Mapping surface plasmon polariton propagation via counter-propagating light pulses

Mapping surface plasmon polariton propagation

via counter-propagating light pulses

Christoph Lemke,1,*

Till Leißner,1 Stephan Jauernik,

1 Alwin Klick,

1 Jacek Fiutowski,

2

Jakob Kjelstrup-Hansen,2 Horst-Günter Rubahn,

2 and Michael Bauer

1

1Institut für Experimentelle und Angewandte Physik, Christian-Albrechts-Universität zu Kiel, 24098 Kiel, Germany 2Mads Clausen Institute, NanoSYD, University of Southern Denmark, Alsion 2, DK-6400 Sønderborg, Denmark

*[email protected]

Abstract: In an interferometric time-resolved photoemission electron

microscopy (ITR-PEEM) experiment, the near-field associated with surface

plasmon polaritons (SPP) can be locally sensed via interference with

ultrashort laser pulses. Here, we present ITR-PEEM data of SPP

propagation at a gold vacuum interface recorded in a counter-propagating

pump-probe geometry. In comparison to former work this approach

provides a very intuitive real-time access to the SPP wave packet. The

quantitative analysis of the PEEM data enables us to determine in a rather

direct manner the propagation characteristics of the SPP.

©2012 Optical Society of America

OCIS codes: (240.6675) Surface photoemission and photoelectron spectroscopy; (240.6680)

Surface plasmons; (320.2250) Femtosecond phenomena.

References and links

1. E. Ozbay, “Plasmonics: merging photonics and electronics at nanoscale dimensions,” Science 311(5758), 189–

193 (2006).

2. N. J. Halas, “Plasmonics: an emerging field fostered by nano letters,” Nano Lett. 10(10), 3816–3822 (2010).

3. M. L. M. Balistreri, H. Gersen, J. P. Korterik, L. Kuipers, and N. F. van Hulst, “Tracking femtosecond laser

pulses in space and time,” Science 294(5544), 1080–1082 (2001).

4. O. Schmidt, M. Bauer, C. Wiemann, R. Porath, M. Scharte, O. Andreyev, G. Schönhense, and M. Aeschlimann,

“Time-resolved two photon photoemission electron microscopy,” Appl. Phys. B 74(3), 223–227 (2002).

5. H. Gersen, T. J. Karle, R. J. P. Engelen, W. Bogaerts, J. P. Korterik, N. F. van Hulst, T. F. Krauss, and L.

Kuipers, “Real-space observation of ultraslow light in photonic crystal waveguides,” Phys. Rev. Lett. 94(7),

073903 (2005).

6. R. J. P. Engelen, Y. Sugimoto, H. Gersen, N. Ikeda, K. Asakawa, and L. Kuipers, “Ultrafast evolution of

photonic eigenstates in k-space,” Nat. Phys. 3(6), 401–405 (2007).

7. A. Kubo, N. Pontius, and H. Petek, “Femtosecond microscopy of surface plasmon polariton wave packet

evolution at the silver/vacuum interface,” Nano Lett. 7(2), 470–475 (2007).

8. M. Bauer, C. Wiemann, J. Lange, D. Bayer, M. Rohmer, and M. Aeschlimann, “Phase propagation of localized

surface plasmons probed by time-resolved photoemission electron microscopy,” Appl. Phys., A Mater. Sci.

Process. 88(3), 473–480 (2007).

9. F. Meyer zu Heringdorf, L. Chelaru, S. Mollenbeck, D. Thien, and M. Horn von Hoegen, “Femtosecond

photoemission microscopy,” Surf. Sci. 601(20), 4700–4705 (2007).

10. W. Swiech, G. H. Fecher, C. Ziethen, O. Schmidt, G. Schönhense, K. Grzelakowski, C. M. Schneider, R.

Frömter, H. P. Oepen, and J. Kirschner, “Recent progress in photoemission microscopy with emphasis on

chemical and magnetic sensitivity,” J. Electron Spectrosc. Relat. Phenom. 84, 171–188 (1997).

