ARTICLE
Dynamically controlled Purcell enhancement of
visible spontaneous emission in a gated plasmonic
heterostructure
Yu-Jung Lu
1,2
, Ruzan Sokhoyan
1
, Wen-Hui Cheng
1
, Ghazaleh Kafaie Shirmanesh
1
, Artur R. Davoyan
1,3,4
,
Ragip A. Pala
1
, Krishnan Thyagarajan
1
& Harry A. Atwater
1,3
Emission control of colloidal quantum dots (QDs) is a cornerstone of modern high-quality
lighting and display technologies. Dynamic emission control of colloidal QDs in an optoe-
lectronic device is usually achieved by changing the optical pump intensity or injection
current density. Here we propose and demonstrate a distinctly different mechanism for the
temporal modulation of QD emission intensity at constant optical pumping rate. Our
mechanism is based on the electrically controlled modulation of the local density of optical
states (LDOS) at the position of the QDs, resulting in the modulation of the QD spontaneous
emission rate, far-
fi
eld emission intensity, and quantum yield. We manipulate the LDOS via
fi
eld effect-induced optical permittivity modulation of an ultrathin titanium nitride (TiN)
fi
lm,
which is incorporated in a gated TiN/SiO
2
/Ag plasmonic heterostructure. The demonstrated
electrical control of the colloidal QD emission provides a new approach for modulating
intensity of light in displays and other optoelectronics.
DOI: 10.1038/s41467-017-01870-0
OPEN
1
Thomas J. Watson Laboratories of Applied Physics, California Institute of Technology, Pasadena, CA 91125, USA.
2
Research Center for Applied Sciences,
Academia Sinica, Taipei 11529, Taiwan.
3
Kavli Nanoscience Institute, California Institute of Technology, Pasadena, CA 91125, USA.
4
Resnick Sustainability
Institute, California Institute of Technology, Pasadena, CA 91125, USA. Yu-Jung Lu and Ruzan Sokhoyan contributed equally to this work. Correspond
ence
and requests for materials should be addressed to H.A.A. (email:
haa@caltech.edu
)
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1
1234567890
T
ailoring light emission of quantum emitters, such as
semiconductor quantum dots (QDs), is a central theme of
nanotechnology
1
. The spontaneous emission decay rate of
a solid state emitter can be modi
fi
ed by coupling the emitter to a
nanostructured environment with a tailored local density of
optical states (LDOS)
2
,
3
. Previously this phenomenon, also
known as the Purcell effect, has yielded a 540-fold increase of the
emission decay rate and a simultaneous 1900-fold increase of
total emission intensity for colloidal QDs coupled to a plasmonic
nanocavity
4
. Typically, the properties of the nanostructured
environment are
fi
xed at the time of fabrication, which also sets
the spontaneous emission decay rate of the emitter. Con-
ventionally, the power radiated from an array of quantum emit-
ters is dynamically modulated (actively controlled) by changing
the optical
5
or electrical
6
pump intensity within a given nanos-
tructured environment. On the other hand, the coupling of
emitters to a local environment with tunable optical properties, as
done in this study, enables the modulation of the emitter decay
rate while keeping the optical pump power constant. Previous
experiments have demonstrated the tunability of the narrowband
emission of an epitaxial QD, which is coupled to a high-quality
factor dielectric cavity
7
,
8
. However, these experiments can be
performed only at cryogenic temperatures, making them less
amenable for immediate practical applications.
Recently, there has been considerable interest in studying active
nanophotonic structures with tunable optical responses
9
. Active
plasmonic structures are especially interesting candidates for
tuning the emission of room temperature broadband solid state
emitters. This is due to the fact that they offer small optical mode
volumes and relatively low-quality factors, thus eliminating the
necessity of careful alignment of cavity and emitter resonances.
Previous works have reported
fi
eld-effect tuning of the carrier
density and Fermi level in graphene
10
, transparent conducting
oxides
11
, and silicon
12
to modulate transmittance and re
fl
ectance
of plasmonic structures in the near- and mid-infrared wavelength
range. Previous work has adopted this tuning mechanism to
electrically modulate the 1.5
μ
m wavelength emission of trivalent
erbium ions coupled to a graphene sheet
13
. In an alternative
approach, a previous study has used the optically induced phase
transition in vanadium dioxide to modulate near-infrared emis-
sion of erbium ions coupled to Salisbury-screen-type
heterostructure
14
. However,
fi
eld effect modulation of the optical
response of a nanophotonic structure has not been demonstrated
at visible wavelengths. This is primarily due to the relatively low
carrier concentrations of previously used doped oxides and
semiconductors
11
,
12
that limit their epsilon-near-zero (ENZ)
wavelengths to the near- or mid-infrared wavelength range.
