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Active Control of Surface Plasmon Waveguides with a Phase Change
Robert E. Simpson,
and Jan Renger
ICFO - The Institute of Photonic Sciences, Mediterranean Technology Park, Avenida Carl Friedrich Gauss 3, 08860 Castelldefels
Engineering Product Development, Singapore University of Technology and Design, 8 Somapah Road, Singapore 487372, Singapore
ICREA - InstitucióCatalana de Recerca i Estudis Avançats, Passeig Lluís Companys 23, 08010 Barcelona, Spain
Photonics Laboratory, ETH Zürich, Hönggerbergring 64, 8093 Zürich, Switzerland
ABSTRACT: The ability to manipulate light propagation at
the nanoscale is of vital importance for future integrated
photonic circuits. In this work we exploit the high contrast in
the optical properties of the phase change material Ge2Sb2Te5
to control the propagation of surface plasmon polaritons at a
Au/SiO2interface. Using grating couplers, normally incident
light at λ= 1.55 μm is converted into propagating surface
plasmons on a Au waveguide. Single laser pulses (λ= 975 nm)
are applied to a thin ﬁlm of Ge2Sb2Te5placed on top of the
device, which, upon transition from its amorphous to
crystalline structural phase, dramatically increases both its refractive index and absorption coeﬃcient, thus inhibiting propagation
of the plasmonic mode. This eﬀect is investigated for diﬀerent interaction lengths between the phase change material and the Au
waveguide, and contrast values in the transmitted intensity up to several tens of percents are demonstrated.
KEYWORDS: phase change materials, surface plasmon, nanophotonics, nonvolatile, chalcogenides
In recent years there has been an ongoing eﬀort to develop
novel photonic circuits with high processing speed and
robustness against fabrication tolerances. Optical data commu-
nication already outperforms their electronic counterparts in
terms of speed and transmission losses. However, due to the
barrier imposed by the diﬀraction limit of light,
optical components usually have dimensions larger than the
wavelength of light and are quite sensitive to changes in their
geometry arising from fabrication tolerances, thus making it
diﬃcult to achieve both high ﬁeld conﬁnement and robustness.
Surface plasmon polaritons (SPPs) emerged as a promising
candidate for solving these drawbacks of conventional photonic
SPPs are hybrid modes that are bound at the metal−dielectric
interface when a light wave is coupled to the oscillation of the free
electrons present in the metal. These waves are conﬁned to the
interface with an electric ﬁeld that decays exponentially in both
surrounding materials. Thus, the light is strongly conﬁned, which
provides a platform to guide it in compact devices
smaller than conventional optical components, such as electro-
optic modulators or optical ﬁbers.
One of the main challenges of plasmonic-based circuits is
ﬁnding a way to control SPP propagation. This problem has been
tackled by changing the optical properties of the surrounding
environment, for instance by exploiting the electro-optic eﬀect,
using quantum dots,
or by externally pumping photochromic
However, the majority of such proof-of-principle
experiments employ modulator designs that weakly eﬀect the
SPP propagation and therefore exhibit a limited SPP modulation
depth. More recently, photonic switches and modulators using
low-dimensional materials such as graphene have also been
that can achieve high modulation frequencies in
the GHz regime but that require the continuous application of
electric ﬁelds and are more prone to long-term instabilities due to
Here, we use the unique features of the prototypical phase
change material (PCM) Ge2Sb2Te5(GST)
nonvolatile control of SPP propagation in Au waveguides.
PCMs on the GeTe-Sb2Te3pseudobinary tie-line exhibit an
extraordinarily large contrast between their two structural
phases. The covalently bonded amorphous phase of GST
corresponds to a disordered material with short-range atomic
order and low electrical conductivity and optical absorption
(ñamorph = 4.7 + 0.2iat λ= 1.55 μm). In contrast, the resonantly
bonded crystalline phase can be seen as a low-band-gap
semiconductor and exhibits an electrical conductivity that is 3
orders of magnitude greater than the amorphous phase and has a
larger optical absorption (ñcryst =7+2iat λ= 1.55 μm).
large refractive index of the crystalline phase is due to the
Received: February 5, 2015
© XXXX American Chemical Society ADOI: 10.1021/acsphotonics.5b00050
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existence of resonant bonds, which can be easily polarized by an
external electric ﬁeld.