11. M. U. Wehner, M. H. Ulm, and M. Wegener, “Scanning interferometer stabilized by use of Pancharatnam’s

phase,” Opt. Lett. 22(19), 1455–1457 (1997).

12. T. Leißner, K. Thilsing-Hansen, C. Lemke, S. Jauernik, J. Kjelstrup-Hansen, M. Bauer, and H.-G. Rubahn,

“Surface plasmon polariton emission prompted by organic nanofibers on thin gold films,” Plasmonics,

doi:10.1007/s11468-011-9301-9.

13. H. Ditlbacher, J. R. Krenn, N. Felidj, B. Lamprecht, G. Schider, M. Salerno, A. Leitner, and F. R. Aussenegg,

“Fluorescence imaging of surface plasmon fields,” Appl. Phys. Lett. 80(3), 404–406 (2002).

14. P. Lalanne, J. P. Hugonin, H. T. Liu, and B. Wang, “A microscopic view of the electromagnetic properties of

sub-lambda metallic surfaces,” Surf. Sci. Rep. 64(10), 453–469 (2009).

15. L. Zhang, A. Kubo, L. Wang, H. Petek, and T. Seideman, “Imaging of surface plasmon polariton fields excited

at a nanometer-scale slit,” Phys. Rev. B 84(24), 245442 (2011).

#165333 - $15.00 USD Received 23 Mar 2012; revised 13 Apr 2012; accepted 16 Apr 2012; published 23 May 2012(C) 2012 OSA 4 June 2012 / Vol. 20, No. 12 / OPTICS EXPRESS 12877

16. B. Wang, L. Aigouy, E. Bourhis, J. Gierak, J. P. Hugonin, and P. Lalanne, “Efficient generation of surface

plasmon by single-nanoslit illumination under highly oblique incidence,” Appl. Phys. Lett. 94(1), 011114

(2009).

17. V. V. Temnov, U. Woggon, J. Dintinger, E. Devaux, and T. W. Ebbesen, “Surface plasmon interferometry:

measuring group velocity of surface plasmons,” Opt. Lett. 32(10), 1235–1237 (2007).

18. P. B. Johnson and R. W. Christy, “Optical constants of the noble metals,” Phys. Rev. B 6(12), 4370–4379

(1972).

1. Introduction

Surface plasmon polaritons (SPP), propagating electromagnetic modes that are bound to a

metal-dielectric interface, are considered as one of the key ingredients of next-generation

nano-photonic devices [1]. This promising perspective is one of the driving forces for the

multitude of current research activities in the field of plasmonics [2]. With regard to high-

speed applications well-founded means for the development of SPP-based devices rely on a

comprehensive knowledge of the spatio-temporal characteristics of ultrashort SPP pulses and

their modifications during formation and propagation. Optical pump-probe schemes in

combination with near-field microscopy [3] and photoemission electron microscopy (PEEM)

[4] techniques provide experimental means to track and visualize the two-dimensional SPP

propagation in space and time at sub-µm lateral and femtosecond temporal resolution. The

potential of both approaches in tracking the ultrafast SPP dynamics has been demonstrated in

numerous works in the recent past [5–9].

For these types of experiments the PEEM technique is operated in an interferometric time-

resolved mode (ITR-PEEM), and usually the properties of the SPP are probed via spatio-

temporal interference while SPP wave packet and the probing laser pulse are co-propagating,

as illustrated in Fig. 1(a). The entanglement of different contributions to the PEEM signal in

this detection scheme implicates, however, a rather indirect data interpretation. In this work

we demonstrate experimentally that a counter-propagating PEEM detection mode, as

illustrated in Fig. 1(b), provides a much more intuitive access to the SPP wave packet

propagation. Particularly the group velocity and the phase velocity of the SPP can be deduced

from these data in a very direct manner.

