of 10
Frequency tunable near-infrared metamaterials
based on VO
2
phase transition
Matthew J. Dicken
1*‡
, Koray Aydin
1*
, Imogen M. Pryce
1*
, Luke A. Sweatlock
2
, Elizabeth
M. Boyd
1
, Sameer Walavalkar
1
, James Ma
2
, and Harry A. Atwater
1†
1
California Institute of Technology, 1200 E. Califo
rnia Boulevard, Pasadena, California, USA
2
Northrop Grumman Aerospace Systems, One Space Park
, Redondo Beach, California, USA
haa@caltech.edu
Abstract:
Engineering metamaterials with tunable resonances
from mid-
infrared to near-infrared wavelengths could have fa
r-reaching consequences
for chip based optical devices, active filters, mod
ulators, and sensors.
Utilizing the metal-insulator phase transition in v
anadium oxide (VO
2
), we
demonstrate frequency-tunable metamaterials in the
near-IR range, from 1.5
- 5 microns. Arrays of Ag split ring resonators (SR
Rs) are patterned with e-
beam lithography onto planar VO
2
and etched via reactive ion etching to
yield Ag/VO
2
hybrid SRRs. FTIR reflection data and FDTD simulat
ion
results show the resonant peak position red shifts
upon heating above the
phase transition temperature. We also show that, by
including coupling
elements in the design of these hybrid Ag/VO
2
bi-layer structures, we can
achieve resonant peak position tuning of up to 110
nm.
© 2009 Optical Society of America
OCIS codes:
(160.3918) Metamaterials; (310.6845) Thin film dev
ices and applications
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1. Introduction
Electromagnetic resonances in subwavelength metalli
c resonators can be used to engineer
optical responses not found in natural materials [1
,2]. Metamaterials have reemerged
following their initial introduction [3], and are t
he subject of intensive investigation for
applications in frequency selective surfaces [4,5]
and transformation optics, e.g. negative-
index materials [6–9], super-lensing [10,11], and c
loaking [12–14]. Active metamaterials
have been demonstrated at terahertz frequencies usi
ng carrier depletion in GaAs [15,16],
photoexcitation of free charge carries in Si [17],
and the metal-insulator phase transition in
vanadium oxide thin films [18]. Typical metamateria
ls consist of arrays of metal structures
embedded in a dielectric with feature sizes much sm
aller than the desired operating
wavelength. In the simplest sense, the unit cell of
a metamaterial array is designed to be an
individual “LC” circuit with resonant frequency,
ω
o
~(LC)
-1/2
[4]. Split-ring resonators (SRRs)
are the basis for many metamaterial designs due to
ease of fabrication and modeling. Each
SRR has a distributed inductance, L, and a distribu
ted capacitance, C, arising from charge
build-up at the notch. The choice of materials and
the resonator dimensions set these two
parameters and determine the resonant frequency of
the metamaterial. Active metamaterial
designs have previously focused on changing the cap
acitance in the SRR gap to modulate the
amplitude of the resonance [15–17]. Integrating mat
erials with tunable electrical or optical
properties allows further control over the resonant
response in metamaterials. Vanadium
oxide is a promising candidate that exhibits a dram
atic change in its complex refractive index
arising from a structural phase transition from mon
oclinic to rutile. Here, we propose an
alternative geometry consisting of self-aligned, hy
brid Ag/VO
2
SRR bi-layers as an approach
to tuning the metamaterial response in the near-IR
by controlling the resonator geometry with
the phase transition. VO
2
undergoes a structural transition from an insulati
ng monoclinic
phase to a metallic rutile phase at 68 °C [19,20].
This phase transition can occur on a sub-
picosecond timescale [21] and can be induced therma
lly, optically [22] or electrically [23]. As
a result of the insulator-to-metal transition, the
conductivity increases by as much as four
orders of magnitude and the optical transmission in
the near-IR decreases significantly [24].
Drastic changes in the optical properties of VO
2
with the phase transition enable control over
(C) 2009 OSA
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#115088 - $15.00 USD
Received 11 Aug 2009; revised 17 Sep 2009; accepted 18 Sep 2009; published 25 Sep 2009
the transmission and reflection properties of nanop
hotonic structures, such as nanoparticles
[24,25], hole arrays [26], and metamaterials [18].
Fig. 1. Hybrid split-ring resonator metamaterials b
ased on vanadium oxide. (a) Schematic of a
self-aligned silver/vanadium oxide metamaterial uni
t cell. The 150 nm thick silver SRR is
fabricated by e-beam patterning and metal lift-off.
70 X 70