Moreover, switching between the two
phases can be triggered externally by applying electrical or laser
The transition from the amorphous state to the crystalline
state typically requires pulses with a duration of hundreds of
nanoseconds, although picosecond-order crystallization times
have been reported for GST.
While PCMs have been
Figure 1. (a) Schematic of the devices used to probe the propagation length of SPPs in Au/PCM hybrid waveguides. (b) Intensity images of three
devices with diﬀerent values of LG(160, 100, and 80 μm) with the GST in the amorphous phase. (c) Logarithm of the normalized intensity at the output
port as a function of the distance between gratings with the GST in the amorphous (black squares) and crystalline (red circles) phases. The output value
is normalized by the intensity scattered at a single groove (A) placed at a ﬁxed distance from the input grating.
Figure 2. (a) Schematic of the cross section in the interaction region (not to scale). (b) Optical microscope image of the plasmonic waveguide with a
GST strip in the middle. The insets show the GST area before and after crystallization using a single laser pulse. (c) Principle of operation of the device. A
probe laser at λ= 1.55 μm is used to convert free space light into propagating SPPs that interact with an 80 nm GST ﬁlm. An independent pump laser at λ
= 975 nm is used to trigger the amorphous to crystalline phase transition in this ﬁlm.
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extensively studied for their application in optical data storage
and phase change random access memories (PCRAM),
these properties also make them a promising candidate to control
and manipulate light in photonic circuits in a nonvolatile manner,
an application area that has started to receive an increasing
amount of attention.
Indeed GST’s large change in optical properties, fast switching
ability, high cyclability, and ability to retain its structural phase for
years without any input energy make it ideal for programming
reconﬁgurable photonic circuits. Herein, we experimentally
demonstrate and simulate the use of GST to perform nonvolatile
switching of telecom frequency SPPs in plasmonic waveguides.
The change in GST’s optical properties upon phase transition
can strongly aﬀect the propagation length of the plasmonic mode
and hence allows for a change in SPP transmission because of the
diﬀerent attenuation for the amorphous and crystalline phase.
Figure 1a illustrates the geometry employed to measure the
diﬀerence in the propagation length in the Au/SiO2/GST/
PMMA/air waveguides for the two phases. Here, the switch is
achieved by the phase change in the 30 nm thin GST layer, which
was separated by a SiO2layer of roughly 150 nm from Au and
covered by a 2 μm thick PMMA layer. The PMMA layer was used
to tune the number and properties of the waveguide mode(s)
and the SiO2to thermally isolate the GST from the Au as well as
to optimize the sensitivity to the phase transition without
substantially sacriﬁcing SPP propagation too much, as explained
in the Supporting Information (Figures S1 to S4). The diﬀerence
of the real part of the mode’s propagation constant (cf. Figure
S2) enables the selective excitation of the SPP mode using the
appropriate spacing of the grooves of the coupling grating in the
metal. Additionally, a small groove-like line defect placed 20 μm
away from the excitation grating (marked by the label A in Figure
1b) is used to probe the SPP intensity, which allows for
normalization and elimination of changes in the incoupling
eﬃciency. Similarly, line defects and outcoupling gratings have
been engineered at diﬀerent LG. The resulting transmission is
measured by imaging the light at λ= 1550 nm by an InGaAs
camera, as shown in Figure 1b. Figure 1c shows the logarithm of
the intensity at the output port normalized by the input
scattering line defect at point A obtained from the intensity
images at various distances LG. The same measurement is
repeated for crystallized GST areas. As shown in the graph in
Figure 1c, the intensity along the waveguide decays exponentially
for both cases. In the case of crystalline GST, the decay is
stronger, which originates from the increased absorption in the
GST layer. For distances greater than 80 μm the losses are too
high and no transmitted intensity could be measured. This strong
diﬀerence in the SPP waveguide-mode decay length can be used
to control optical signals at customized contrast simply by
choosing the adequate length of the active region.
