire laser produces 15 fs pulses centered at 800 nm at a repetition rate of 90 MHz. A slit within the laser cavity is used for fundamental wavelength (780−840 nm range) and bandwidth tuning. Because of the bandwidth limitation, most fundamental (∼800 nm) pulses used in this study have pulse durations of 40 fs, after recompression in a fused silica prism pair. Approximately, 50% of the pulse is split to produce the second harmonic in a 200 μm thick beta-barium borate crystal. In contrast to red laser pulses, the blue pulses are not recompressed and have pulse durations near 80 fs. A variable delay line controls the relative timing between the red (∼800 nm) and blue (∼400 nm) pulses. p-Polarized laser pulses are recombined on a dichroic beam splitter and directed collinearly onto the sample at a 75° angle of incidence with respect to the surface normal. The spot sizes of the separate beams are adjusted such that the red pulse spot size is roughly 50% smaller than the blue pulse spot size at the sample position. A typical spot size for the red laser is 40 μm × 120 μm at the sample. The oblong spot is caused by the steep angle of incidence required by our PEEM geometry. The power dependences are determined by measuring the photoelectron yield as a function of incident laser power. Two-color power dependence experiments require holding one pulse at a constant power while varying the power of the second pulse. For the two-color power-dependent measurements, the background signal induced by the nonvaried laser is subtracted. Typically, 50−100 mW of ∼800 nm power is used in combination with 6−12 mW of ∼400 nm laser light for most experiments described herein. The samples were fabricated using a freshly cleaved mica substrate followed by dc-magnetron sputtering of a silver target to form a 100 nm thick polycrystalline silver film with 2−3 nm rms surface roughness. The polycrystalline nature of the films likely leads to some dispersion in the work function (and therefore electron yield) and may affect the propagation length of the nascent SPPs. The patterns are etched into the film using focused ion beam milling and characterized by scanning electron microscopy. The etched patterns are 2 μm × 10 μm or 2 μm × 40 μm trenches. Finite-difference time-domain (FDTD) calculations were performed using commercial software (Lumerical Inc.) for ppolarized blue excitation of a 2 μm wide trench etched into a 100 nm silver film. Our calculations employ a total field

Figure 1. Experimental schematic of the two-color pump−probe experiment (top) and a typical PEEM image (bottom, 50 μm field of view). The red (∼800 nm) and blue (∼400 nm) p-polarized pulses are incident from the left at a 75° angle of incidence relative to the surface normal. The interaction of the pulses with a 2 μm × 40 μm trench structure initiates SPPs that propagate toward the right (positive z direction). SPPs initiated with blue excitation interfere with the blue laser field beyond the trench to produce narrowly spaced interference fringes as marked with arrows, whereas SPPs initiated with red pulses interfere with the red laser field to produce the larger-spaced fringes marked by the dashed black ovals.

nm) and blue (∼400 nm) p-polarized pulses interact with a 2 μm × 40 μm trench structure and initiate SPPs that propagate along the surface. SPPs initiated at the trench interfere with the incident and scattered laser fields beyond the trench to produce interference fringes whose spacing is determined by the excitation wavelength. The interference of the blue laser pulses with the blue pulse-initiated SPP is manifest in the closely spaced (∼2.5 μm) fringes evident immediately after the trench. The longer wavelength red pulse-initiated SPP interference with the red laser pulse produces the larger spaced (∼15 μm) fringes marked in Figure 1. The resulting interference between the laser pulses and SPPs launched from the two different color pulses yields an enhancement in photoelectron yield and allows SPP visualization in real time with nanometer precision. B

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result from these power dependences is that the signal is linear with respect to the blue pulse power and nearly linear with respect to the red pulse power. This is consistent with a silver work function that can be overcome on average with two blue photons, three to four red photons, or one red and one blue photon. Some work function variation over the interrogated region is expected because of the polycrystalline nature of the thin film. A simple, but incomplete explanation of this result is that once one pulse excites an SPP, a second pulse leads to electron ejection. In reality, the power dependences are determined by the total nonlinear polarization with contributions from terms dependent on one or both laser fields. At the power levels used in our experiments, these terms lead to a nearly linear result for the power dependence of either field. Previous reports from single-color PEEM experiments demonstrate that the photoelectron signal (S) may be described as the time-integrated polarization field of appropriate order given by32,36

