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Enhanced Propagating Surface Plasmon Signal Detection Y Gong, Alan G. Joly, Patrick Z. El-Khoury, and Wayne P Hess ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00636 • Publication Date (Web): 07 Nov 2016 Downloaded from http://pubs.acs.org on November 9, 2016

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ACS Photonics

Enhanced Propagating Surface Plasmon Signal Detection

Y. 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, USA

*

[email protected]

Abstract: Overcoming the dissipative nature of propagating surface plasmons (PSPs) is pre-requisite to realizing functional plasmonic circuitry, in which large bandwidth signals can be manipulated over length scales far-below the diffraction limit of light. To this end, we report on a novel PSP enhanced signal detection technique achieved in an all-metallic substrate. We take advantage of two strategically spatio-temporally separated phase-locked femtosecond laser pulses, incident onto lithographically patterned PSP coupling structures. We follow PSP propagation with joint femtosecond temporal and nanometer spatial resolution in a time-resolved non-linear photoemission electron microscopy scheme. Initially, a PSP signal wave packet is launched from a hole etched into the silver surface from where it propagates through an open trench structure and is decoded through the use of a timed probe pulse. FDTD calculations demonstrate that PSP signal waves may traverse open trenches in excess of 10 microns in diameter, thereby allowing remote detection even through vacuum regions. This arrangement results in a 10X enhancement in photoemission relative to readout from the bare metal surface. The enhancement is attributed to an all-optical homodyne detection technique that mixes signal and reference PSP waves in a non-linear scheme. Larger readout trenches achieve higher readout levels, however reduced transmission through the trench limits the trench size to 6 microns for maximum readout levels. However, the use of an array of trenches increases the maximum enhancement to near 30X. The attainable enhancement factor may be harnessed to achieve extended coherent PSP propagation in ultrafast plasmonic circuitry.

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Key words: Propagating Surface Plasmon, Plasmon Coupling, Plasmon Imaging, Homodyne Detection, inteferometric transient photoemission electron microscopy

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The drive to produce increasingly miniaturized electronic devices requires innovations on the micro- or nanoscale, often colliding with fundamental material limitations. Propagating surface plasmons (PSPs)

1, 2

arising from light interacting with tailored metal nanostructures are

promising in this regard, as in principle, they can be harnessed to achieve nanoscale circuitry featuring transit speeds approaching the speed of light.3 PSPs are by definition polaritons, or

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photons coupled to electrons, that propagate along the interface between a metal and a dielectric 4-8

and can be confined to ultra-small dimensions, well below the diffraction limit of the light

used to excite them.9, 10 For instance, PSPs have been propagated through nanometer diameter wires, which essentially act as waveguides, over relatively modest distances of 14 microns.11

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Using PSPs as carriers has the added advantage of higher transmission rates, in the range of THz, instead of Mhz to GHz frequencies characteristic of traditional electronic circuits. Due to momentum mismatch, however, PSPs cannot be directly excited by photons incident on a flat metal surface; PSPs are either generated in the so-called Kretchsmann configuration

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, or

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through nanoscale coupling structures on an otherwise flat metallic surface.13-15

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One problem with PSP transmission, analogous to light transmission in a fiber, is that of inefficient propagation over long distances as a result of their dissipative nature. This makes remote PSP signal readout problematic unless either in-line amplification of the signal or else more efficient signal detection can be achieved. It is therefore crucial to understand the physics of PSP propagation and to explore the feasibility of achieving higher PSP signal throughput. If PSPs are to perform as electronic signals, then when amplification is not an option more efficient means of detection of the otherwise dissipative signals must be found. Figure 1 displays a schematic representation of this concept, whereby a signal is encoded via an optical pulse or pulse train at a coupling structure (hole), transmitted along a wire or narrow channel and then 3



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split into a variety of replicas in a multiplexor after which each channel may be decoded independently. The original signal can be severely attenuated due to dissipation and division, thereby making readout a challenging task. Currently, readout of a PSP-encoded signal could be accomplished either through out-coupling to the vacuum as a photon, or else energy transfer to a fluorescent molecule and subsequent detection of the fluorescence 11. The inherent constraint in these approaches is that the detected signal is linear with respect to the PSP field. For severely attenuated PSP fields, this becomes the limiting factor for detection.