m arrays of these structures sit on
60 nm of VO
2
. A 10 nm chromium mask protects the silver SRR dur
ing etching. (b-d) SEM
images of the three coupled-SRR metamaterial arrays
studied in this work. These designs yield
multiple resonances in the near-IR spectrum.
2. Ag/VO
2
active metamaterials: simulations and experiments
2.1 Unit cell design
The active metamaterial device design presented her
e consists of self-aligned Ag/VO
2
SRR
arrays (Fig. 1a). The VO
2
thin films are grown epitaxially by pulsed laser d
eposition on c-
plane Al
2
O
3
substrates at 500 °C. A vanadium metal target is u
sed as the source material and
deposition takes place in 12 mTorr oxygen. 60 nm th
ick VO
2
films are deposited with a 300
mJ laser pulse at a rate of 10 Hz. Utilizing the in
sulator-metal phase transition in VO
2
, we can
control the effective dimensions of the unit cell a
s well as the optical properties of the hybrid-
SRR arrays in both phases. Coupled asymmetric split
-ring resonators are employed to reduce
the spectral linewidth at the resonance frequency (
Fig. 1b-d). A narrower resonant peak will
increase the tuning figure of merit (FOM), the rati
o of the tuning range to the full width at half
maximum (FWHM) of the resonant peak. In this work,
we investigate four self-aligned
hybrid-SRR structures including two coupled SRR sys
tems and one SRR coupled to a
nanowire.
(C) 2009 OSA
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Received 11 Aug 2009; revised 17 Sep 2009; accepted 18 Sep 2009; published 25 Sep 2009
Fig. 2. (a). Near-IR reflection spectra of 150 nm t
hick Ag metamaterial arrays on 60 nm VO
2
thin films. As the temperature is increased above t
he insulator-metal transition temperature of
VO
2
, the near-IR spectral reflection properties of the
SRR array become similar to those of a
non-patterned metal-phase VO
2
film (inset). (b) Simulated reflection spectra for
the same
structure using FDTD. Magnetic field intensity, |H
z
|
2
plots at the resonant peak position, show
the resonance disappear as the structures become el
ectrically shorted by the metal-phase VO
2
(inset). (c) Variable angle spectroscopic ellipsome
try is used to model the complex optical
constants of VO
2
thin films. Multi-oscillator ellipsometric models
were used to extrapolate to
the near and mid-IR to aid in FDTD simulations.
2.2 Active Ag SRR arrays on planar VO
2
substrates
The metamaterial arrays are fabricated on a 60 nm t
hick planar VO
2
film using standard
electron beam lithography and evaporation of 150 nm
of silver and 10 nm of chromium to act
as an etch mask. SRR arrays are tested using the re
flection mode of an FTIR microscope
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using a light source in the range of 1.5 – 8 micron
s. Reflection data were normalized using an
optically thick gold standard. Electromagnetic radi
ation, normally incident to the SRR array
(along the
z-
direction) with the incident E-field polarized perp
endicular to the SRR arms, was
used to efficiently couple to the resonators and in
duce circulating currents in the structures
(Fig. 1a). Two resonant reflection peaks, 2.1 μ m (e
lectric resonance) and 3.6 μ m (magnetic
“LC” resonance), were observed as shown in Fig. 2a.
The sample was subsequently heated to
temperatures above the insulator-metal phase transi
tion temperature and then cooled using a
commercial heating stage comprised of a silver bloc
k with a tiny aperture milled in the center.
The resulting spectra show a continuous change from
two distinct metamaterial resonant
peaks to a broad reflection indicative of metallic
VO
2
. Although there is a slight hysteresis,
the resonances are recovered upon cooling below the
phase transition temperature. We also
measured the reflection from a 60 nm thick VO
2
film on sapphire substrate at room
temperature and 85 °C. The reflection intensity at
3.6 μ m increased from 0.36 to 0.75,
indicating a drastic change in complex refractive i
ndex of the VO
2
thin film upon phase
change (Inset of Fig. 2a).
The resonant response was modeled using Lumerical,
a commercially available finite-
difference time-domain (FDTD) simulation software.
A unit cell of the investigated structure
is simulated using periodic boundary conditions alo
ng the
x
and
y
axes and perfectly matched
layers along the axis of propagation of the electro
magnetic waves (
z
axis). A broadband plane
wave is incident on the unit cell along +
z
direction, and reflection is monitored by a power
monitor that is placed behind the radiation source.
Electric and magnetic fields are monitored
by frequency profile monitors. Figure 2b shows the
simulated response of an Ag SRR array
on a planar VO
2
thin film in both the insulator and metallic phase
s. To accurately simulate the
metamaterial response, we measured the complex refr
active index of the VO
2
thin films in
both phases (Fig. 2c) using variable angle spectros
copic ellipsometry. In the insulator phase,
the VO
2
optical constants are fit to a Tauc-Lorentz model.
In the metal phase, VO
2
acts like a
Drude metal with strong absorption. These models we
re used to extrapolate the results to the
near and mid-IR to aid in FDTD simulations. The sim
ulated reflection spectra in both VO
2
phases agree well with the measured data.
We calculated the magnetic field intensity, |H
z
|
2
at
λ
= 3.6