Figure 2 displays PEEM images obtained with a single red pulse, a single blue pulse, and a combination of red (805 nm,

∞

S = C0 ×

∫−∞ dt |Plaser + Pplasmon|n

(1)

where n is 4 for blue (two-photon) excitation and 6 for red (three-photon) excitation. C0 is a proportionality constant that accounts for the overall detection efficiency. For a two-color experiment, the laser and plasmon fields are simply the sum of the single-color fields. The two-color power dependences infer that the signal must be linear in the blue field and essentially linear in the red field. Furthermore, the two-photon response to the red photon field does not produce photoelectrons as the combined photon energy is not great enough to overcome the work function. Therefore, we partition eq 1 into the sum of a two-photon blue-only response, a three-photon red-only response, and combined red-plus-blue two-photon response given by

Figure 2. PEEM images (A,C,E) and corresponding line profiles (B,D,F) obtained following red-only (A,B), blue-only (C,D), and redplus-blue (E,F) excitations incident upon a 2 μm × 40 μm trench etched into a 100 nm silver film. Laser pulses are incident from the left, as shown in Figure 1. The line profiles are obtained along the propagation direction, as shown by the dashed yellow line in (A). X = 0 is defined as the trailing edge of the trench.

∼40 fs) and blue (402.5 nm, ∼80 fs) pulses. As previously noted, PEEM images recorded near plasmonic coupling structures (slits and holes) are dominated by the interference between the nascent SPP launched from the structure edge and the residual laser field impinging at and beyond the coupling structure.23 This interference between the two waves produces regions of high electron emission (bright) and regions of negligible emission (dark). Figure 2 also displays line profiles along the propagation direction for three cases (Figure 2B,D,F). Three main features are highlighted. First, the interference pattern obtained when the red and blue pulses overlap is not a sum of the single pulse patterns, indicating higher-order nonlinearity. Second, the photoelectron yield resulting from the overlapped red and blue pulses is significantly greater (between 10× and 25×) than the yield of either pulse alone (i.e., the single-color measurements). Third, although the blue-only interference pattern damps to near zero after propagating for ∼15 μm, under combined red and blue excitation, recurrences can be observed as small modulations upon the larger red recurrence located over 25 μm away from the trench. Power-dependent single-color experiments indicate that on average, two blue or three or more red photons are needed to overcome the work function of the silver film. The picture changes when the sample is excited by both red and blue pulses (see Supporting Information S1). At a constant red input, the blue power dependence is very close to linear, with a value of 1.1 ± 0.1. At a constant blue input, the red power dependence is slightly greater, about 1.4 ± 0.2. The primary

∞

S(z ) = C 0 ×

∫−∞ dt{Ci × {|a1Pr + a2Pb + b1Prpl + b2Pbpl|4

− |a1Pr + a 2Prpl|4 − |b1Pb + b2Pbpl|4 } + C2 × |b1Pb + b2Pbpl|4 + C3 × |a1Pr + a 2Prpl|6 } (2)

where C2 and C3 represent the second- and third-order singlecolor weighting factors and Ci represents the interaction term contribution. The coefficients a1, a2, b1, and b2 represent the relative polarization field amplitudes for the four fields and take into account the relative coupling between the laser and plasmon fields. The fields Pb, Pbpl, Pr, and Prpl denote the blue laser, blue plasmon, red laser, and red plasmon polarization fields, respectively. The laser field is given by 2

z

2

E L(z , t ) = Al × e−1/ τ (t − c ) × e−z

2

/σ 2

× (e i(kL·z − ωt ) + e−i(kL·z − ωt ))

(3)

where t denotes time, z is the plasmon propagation direction along the surface (see Figure 1), Al is the field amplitude, kL is the laser wave vector in the propagation direction, ω is the angular frequency, τ is the 1/e pulse width, σ is the 1/e spatial width in the z direction, and c is the speed of light. The wave vector kL is related to the laser wave vector k0 through kL = k 0 sin θ C

