Plasmonics
DOI 10.1007/s11468-014-9756-6
Normal-Incidence Photoemission Electron Microscopy
(NI-PEEM) for Imaging Surface Plasmon Polaritons
Philip Kahl & Simone Wall & Christian Witt & Christian Schneider & Daniela Bayer &
Alexander Fischer & Pascal Melchior & Michael Horn-von Hoegen & Martin Aeschlimann &
Frank-J. Meyer zu Heringdorf
Received: 8 April 2014 / Accepted: 7 July 2014
# Springer Science+Business Media New York 2014
Abstract We introduce a novel time-resolved photoemissionbased near-field illumination method, referred to as femtosecond normal-incidence photoemission microscopy (NIPEEM). The change from the commonly used grazingincidence to normal-incidence illumination geometry has a
major impact on the achievable contrast and, hence, on the
imaging potential of transient local near fields. By imaging
surface plasmon polaritons in normal light incidence geometry, the observed fringe spacing directly resembles the wavelength of the plasmon wave. Our novel approach provides a
direct descriptive visualization of SPP wave packets propagating across a metal surface.
Keywords Surface plasmon polariton . Nonlinear
photoemission microscopy . Normal incidence
Introduction
The imaging of surface plasmon polaritons (SPPs) has attracted
significant experimental attention over the past few years. SPPs
are the combination of a collective oscillation of the free
electron gas in the metal and an electric and magnetic field at
P. Kahl : S. Wall : C. Witt : M. Horn-von Hoegen :
F.<J. Meyer zu Heringdorf (*)
Faculty of Physics and Center for Nanointegration
Duisburg-Essen (CENIDE), University of Duisburg-Essen,
47048 Duisburg, Germany
e-mail: meyerzh@uni-due.de
C. Schneider : D. Bayer : A. Fischer : P. Melchior : M. Aeschlimann
Department of Physics and Research Center OPTIMAS,
University of Kaiserslautern, 67663 Kaiserslautern, Germany
Present Address:
S. Wall
The Fraunhofer Institute for Microelectronic Circuits and Systems,
47057 Duisburg, Germany
the interface caused by the moving charges. Hence, propagating SPPs are accompanied by a strong electric field of optical
frequency that is created by the collective electron density
oscillation. Motivated by the potential to use SPPs in future
broadband and ultrafast nanophotonic devices, a variety of
optical functional units for steering and manipulating guided
SPP modes have been evaluated [1, 2]. Several sophisticated
microscopy techniques were developed to reveal the reliability of passive and active plasmonic elements. In addition to
several near-field microscopy methods, nonlinear photoemission electron microscopy (PEEM) [3, 4] has, in the last
years, been shown to be an ideal method for the characterization of SPPs [5, 6] and even for an indirect observation of the
time-dependent propagation of SPPs across a surface [7–10].
The simultaneous presence of the electric fields of a femtosecond laser pulse and an SPP leads to a time-dependent
interference pattern, the temporal integral of which PEEM is
able to detect. The PEEM approach, however, is hampered by
the fact that conventional PEEM setups exploit grazingincidence geometries in which the incidence direction of
the laser pulses and the surface normal comprise an angle
of ~65–74° (grazing-incidence or GI-PEEM). The obtained
contrast in these experiments depends strongly on the angle of
incidence because an SPP and a laser pulse generally propagate in different directions and the grazing-incidence
geometry breaks the symmetry. The resulting observed microscope images can be interpreted as a Moiré pattern [8, 11],
and the periodicity of the pattern differs strongly from the SPP
wavelength. For the case where light and SPPs propagate in
the same direction (when projected into the surface plane), a
pattern with micrometer spacing is observed [11–13]. In the
other extreme, where light and SPPs propagate in opposite
directions, the pattern spacing is about half of the SPP wavelength [12–14].