2. Experimental

The ITR-PEEM experiments were performed with a photoemission electron microscope (IS

PEEM, Focus GmbH) [10] mounted in an ultrahigh vacuum µ-metal chamber (base pressure

1× 10−10 mbar) and providing a lateral resolution of better than 40 nm. The interferometer is

an actively stabilized Mach-Zehnder interferometer following a design described in detail in

reference [11]. The stability of the interferometer allows us to adjust the temporal delay

between the two excitation laser pulses with a timing accuracy of better than 30 attoseconds.

The interferometer is also used for characterization of the laser pulse profile via second

harmonic generation interferometric autocorrelation measurements. These measurements are

performed in parallel to the ITR-PEEM experiments. Laser pulses are delivered by a

commercial Ti:Sapphire laser system providing 15 fs pulses at 816 nm and a pulse energy of 6 nJ. The laser pulses are p-polarized and hit the sample at an angle θ = 65° with respect to the

sample surface normal. More details of the experimental setup are described in [12].

For the experiments in the counter-propagating scheme we used 60 nm thick gold films

evaporated onto a silicon substrate (see Fig. 1(f)). A 2.25 µm wide and 140 nm high gold bar

was fabricated on top of the gold film by means of electron beam lithography, gold deposition

via evaporation, and lift-off. The two edges of the gold bar can provide the wave vector

required to overcome the wave vector mismatch between laser field and SPP [13] and are

used in the experiment as defined and localized sources for SPP wave packet emission. For

some of the experiments a 45 nm thick film of para-Hexaphenylene (p6P) molecules was

evaporated onto the gold film. Reference measurements in the co-propagating scheme were

#165333 - $15.00 USD Received 23 Mar 2012; revised 13 Apr 2012; accepted 16 Apr 2012; published 23 May 2012(C) 2012 OSA 4 June 2012 / Vol. 20, No. 12 / OPTICS EXPRESS 12878

performed at a flat rectangular gold pattern on silicon (see Fig. 1(e)). In this case, the wave

vector required for SPP excitation was provided by an edge of the gold pattern.

Prior to the PEEM measurements, the sample was covered under UHV conditions with a

small amount of Cesium (coverage << 1 monolayer) from a well degassed resistively heated

SAES getter source. This treatment is required to lower the work function of the gold surface

from a value of about 5.5 eV to about 3 eV to facilitate a two-photon photoemission process

with the 816 nm light pulses.

Fig. 1. Comparison of co-propagation (left) and counter-propagation (right) PEEM detection of

SPP wave packet propagation; (a),(b) scheme of the co-propagation (counter-propagation)

detection mode: SPP and probing laser pulse are propagating in the same (opposite) direction;

(c) co-propagation PEEM data of SPP propagation along a planar gold surface for temporal

excitation-probe delays of 0 fs and 40 fs; (d) corresponding data for the counter-propagating

PEEM imaging mode; (e), (f) scheme of the gold structures as used for the PEEM experiments

shown in (c) and (d);

3. Results and discussion

Figures 1(c) and 1(d) compare ITR-PEEM data of SPP wave packet propagation in co-

propagating and counter-propagating PEEM detection mode for two different temporal delays

τ between the applied laser pulses. The characteristic beating pattern and its modulation as

function of delay observed in the co-propagation mode results from the superposition of SPP

induced polarization field and laser pulse and agrees qualitatively well with data reported for

silver films by other groups before [7, 9]. The periodicity of the beating pattern (7.25 ± 0.15

µm) is given by the wave vector mismatch between laser field and SPP and, therefore, allows

determination of the wave vector kSPP and the phase velocity vp,SPP of the SPP. Two different

#165333 - $15.00 USD Received 23 Mar 2012; revised 13 Apr 2012; accepted 16 Apr 2012; published 23 May 2012(C) 2012 OSA 4 June 2012 / Vol. 20, No. 12 / OPTICS EXPRESS 12879

contributions in the imaged superposition field have to be distinguished: A dominating static

(delay-independent) term that starts right at the excitation edge of the gold film and that is

damped in the direction of SPP propagation. It is formed by the interference between SPP

wave packet and the laser pulse responsible for its excitation. Additionally, in the very

vicinity of the edge, this signal may also be affected or even dominated by contributions from

the excitation of so-called quasi-cylindrical waves (cw waves) [14]. An estimation based on

Eq. (20) given in reference [14] yields a maximum distance of ≈3.5 µm at which the

amplitude of the cw wave should dominate the SPP field. Indications for the presence of cw

contributions in the PEEM signal for plasmon excitation at a metallic edge have been reported

in reference [15].