Here we use degenerately doped n-type titanium nitride (TiN)
15
–
20
, with an ENZ wavelength in the visible spectrum, to
demonstrate the
fi
eld effect tunable optical response of a gated
TiN/SiO
2
/Ag heterostructure at visible wavelengths. We embed
InP/ZnS core
–
shell colloidal QDs in the SiO
2
layer of the tunable
TiN/SiO
2
/Ag heterostructure and study emission properties of
the QDs while biasing the TiN and Ag layers with respect to each
other. We observe a temporal modulation of the QD spontaneous
emission rate, far-
fi
eld emission intensity, and quantum yield that
occurs due to the modulation of the LDOS at the position of the
QDs. The optoelectronic device fabricated in this work exem-
pli
fi
es how conventional electronic components, in our case, a
metal-oxide-semiconductor capacitor, can be adapted to the
fi
eld
of nanophotonics to yield the bias-controlled modulation of the
PL intensity of
fl
uorophores. Our proof-of-principle experiment
demonstrates an active plasmonic mechanism for modulating
visible light that is extensible to other types of quantum emitters.
Results
Field effect modulation of dielectric permittivity of TiN
. The
schematic of our TiN/SiO
2
/Ag heterostructure is shown in Fig.
1
a.
As seen in the high-resolution transmission electron microscopic
image, the fabricated heterostructure consists of an optically thick
Ag, a 9 nm-thick insulating SiO
2
spacer, and a 7 nm-thick con-
ductive TiN layer (Fig.
1
b). Visible-emitting colloidal QDs with
diameters of 4
–
5 nm are embedded in the insulating SiO
2
spacer
(Fig.
1
c). In our work, we use InP/ZnS core
–
shell colloidal QDs
(hereafter, InP QDs), which are of greater application interest
because they are free of heavy metals and may be bene
fi
cial in
considering health and environmental concerns. When a bias is
applied between TiN and Ag, a charge depletion or accumulation
layer is formed in the TiN at the TiN/ SiO
2
interface (Fig.
1
d and
Supplementary Fig.
1
). This results in a modulation of the
complex refractive index of TiN and, therefore, also modulation
of the LDOS at the position of QDs embedded in the SiO
2
layer.
ext
PL
–
+
TiN
HR-TEM
InP
ZnS
TiN
InP QDs in SiO
2
TEM
Ag
Ag
Ag
80 nm
0
ENZ
TiN
V
G
> 0 V
V
G
= 0 V
V
G
< 0 V
–1
+
–
SiO
2
SiO
2
9 nm
7 nm
N
(cm
–3
)
Re(
)
V
G
InP QDs
Modulated TiN
a
b
c
d
Fig. 1
Concept of gated TiN/SiO
2
/Ag plasmonic heterostructure for active control of spontaneous emission.
a
Schematic of the gated plasmonic
heterostructure that consists of 80 nm-thick Ag and 9 nm-thick SiO
2
layers in which InP quantum dots (QDs) are embedded, followed by a 7 nm layer of
TiN. The
fi
lling factor of the QDs in SiO
2
is 9%.
b
Cross-sectional transmission electron microscopy image of the fabricated heterostructure. The image
shows that the deposited TiN
fi
lm is conformal and smooth. The scale bar is 10 nm.
c
High-resolution transmission electron microscopic image of InP QDs
with the diameter of 4
–
5 nm. The PL emission of the QDs peaks at 630 nm. The scale bar is 5 nm.
d
The proposed physical mechanism of modulation of
optical response. When Ag is biased positively (negatively) with respect to TiN, a charge accumulation (depletion) layer is formed in TiN at the TiN/S
iO
2
interface. Charge accumulation reduces the real part of the dielectric permittivity of the TiN
ARTICLE
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Depending on the deposition conditions, TiN can either be in an
optically dielectric phase (Re(
ɛ
)
>
0) or in an optically plasmonic
phase (Re(
ɛ
)
<
0) in the visible wavelength range (Fig.
2
).