In the present work, we present a novel approach to image
SPPs using normal-incidence photoemission microscopy
Plasmonics
(NI-PEEM). Under normal-incidence illumination, a cylindrical symmetry for the imaging conditions is obtained and a
“direct” imaging of the SPP wave becomes possible, regardless of the propagation direction of the SPP. Unfortunately,
this desirable situation is difficult to realize experimentally,
since within the PEEM devices the optical beam path for
normal incidence is usually blocked by electron optics. We
will present two experimental concepts for the realization of
normal incidence, and we will discuss the differences between
conventional grazing-incidence GI-PEEM and normalincidence NI-PEEM.
Experimental Setup
In the past decades, two rather different types of PEEM
concepts have been invented and commercialized. Both setups
are sketched in Fig. 1. In one case (ELMITEC GmbH design,
operated in Duisburg), PEEM is just one of many contrast
mechanisms of a low-energy electron microscope (LEEM)
(see Fig. 1a). The LEEM consists of an electron gun, a
magnetic beam splitter (sector field), and an imaging column.
In the PEEM mode, electrons are emitted at the surface and are
deflected by a magnetic prism into the imaging column. In the
LEEM, the realization of normal incidence is achieved by
guiding the beam along the accessible beam path B of
Fig. 1 Schematic LEEM and PEEM setups. The laser beams are
sketched by blue dashed lines. In both experimental configurations, the
GI geometry (beam paths A and C) is still possible. a In the LEEM, NI
illumination (beam path B) is achieved by guiding the laser beam through
Fig. 1a normal to the surface. Quite contrary, dedicated
PEEM instruments are usually built in a linear fashion as
shown in Fig. 1b (FOCUS GmbH design, operated in
Kaiserslautern). Here, the realization of normal incidence is
hindered by the geometry of the electron optics but can be
achieved by inserting a small metallic mirror inside the electron optics column, as close as possible to the optical axis of
the electron path. The mirror is located in the back focal plane
of the objective lens where it has minimal influence on the
PEEM image. With this geometry, an incidence angle of <4°
with respect to the surface normal can be realized (beam path
D of Fig. 1b).
As a proof of principle for our approach and to demonstrate
the potential of NI-PEEM, we imaged SPPs in epitaxial silver
islands. SPPs are selectively excited at the edges of these
islands because here the necessary momentum conversion from light to SPP can take place. The samples
were prepared in-situ by self-assembly of Ag on a clean
Si(111) surface [15] in the LEEM. The Ag was deposited from a home-built electron beam evaporator [16, 17]. One
of the samples was then transferred to air and shipped to
Kaiserslautern.
For nonlinear photoemission experiments, ultra-short laser
pulses from a Ti:Sapphire oscillator (LEEM setup:
FEMTOLASERS, FEMTOSOURCE compact; PEEM setup:
Spectra-Physics, Tsunami) were used. While the two laser
the magnetic sector field; b in the linear PEEM, NI illumination (beam
path D) is achieved by reflecting the laser off a small mirror, which is
inserted into the back focal plane of the objective lens
Plasmonics
systems differ in pulse duration and peak power, the specific
properties of the laser pulses are not crucial for the findings
discussed below. In brief, both lasers produce <25 fs short laser
pulses at a central wavelength of 800 nm with a repetition rate of
80 MHz. The pulses are frequency doubled in a beta-barium
borate (BBO) crystal. The average power of the frequency
doubled 400 nm laser pulses amounts to >75 mW. In all cases,
the laser is focused on the surface, while the used focal lengths
and the achievable minimum spot sizes are different for the
PEEM setup and the LEEM setup; they also differ between NI
illumination and GI illumination. As a rule of thumb, we achieve
spot sizes of approximately 200 μm with focal lengths of the
focusing lenses ≥200 mm, corresponding to power densities of
>108 W/cm2 at the sample surface. For further details of the laser
setups of the LEEM and PEEM configurations under grazingincidence conditions, see [18] and [19], respectively.