It can be shown that the detected amplitude decay of the superposition field is governed

by the damping length of the SPP and, furthermore, by the mismatch in the group velocity

between SPP and laser pulse: As the laser pulse passes the SPP wave packet the intensity of

the probed superposition field gradually decreases.

The other contribution to the co-propagation PEEM signal arises from the interference

between second laser pulse and the SPP wave packet excited by the first laser pulse. It

consequently exhibits a distinct dependence on the temporal delay τ between the laser pulses,

which is adjusted by the interferometer. This signal is the actual probe of the SPP propagation

in this experimental scheme. The most distinct feature that can be associated with this

contribution is the increase in relative photoemission intensity at large distances from the

excitation edge as can be seen in Fig. 1(c) in the comparison of the data recorded at τ = 40 fs with the data recorded at time-zero: for large delays τ the second laser pulse probes the

propagating SPP wave packet at later times, i.e. more distant from the SPP excitation edge. It

is rather evident that the entanglement of the co-propagation signal and the dominating static

background in these data makes a quantitative analysis with respect to the SPP propagation

dynamics difficult.

In the counter-propagating SPP imaging mode the propagation signal is much less

affected by the static background as demonstrated in Fig. 1(d). Now, the laser field is incident

from the right and probes the SPP wave packet propagating in opposite direction. The most

distinct difference in comparison to Fig. 1(c) is the significantly shorter decay length of the

static superposition background (note the different length scales in Fig. 1(c) and Fig. 1(d)).

This is a direct consequence of the propagation of laser field and SPP wave packet in opposite

directions which guarantees that both fields overlap for a very short time, only, virtually given

by the temporal width of the excitation laser pulse. One observes, furthermore, a reduction in

the period of the beating pattern to a value of 423 nm ± 10 nm as the relative orientation of the

interacting wave vectors is changed. We would like to note that interference patterns arising

from the counter-propagation of the excitation laser field and the SPP wave have been

observed before for instance in a study on a 100 nm thick gold film using scanning near-field

optical microscopy [16]. Furthermore, recent FDTD simulations on the SPP excitation at

defined nanometer-scaled slits in a silver film provided also evidence for the existence of

such a signature in ITR-PEEM experiments [15].

#165333 - $15.00 USD Received 23 Mar 2012; revised 13 Apr 2012; accepted 16 Apr 2012; published 23 May 2012(C) 2012 OSA 4 June 2012 / Vol. 20, No. 12 / OPTICS EXPRESS 12880

Fig. 2. (a) Delay-path diagram of SPP propagating at the gold-vacuum interface deduced from

the ITR-PEEM measurements, (b) corresponding delay-path diagram as deduced from an

analytic simulation (c) delay-path diagram of SPP propagating at the gold-p6P interface; note

that in the latter case the second SPP wave packet excited at the left hand edge of the gold bar

is not observable; the green dashed line in the graph marks a trace along an interference

maximum; the red dashed line follows the envelope maximum of the SPP-laser superposition

signal.