Moreover, one can identify a carrier concentration that yields
ENZ-TiN (
−
1
<
Re(
ɛ
)
<
1), which is crucial for observation of
re
fl
ectance modulation (Supplementary Fig.
4
).
Dynamic control of emission rate of QD via LDOS modula-
tion
. The optical measurements performed on the ENZ-TiN/
SiO
2
/Ag heterostructures at the QD emission wavelength of
λ
=
630 nm show a relative re
fl
ectance modulation of 18% (normal-
ized to the re
fl
ectance at zero bias) when the gate voltage is varied
between
−
1 and +1 V (Fig.
3
a). Additionally, we demonstrate that
the device has a modulation speed exceeding 20 MHz (Supple-
mentary Fig.
5
). The re
fl
ectance is modulated by changing the
carrier density under
fi
eld effect gate control in the TiN layer.
This in turn modulates the real part of dielectric permittivity of
the TiN from positive to negative values through the so-called
ENZ regime, which marks the borderline between dielectric and
plasmonic media (Fig.
3
b). Moreover, when we replace the top
TiN electrode by a titanium (Ti) electrode of identical thickness,
we observe no re
fl
ectance modulation under an applied bias
(Fig.
3
c, d). This shows that the TiN plays a crucial role in the
observed optical modulation (for re
fl
ectance calculations, see
Supplementary Fig.
6
).
Having established gate-tunable re
fl
ectance, we study emission
modulation of InP QDs embedded in the SiO
2
spacer layer of the
TiN/SiO
2
/Ag plasmonic heterostructure (Fig.
1
a). The depen-
dence of the photoluminescence (PL) intensity on applied bias at
the QD peak emission wavelength of
λ
=
630 nm is shown in
Fig.
4
a (for optical setup, see Methods). As seen in Fig.
4
a, the PL
intensity monotonically increases with positive bias and mono-
tonically decreases with negative bias. At
λ
=
630 nm, the PL
intensity change (normalized to the PL intensity at zero bias) is as
high as 15% when gate voltage is varied between
−
1 and +1 V. On
the other hand, for QDs embedded in the Ti/SiO
2
/Ag passive
heterostructure (the same device was used for Fig.
3
c), no PL
intensity modulation is observed under applied bias (Fig.
4
b). In
our experiments, we observe no shift of the wavelength of PL
peak intensity under applied bias in both cases when QDs are
embedded in a tunable TiN/SiO
2
/Ag or passive Ti/SiO
2
/Ag
heterostructure (Supplementary Figs.
7
and
8
). Hence, at applied
electric
fi
elds of 1.1 MV cm
−
1
, our InP QDs show no quenching
or red-shift of emission, which is characteristic for cadmium-
based core
–
shell colloidal QDs
21
,
22
. This is attributable to the
large bandgap difference between InP core and ZnS shell
materials
21
. Thus the electron and hole wave functions are well
con
fi
ned to the core of the core/shell QDs with an external
electric
fi
eld. The fact that we observe both an increase and
decrease of PL intensity, depending on the polarity of the electric
fi
eld, provides additional evidence that the observed modulation
of PL intensity is not caused by the change of the internal state of
the QDs under applied bias. If this were the case, the observed
modulation of the PL intensity would depend only on the
magnitude of the electric
fi
eld and not on its direction.
The measured PL intensity spectrum shows signi
fi
cant
modulation only around the central emission wavelength of
λ
=
630 nm. This seems to contradict the theoretical prediction of
the broadband LDOS modulation under applied bias (Fig.
4
d).
The apparent contradiction is resolved by recalling that the
measured PL intensity spectrum originates from an inhomogen-
eously broadened QD ensemble. Note that the relative PL
intensity modulation of each QD does not depend on the
emission wavelength (Supplementary Fig.
7
c). In the measured
ensemble, the size distribution of the QDs is such that a large
fraction of the individual QDs emits around the wavelength of
λ
=
630 nm, yielding a brighter PL signal at
λ
=
630 nm.
Once we have demonstrated the PL intensity modulation of the
QDs, we perform time-resolved PL measurements to identify the
lifetime of the QDs embedded in the TiN/SiO
2
/Ag heterostruc-
tures. When no electrical bias is applied, the measured lifetime of
the InP QDs is 390 ps. This shows that embedding the QDs into
the TiN/SiO
2
/Ag plasmonic heterostructure results in 28-fold
reduction of the QD lifetime as compared to the lifetime of the
QDs on bare silicon (Supplementary Fig.