Figure 1. Schematic representation of a surface plasmon nanocircuit. The original signal is encoded as an optical pulse train at the hole coupling structure by the pump pulse. Following propagation and multiplexor division, the attenuated signal is then decoded remotely by the probe beam at the trench.

While PSP amplification has been an active area of research in recent years, leading to the demonstration of active gain and the realization of SPASERS,16, 17 PSP detection schemes are not

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as widely investigated, even though enhanced detection capabilities can compensate for an


inability to amplify the PSP. Standard detection enhancement techniques include both homodyne and heterodyne detection, which nonlinearly mix the desired signal with a reference resulting in an increased cross-term signal that can be more easily detected. Herein we describe a simple 4


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scheme for encoding PSP signals followed by enhanced remote detection hundreds of microns from the source in a method reminiscent of homodyne detection. Our arrangement utilizes nonlinear photoemission electron microscopy (PEEM) to visualize the propagation and enhanced detection of a signal transported in the form of a PSP in real space-time. The detection scheme relies on non-linear mixing between the PSP and laser readout fields resulting in signal enhancement relative to the undetected and dissipated PSP wave. Our procedure and analysis can be generalized to most detection schemes exploited to detect PSP signal waves. In fact, our approach works with traditional linear detection schemes, as it incorporates an additional homodyne field to enhance detection. Experimental:

Our experimental scheme is depicted in figure 2a. Hole and rectangular trench structures are milled into 100 nm thick silver thin films on mica substrates using focused ion beam lithography. The hole diameter is fixed at 8 µm, while the width of the trench is varied from 100 nm to 10 µm. The middle of the trench is located 100 µm from the hole center. The photoemission electron microscope used is described elsewhere.18 Briefly, interferometrically-


locked femtosecond laser pulses (90 MHz repetition rate) are generated using a commercial Titanium-Sapphire femtosecond oscillator that produces dispersion-compensated 15 fs pulses (FWHM) measured at the sample using time resolved multi-photon photoemission. The ppolarized 780 nm central wavelength laser beam is split into a ‘pump’ and ‘probe’ pair for timeresolved measurements with roughly 50 mW average power per beam. The incident laser sources are focused onto the sample surface at an incident angle of 73 degrees with respect to the surface normal, leading to highly elliptical laser spots on the sample surface (~100 µm x 30 µm Gaussian FWHM), as illustrated in figure 2a. As the work function of silver is above the energy of a single 5


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excitation photon, at least three photons are required to induce photoelectron emission recorded using PEEM. The signal wave is encoded through the interaction of the pump beam with the hole coupling structure generating a PSP wave that subsequently propagates from the coupling site. At subsequent delay times (larger travel distances), the signal may be modified and decoded through the interaction with the spatially separated probe beam, as described in the below. Numerical simulations were performed using a commercial FDTD package (Lumerical FDTD Solutions). The calculations model a 100 nm deep trench of varying widths etched in a

100 nm thick silver film on a dielectric substrate and span a total distance of 66 microns in the propagation direction. A plane wave source is used, injected at a 73 degree angle of incidence. The trench size and simulation dimensions are chosen to provide an accurate reproduction of the experiment, without resulting in prohibitively long simulation times. Model laser pulses of 80 fs are used both to ensure that the angular dispersion in injected wave vector is minimized, and to ascertain that the interference of the pump and plasmon fields occurs over the full region of the simulation. Field intensities are determined by monitors placed directly before and after the trench. Results and Discussion:

Figure 2b shows a time-resolved PEEM (tr-PEEM) image generated when both the pump and probe laser pulses impinge on the sample surface at a relative time delay set such that the probe beam and pump-launched PSP overlap temporally. The pump laser spot is centered on the 8 µm hole coupling structure to launch the PSP that then propagates in the x-direction. The probe laser spot is spatially offset from the pump laser spot and centered on the 1 µm wide trench. The pump and probe pulses are spatially separated to enable recording time-resolved images of the