Using femtosecond laser pulses of a photon energy that is
smaller than the work function of the surface, SPPs can be
imaged via a plasmon-enhanced two-photon photoemission
process (2PPE) [4, 5]. Here, a part of the laser pulse is used to
excite SPPs, while the remaining part of the laser pulse is used
to probe the SPP state [5, 7]. At low pulse energy, 2PPE
PEEM detects the temporal integral of the fourth power of
the superposition of the light and SPP field.
!
vector k M and the corresponding Moiré wavelength (or fringe
spacing) λM : For calculating the Moiré wavelength, the in-plane
!∥ !
component k L ¼ k L ⋅sinðθÞ of the incidence light wave vector
!
k L is needed. Here, θ is the incidence angle of the laser pulse
!
relative to the surface normal. For grazing incidence, k SPP and
!∥
k L can point in different directions and comprise an in-plane
angle α . The Moiré condition then yields a fringe spacing.
λ∥L λSPP
λM ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
λ∥L þ λSPP 2 −2λ∥L λSPP ⋅cosðαÞ
This result, that λM depends on α , is, for instance, visible
in Fig. 2a, where a narrow spacing of Moiré fringes is visible
at the top and bottom of the Ag island.
Figure 3 further analyzes the impact of θ and α on the
fringe spacing λM . All possible values for λM from Moiré
theory lie within the shaded area. For Fig. 3, λSPP and λM
were calculated using dielectric constants from the literature
[20] for SPP excitation with 400 nm laser pulses. For grazing
incidence, patterns with a vastly different fringe spacing can
Results and Discussion
Figure 2 shows four examples for time-integrated 2PPE SPP
imaging of Ag islands with the aforementioned beam paths A–D
depicted in Fig. 1. In Fig. 2, panels a, b, and c each show a
different Ag island, while the island shown in panel d is the same
as the one shown in panel c. To mark the locations of the Ag
islands, we indicated their outlines in Fig. 2 by black dashed
lines. A direct comparison of absolute intensities between panels
a and d is of no avail, since in GI the overall intensity is
polarization dependent, the SPP strength depends on the specifics of the particular island investigated, and the absolute
intensity sensitively depends on the achievable laser intensity,
laser spot size, and sample history. All images have thus simply
been optimized for contrast of the fringe pattern. In panels a and c
of Fig. 2, the micrometer fringe spacing (beating pattern) observed under GI condition is clearly visible. Under NI condition,
the contrast in panels b and d is entirely different. In the following, the various aspects of the striking differences in the pattern
formation between GI-PEEM and NI-PEEM will be discussed.
For grazing incidence, the contrast mechanism for SPP
imaging has been extensively discussed in the literature [5,
7, 11]. In brief, the observed periodic brightness modulation
exhibits a pattern with a periodicity that depends not only on
the used laser wavelength but also on the direction of the laser
pulses and the propagation direction of the SPP. The contrast
can be described by a Moiré pattern [12, 13] with the Moiré wave
Fig. 2 2PPE PEEM images of SPPs propagating across Ag islands for all
possible beam paths from Fig. 1. The outlines of the islands are marked
by the black dashed lines. Note that the Ag islands shown in a, b are
different from each other and also differ from the Ag island shown in c, d.
LEEM setup: a GI illumination with Moiré pattern and b NI illumination
where the fringe spacing resembles the SPP wavelength. Linear PEEM
setup: c GI illumination, the contrast is comparable to a, and d NI
illumination, the contrast is comparable to b. The illumination wavelength is λL =400nm in all cases. The color scaling and brightness of each
panel have been adjusted individually to give optimal contrast of the
fringe pattern
Plasmonics
!∥
be observed. For instance, if α=0°, i.e., when k L is parallel
!
to k SPP , the fringe spacing amounts to several micrometers.
!∥
In the other extreme, with α=180°, when k L is antiparallel to
!
k SPP , the fringe spacing is approximately half the SPP
wavelength. The latter has been referred to as “counter-propagation” geometry [14].
!∥
In normal incidence, θ=0° applies, and k L =0 as well. From
! !