(4) DOI: 10.1021/acs.jpcc.8b05849 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 3. Total polarization fields (A,B) obtained using eqs 2, 3, and 5 for a delay time of −167 (A) and −54 fs (B) between red and blue excitation pulses, where negative delay times correspond to the blue pulse preceding the red pulse. The left axis represents the propagation distance from the trench coupling structure. The lower traces (C,D) show corresponding experimental line profiles obtained from PEEM images (blue, experiment) as well as the time-integrated polarization field depicted in the upper plots (red, calculated). The trailing edge of the coupling element is located at 0 μm. The red excitation wavelength is 840 nm and the blue wavelength is 420 nm.

Figure 4. Line profiles obtained from PEEM images following red-plus-blue excitation of a 2 μm × 40 μm trench at different time delays between the two pulses. A negative time delay denotes that the blue pulse is incident upon the sample preceding the red pulse. The insets show the region between 10 and 30 μm from the trench. A clear modulation is observed upon the red interference maximum located at 22 μm at delays of −53 and 0 fs, but not at delays of −120 or 53 fs. The red and blue excitation wavelengths are 840 and 420 nm, respectively.

where θ is the angle of incidence upon the sample surface, near

where Apl is the field magnitude, γ is the decay constant, vg is the group velocity, and kpl is the plasmon wave vector given by ÄÅ ÉÑ ÅÅ ω ÑÑ εm Å ÑÑ Å k pl = ReÅÅ Ñ ÅÅ c (1 + εm) ÑÑÑ (6) ÅÇ ÑÖ

75° for our experimental setup. A short laser pulse initiates a plasmon wave packet which travels along the surface according to Epl(z , t ) = A pl ×

y i −1/ τ 2jjjt − vz zzz g{ e k

in which εm is the frequency-dependent silver dielectric function and ω denotes the angular frequency of the laser light. Equations 3 and 5 represent two-dimensional plots with time on one axis and propagation distance (z) on the other

2

× e −t / γ

× (e i(k pl·z − ωt ) + e−i(k pl·z − ωt ))

(5) D

DOI: 10.1021/acs.jpcc.8b05849 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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group velocity can be determined using eq 5. This is possible because the red fields serve as the mixing fields, amplifying the blue interference pattern only when all four fields are present. In this regard, the key feature of Figures 3 and 4 is the strong fast modulation upon the red interference maximum at 22 μm. As discussed, these modulations will be present for the situations where the blue pulse precedes the red pulse; however, no modulation can occur when the red pulse precedes the blue pulse. This is the case because the group velocity (slope) of the blue laser-initiated plasmon is smaller than the red laser-initiated plasmon group velocity (slope), leading to an asymmetric response about zero time. Physically, this means that for interference to occur, the blue plasmon must be launched before the red plasmon, as it travels at a slower speed. Thus, the time delay between the pulses dictates at what distance from the trench the red-initiated plasmon will overtake the blue-initiated plasmon and result in interference. Mixing this interference field with the laser fields results in an increased photoelectron yield, allowing the interference to be observed where the red fields interfere (∼22 μm), well past the point where the blue-only signal decays. Similarly, if the redinitiated plasmon is launched first, the blue-initiated plasmon will never catch up; therefore, no interference is observed at the second red maximum. However, even without the interference from the blue-initiated plasmon, photoelectron enhancement is still observed, as the photoelectron signal is derived from the interference from the combinations of all fields and does not necessarily require the blue plasmon field. Comparing simulations using eq 2 to our measurements, we determine that the blue plasmon group velocity is 0.70c ± 0.07c at 420 nm. In practice, the large number of parameters and the modulation depth of the blue plasmon interference limit the accuracy of the measurement. As the blue wavelength is tuned toward 400 nm, the modulation depth decreases because of increased damping. This makes accurate determination of the group velocity more difficult. Measurements at 400 nm result in a slightly slower group velocity of 0.6c ± 0.1c. This value compares well with a previous single-color (400 nm) measurement of blue plasmon propagation on silver thin films.28 Additionally, we have calculated the blue-initiated plasmon group velocity using FDTD simulations and the dielectric constants from the work of Yang et al.35 Our FDTD results indicate that at 420 nm, the calculated group velocity is 0.65c, slowing to 0.56c at 400 nm, comparing well with the experimentally measured values. Our experimental setup incorporates red dispersion compensation but does not incorporate analogous compensation for the blue pulse. This dictates that a temporally broad blue pulse with linear dispersion (chirp) is utilized for the measurements and must be taken into account in the simulation. In practice, we measure the red pulse duration at the sample using a collinear phase-locked autocorrelation method. We then measure the cross-correlation on the bare metal surface (no trench or coupling element) between the red and blue pulses. We also separately measure the spectrum of both input pulses. This information is then simulated using eqs 3 and 5 to determine the linear chirp on each beam, which is then used as an input in the wave packet simulations. The red pulse displays a small amount of dispersion, which does not affect the simulation significantly. However, the blue pulse is significantly chirped because of the lack of compensation within the experimental setup. Figure 5 displays the impact of