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launched PSP wave propagating across the sample.3 Clear photoemission interference patterns can be observed along the propagation direction. As discussed in prior reports,19-23 these images are governed by the interference between the laser pulse and the laser-irradiated surface-bound 3

PSP wave propagating far beyond the coupling site. Two coupling structures, both hole and

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trench, govern the recorded PEEM images. Figure 2c shows such an interference pattern generated when the probe beam alone is incident onto the sample and is centered on the trench. Figure 2d displays the tr-PEEM image generated when the probe-only signal is subtracted from the pump plus probe measurement. The resulting image is determined by effects from the holecoupled PSP in addition to other signals arising from the interaction between the hole-coupled PSP and the trench. The line profile in Figure 2d demonstrates that the hole-coupled PSP wave both traverses the trench, and its amplitude is enhanced by about a factor of 3 relative to the same signal recorded before the trench.

Figure 2. (a) Schematic representation of the experimental construct, depicting laser pulse positions, the PSP wave generation source (hole) and the etched material discontinuity. Laser pulses propagate from the lower left to upper right corner of the figure. (b) A tr-PEEM image showing the interference fringes between the pump-generated PSP

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from the hole and the probe-generated PSP located at the trench. (c) A tr-PEEM image displaying the background photoemission from probe only excitation; a faint half-elliptical signal is evident before the trench and a strong plasmon-probe interference pattern is apparent after the trench. (d) tr-PEEM image generated by subtracting the probe-only image in panel (c) from the image in panel (b). The result is a tr-PEEM image showing the propagation of the hole-generated PSP. The line profile along the propagation direction indicates that the enhancement after the trench is approximately a factor of three.

The PSP wave packet travels at about 95 percent of the speed of light in vacuum.3 As the


distance between the launch and detection points increases, the PSP signal wave will begin to lag with respect to the laser field due to a difference in their relative group velocities. Figures 3b and 3c display the time-resolved photoemission yield, measured at the indicated positions in the trPEEM image (figure 3a) for a trench of 100 nm in width. Time zero is set such that the pump and probe pulses arrive simultaneously at the monitored position. The photoemission intensity displays a twin-peaked interferogram structure associated with the temporal overlap of pump and probe pulses in conjunction with the pump-pulse-launched hole PSP wave overlapped with the probe pulse as previously discussed in detail .3


The PSP signal read-out amplitude is proportional to the maximum of the fitted interferogram peak generated by the overlap of the hole-coupled PSP and the probe pulse at positions before (Figure 3b) and after (Figure 3c) the trench. We define readout enhancement as the PSP signal amplitude at the first interference maximum after the trench, divided by the PSP signal amplitude measured immediately before the trench. The signal readout enhancement is our direct experimental observable and incorporates PSP field attenuation after traversing the trench and any subsequent modification. Figure 3 reveals three distinctive features of this experiment. First, the encoded PSP clearly traverses the 100 nm trench. Second, the encoded signal on the far side of the trench is clearly larger than before the trench. Third, the interferogram consisting of

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overlap between the pump and probe pulses (zero time feature) also is larger when measured after the trench relative to before the trench.

Figure 3. (a) tr-PEEM image of a PSP wave propagating across a 100 nm trench. The red arrows indicate selected positions before and after the trench where we monitored the photoemission yield as a function of time. The field-ofview is 150 microns. Laser pulses propagate from the lower left to upper right corner of the figure. (b),(c) Time dependent photoemission yield measured at positions marked in (a). The relative intensity of the PSP wave before and after the trench is indicated by length of the solid arrows in b and c.

These results are not specific to small trenches. We have measured the enhanced signal detection from trench widths between 100 nm and 10000 nm and have observed that the signal PSP traverses the trench and can then be detected using our scheme in all cases (see supporting information). In order to understand how the PSP wave packet traverses the free space region between the trench walls, Finite Difference Time Domain (FDTD) calculations were performed on different size trench widths between 100 and 14000 nm. The FDTD calculations span a region large enough that the PSP field may be explicitly measured due to the time lag between it and the excitation field. Figure 4 displays the results from the FDTD calculation where the vertical axis

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represents the field measured at the Ag film surface on the far side of the trench relative to the corresponding field measured at the near side.