!∥
! !
the Moiré condition kM ¼ k SPP k L , it follows that k M =k SPP,
!
and the fringe spacing λM ¼ 2π=j k M j mathematically becomes the SPP wavelengthλSPP ¼ 2π=k SPP . In Fig. 3, the data
points for θ=0° and θ=74° were obtained with the LEEM setup
(blue squares). The data for θ=4° and θ=65° was obtained with
the PEEM setup (red triangles). The marked data points in
Fig. 3 arise from an analysis of the various excitation edges
of the Ag islands in Fig. 2. Apparently, the Moiré concept is
suitable for explaining both the grazing-incidence illumination
and normal-incidence illumination contrast.
It is surprising, however, that the Moiré concept works so
well. After all, the experimental data is the temporal integral of
a nonlinear response of the superposition of rapidly varying
electric fields. Figure 4 considers these electric fields at different times and illustrates that the superposition of the electric
fields at the surface must yield the SPP wavelength. The blue
arrows indicate the strength of the electric fields of the laser
pulse. In normal incidence (NI-PEEM), the phase fronts of a
linearly polarized Gaussian laser pulse are parallel to the
surface plane. Accordingly, the electric field vector of the laser
pulse must be the same for the entire illuminated surface area
at a certain time (for each time, all blue arrows in Fig. 4 have
the same length). For the propagating SPP wave packet, the
electric field vector depends on time and position, as the
SPP’s field is caused by the periodic displacement of
Fig. 3 Pattern periodicity λM as a function of the incidence angle of light,
θ. The blue square-shaped data points were acquired with the LEEM setup.
The red triangle-shaped data points were acquired with the linear PEEM
setup. The gray-shaded area within the black dashed lines indicates the
theoretical possible values for the fringe spacing, depending on the angle α
between SPP and light direction as projected into the surface plane
electrons. The red areas in Fig. 4 represent positions with
a lower electron density, while green areas indicate a
higher electron density.
The electric field lines (gray) connect the areas of different
electron density. The red arrows in Fig. 4 indicate the local
electric field strength of the SPP. Above the green and red areas,
the field strength is minimal, while the maximum field can be
found just on the border between the regions of opposite
charge. For the following discussion, we assume that the
strengths of the near field of the SPP and the near field of the
laser are of the same order. If that is not the case, additional
photoelectrons add incoherent background intensity to the SPPrelated contrast. Such photoelectrons are generated by that part
of the stronger one of the two individual fields that cannot
entirely be annihilated by the other field via destructive
Fig. 4 Schematic explanation for the observation of the SPP wavelength in NI-PEEM. The blue arrows indicate the direction of the
electric field of the laser EL, which is at any given time constant across
the surface. The red arrows indicate the electric field strength ESPP of
the SPP which is modulated with the SPP wavelength λSPP. The SPP is
in part a modulation of the electron density ρ. For different observation
times (from top to bottom), the positions where EL and ESPP interfere
constructively do not change. The black arrows indicate the propagation of the SPP
Plasmonics
interference. As this background intensity is integrated in time
by the detector, it smears out, and any wave characteristic is
lost. Therefore, such background does not influence the periodicity of the photoelectron yield modulation, which is
discussed here. The result of the superposition of the near fields
of the SPP and the laser can then be discussed as three principal
cases. For the case of positions like the one marked by “A” in
Fig. 4, the fields of the laser pulse and the SPP are always of the
same phase, which will result in constructive interference. One
will find the largest electron yield at these positions. In the case
“C” of Fig. 4, the fields of the laser pulse and the SPP are
always of opposite phase, which will result in destructive
interference and minimal electron emission. All other cases,
like the case “B” in Fig. 4, will fall between these extremes and
result in an electron emission yield that is smaller than the one
in case “A” and larger than the one in case “C.” The five
scenarios in Fig. 4 show the situations at different times within
a laser cycle T ¼ 2π=ωL . Interestingly, as the SPP and the laser
pulse exhibit the same frequency ωL (but not the same wavelength) and are phase-locked, the overall electron yield changes
as a function of time, while the maximum emission yield will
always be at location “A” and the yield will always be minimal
at location “C.” One can conclude from these considerations
that in regions which can be found at x0 þ n⋅λSPP , n∈N ,
one finds maximum photoemission yield. In the regions at
x0 þ ð2n þ 1Þ=2⋅λSPP , there is minimum photoemission
yield. The spacing between successive maxima regions
and successive minima regions is exactly λSPP . While the
experimental signal is still a superposition of the electric fields
of SPP and laser radiation, it nevertheless resembles a direct
descriptive visualization of the phase fronts of the SPP pulse.