m (inset of Fig. 2b) for the
SRR array in the insulator (right) and metallic pha
ses (left) of VO
2
. The resonant behavior in
the insulator phase is evident from the magnetic fi
eld profile, in which the magnetic field is
concentrated at the center of SRR due to the excita
tion of circular surface currents at the
resonance frequency. However, in the metallic VO
2
phase the LC circuit of each SRR is
shorted and the array no longer resonates. Although
the resonant behavior of the metamaterial
can be switched with the phase transition of a VO
2
thin film, this switch is essentially
bimodal. In the metallic phase the sample is highly
reflecting and behaves like a mirror at IR
wavelengths, meaning that the spectral selectivity
required for practical applications is not
achieved. To overcome this problem, we propose a se
lf-aligned SRR design composed of a
stack of Ag and VO
2
layers, whose resonant frequency could be modulate
d thermally.
2.3 Active Ag/VO
2
hybrid-SRR metamaterials
The samples are etched using reactive ion etching t
o yield self-aligned hybrid-SRR arrays.
The 100 Watt CF
4
/O
2
plasma etches the VO
2
film not masked by the Cr/Ag SRR structures to
yield hybrid Ag/VO
2
SRR elements. Figure 3a shows experimental reflect
ion spectra for self-
aligned Ag/VO
2
SRR structures with dimensions of 450 × 400 nm and
150 nm width SRR
arms (inset). The resonant reflection peak is obser
ved at 2.52 μ m, compared to 3.6 μ m before
etching. The blue shifting of the metamaterial reso
nance is a direct effect of replacing much
of the underlying high dielectric constant VO
2
substrate with air. Upon heating above the
insulator-metal phase transition temperature to 100
°C, the resonant reflection response of the
new hybrid-SRR structure red shifts by
= 100 nm to 2.62 μ m. A schematic of the self-
aligned hybrid-SRR unit cell is shown in Fig. 1a.
(C) 2009 OSA
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Received 11 Aug 2009; revised 17 Sep 2009; accepted 18 Sep 2009; published 25 Sep 2009
Fig. 3. Active self-aligned hybrid-SRR arrays. (a)
Near-IR reflection spectra of self-aligned
150 nm Ag / 60 nm VO
2
hybrid-SRR arrays. The magnetic resonant peak shif
ts by 100 nm
upon heating as the VO
2
undergoes a phase transition. (b) FDTD simulations
of the self-
aligned hybrid-SRR array. Simulations show a resona
nt peak shift of the same magnitude as
experiment. A bi-layer VO
2
model (15 nm thick metal phase, 45 nm thick insula
tor phase) is
used to predict the experimental results (Inset) Si
mulated field intensity profiles (|H
z
|
2
, and
|E
x
|
2
) at
λ
= 2.5