Figure 4. FDTD results displaying field transmission through trenches of widths between 0 and 14 µm. The fitted line is merely to guide the eye.

Figure 4 demonstrates that the PSP field indeed traverses the trench with appreciable amplitude, even at trench widths greater than 10 microns. Our FDTD calculations indicate that the PSP field transmission varies between 93% for a 100 nm trench to 11.5% for a 14 µm trench. This at first may seem surprising but it is a manifestation of classical Maxwell’s equations as implemented in the FDTD calculations. The PSP wave, upon reaching the near edge of the trench, does not extinguish immediately. Instead, the FDTD calculations show field strength throughout the free-space region within the trench. In essence, the PSP wave at the near-edge of the trench radiates into the vacuum region where it undergoes attenuation with distance, analogous to a dipole antenna. The field that reaches the opposite trench wall is then re-coupled into the surface producing an attenuated, but essentially exact replica of the near-edge wave packet. Thus, a nanocircuit produced using this structure would allow signals to cross in space 10


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where one signal is transmitted along the trench and the other is transmitted across the trench, provided they were adequately separated in time. In addition, small gaps or imperfections in the metallic surface would not completely extinguish PSP signal waves in a nanophotonic device as the PSP wave has the ability to traverse fairly large open regions. In order to understand our results displayed in figure 3, we analytically model the measured third-order photoelectron intensity, Ip, using wave packet propagation techniques. The PEEM detected signal is proportional to the time-integrated sixth-order of the polarization field 19, 22, 23

produced by the linear superposition of the laser and PSP fields


I = C ∗

dt ∙ |P + P + P | (1)

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Here, P is the (coherent) polarization state induced by the laser pump and probe fields, P and

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P are the polarization states of the PSPs originating at the hole and trench structures,


respectively, and

is a proportionality constant that accounts for the overall detection

efficiency (see supporting information). It is important to recognize that there are inherently two types of interactions between the laser fields and the silver surface. The first type of interaction is generation of the PSP signal wave at coupling structures such as the hole and trench. The second type of interaction involves modification of the surface polarization by the laser fields, but does not result in PSP excitation. This type of interaction can be responsible for ionization, producing the photoelectrons that govern the recorded PEEM images. In this regard, the experiment has the inherent complication that the probe beam both excites PSP modes at the trench, and also induces subsequent photoelectron emission. The polarization fields may nonetheless be modeled as wave packets that depend linearly on the input fields as

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!" (#, %) = & ∙ { E (x, t) ∙ *+ ,-. /∙0,12,34 + 5. 5. 7 + E (x − # , t − dt) ∙ *+ ,-. (//9 )∙0,12,3(4:4) + 5. 5. 7} (2) !=>?@ (#, %) = A ∙ e

F I CDEC H G I J

!OP@1QR (#, %) = S ∙

∙ [+ ,-L /,34 + 5. 5. ] ∙ + ,N

(//E I T4 V U I W e

(3)

∙ *+ ,-L(//E),34 + 5. 5. 7 ∙ + ,N (4)

in which θ is the angle of the pump and probe beams with respect to the surface normal, x is the PSP propagation direction along the surface, x0 is the probe beam x coordinate, xt is the trench x coordinate, kl and kp are the laser and PSP wave vectors, dt is the delay between pump and probe pulses, v is the PSP group velocity, ω is the laser angular frequency, ϕ the phase of the PSP relative to the input field, and α,β,δ are coupling constants relating the polarization to the input fields. E0(x,t) is the field envelope function which contains the propagating spatial and temporal profile of the laser pulse. We note that field attenuation due to either population or coherence loss is not included in the above formalism, but can be accounted for in a straightforward fashion. That said, we have previously determined that under our experimental conditions, the 1/e decay length is greater than 80 microns in a similar system.3 Therefore, the aforementioned effects are


not expected to significantly impact our analysis. Equations (1)-(4) specify both the temporal and spatial dependence of the combined PSP and laser fields and may be used to simulate either tr-PEEM images measured at a given pumpprobe delay (see Figure 2b,c), or time-resolved photoelectron emission as a function of pumpprobe delay (see Figure 3b,c). Figures 5a and b establish that the physics described by equation (1) accounts for our experimental observation. In figures 5a and b, the experimental time-