The difference between normal incidence (NI-PEEM)
and grazing incidence (GI-PEEM) does not just concern
the modulated intensity pattern formation. For linearly
polarized laser pulses of the same polarization, an SPP
wave in normal incidence generally propagates in a direction that is different from the direction the SPP would
have in grazing incidence. In both cases, the polarization
of the laser pulses affects the edge of the island at which
an SPP can be excited, as an electric field perpendicular
to the island edge is needed. For grazing incidence, the
direction of SPP propagation was found [11] to follow Snell’s
!∥
!
law of refraction j k L j⋅sinðη1 Þ ¼ j k SPP j⋅sinðη2 Þ. As before,
!∥
k L represents the projection of the wave vector of the laser
!
pulses into the surface plane and k SPP describes the wave
vector of the excited SPP wave. The incidence angle of the
projected light vector ðη1 Þ and the “refracted” angle of the SPP
ðη2 Þ are measured with respect to the normal of the excitation
edge of the Ag island. The consequence of Snell’s law under
grazing-incidence conditions is that the Moiré pattern is always
parallel to the excitation edge, independent of the propagation
direction of the SPP.
!∥
In normal incidence, we already discussed that k L ¼ 0; with
the consequence from Snell’s law that η2 must be zero as well:
SPPs always propagate perpendicularly away from the edge at
which they were excited. The polarization of the laser pulses is
then the decisive factor at what edge of a Ag island an SPP is
started. Figure 5 shows a Ag island under normal-incidence
conditions for different in-plane polarizations. In Fig. 5a, the
polarization of the laser is perpendicular to the horizontal edges
of the island ϕ = 0° and is successively turned by Δϕ=30° in
images b–f of Fig. 5. The excitation of SPPs is strongest when
the polarization of the laser is perpendicular to the respective
island edge. Contrarily, SPPs are not excited at edges which are
parallel to the polarization.
In addition to the difference in propagation direction between
grazing incidence and normal incidence, another more technical
Fig. 5 2PPE-PEEM images of a Ag island under normal incidence
illumination. The laser polarization lies within the surface plane and is
successively rotated with respect to the orientation of the Ag island. In
each panel, the electric field direction is indicated by the black doubleheaded arrow. The SPP excitation strength is proportional to the projection of the electric field of the laser onto the excitation edge
Plasmonics
factor makes normal incidence the superior technique: Most of
the plasmonic work is carried out on Ag or Au surfaces. The
Brewster angle for illumination with λ¼ 400 nm light is 64.5°
for Ag [20], and it is 67.7° for Au [20]. As such, the Brewster
angles for these plasmonic materials are very close to the commonly used incidence angles in grazing-incidence setups. The
consequence is a strong dependence of the overall nonlinear
photoemission yield on the polarization of the laser pulses.
Small polarization-dependent contrast differences are thus hidden in a dramatic change of the overall intensity. This problem
does not exist in normal incidence, as the surface reflectivity is
independent of the polarization (for isotropic media).
Conclusions and Outlook
We presented a novel experimental approach for the imaging
of SPPs with nonlinear PEEM. For most imaging cases, the NI
setup provides improved contrast compared to the commonly
used GI geometry. The main ramifications of the differences
between NI and GI imaging are summarized in Fig. 6. The left
column contains sketches of the two illumination geometries,
while the three remaining columns illustrate the differences
Fig. 6 Comparison of GI (upper panels) and NI (lower panels) imaging.