m for hybrid metamaterials show resonances both for
insulating and metallic
VO
2
phases.
Simulations of the metamaterial array show a simila
r red shift (
100 nm) with an
associated overall decrease in the reflection due t
o the increased absorption in the VO
2
metallic phase (Fig. 3b, blue line). However, this
initial simulation result does not agree well
with the measurements, in which the reflection peak
intensity is not drastically reduced.
Additional simulations were performed to investigat
e the behavior observed in the
experiment. The experimental results suggest that t
he nanostructured VO
2
is not a
homogeneous metallic rutile phase, but represents a
nanocomposite phase where the
semiconductor and metallic phases co-exist. Other r
esearchers have reported that the VO
2
phase transition occurs via nanoscale domain switch
ing rather than a bulk homogeneous
phase transformation [27], and the phase transition
and optical properties of nanoscale VO
2
structures have been shown to differ from those see
n in continuous thin films [24]. Given that
the actual switching mechanism is likely inhomogene
ous for patterned nanoscale structures,
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and that characterization of the actual domain stru
cture is non-trivial, we employed the
simplest model possible that provides good quantita
tive agreement with experimental
observations: we assume that the heated 60 nm thick
VO
2
film is a bilayer composed of a 15
nm thick VO
2
film in the metallic phase at the Ag/VO
2
interface and a 45 nm thick VO
2
film
in the semiconductor phase at the VO
2
/sapphire interface (Fig. 3b, light red line). The
increase
in peak intensity for the bilayer model approaches
to that of observed in the experimental
data. Based on the experiments and simulations, we
conclude that the optical thickness of the
VO
2
layer is different than the geometric thickness. I
t is also important to note that the optical
properties of VO
2
films in both phases were measured up to 2.2 μ m, a
bove which we
extrapolated the complex refractive index data by f
itting to the ellipsometric models as shown
in Fig. 2c.
Simulated magnetic and electric field intensity plo
ts (|H
z
|
2
, and |E
x
|
2
) of the self-aligned
SRR structure (
λ
= 2.5

m) for both the insulator and metallic VO
2
phases are shown as insets
in Fig. 3b. The resonant behavior is evident in the
field profiles. At the resonant wavelengths,
the electric field is strongly localized between th
e edges of the SRR arms due to the circular
currents on the SRR. Since VO
2
has higher losses in the metallic phase, the calcu
lated
intensities of the magnetic and electric fields are
lower compared to those in the insulator
phase. When the active material is in the insulator
phase, each unit cell of the metamaterial
array acts as a 150 nm thick Ag SRR on a 60 nm thic
k VO
2
substrate. Upon heating above the
phase transition temperature, the effective optical
thickness of the resonator element changes
due to the phase transition which causes the effect
ive capacitance and inductance of the SRR
element to increase and the resonant frequency to d
ecrease. The proposed hybrid metal/VO
2
design is applicable to any resonator element in wh
ich the resonant behavior depends on the
effective optical thickness.
2.4 Active coupled asymmetric Ag/VO
2
hybrid-SRR metamaterials
Frequency selective metamaterial surfaces could be
used as modulators, filters and sensors at
IR wavelengths. For practical applications, it is d
esirable to tune the resonant wavelength by a
line-width of the resonance peak (FWHM). As seen in
Fig. 3a, the resonance line-width of an
ordinary SRR array is fairly broad (FWHM
1.25 μ m) and
100 nm tuning is relatively
small compared to the resonance bandwidth. In order
to increase the FOM, the ratio of
to
the FWHM of the resonant peak, one could either inc
rease the wavelength tuning range (
)
or decrease the line width of the resonance. Here,
we demonstrate spectral line width
narrowing of metamaterial resonances by introducing
coupled asymmetric SRRs at near-IR
wavelengths.
Figure 4 shows the experimental reflection spectra
for coupled, self-aligned, Ag/VO
2
hybrid-SRR structures in the near-IR range from 1.5
– 5.0 microns. First, a SRR structure
coupled to a nearby nanowire (Fig. 1b) is investiga
ted in Fig. 4a. We observe two resonant
reflection peaks,
λ
1
and
λ
2
, that can be attributed to electrical and magnetic
resonances,
respectively. Magnetic and electric field intensiti
es in the insulator phase are plotted in the
inset. The reflection peak at
λ
1
= 1.8