axis. A similar representation has been used to determine SPP characteristics on gold surfaces following 800 nm excitation.30 The slope of each field in this plot is related to the group velocity for that field. Using eqs 3 and 5, we construct total polarization fields for each time/space point as the superposition of all four fields (Pr, Prpl, Pb, Pbpl) with the associated weighting coefficients. Integrating over the time axis provides the final signal intensity (eq 2) as a function of distance (z), which we then compare to the experimental line profiles. Figure 3 displays the total time-dependent polarization field (Figure 3A,B) as well as the time-integrated polarization and measured output signal for two different time delays (Figure 3C,D) using the realistic parameters determined for our experimental configuration and simulated using eqs 3 and 5. Time zero is separately determined by a cross-correlation between the pulses performed on the bare metal surface. Negative delay times correspond to the blue pulse preceding the red pulse. In Figure 3A, the two blue fields (laser and plasmon) interact to form an interference pattern at a time delay of −167 fs, whereas the two red fields form a separate interference pattern at time zero. The blue field interference pattern decays significantly faster than the red field interference pattern. This decay primarily reflects two sources of damping. The first source is the intrinsic plasmon decay because of losses from absorption and scattering. The second source is the group velocity mismatch between the blue laser field and the blueinitiated plasmon. As the two blue fields propagate, the velocity mismatch results in decreasing overlap with distance, and thus the superposition intensity also decreases. In Figure 3A, the two patterns are sufficiently time-displaced such that the signal is the time integral of the sum of the individual fields (Figure 3C). As the delay time decreases, the fields begin to interact as dictated by their individual widths and group velocities (slope) in the space−time plot. At a delay of −54 fs (Figure 3B), there is interaction among all four fields, resulting in interferences between the red interference pattern and the blue pattern. An additional modulation can be observed on the polarization field when all four fields are present. This additional modulation is the result of the mixing of nondegenerate fields and is not present when the fields do not overlap appreciably. The resulting time-integrated field is shown along with the experimental trace (Figure 3D). Figure 4 displays the line profiles for four different time delays between an 840 nm red pulse and a 420 nm blue pulse impinging on a 2 μm × 40 μm trench etched into silver. Initially, the two pulses are separated in time, and the signal is the sum of the two single-color responses (Figure 4A). Once again, as the two pulses overlap, there is a very large photoelectron yield enhancement in the total signal encompassing both the blue and red recurrences. More importantly, the blue interference pattern is easily visible at the second red recurrence when the blue pulse precedes the red pulse (−53 fs delay, Figure 4B), decreases noticeably when the pulses overlap at zero delay (Figure 4C), and disappears when the red pulse precedes the blue pulse (53 fs delay, Figure 4D). The slopes of the fields in Figure 3 yield the group velocity; therefore, the speed of the blue plasmon can be determined even though the plasmon is rapidly damped. To determine the blue plasmon group velocity, we first independently measure the red plasmon group velocity on our silver samples using a method developed previously.32 The measured value of 0.93c is very close to the analogous measurement on gold.32 Using this value, the blue plasmon E