#165333 - $15.00 USD Received 23 Mar 2012; revised 13 Apr 2012; accepted 16 Apr 2012; published 23 May 2012(C) 2012 OSA 4 June 2012 / Vol. 20, No. 12 / OPTICS EXPRESS 12881

The efficient suppression of the static background signal in this imaging mode as

observed in our study enables one to obtain a much more distinct view onto the propagation

of the SPP wave packet. As seen in the image recorded at τ = 40 fs, the signal arising from the

interference between second laser pulse and propagating SPP wave packet excited by the first

laser pulse is now well separated from the static background and clearly resolved. Note that

the identical temporal delay applied to the two detection modes does not correspond to the

same SPP propagation distance. Counter-propagation snapshots recorded at varying temporal

delays τ between the two laser pulses allow one to directly follow the propagation of the SPP

wave packet. The results of such a pump-probe scan are summarized in Fig. 2(a) in terms of a

delay-path diagram. For this depiction photoemission intensity line profiles were deduced for

each delay τ by integration along the vertical axis of the PEEM image. In addition, the

displayed data are background corrected by subtraction of reference data recorded at τ = 0 fs

and each line profile is normalized to its maximum. At small propagation distances some

residuals of the static superposition background are still visible because of a non-perfect

background correction. Furthermore, at these distances the signal may also be affected by the

presence of cw waves.

The relevant signature in the delay-path diagram arises from the superposition signal

between first SPP and second laser pulse. It exhibits a linear slope which is governed by the

phase and the group velocity of the SPP. In more detail, the depiction shows that the

superposition signal contains two distinct contributions (see blue arrows), a dominating

leading beating pattern, followed by a weaker second beating pattern. We will see later that

the two signals arise from different SPP excitation processes at the gold bar. For now, we will

concentrate on the analysis of the dominant leading part of the cross-correlation signal. The

slope τ / xp of the straight line defined by an individual beating maximum resulting from

constructive interference (see for instance green dashed line in Fig. 2(a)) is governed by the

phase velocity of SPP and laser pulse. It can easily be shown that

,

sin( ) 1

p Vac p SPPx c v

τ θ = +

(1)

Here, vp,SPP is the phase velocity of the SPP and cVac is the velocity of the incident laser pulse.

The sine in the numerator of the first term projects cVac onto the surface plane and accounts

for the incidence angle θ of the laser pulse.

Correspondingly, the slope of the envelope maximum of the beating signal, τ / xg (see red

dashed line in Fig. 2(a)), is related to the group velocity vg,SPP of the SPP wave packet:

,

sin( ) 1

g Vac g SPPx c v

τ θ = +

(2)

The quantitative analysis of the data in Fig. 2(a) under consideration of the 816 nm wave

length of the excitation laser pulse yields vp,SPP = (0.979 ± 0.025)⋅cVac and vg,SPP = (0.939 ±

0.035)⋅cVac. xg was determined from Gaussian fits to the signal envelope following the beating

maxima as shown in Fig. 3. To avoid potential distortions of the fit by cw wave contributions

to the signal, we restricted the analysis to propagation distances > 9 µm. The numbers

obtained from this analysis are in good agreement with experimental data reported by

Temnov et al. [17] (vg,SPP ≈0.939⋅cVac) as well as values calculated under consideration of the

dielectric function of gold [18] (vp,SPP = 0.98⋅cVac and vg,SPP = 0.95⋅cVac). The mismatch

between vp,SPP and vg,SPP, which becomes in the experiment evident from the clear difference

in the slopes τ / xp and τ / xg, is indication for the dispersive character of the SPP propagation

and the continuous shift of the carrier-to-envelope phase as the plasmon propagates along the

gold surface.

#165333 - $15.00 USD Received 23 Mar 2012; revised 13 Apr 2012; accepted 16 Apr 2012; published 23 May 2012(C) 2012 OSA 4 June 2012 / Vol. 20, No. 12 / OPTICS EXPRESS 12882

Fig. 3. (a) signal envelope of the laser-SPP superposition pattern for different temporal delays

between 65 fs and 120 fs; for better illustration, the signals have been normalized to the

envelope maxima; the full lines show Gaussian fits to the data to determine the position xg of

the envelope maxima; (b) shows a plot of xg versus τ inclusive linear fit.