m is due to the electric dipole resonance of the
nanowire. The magnetic fields are localized around
the nanowire due to the surface current
flowing parallel the nanowire. At the electric dipo
le resonance wavelength, simulations
predict strong localization of electric fields at t
he ends of the nanowire. The resonant
wavelength,
λ
1
, does not change when the VO
2
switches phase. The wavelength of the
nanowire dipole electric resonance is determined by
the nanowire length and does not depend
significantly on the thickness of the metal.
(C) 2009 OSA
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Fig. 4. Active coupled asymmetric Ag/VO
2
hybrid-SRR metamaterials (a) Electric and
magnetic resonances are seen in the SRR coupled to
a bar. The magnetic resonance at 3.0

m
shifted by 100 nm to 3.1

m. Simulated field plots (inset) at the resonant wa
velengths show
|H
z
|
2
(top) and |E
x
|
2
(bottom) of the self-aligned structures on insulat
ing VO
2
. (b) Face-to-face
coupled structures show an enhanced resonance at th
e shorter wavelength,
λ
1
= 2.1

m, and
decreased reflection resonance at
λ
2
= 3.1

m. (c) Back-to-back coupled structures have
magnetic resonant reflection peaks at
λ
1
= 2.3

m and
λ
2
= 2.8

m which shift by 65 nm upon
VO
2
switching.
The resonant peak at the higher wavelength,
λ
2
= 3.0

m, is due to the magnetic
resonance, as evident by the characteristic magneti
c resonance field responses, (i) large
magnetic response at the center of SRR and (ii) str
ong electric field localization at the SRR
gap. The magnetic resonance wavelength is red-shift
ed by
2
= 100 nm as the VO
2
changes
(C) 2009 OSA
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phase from insulator to metal. Introducing nanowire
s as an additional element to the
metamaterial unit cell resulted in narrower resonan
ce peaks than those observed for ordinary
SRR arrays. The FWHM of the resonance peaks at 1.8
and 3.0

m were measured to be 0.5
and 0.7

m, respectively. The FOM of ordinary SRR array was
0.08, whereas the FOM for
the nanowire coupled SRRs is 0.14, due to the spect
ral line-width narrowing of the
metamaterial resonance.
Breaking the structural symmetry of metamaterials h
as been shown to yield narrower
metamaterial resonances at microwave frequencies [2
8]. Coupling a nanowire to a single SRR
is a way to introduce asymmetry to the resonant ele
ments. Another approach is to couple two
SRRs of different sizes (asymmetric SRRs) by proper
ly designing the unit cell. Figure 4b
shows the reflection spectra of two face-to-face SR
Rs of different dimensions (Fig. 1c) in the
insulator and metallic phases. The resonant peaks f
or these SRRs are
λ
1
= 2.1