DOI: 10.1021/acs.jpcc.8b05849 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 5. PEEM image line profiles obtained following red-plus-blue excitation of a 2 μm × 40 μm trench at different time delays between the two pulses. The influence of linear dispersion within the blue pulse is especially obvious in the recurrences near 9, 12, and 14 μm. The dashed red lines indicate the position of the recurrences when there is no pulse overlap (−120 fs delay). As the delay between the pulses decreases and the fields overlap, the interference spacing decreases. The red excitation wavelength is 840 nm and the blue wavelength is 420 nm.

probe (blue) pulse linear dispersion. The effect of chirp is twofold: it increases the time duration of the probe pulses and provides a linear frequency sweep that serves to change the interference spacing as the pulses overlap. In Figure 5, as the pulses begin to overlap, the spacing of the peaks decreases in the area near 10 μm from the trench. The peak spacing reverts to the original value at time delays near zero, depending on the field time duration and the group velocity of the blue plasmon. These changes can be accounted for explicitly using eqs 3 and 5. The simulations are capable of reproducing the changes in the peak positions and confirm that the blue pulse has a positive linear dispersion as expected. Finally, two-color (red/blue) imaging of surface plasmons using PEEM has some advantages over traditional single-color measurements. First, it provides a way to amplify weaker signals through a heterodyne detection scheme. Both the blue plasmon/laser and red plasmon/laser interference fringes are enhanced significantly relative to single-color measurements. Figure 6 displays a demonstration of the photoemission yield increase on silver films using two-color excitation. The top panel is a PEEM image using red-only excitation with a laser spot size that covers most of the field of view. The middle image is the analogous blue-only excitation image, and the bottom image is the combined red-plus-blue excitation image. All three images are taken under identical conditions. It is clear that the addition of the blue photon to the red response drastically increases the photoelectron yield, such that a formerly weak signal becomes easily observable and the full lateral spatial extent of the red plasmon becomes visible. The addition of the blue photon provides a more efficient way to visualize the red plasmon propagation from less efficient coupling structures. These advantages come with a cost however; eq 2 contains numerous terms which do not contribute to the interference peaks but ultimately provide only background emission, thereby lowering the dynamic range. This additional background is clearly observed in Figure 6 as an emission from the bare metal surrounding the SPP interferences. Importantly, many of these terms are dependent on the blue laser field but not on the blue plasmon field. This explains why the full image is enhanced even though the blue plasmon field has decayed to near zero within 20 μm. In this

Figure 6. PEEM images under identical conditions following red-only (top), blue-only (middle), and red-plus-blue (bottom) excitations of a 2 μm × 40 μm trench etched into silver and located at x = 0, y = 0 μm. In this case, both the red and blue spot sizes are roughly the size of the field of view.

sense, the blue laser field acts to increase the photoemission yield from the red laser/plasmon interference regions.

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CONCLUSIONS In summary, two-color time-resolved PEEM measurements of SPP propagation on silver thin film surfaces show that photoemission yield is not simply a sum of the two singlecolor results, but depends intrinsically on the interplay between the two laser and two laser-launched plasmon polarization fields. Signal enhancements between 10 and 25 times are observed, and near-linear power dependences are measured for both the red and blue photons. The time-delayed PEEM images display additional complexity, but allow for the determination of the blue plasmon group velocity, even though the plasmon is strongly damped. Finally, the effects of linear dispersion on the time-resolved images are elucidated F

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and accounted for using numerical simulations that reproduce most of the experimentally observed features.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b05849.

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Measured one- and two-color power dependences for 100 nm thick silver thin films (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Patrick Z. El-Khoury: 0000-0002-6032-9006 Wayne P. Hess: 0000-0002-3970-9282 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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 PNNL. PNNL is operated by Battelle Memorial Institute for the United States Department of Energy.

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REFERENCES

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DOI: 10.1021/acs.jpcc.8b05849 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.8b05849 J. Phys. Chem. C XXXX, XXX, XXX−XXX