The first column shows a schematic side view of a silver island under
laser illumination (blue) for the specific incidence geometries. The second
column shows the origin of the fringes observed in nonlinear
between NI and GI imaging as discussed above. The upper
row of sketches shows the situation for GI geometry while the
lower images illustrate the NI situation. We find three major
differences for the contrast. First, illustrated in the second
column from the left, the fringe spacing in GI depends on
the laser’s incidence angle relative to the surface normal.
Typically, a fringe spacing of several micrometers is observed
when the SPP and laser pulse (as projected into the surface
plane) propagate in the same direction. In NI, however, the
fringe spacing always directly resembles the SPP wavelength.
Second, illustrated in the second column from the right, the
propagation direction of SPPs is different for NI and GI.
Furthermore, the interpretation of GI contrast is counterintuitive.
For various angles between the projection of the laser pulses
into the surface plane and the excitation edge, the Moiré fringes
will always be parallel to the excitation edge. Different SPP
directions are reflected in a changed fringe spacing of the
observed pattern. The SPP propagation direction must be calculated from the measured Moiré pattern and the known laser
incidence geometry and laser polarization. In NI, the fringes are
still aligned parallel to the excitation edge. We showed, however, that the SPP always propagates in the direction normal to the
excitation edge. This implies that the fringes observed in NI are
photoemission microscopy. The third column illustrates when and why
the fringe pattern is aligned parallel to the island edge. The fourth column
indicates what happens when C60 is deposited onto the silver islands
Plasmonics
a direct descriptive visualization of the phase fronts of the
propagating SPP and that, in NI, the excitation edge can be
freely chosen by turning the polarization of the laser pulses.
The right column in Fig. 6 illustrates the third major difference between NI and GI contrast and discusses an aspect that has
not yet been addressed: while it is in many cases advantageous
that in NI-PEEM the SPP propagation direction is always perpendicular to the excitation edge, it also removes one degree of
freedom from the experiment. Kirschbaum et al. have manipulated the surface dielectric function of Ag by deposition C60 in
GI-PEEM and reported a coverage-dependent change of direction of the SPP relative to the island edge [21]. Using the results
and equation from the referenced work, we calculate that two
monolayers of C60 can change the refracted angle of the SPP
by up to 6°. This allows for sensor device concepts that
depend on a particular SPP propagation direction. Such
surface dielectric function-dependent propagation direction of
SPPs does not occur in NI geometry, since the SPP propagation direction is always normal to the excitation edge.
We conclude that for most investigations of SPP wave packets
the NI illumination geometry is advantageous, since it provides
an easily interpreted high-resolution “snapshot” of the SPP wave.
In particular, for time- and phase-resolved measurements, NI will
provide a rather direct imaging of propagating SPP wave packets
by means of a cross-correlation signal. For other situations,
however, the GI setup may be better suited, since it allows a
coverage-dependent manipulation of the SPP propagation direction. Depending on the scientific question, the laser incidence
geometry for a PEEM experiment must be carefully chosen. Our
work constitutes the basis for such a choice.
Acknowledgments Financial support from the Deutsche
Forschungsgemeinschaft through SFB616 “Energy Dissipation at
Surfaces” and SPP1391 “Ultrafast Nanooptics” is gratefully
acknowledged.