The design of our nonvolatile plasmonic switch, sketched in
Figure 2a and c, consists of two grating couplers to convert
propagating waves into waveguide modes or SPPs at the input
port and vice versa at the output port. The two ports are
connected by a narrow (2 μm) SPP waveguide made of gold, as
illustrated in Figure 2b. Additionally, part of the SPP waveguide is
covered by the active PCM material. This allows to control the
transmitted intensity by changing from the low-loss amorphous
phase to the crystalline phase, which has higher propagation
losses. The same scheme can be used to electrically drive the
phase transition in the GST layer using the tapered design with
two electrodes. This design increases the electrical current
density at the region where the GST and the Au waveguide cross
over, thus providing a hybrid platform to optically or electrically
Figure 3. (a, b) Intensity images of the scattered and transmitted laser light at λ= 1.5 μm as seen by the imaging camera (the dashed lines are an outline
of the actual device). The intense spot on the left side corresponds to the focus placed on the in-coupling grating converting propagating light into
waveguide modes and SPP modes. The light propagates along the waveguide nearly radiationless and is out-coupled and detected. The intensity at the
out-coupling grating changes strongly upon changing the GST phase, leading to a strong diﬀerential signal (c).
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The devices shown in Figure 2b have been fabricated using
multiple lithographic steps. The metallic pads containing the
gratings and waveguide parts have been structured by ﬁrst
employing a positive tone e-beam resist. Subsequently, 30 nm
thick Ti and then 60 nm thick Au ﬁlms are deposited by thermal
evaporation. The Ti layer serves as an adhesion layer and
suppresses the coupling of SPPs at the lower interface between
the Au and the SiO2substrate, which would otherwise contribute
to the transmitted intensity. After lifting-oﬀthe residual
photoresist at the areas unexposed by the electron beam, a top
layer of 150 nm of SiO2was radio frequency (RF) sputtered from
a high-purity target in an Ar atmosphere of 0.5 Pa. The second
lithographic step deﬁnes the GST strips in the center of the
waveguides, where we induce the phase change. The 80 nm thick
GST ﬁlms were deposited by RF co-sputtering from stoichio-
metric GeTe and Sb2Te3targets. Energy dispersive X-ray (EDX)
measurements performed on GST ﬁlms that were prepared
under the same conditions veriﬁed that the composition was
equal to the stoichiometric Ge2Sb2Te5, with an error smaller than
5 at. %. Additionally, X-ray diﬀraction (XRD) measurements
conﬁrmed the as-deposited ﬁlms to be in the amorphous phase.
Finally a 20 nm thick capping layer of Si3N4was deposited by dc
sputtering from a Si target in a reactive Ar:N2atmosphere at 0.5
Pa. The Si3N4cap is necessary to protect the GST from oxidation
and to conﬁne heat in the structure during the switching process.
In a ﬁnal step, the remaining resist was removed and the whole
device was covered with a 2 μm layer of PMMA.
The optical micrograph of a device employing a 2 μm wide
SPP waveguide is shown in Figure 2b. The in- and out-coupling
gratings have been placed on larger gold pads to reduce the direct
transmission of the incident probe laser and to funnel the SPPs to
the narrow SPP waveguide.
The light-to-SPP conversion can
be controlled using the number of grooves and their shape.
Linearly polarized light (λ= 1.55 μm) from a laser diode is
focused using a 20×objective onto the in-coupling grating on the
left in Figure 3. The grating periodicity, G=1μm, is designed to
couple the incident light, which has no in-plane momentum, into
SPPs by fulﬁlling the condition kSPP =2π/iG, where iis an
The actual value of kSPP was found by numerically
solving the eigenvalue problem for the stratiﬁed media using
The out-coupling grating on the right is used to
convert the transmitted SPPs into far-ﬁeld radiation, where it is
collected using a 50×objective and imaged using an InGaAs
camera, as shown in Figure 3. The SPP waveguide width is 2 μm
and its length is 25 μm, which is smaller than the SPP
propagation length for Au at λ= 1.55 μm(ΛSPP ≈75 μm for Au
embedded in a dielectric slab).