10
). At applied electrical
bias of +1 V, the lifetime of the QDs decreases by 12%, while at
the bias of
−
1 V, the lifetime increases by 18% (Fig.
4
c). Our
calculations indicate that the optical frequency electric
fi
eld
radiated by a QD is tightly con
fi
ned in the SiO
2
layer and shows a
considerable enhancement at the interface with TiN (Fig.
4
d). As
a result, the LDOS in the middle of the SiO
2
layer is sensitive to
the modulation of the complex refractive index of the TiN
fi
lm
(Supplementary Fig.
11
). The observed lifetime as well as the PL
intensity and re
fl
ectance modulation reported in our work cannot
be fully explained by changing the carrier concentration in the
Drude term of the Drude
–
Lorenz model that describes the
dielectric permittivity of TiN. At high carrier concentrations (
N
=
1.8 × 10
22
cm
−
3
for ENZ-TiN), a number of different effects
may contribute to the observed optical modulation, such as the
10
a
b
ENZ
Au
Au
N
= 5.9×10
20
cm
–3
N
= 5.9×10
20
cm
–3
N
= 2.6×10
21
cm
–3
N
= 2.6×10
21
cm
–3
N
= 4.1×10
22
cm
–3
N
= 4.1×10
22
cm
–3
N
= 1.8×10
22
cm
–3
N
= 1.8×10
22
cm
–3
Ag
Ag
Re(
)
5
0
–5
–10
–15
–20
10
8
6
4
2
0
500
600
700
Wavelength (nm)
Im (
)
500
600
Wavelength (nm)
700
Fig. 2
Optical properties of TiN
fi
lms. Measured
a
real and
b
imaginary
parts of the complex dielectric permittivity of TiN thin
fi
lms. The fabricated
TiN
fi
lms are n-type with carrier densities ranging from 5.9 × 10
20
to 4.1 ×
10
22
cm
−
3
. Depending on the carrier density, the fabricated TiN
fi
lms can be
optically dielectric (Re(
ɛ
)
>
0) or optically plasmonic (Re(
ɛ
)
<
0). The gray
dotted line in
a
denotes Re(
ɛ
)
=
0. For a carrier density of 1.8 × 10
22
cm
−
3
,
Re(
ɛ
) is in the epsilon-near-zero (ENZ) region (
–
1
<
Re(
ɛ
)
<
1) over the
entire wavelength range. By applying a voltage between the TiN and Ag
(Fig.
1
), the interfacial TiN
fi
lm region undergoes a transition from an
optically dielectric to an optically plasmonic state or vice versa. For
comparison, we also plot the dielectric permittivity values for gold and
silver (Johnson and Christy)
31
, two standard plasmonic materials, which are
shown in dashed lines
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3
dependence of the electron effective mass on the applied voltage
due to the nonparabolicity of the conduction band and the
dependence of the electron mobility on the applied bias. In our
theoretical calculations (Fig.
4
d), we assume that the dielectric
permittivity of the 1 nm-thick accumulation layer of TiN is given
by the measured dielectric permittivity of the TiN
fi
lm with
carrier concentration of
N
=
4.1 × 10
22
cm
−
3
(Fig.
2
). Strictly
speaking, this assumption is not accurate; however, it allows us to
estimate the sensitivity of the calculated LDOS with respect to the
variation of the refractive index of the 1 nm-thick accumulation
layer (Fig.
4
d).
To summarize, in our experiment we observe either a
simultaneous increase of the PL intensity and total decay rate
of emission
Γ
tot
or a simultaneous decrease of the PL intensity
and
Γ
tot
. However, the observed PL intensity modulation it not
necessarily caused the actively changed LDOS. For example,
variation of excitation
fi
eld intensity under applied bias could also
result in modulation of PL intensity. To investigate this
possibility, we measure the absorbance spectrum of the ENZ-
TiN/SiO
2
/Ag plasmonic heterostructure at a laser excitation
wavelength of
λ
=
375 nm as a function of applied voltage
(Supplementary Fig.
13
). We observe a slight decrease of the
absorbance at positive voltages that implies increased excitation
intensity, and consequently, increased PL intensity. The observed
PL intensity modulation can also be attributed to the reduction or
increase of the absorbance of the ENZ-TiN/SiO
2
/Ag plasmonic
heterostructure at QD emission wavelengths (Supplementary
Fig.
