# SURFACE PLASMON PROPAGATION IMAGING
## BY A PHOTON SCANNING TUNNELING MICROSCOPE

F. DE FORNEL<fnref n="1" />*, P. DAWSON<fnref n="2" />, L. SALOMON<fnref n="1" />, B. CLUZEL<fnref n="1" />

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This paper describes the first direct observation of surface plasmon propagation on a metallic thin film deposited on a glass slide, thus facilitating the first direct measurement of the propagation length. This was achieved using photon scanning tunneling microscopy (PSTM). Subsequently, near-field observations of the surface plasmon buried at the metal-glass interface, generated from a discontinuity in the metallic thin film, were reported.

https://doi.org/10.1051/photon/202111332

In the 1990's, near-field optical microscopies took off promising to become a unique tool for exploring optical phenomena at the nanoscale. At that time, the photon scanning tunneling microscope (PSTM) [1,2] which was the analog for photons of the electronic scanning tunneling microscope (STM) demonstrated these capabilities. Using this technique, it became possible to measure the optical near

field in the vicinity of subwavelength structures illuminated under various conditions. Among them, the PSTM has proven to be a powerful tool for the study and the analysis of surface plasmons which are resonant electromagnetic modes associated with the collective oscillation of an electron plasma at the interface between a metal and a dielectric [3]. Indeed, for a thin film of silver deposited on glass, the metal-air surface plasmon excited

in Kretschmann configuration [4] has an extension in air of a few hundred nanometers in the visible range, making it an ideal candidate for optical near-field measurement with a PSTM. In the seminal paper of Dawson et al [5], the propagation of a surface plasmon was shown for the first time and its damping was directly quantified by viewing its propagation length. These results were further completed by the demonstration of the contribution of

<footnote n="1">¹ Laboratoire Interdisciplinaire Carnot de Bourgogne, Université de Bourgogne, 9 Avenue Alain Savary, 21078 Dijon, France</footnote>

<footnote n="2">² Centre for Nanostructured Media, School of Mathematics and Physics, Queen's University, Belfast, BT7 1NN, UK</footnote>

<footnote>*ffornel@u-bourgogne.fr</footnote>
a buried metal-glass plasmon in the
near- field pictures thanks to succes-
sive improvement in near-field mea-
surement techniques.

DIRECT OBSERVATION OF THE
METAL-AIR SURFACE PLASMON

Figure 1 shows the schematic of the
seminal experiment performed at
the University of Bourgogne. A 50 nm
thin silver film was illuminated by two
Helium:Neon lasers in oblique inci-
dence. A first, green one at λ₁=543,5nm
was unfocussed to yield a spot size on

mm² scale on the silver film while the
second one at λ₂ = 632.8 nm was tightly
focused into a ~10 µm diameter spot at
the silver film. The angle of incidence
of the red beam was chosen so that it
excited the silver-air surface plasmon
by phase matching the parallel (to the
sample interface) component of wave-
vector of the incident light to that of the
surface plasmon. In plasmonics this
phase matching technique is well known
as the Kretschmann configuration [4].
The angle of incidence of the green la-
ser was likewise chosen to excite

**Figure 1.** Schematic of the experiment: a silver film evaporated on a glass prism was illuminated by two lasers. The green one acts as a pilot for scanning a tapered optical fiber in the near-field of the silver film while the red one excites the silver-air surface plasmon in Kretschmann configuration.

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surface plasmons at the silver-air interface at that wavelength. An optical probe, obtained by tapering an optical fiber, was brought into the optical near field of the metal film in order to 'collect the surface waves' (i.e. by scattering the surface-bound optical field of the surface plasmons to photons travelling up the tapered optical fiber) while scanning the metal film with help of a home-made stack of piezoelectric tubes.

In this configuration, the near-field probe simultaneously detects signals coming from the two lasers at 632.8 nm and 543.5 nm. A fiber optic coupler then separates the signal in two arms. On one arm, a spectral

**Figure 2.** a) PSTM image of the optical near field intensity of the incident beam, $\lambda_1 = 543.5$ nm, on a simple prism illuminated under total internal reflection. b) PSTM image recorded at $\lambda_2 = 632.8$ nm at the surface plasmon excitation angle for red light on a section of the same prism covered by a 50nm thin silver film. The right side of the image displays a clear exponentially decaying tail explicitly demonstrating the propagation and damping of the surface plasmon. All images are 40 µm x 40 µm.

**Figure 3.** a) Scheme of the experiment. A semi-infinite thin gold film of thickness $h=55$ nm is vacuum deposited onto a glass prism. The illumination is carried out at various incident angles $\theta$, by a collimated laser beam preliminary linearly polarized. The optical signal is collected via the probe and a photomultiplier tube (PMT). b) PSTM image operating in a constant intensity mode for an angle of incidence of 52°.

filter selected the green laser which acts as a pilot for driving a feed-back loop allowing the probe to scan the surface without touching it. For this reason, the green laser spot is large (mm scale) in order to generate a surface intensity distribution on the topside of the silver film that is (near-) constant on the ~10µm scale of the surface plasmon propagation length. The green signal is used as the input signal for the feedback loop of the piezoelectric scanners to allow for scanning the silver film at a constant height. On the second arm, the red laser exciting the surface plasmon is filtered out and then sent to a photomultiplier while the

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This was the first direct observation of the propagation of a surface plasmon on top of a metal film by near-field imaging paving the way to many other achievements and realizations in this field and development of commercial near-field microscopes for studying the optical near-field at nanoscale.

probe is scanning the surface. At 632.8 nm, the typical images reported in [5] are displayed in Fig. 2. It shows the original near-field pictures obtained in the same conditions on a section of the glass slide without the silver film (Fig.2a) and with the silver film (Fig.2b). Without the silver film, the measured laser spot is approximately circular with a ~ 10 µm diameter, while with the silver film the right side of the imaged spot appears elongated with an

exponential decay directly mapping the surface plasmon excitation, propagation and attenuation.