The principle of operation is as follows. A 5 μm wide GST strip
is structured on top of the SPP waveguide and separated by a
SiO2layer in order to force the SPP to sense the presence of the
GST. The thickness of the SiO2ﬁlm (150 nm) is chosen to obtain
a strong mode overlap with the 80 nm GST ﬁlm, to improve heat
trapping in the GST layer, and to prevent chemical reactions
between the Au and the GST. In the initial amorphous state, the
ﬁlm has low absorption and behaves like a dielectric, allowing the
SPP to propagate along the waveguide, while in the crystalline
state the SPPs are reﬂected and attenuated due to the strong
absorption present in this layer. The structural phase transition in
GST is triggered using a control laser (λ= 975 nm) focused on
the GST area down to a spot size of 4 μm using the same
objective as the probe laser. The focused laser spot size is larger
than the GST-Au crossover area to ensure that the GST ﬁlm
which is directly above the SPP waveguidecompletely
crystallizes for widths below 4 μm, as can be seen in the inset
of Figure 2b. By applying single laser pulses (tfwhm = 300 ns, trise =
80 ns, P= 23 mW), the temperature inside the ﬁlm increases
above the crystallization temperature of the PCM, which for this
heating rate is expected to be Tcryst > 625 K,
polycrystalline region is obtained, corresponding to the brighter
area in the inset in image Figure 2b due to the increased refractive
index and absorption. At λ= 975 nm the absorption of GST is
high (k= 1) and the penetration depth of the laser at this
wavelength is approximately damor = 77 nm, which is smaller than
the ﬁlm thickness. Thus, most of the incident laser power will be
absorbed by the 80 nm thick ﬁlm, which will then crystallize.
Although not shown in this work, for a full switching cycle a
remorphization process is also present, which is much more
diﬃcult to achieve. In this case the penetration depth of the laser
in the crystalline GST is dcryst =40nm(k=2atλ= 975 nm), and
even more energy will be absorbed. However, much higher
temperatures and cooling rates are necessary in order to melt the
crystalline phase and quench the resulting liquid state to obtain
the amorphous phase, and this could be achieved using the Au
waveguide as a heat-sinking structure to rapidly cool the molten
The transmitted λ= 1.55 μm light before and after the GST
phase change is shown in Figure 3a and b. After irradiating the
GST, the transmitted signal is visibly and quantiﬁably less
intense, as shown in Figure 3b. To quantify the contrast due to
the GST phase change, we integrated the intensity in the
rectangular area on top of the out-coupling grating, as indicated
by the white rectangle in Figure 3a and b for the amorphous
phase Iaand after crystallization Icrys. The data have been
background corrected by measuring the thermal and readout
noise in the close vicinity of the grating, as indicated by the yellow
rectangle in Figure 3a and b, resulting in values for Ibg
the insertion loss of the device depends only on the coupling
eﬃciency of the input grating and is thus independent of the
phase of the GST ﬁlm, these values can be used to calculate the
contrast of the device, which is given by
The strong SPP contrast by the GST phase transition is readily
visualized by taking the diﬀerence of the camera images as shown
in Figure 3c. The majority of the image pixels (nearly) cancels,
and a large diﬀerence in the intensity is found only at the position
of the out-coupling grating, which supports our intention to
inhibit SPP propagation by inducing a phase change in the GST
layer. In this case the contrast of the device is approximately 31%.
The experimental error, obtained by calculating the diﬀerence in
the average intensity of the same image taken at two diﬀerent
times, is smaller than 0.5% for both phases.