13
). However, we would like to point out that the absorbance
modulation at the QD emission wavelength and modulation of
the total decay rate of a QD are interrelated since the QDs are
placed in the immediate vicinity of the TiN layer (Fig.
1
). In what
follows, we further investigate how the LDOS modulation
contributes to the observed PL modulation.
Active control of quantum yield of QDs
. Once a QD has been
excited by absorbing a photon, it may undergo a transition to the
ground state either by far-
fi
eld photon emission (radiative path-
way) or via non-radiative processes, such as energy transfer via
non-radiative dipole
–
dipole coupling
23
. An important metric to
quantify the emission properties of
fl
uorophores is the quantum
yield, which is the probability of an excited QD to relax via the
radiative pathway. When no electrical bias is applied, the esti-
mated quantum yield of the InP QDs embedded in the SiO
2
layer
of plasmonic heterostructure is 15% (Supplementary Fig.
12
).
Our measurements show that under positive bias the radiative
emission decay rate (
Γ
rad
) increases by 15%, while under negative
bias
Γ
rad
decreases by 11% (Fig.
5
a). This amounts to a relative
modulation of
Γ
rad
of 26% when the applied gate voltage varies
between
−
1 and +1 V (see Methods for further details). The
V
G
= 0 V
=630nm
= 630nm
V
G
= 1V
V
G
= –1V
1.0
0.5
0.0
V
G
(V)
–0.5
–1.0
50
60
R
(%)
ENZ
ENZ
TiN
Ti
–
–
7 nm
7 nm
Ag
+
+
Re(
)
Re(
)
Ag
SiO
2
SiO
2
×10
22
N
(cm
–3
)
1
400
1200
Wavelength (nm)
Optically plasmonic
Optically dielectric
ENZ
1000
800
600
2
3
4
3
2
0
Re(
)
1
–1
–4
–3
–2
5
500
Wavelength (nm)
800
700
600
80
TiN/SiO
2
/Ag
60
80
0 V
= 630nm
70
1.0
0.5
0.0
V
G
(V)
V
G
= 1 V
V
G
= –1 V
–0.5
–1.0
R
(%)
40
20
0
500
Wavelength (nm)
800
700
600
Reflectance(%)
80
Ti/SiO
2
/Ag
60
40
20
0
Reflectance(%)
a
c
b
d
Fig. 3
Re
fl
ectance modulation under applied gate bias:
fi
eld-effect induced dielectric permittivity change in TiN.
a
Measured re
fl
ectance spectrum of a
gated TiN/SiO
2
/Ag plasmonic heterostructure for different applied voltages. Here Ag is biased with respect to TiN, and the applied voltage varies from
−
1
to 1 V in 0.2 V steps. The inset of
a
shows the heterostructure re
fl
ectance as a function of voltage for a wavelength of
λ
=
630 nm. The re
fl
ectance
increases from 67 to 82% when the applied bias varies from
−
1to1V.
b
Calculated real part of the TiN dielectric permittivity as a function of wavelength
and carrier concentration. The vertical dashed line shows the operation wavelength of our device (
λ
=
630 nm), while the horizontal dashed line marks the
carrier concentration of TiN. The two dashed lines intersect in the epsilon-near-zero (ENZ) region of the dielectric permittivity of TiN, indicatin
g that under
an applied bias TiN undergoes a transition from an optically dielectric to an optically plasmonic phase or vice versa.
c
Measured re
fl
ectance spectrum of the
gated Ti/SiO
2
/Ag control heterostructure. The inset of
c
shows the re
fl
ectance of the heterostructure as a function of electrical bias applied between Ti
and Ag for a wavelength of
λ
=
630 nm. No visible re
fl
ectance modulation of the Ti/SiO
2
/Ag control heterostructure is observed when the applied bias
varies between
−
1 and 1 V.
d
Spatial distribution of Re(
ɛ
) in the designed heterostructures. In the TiN/SiO
2
/Ag heterostructures, the carrier density of TiN
is lower than in metals, so the direct-current (DC) electric
fi
eld penetrates into the TiN
fi
lm, resulting in a graded spatial variation of epsilon. In the Ti/
SiO
2
/Ag control heterostructures, the DC electric
fi
eld is unable to penetrate into the metallic electrodes due to the inherently higher metallic Ti carrier
density. Hence, the Re(
ɛ
) exhibits an abrupt change at the Ti/dielectric interface
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measured voltage-dependent total emission decay rate (Fig.