m and
λ
2
= 3.1

m. Simulated field plots (inset) show the signal at
λ
1
is due to resonances in both SRRs,
whereas the signal at
λ
2
is due only to the SRR with longer arms. In the ca
se of coupled SRR
arrays, the shorter wavelength response is due to t
he SRR with shorter arms. The total shift
upon phase change in the VO
2
is
2
= 110 nm. Figure 4c shows the reflection spectra f
or an
array of back-to-back coupled SRRs (Fig. 1d) with t
he same dimensions as the face-to-face
structures. Coupling the SRRs in this manner narrow
ed the wavelength separation of the two
resonant peaks:
λ
1
red shifted by 200 nm to 2.3

m and
λ
2
blue shifted by 300 nm to 2.8

m.
The change in the magnetic resonant reflection peak
s for these coupled structures is
1
=
2
= 65 nm. We observed the hybridization of the reson
ant wavelength of a single SRR,
λ
= 2.52
μ m, to shorter and longer wavelengths,
λ
1
and
λ
2
. Different coupling schemes resulted in
different peak positions in the reflection spectra.
The resonant peak shift depends on the
coupling strength of resonator elements, and in the
case of strong coupling, resonant peaks
show larger shifts [29]. Since the electric fields
are strongly localized at the gap of SRRs, the
coupling of face-to-face SRRs is stronger than back
-to-back coupled SRRs. We observed
larger resonance wavelength separation in the case
of face-to-face SRRs.
3. Summary
Metamaterial resonances can be engineered using dif
ferent coupling and hybridization
mechanisms as reported here. The three geometries i
nvestigated here show reflection
resonances of different magnitudes, at wavelengths
dependant on the size and degree of
coupling between SRRs. In all cases, the magnetic r
esonance reflection peaks of self-aligned
structures red shift as the overall geometry change
s with the VO
2
insulator-metal phase
transition. Here, we proposed a sandwiched VO
2
active layer between an Ag SRR and
sapphire substrate, which caused wavelength tuning
of ~100 nm. Incorporating VO
2
nanostructures at different resonant metamaterial e
lements such as the SRR gap or SRR arms
by two-step electron-beam lithography and processin
g could yield better device performances
and higher tuning ratios. In this work the phase ch
ange was induced thermally simply by
heating the metamaterial array, but optical and ele
ctrical phase changes are also possible in
VO
2
.
In conclusion, we have demonstrated the first activ
e metamaterial at near-IR wavelengths
based on an Ag/VO
2
design allowing us to change the optical geometry
of the resonant
element with the heat induced phase transition of V
O
2
. Resonance hybridization with
additional coupling elements such as nanowires or d
ifferent size SRRs led to narrower
metamaterial resonant peaks, thus increasing the fr
equency-tuning figure of merit.
Acknowledgements
We acknowledge financial support from the National
Science Foundation under Grant DMR
0606472, and the Army Research Office; portions of
this work were performed in facilities
sponsored by the Center for Science and Engineering
of Materials, and NSF MRSEC. We
gratefully acknowledge critical support and infrast
ructure provided for this work by the Kavli
(C) 2009 OSA
28 September 2009 / Vol. 17, No. 20 / OPTICS EXPRESS 18338
#115088 - $15.00 USD
Received 11 Aug 2009; revised 17 Sep 2009; accepted 18 Sep 2009; published 25 Sep 2009
Nanoscience Institute at Caltech. I. M. P. acknowle
dges the support of a National Science
Foundation Graduate Fellowship. We also gratefully
acknowledge the help of Professor
George Rossman for IR facilities and measurements.
* These authors contributed equally to this work.
‡ Present address: Booz Allen Hamilton, Arlington V
A, 22203.
(C) 2009 OSA
28 September 2009 / Vol. 17, No. 20 / OPTICS EXPRESS 18339
#115088 - $15.00 USD
Received 11 Aug 2009; revised 17 Sep 2009; accepted 18 Sep 2009; published 25 Sep 2009