References
1. Ozbay E (2006) Plasmonics: merging photonics and electronics at
nanoscale dimensions. Science 311(5758):189–193
2. MacDonald KF, Samson ZL, Stockman MI, Zheludev NI (2009)
Ultrafast active plasmonics. Nat Photonics 3(1):55–58
3. Schmidt T, Heun S, Slezak J, Diaz J, Prince KC, Lilienkamp G,
Bauer E (1998) Speleem: combining LEEM and spectroscopic imaging. Surf Rev Lett 5:1287–1296
4. Wiemann C, Bayer D, Rohmer M, Aeschlimann M, Bauer M (2007)
Local 2PPE-yield enhancement in a defined periodic silver nanodisk
array. Surf Sci 601(20):4714–4721
5. Chelaru LI, Meyer zu Heringdorf F-J (2007) In situ monitoring of
surface plasmons in single-crystalline Ag-nanowires. Surf Sci 601:
4541–4545
6. Lemke C, Schneider C, Leißner T, Bayer D, Radke JW, Fischer A,
Melchior P, Evlyukhin AB, Chichkov BN, Reinhardt C, Bauer M,
Aeschlimann M (2013) Spatiotemporal characterization of spp pulse
propagation in two-dimensional plasmonic focusing devices. Nano
Lett 13(3):1053–1058
7. Kubo A, Pontius N, Petek H (2007) Femtosecond microscopy of
surface plasmon polariton wave packet evolution at the silver/
vacuum interface. Nano Lett 7:470–475
8. Meyer zu Heringdorf F-J, Buckanie NM (2010) Nonlinear photoemission microscopy with surface plasmon polaritons. Microsc
Microanal 16:502–503
9. Schmidt O, Bauer M, Wiemann C, Porath R, Scharte M,
Andreyev O, Schönhense G, Aeschlimann M (2002) Timeresolved two photon photoemission electron microscopy. Appl
Phys B 74(3):223–227
10. Bauer M, Wiemann C, Lange J, Bayer D, Rohmer M, Aeschlimann
M (2007) Phase propagation of localized surface plasmons probed by
time-resolved photoemission electron microscopy. Appl Phys A
88(3):473–480
11. Buckanie N, Kirschbaum P, Sindermann S, Meyer zu Heringdorf F-J
(2013) Interaction of light and surface plasmon polaritons in Ag islands
studied by nonlinear photoemission microscopy. Ultramicroscopy 130:
49–53
12. Amidror I (2000) The theory of the Moiré phenomenon. Kluwer,
Dordrecht
13. Hermann K (2012) Periodic overlayers and Moiré patterns: theoretical studies of geometric properties. J Phys Condens Matter 24:
314210
14. Lemke C, Leißner T, Jauernik S, Klick A, Fiutowski J, KjelstrupHansen J, Rubahn H-G, Bauer M (2012) Mapping surface plasmon
polariton propagation via counter-propagating light pulses. Opt
Express 20:12877–12884
15. Wall D, Tikhonov S, Sindermann S, Spoddig D, Hassel C, Horn-von
Hoegen M, Meyer zu Heringdorf F-J (2011) Shape, orientation, and
crystalline composition of silver islands on Si(111). IBM J Res Dev
55:9:1–9:6
16. Kury P, Hild R, Thien D, Guenter H-L, Meyer zu Heringdorf F-J,
Horn-von Hoegen M (2005) Compact and transferable threefold
evaporator for molecular beam epitaxy in ultrahigh vacuum. Rev
Sci Instrum 76:83906
17. Meyer zu Heringdorf F-J, Belton A (2004) Flexible
microprocessor-based evaporation controller. Rev Sci Instrum
75(12):5288–5292
18. Meyer zu Heringdorf F-J, Chelaru L, Möllenbeck S, Thien D, Hornvon Hoegen M (2007) Femtosecond photoemission microscopy. Surf
Sci 601:4700–4705
19. Scharte M, Porath R, Ohms T, Aeschlimann M, Krenn JR,
Ditlbacher H, Aussenegg FR, Liebsch A (2001) Do Mie plasmons
have a longer lifetime on resonance than off resonance? Appl Phys B
73(4):305–310
20. Johnson PB, Christy RW (1972) Optical constants of the noble
metals. Phys Rev B 6:4370–4379
21. Kirschbaum P, Buckanie N, Meyer zu Heringdorf F-J (2012)
Impact of C60 adsorption on surface plasmon polaritons on
self-assembled Ag(111) islands on Si(111). Plasmonics 7(2):
229–233