4
c)
and radiative decay rate can be used to determine the variation of
QD quantum yields
η
=
Γ
rad
/
Γ
tot
under applied bias. We observe
a 35% relative increase of the quantum yield at an applied bias of
+1 V and 21% relative decrease of the quantum yield at an applied
bias of
−
1 V (Fig.
5
b). This in situ control of quantum yield is a
unique consequence of the bias-induced modulation of LDOS.
We emphasize that the LDOS, radiative emission decay rate, and
quantum yield do not depend on the absorbance at the excitation
wavelength of
λ
=
375 nm.
To summarize, we observe that at positive bias the increase in
the PL intensity is always accompanied by increases in both total
and radiative emission decay rates:
Γ
tot
and
Γ
rad
(Figs.
4
and
5
).
This implies that the LDOS modulation also contributes to an
increase in PL intensity under applied bias along with reduced
absorption of the ENZ-TiN/SiO
2
/Ag plasmonic heterostructure at
the QD emission wavelength
λ
=
630 nm and slightly increased
excitation intensity at
λ
=
375 nm. Although, as mentioned above,
at the QD emission wavelength the LDOS modulation and the
absorbance modulation cannot be fully decoupled. In previously
reported cases of bias-induced LDOS modulation, an increase of
the LDOS has always been accompanied by a decrease in the PL
intensity, which implies that the applied bias primarily affects the
non-radiative decay rate
13
,
23
. In contrast, in our work we observe
a simultaneous increase in LDOS and PL intensity.
Discussion
The ability to electrostatically control the spontaneous emission
rate, intensity, and quantum yield of colloidal QDs via modula-
tion of LDOS may lead to a number of applications, since this
represents a means to dynamically modulate emitted intensity
without electrical injection of carriers into the QD emitters. Thus
more optimized structures featuring electrostatic LDOS mod-
ulation of optically pumped QDs have potential for application in
optically ef
fi
cient ultrathin displays with long durability, reduced
pixel size, and large viewing angle. We note that modern com-
mercially available QD displays typically use blue light emitting
diodes (LEDs) to optically pump colloidal QDs, which down-
convert the blue LED emission into red and green light
24
. The
resulting QD light emission is used for backlighting, while the
image is formed by the same principles as in regular liquid crystal
displays
25
. Using colloidal QDs with narrowband emission at
controllable wavelengths has enhanced the color gamut of mod-
ern displays, enabling generation of more realistic and vivid
colors. The proof-of-concept experiment demonstrated here
suggests that emission control of optically pumped QDs may be
directly used for image formation, potentially eliminating neces-
sity of the liquid crystal light modulator. Unlike the case of QD-
LEDs, where both QD excitation and emission intensity mod-
ulation are achieved via current injection into the QDs, our
approach decouples the excitation and modulation mechanisms.
5
0
–5
–10
–15
20
0
–20
10
0
500
600
Wavelength (nm)
Optically plasmonic TiN
ENZ TiN
Modulated
LDOS enhacement
Normalized
⏐
E
⏐
Ag (80 nm)
+
–
TiN (7 nm)
SiO
2
(9 nm)
InP (D = 4.5 nm)
700
800
1.0
0.5
0.0
Ag
Ag
SiO
2
TiN
TiN
SiO
2
V
G
>0
V
G
<0
~
–1.0
–0.5
0.5
500 600
(nm)
=630 nm
700 800
500 600
(nm)
700 800
1 V
0 V
–1 V
1 V
0 V
–1 V
460 ps
390 ps
344 ps
1 V
0 V
–1 V
1.0
0.0
PL (a.u.)
Intensity (a.u.)
PL (a.u.)
V
G
(V)
–1.0
–0.5
0.5
1.0
0.0
V
G
(V)
–1.0
–0.5
0.5
1.0
0.0
V
G
(V)
V
G
V
G
a
b
c
d
TiN/SiO
2
/Ag
=630 nm
tot
=1/
TiN/SiO
2
/Ag
=645 nm
Ti/SiO
2
/Ag
Δ
I
PL
/
I
0
PL
(%)
Δ
I
PL
/
I
0
PL
(%)
Δ
/
0
(%)
5
0
–5
–10
–15
10
1.0
0.1
012
Time (ns)
34
Fig. 4
Gate-tunable spontaneous emission of quantum dots (QDs) via modulation of the local density of optical states (LDOS).