Using a ﬂuence of 1.8 W m−2, the total energy required for the
switching process is 6.9 nJ, which is comparable to other
Other approaches to switch/modulate
SPPs have been shown to need less energy;
schemes are not nonvolatile and require a continuous external
stimulus, thus making it diﬃcult to design reconﬁgurable
For future modulating devices based on the same scheme it
would be necessary to perform several crystallization/ream-
orphization cycles. Typically for optical data storage applications
GST can withstand 105cycles, while for electrical data storage the
cyclability is even higher (107cycles).
Interestingly, the number
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of cycles increases with decreasing energy required to trigger the
The GST volumes used in this work are huge
(0.08−0.8 μm3), and we expect cycleabilities smaller than 105
cycles. With regard to the maximum frequency achievable by the
device a full modulation cycle would include both an
amorphous/crystalline and a crystalline/amorphous transition.
The minimum time required to trigger these transitions will
ultimately limit the modulation speed of the device. In our
devices crystallization is achieved using 300 ns pulses. Ream-
orphization of GST usually requires fast quench rates on the
order of 109K/s; thus cooling the material from the melting
temperatue (1000 K) to room temperature (300 K) would take
approximately 700 ns. A full modulation cycle could be achieved
in approximately 1 μs, obtaining a modulation frequency of 1
The contrast is based on the fact that the SPPs have
signiﬁcantly diﬀerent absorption lengths for the two GST
phases, which allows one to engineer the contrast. In order to
evaluate the inﬂuence of the GST area on the performance of the
device, the same experiment was repeated using diﬀerent GST
widths (0.5, 1, 1.5, 2, 2.5, and 3 μm) for a ﬁxed Au waveguide
width of 2 μm (Figure 4) . For each PCM length the
measurement has been performed on three diﬀerent devices.
The experimental points in Figure 4 indicate the mean value of
the contrast, and the error bars its standard deviation. For the
wide GST strip (w=5μm), full crystallization could not be
achieved due to the spot size of the laser being smaller than the
overlap area, obtaining an experimental contrast smaller than the
simulated one. As expected, the contrast increases for wider
active regions of GST due to a higher interaction length, which
leads to larger cumulative absorption of the light propagating in
the SPP waveguide mode.
In conclusion, we have demonstrated inhibition of SPP
propagation in a Au/SiO2interface by exploiting the high
contrast in the optical properties of the phase change material
Ge2Sb2Te5. The attenuation in the transmitted intensity strongly
depends on the interaction length between the Au waveguide and
the GST ﬁlm, obtaining a contrast of more than 30% for
suﬃciently wide GST strips, using low switching energies to
inhibit SPP propagation in a nonvolatile manner. The results
demonstrate that the combination of surface plasmon polaritons
with phase change materials, such as GST, allows for designing
and fabrication of novel active devices such as plasmonic switches
or reconﬁgurable optical circuits.
Finally, we expect that larger contrasts could be achieved either
by using larger overlap areas or working at other wavelengths
where GST exhibits even higher absorption and contrast, while
an improved thermal design could reduce the crystallization time
from hundreds of nanoseconds down to the picosecond time
scale, thus reducing the switching energy. Moreover, smaller
devices that incorporate highly eﬃcient phase change material
structures and new switching paradigms, such as coherent
could present a viable way to reduce the
amount of energy deposited in the structure and extend the
cycling endurance of the device.
Simulation of the characteristic eigenmodes in the plasmonic
waveguide for the armophous and crystalline phases of GST.
Inﬂuence of the SiO2and PMMA thicknesses in the propagation
constants of the diﬀerent modes. The Supporting Information is
available free of charge on the ACS Publications website at DOI:
The authors declare no competing ﬁnancial interest.
We acknowledge ﬁnancial support from the Spanish Ministry of
Economy and Competitiveness (MINECO) and the Fondo
Europeo de Desarrollo Regional (FEDER) through grant
TEC2013-46168-R, the European Research Council through
grant 259196 (PLASMOLIGHT), and FundacióPrivada
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