From the image of Fig. 2b) the propagation length was measured to be 13.6µm, which is less than that predicted for a thin silver film of this thickness. The difference was attributed to re-radiation of the surface plasmon back into the prism as it propagates, contamination of the silver surface with a thin, optically

## SURFACE PLASMON POLARITON

A surface plasmon polariton (SPP) is an electromagnetic excitation that propagates in a wave like fashion along the planar interface between a metal and a dielectric medium. Thus, a SPP is a surface electromagnetic wave, whose electromagnetic field is confined to the near vicinity and whose amplitude decays exponentially with increasing distance into each medium from the interface of the dielectric-metal interface. For a film deposited on glass, plasmons at each interface can exist.

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dissipative silver sulphide layer and
a possible angular shift between
the theoretical and actual angle of
incidence. This was the first direct
observation of the propagation of a
surface plasmon on top of a metal
film by near-field imaging paving the
way to many other achievements and
realizations in this field and the de-
velopment of commercial near-field
microscopes for studying the optical
near-field at nanoscale.

However a metallic thin film has
two interfaces and further studies
were then conducted to explore the
surface plasmon buried at the me-
tal-glass interface, which is not ex-
cited in Kretschmann configuration.

BURIED SURFACE
PLASMON OBSERVATION

In this second experiment, a se-
mi-infinite metallic film was de-
posited on glass prism as shown in
Fig. 3. It is noteworthy that here,
gold was preferred to silver to avoid
sulfurization. Under collimated il-
lumination, the edge of the metal
film scatters the incident laser and
generates both the metal-air and
the metal-glass surface plasmons.

First measurements in this configu-
ration were reported in [6] which was
employing the same laser for exciting
surface plasmons as well as for driving
the piezoelectronic scanners through
a feedback loop. In this configuration,
the height of the probe during the scan
was modulated by the detected light
intensity. Recorded near-field images
showed both an exponential decay re-
lated to the propagation and damping
of the gold:air surface plasmon and
but also the presence of oscillations
whose period corresponds to an inter-
ference between the incident light and
the gold:air plasmon. These results
were in very good agreement with nu-
merical simulations. However, these
latter also predicted that interferences
between the glass-to-metal plasmon
and the incident beam should also be
visible in the measurements but was
not observed experimentally. This was
finally achieved by Brissinger *et al.* [7]

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**Figure 4.** Interference periods $\Lambda_1$ in near-field images obtained in the experiment described in Fig.3a by varying the angle of incidence. $\Lambda_1$ and $\Lambda_2$ are associated with the interferences of the incident wave and the plasmonic waves, respectively at the m/a and the m/g interfaces. $\Lambda_3$ is associated with the interferences between both surface plasmon waves and $\Lambda_4$ between the incident wave and the back reflection at the prism/air interface. The solid and dotted lines are the values from the numerical simulations and the crosses are recovered from the near-field optical images.

If the photon scanning
tunneling microscope in
constant optical intensity
mode is no longer used, it
has nevertheless allowed the
direct and first imaging of
the surface plasmon excited
at oblique incidence in
Kretschmann configuration.

many years after, benefiting from
a shear-force feedback for driving
the near-field probe with an impro-
ved reliability.

At that point, the angle of incidence
was tuned from 30° to 60° and both sur-
face plasmons were finally visible in
the near-field images. Interferences
were in very good agreement with
numerical predictions. It should be
noted that the wavector of the buried
surface plasmon, that of the metal:
glass interface, was recovered expe-
rimentally (1.63±0.08) ω/c quantita-
tively compared to its theoretical value
of 1.59ω/c.

CONCLUSION

If the photon scanning tunneling
microscope in constant optical in-
tensity mode is no longer used, it
has nevertheless allowed the direct
and first imaging of the surface plas-
mon excited at oblique incidence in
Kretschmann configuration As such
these experiments have paved the way
to the emergence of plasmonics as
an entire field of research which has
enabled applications in optoelectro-
nics, medicine, chemistry, or sensing
to cite just a few. In addition, current
improvements in the stability of fee-
dback loops, piezoelectric scanners,
nanotechnologies used for shaping
near-field probes, as well as recent
developments in laser technologies
have allowed the near-field micros-
cope to move beyond the walls of the
laboratory and to become a standard
imaging tool for materials science. ●

REFERENCES

[1] R.C. Reddic, R.J. Warmack, T.Ferrel, Phys. Rev. B 39, 767-770 (1989)

[2] D. Courjon, K. Saraeyeddine, M. Spajer, Opt. Commun. 71, 23 (1989)

[3] A. Zayats, I. Smolyaninov, A. Maradudin, Phys. Rep. 408, 131-314 (2005)

[4] E. Kretschman, H. Rather, Z. Naturforsch, A23, 2135 (1968)

[5] P. Dawson et al, Phys. Rev. Lett. 72, 2927 (1994)

[6] L. Salomon et al., Phys. Rev B 65, 125409 (2002)

[7] D. Brissinger et al, Opt. Express 19, 17750 (2011)
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### REFERENCES

[1] J. Reichert et al., Opt. Commun. **172**, 59 (1999)

[2] R. W. P. Drever et al., Appl. Phys. B. **31**, 97 (1983)

[3] E. Oelker et al., Nat. Photonics **13**, 714 (2019)

[4] M. Giunta et al., Nat. Photonics **14**, 44 (2020)