Above, it has been mentioned that the second laser-SPP superposition signal following the

main peak in Fig. 2(a) at significantly reduced amplitude can be associated with a second SPP

wave packet that has been excited at the gold bar. A quantitative analysis of the pulse

envelope of this wave packet signature shows that it follows the main peak at a distance of 4.5

µm, which exactly corresponds to twice the width of the gold bar. We suggest that we

observe here a SPP wave packet that was excited at the gold bar in co-propagating direction

and that was then back-reflected at the left hand edge of the gold bar. A corresponding

beating pattern is indeed observed on top of the gold, as seen for instance in Fig. 1(d). The

reduced amplitude of the signal in comparison to the preceding wave packet is associated

with a finite reflectivity of the left hand edge and a finite transmission probability as the pulse

penetrates the right hand edge of the gold bar.

To further support our data interpretation we performed analytic model simulations of

ITR-PEEM data under consideration of the relevant parameters used in the experiment, such

as sample geometry, excitation and probing geometry, and laser pulse width and wave length.

To account for the propagation properties of the SPP wave packet dielectric data tabulated in

[18] were used. The results of the simulation are summarized in the delay-path diagram in

Fig. 2(b). A comparison with Fig. 2(a) clearly shows that the relevant signatures considered in

this work are quantitatively well reproduced. The analysis of the simulated data yields correct

values for group and phase velocity and also the two-peak structure in the delay-path diagram

agrees with the experimental data.

For illustration of the sensitivity of the counter-propagating PEEM mode to changes in the

propagation properties of the SPP, Fig. 2(c) shows data recorded for a gold film covered with

a 45 nm thick p6P film. The dielectric response of the organic overlayer acts dispersive onto

the SPP wave packet propagation and consequently modifies vp,SPP and vg,SPP. The distinct

changes in the slopes of the phase trace (dashed green line) and the envelope maximum trace

(dashed red line) in comparison to the results for the gold-vacuum interface (Fig. 2 (a)) are

characteristic for these modifications. The quantitative analysis of the p6P data yield vp,SPP =

(0.88 ± 0.04)⋅cVac and vp,SPP = (0.60 ± 0.04)⋅cVac and indicates the enhanced dispersive

character of this interface in comparison to the gold-vacuum interface. Note that for the p6P

sample a second SPP pulse is not resolved in the data. A likely explanation is a reduction in

reflectivity and/or transmission of the gold edges by the presence of the p6P film.

4. Conclusion

The spatio-temporal characterization of propagating surface plasmon polaritons becomes

important as potential applications rely more and more on SPP pulse profiles in the low

femtosecond time regime. In this work a PEEM-based scheme has been presented that

#165333 - $15.00 USD Received 23 Mar 2012; revised 13 Apr 2012; accepted 16 Apr 2012; published 23 May 2012(C) 2012 OSA 4 June 2012 / Vol. 20, No. 12 / OPTICS EXPRESS 12883

provides an efficient experimental access to the characterization of relevant propagation

properties of SPP wave packets. The present experimental configuration is particularly

suitable for the study of two-dimensional systems such as for instance multilayer systems,

where the interaction of SPP modes located at different interfaces becomes relevant. A further

aspect that was not addressed in this study is the effect of SPP pulse broadening in a strongly

dispersing environment. The signal probed in the counter-propagating PEEM scheme is

essentially a cross correlation signal so that a data analysis should in principle also provide

information of SPP pulse broadening effects. Even more, the phase sensitivity of the used

interferometric approach could in future experiments allow for quantitative insights into the

phase modulation of the SPP wave packet.

Acknowledgments

This work was funded by the Deutsche Forschungsgemeinschaft through Priority Program

1391 “Ultrafast Nanooptics” as well as the Danish Council for Independent Research (FTP –

Project No. 09-072949 ANAP).

#165333 - $15.00 USD Received 23 Mar 2012; revised 13 Apr 2012; accepted 16 Apr 2012; published 23 May 2012(C) 2012 OSA 4 June 2012 / Vol. 20, No. 12 / OPTICS EXPRESS 12884