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resolved photoemission yield for a 6 µm trench is compared to a simulated trace computed using equations (1)-(4) at positions before and after the trench. The parameters used in the simulation closely mimic the actual experimental scheme. To account for transmission through the trench region and subsequent re-coupling at the far wall, the hole-coupled PSP wave packet amplitude is diminished by an amount determined by the FDTD transmission calculations (figure 4). In order to fit the experimental data, an additional phase shift is added to account for out-coupling and subsequent re-coupling of the PSP field. This phase shift is constant across all trench widths. Excellent agreement in both shape and relative PSP signal amplitude is observed between the experimental and analytically modeled traces, particularly in terms of the magnitude of the enhanced detected signal, which, for a 6 µm trench, is almost an order of magnitude.

Figure 5. Experimental and simulated results for a 8 µm hole and 6 µm trench configuration. (a) Simulated (upper, blue) trace and experimental (lower, green) time-resolved PEEM intensity measured before the coupling trench. (b) Simulated (upper, blue) trace and experimental (lower, red) time-resolved PEEM intensity measured after the coupling trench. The enhancement factor, of nearly a factor of ten, is defined as the ratio of amplitudes of the two PSP signals located at positions before (time delay ~ 30 fs) and after the trench (time delay ~ 38 fs). Signals are offset for clarity.

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The mathematical origin of the PSP detection enhancement may be understood as follows: when Equation (1) is expanded the signal can be related to the sum of single polarization field terms (e.g. |PL|6) and cross-terms or coherences between the different polarization fields (e.g. |!=>?@ |Y ∙ |!" |Z), including linear and higher order terms (see the supporting information section).

A coherent interaction between PHole and PL results in static (for PL representing the pump field) and transient (for PL representing the probe field) PSP-laser interference fringes, as previously described.3 Coherence between PHole and PTrench results in a polarization field that is the sum of


linear and non-linear multiplicative combinations of the two PSP polarization fields governed by equation (1). The resulting superposition of polarization fields is directly responsible for the enhanced detection after the trench. The addition of non-linear interference terms increases the signal level of the attenuated input field (see the supporting information section). Thus enhanced detection of the encoded PSP signal wave is achieved through an optical implementation of homodyne detection. Traditional optical homodyne detection mixes reference and signal fields and uses a square-law detector, therefore [R ∝ |]0,^ ∙ + ,(34_) + ]P@` ∙ + ,34 |Y

where Ih is the detected signal, Esig and Eref are the signal and reference field amplitudes, ω the field frequency and φ is the relative phase between the fields. Upon expansion this yields ∗ ∗ [R ∝ |]0,^ |Y + |]P@` |Y + ]0,^ ]P@` ∙ + ,_ + ]P@` ]0,^ ∙ + ,_

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If Esig is small, then the first term can be ignored. The result is that the detected phase-dependent signal level is linear in both the reference and signal fields and also preserves the optical phase. Thus, using a larger reference field increases the detected signal. In our case, the detected signal is more complicated, arising from the third order interactions inherent in equation (1). In addition, the reference field is a superposition of both the probe laser field and the probe-launched PSP field. This results in multiple cross-terms between the reference field and signal making the power dependence more complicated but greater than linear in each field. This model explains the increase in the PSP wave packet signal intensity observed in figure 2. The increase in the pump-probe interferogram intensity can also be understood by noting that upon striking the silver surface, the probe beam produces both a static polarization and a dynamic PSP wave packet. The probe-induced PSP wave packet is essentially overlapped with the static polarization as the relative time delay between them is near zero at the origin point. This is the reference oscillator field stated above. Thus, the pump beam is temporally overlapped with both static and dynamic polarization states, resulting in enhancement of the pump-probe correlation signal as well. In contrast, the pump-probe correlation signal before the trench is absent the probe-induced PSP wave and therefore has lower amplitude relative to the signal after the trench. We have used this model to simulate the experimental results for a variety of different trench widths simply by adjusting the parameter related to the trench polarization field strength PTrench and modifying the PSP wave packet transmission across the trench using the FDTD-