a
Modulation of the
photoluminescence (PL) intensity of InP QDs embedded in the gated TiN/SiO
2
/Ag plasmonic heterostructure for wavelength of
λ
=
630 nm. We observe a
10% PL relative intensity increase when the gate voltage
V
G
is varied from 0 to +1 V. When
V
G
is varied from 0 to
−
1 V, we observe a 5% PL relative
intensity decrease. The inset of
a
shows the PL intensity spectra for different gate voltages.
b
PL intensity of QDs embedded in a gated Ti/SiO
2
/Ag
heterostructure for wavelength of
λ
=
645 nm. No modulation of PL intensity is observed under an applied bias (within the PL intensity measurement error,
which is the standard deviation of the PL intensity measured at the same excitation spot of the sample at zero bias). The PL spectra for different gate
voltages, plotted in the inset of
b
, also shows no modulation under applied bias.
c
PL lifetime of QDs embedded in the gated TiN/SiO
2
/Ag plasmonic
heterostructure. When the gate voltage
V
G
is increased from 0 to +1 V, the QD lifetime decreases by 12%. When
V
G
is varied from 0 to
−
1 V, the QD
lifetime increases by 18%. The inset of
c
shows the PL intensity as a function of time for different gate voltages
V
G
.
d
Calculated LDOS enhancement
spectra at the position of a QD (averaged over QD dipole orientations) for different carrier densities in a 1 nm-thick modulated TiN layer. The black cu
rve
corresponds a homogeneous TiN
fi
lm, which is in the epsilon-near-zero (ENZ) region. The red curve corresponds to a TiN
fi
lm with a 1 nm-thick modulated
TiN layer that is plasmonic but far from the ENZ region. The top panels show the simulated spatial distribution of the optical frequency electric
fi
eld
|
E
|
radiated by a QD (
λ
=
630 nm). Both the calculated LDOS and optical
fi
eld intensity
|
E
|
in the SiO
2
gap increase with gate voltage
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5
As a result, our method does not lead to deterioration of the
emission properties of QDs typically observed in QD-LEDs.
Moreover, due to capacitive nature of the device architecture used
for our modulation mechanism, there is no need to place QDs
between p- and n-type materials. This eliminates the necessity of
the careful band alignment of the constituent materials.
When considering display applications, it is crucial to have
large relative modulation of PL intensity. An obvious improve-
ment would be to replace SiO
2
by a gate dielectric with higher DC
permittivity, such as Al
2
O
3
or HfO
2
. The relative modulation
strength of our device can further be improved by incorporating a
TiN layer into a plasmonic cavity that can be formed, for
instance, by placing an appropriately designed patch antenna on
top of the Ag/SiO
2
/TiN heterostructure
4
,
26
. Previous research has
shown that the lifetime of a quantum emitter embedded in the
dielectric gap of the patch antenna resonator is very sensitive to
gap thickness
26
. If the resonant wavelength of the cavity is aligned
with the emission wavelength of the emitter, this con
fi
guration
will yield a large modulation of the PL intensity under an applied
bias. Moreover, using a patch antenna resonator can increase
emission intensity by orders of magnitude
4
, thus making the
whole device more energy ef
fi
cient. The energy ef
fi
ciency of our
device can be further improved by using QDs with a higher
quantum yield and by using a QD ensemble with narrower
spectral width of PL intensity (for a more detailed discussion, see
Supplementary Note
7
).
Our
fi
ndings can be readily used for visible light commu-
nication systems such as Li-Fi, a counterpart of Wi-Fi that utilizes
the visible spectral range
27
,
28
. Li-Fi denotes the proposed future
visible light communication scheme, in which temporally
modulated LED lamps are likely candidates as sources for an
optical wireless communication system to transmit information
encoded via intensity variations in the emitted light. Cadmium-
free QDs are currently used for improving the quality of light of
LED light bulbs by making the emitted light more pleasant for the
human eye. In many lighting devices, QDs absorb blue LED light
and re-emitted as light downshifted in wavelength to achieve a
desired lamp emission color characteristic. It has been suggested
that light intensity modulations could be achieved by varying the
intensity of the blue LED pump
27
,
28
. Instead of varying the
intensity of the LED pump light source, we propose to dynami-
cally control the emission properties of QDs via LDOS modula-
tion. Our proposed approach would enable the realization of a
more
fl
exible visible light communication system, where, for
example, an actively tunable wavelength multiplexing device
could be enabled by separately modulating the emission intensity
of QDs with different emission wavelengths. We would like
emphasize that, within the context of visible light communication
systems, a large amplitude modulation of the emitted light is not
necessary
27
,
28
.