calculated values. As the trench width increases, two effects compete to yield the overall signal detection level. The first is the transmission coefficient across the trench which decays with trench width according to Figure 4. The second factor is the trench-induced field enhancement, 15



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which increases quickly with trench size from 100 nm to about 10 microns and then drops slightly at the larger trench widths as calculated using FDTD simulations (see the supporting information section). The field enhancement is a function of both the PSP field strength and interference between the forward-launched PSP wave with cylindrical waves or other fields generated at the coupling trench.23 Our results suggest that initially, the increase in trench-


coupled PSP field strength outweighs the attenuation due to free-space propagation across the trench, resulting in an overall net increase in the enhancement with trench width. At larger trench widths, the PSP polarization field does not increase as dramatically, and the free-space attenuation becomes the dominant factor. The overall detection enhancement we measure is a result of these competing effects is shown in figure 6 and shows detection maximum for a 6 µm trench.

Figure 6. Experimental signal detection enhancement as a function of trench width. The maximum detection enhancement results from a 6 micron trench.

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Our results point to at least two different methods to increase the signal detection level. The first is to increase the probe laser intensity. The highly non-linear nature of equation (1) results in complicated laser power dependences, however we have previously shown that at lower power levels like those used in these experiments, the PSP wave shows a near-linear power dependence with respect to the pump laser, and a near-quadratic power dependence on the probe laser 3. Thus, simply increasing the probe power results in a non-linear increase in the detected PSP signal. The second method is to increase the probe-PSP coupling efficiency resulting in a larger reference field and therefore larger pump-induced PSP, probe-induced PSP cross-terms. Both of these methods effectively increase the reference field. Toward this goal, we have replaced the simple trench structure with an array of horizontal trenches similar to a diffraction grating. In principle, this array will allow greater transmission of the PSP signal through the continuous regions, while also allowing for large reference fields resulting from PSPs produced from each trench.

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Figure 7. (a) A tr-PEEM image showing the interference fringes between the pump-generated PSP from the hole and the probe-generated PSP from the array of horizontal trenches. The hole structure is outside field of view. Laser pulses propagate from the lower left to the upper right corner of the figure. The inset is a magnified PEEM image of photoemission from array of horizontal trenches. (b),(c) Time dependent photoemission yield measured at positions marked in (a). The relative intensity of the PSP wave before and after the trench is indicated by separations of the solid arrows in b and c. Signal detection enhancement of nearly 30X is observed.

Figure 7a displays an tr-PEEM image of such a trench array along with the corresponding PSP signal enhancement (Fig 7b,c). The length of each trench is 6 µm while the trench width is about 100 nm and trench separation of 400 nm. This type of structure produces a detection enhancement of nearly 30, under near identical conditions to those above. In conclusion, a simple technique to increase PSP signal readout level has been described. This technique relies on homodyne detection where the reference signal is derived from the sum of laser and PSP fields at an auxiliary readout position. PSPs have the advantages of travelling near the speed of light, and the ability to traverse gaps and trenches. This quality allows for imperfections in the nanocircuit fabrication as PSPs can easily jump open areas of modest size. In addition, the ability to jump channels permits multiple level circuitry where PSP signals may be transmitted both on the surface and in trenches. This detection scheme may be employed in most readout geometries since by design it provides the fields necessary for homodyne detection. Supporting Information Raw interferometric PEEM data for different trench widths, the derivation of signal enhancement, PSP field enhancement as a function of trench width; this information is available free of charge on the ACS publications website at http://pubs.acs.org Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest.

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

References

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