Methods
Sample fabrication
. We formed a dilute solution of commercially available InP/
ZnS core
–
shell QDs (2 mgmL
−
1
in toluene) in isopropanol, with a volume ratio of
1:20 and a quantum yield of 17%. Next, we fabricated TiN/SiO
2
/Ag plasmonic
heterostructures with QDs embedded in the SiO
2
layer by depositing 80 nm of Ag,
followed by 4 nm of SiO
2
on a mica substrate via electron-beam evaporation at a
chamber pressure of 10
−
7
torr. We then spin-coated the diluted QD solution for
30 s at 4000 rpm to spread the solution evenly and subsequently deposited another
4 nm of SiO
2
via electron-beam evaporation. In this way, we obtained a layer of
QDs embedded in the SiO
2
layer. As a
fi
nal step, we sputtered 7 nm TiN
fi
lm with
DC magnetron sputtering at a chamber pressure of 7.5 × 10
−
7
torr and at room
temperature. In accordance with previously reported results
16
–
18
, varying argon/
nitrogen
fl
ow rate, DC power, growth temperature, and growth substrate strongly
in
fl
uenced the optical properties of the deposited
fi
lms. Using Hall measurements,
we identi
fi
ed the growth conditions enabling the carrier concentration of the
fabricated TiN
fi
lms to lie between
N
=
5.9 × 10
20
cm
−
3
and
N
=
4.1 × 10
22
cm
−
3
,
while the mobility ranged from 0.059 to 5.8 cm
2
V
−
1
s
−
1
(Supplementary Tables
1
and
2
). Each deposition step was undertaken with an appropriate face mask made
of stainless steel to permit easy bias application con
fi
guration in the
fi
nal device.
The working area of the device was 1 × 1 mm
2
. The thicknesses of the constituent
layers in the fabricated TiN/SiO
2
/Ag heterostructure were determined via cross-
sectional transmission electron microscopic analysis. To prepare the sample for the
electron microscopic analysis, we used dual-beam focused gallium ion-beam (FIB,
FEI model Nova 200) to FIB-cut the heterostructure normal to the interface. The
diameter of the QDs was 4
–
5 nm, as determined by high-resolution transmission
electron microscopy (FEI Tecnai TF20 STEM). To determine the material para-
meters of the sputtered TiN
fi
lms, we fabricated additional large-area control
samples that consisted of 35 nm of SiO
2
and about 30 nm of TiN deposited on Si
substrate under the same growth conditions as the ones used to fabricate our
fi
nal
devices. We used large-area control samples for both Hall and ellipsometry (J.A.
Woollam Co. model VASE) measurements. Since the optical properties of TiN
fi
lm
sensitively depend on the choice of the substrate
19
, our control samples used for
both ellipsometry and Hall measurements were deposited on SiO
2
.
Optical measurements
.Re
fl
ectance spectra were taken under normal incidence
with an objective 5× (Olympus, with a numerical aperture of 0.14) focusing a
supercontinuum laser (Fianium) down to a small spot of 3
μ
m in diameter. The
re
fl
ected signal was measured by a silicon photodetector. The micro-PL and time-
resolved PL measurements were taken on an inverted microscope (Zeiss, Inc.)
equipped with a spectrometer consisting of a monochromator and a liquid-
nitrogen-cooled CCD camera. For PL lifetime measurements, a 375 nm picosecond
laser diode (70 ps pulse duration, 40 MHz repetition rate; PicoQuant) excitation
source was used, and a 375 nm band pass
fi
lter was placed after laser source to
purify the laser beam. A 100× objective lens with a numerical aperture of 0.9 (Zeiss,
Inc.) was used to focus the pulsed laser to a small spot of 1.6 × 10
−
6
cm
2
with an
estimated peak power density of 7.5 kW cm
−
2
. We measured QD lifetime by using
a time-correlated single photon counting module (PicoHarp 300, PicoQuant) and
single photon avalanche diode detector (PDM 50 T, MicroPhoton Devices)
29
.We
used periodic pulsed laser excitation and collect photons from multiple excitation
and emission cycles. This approach allowed us to construct a histogram of number
of photon counts at different photon arrival times. The time resolution of our
lifetime measurement setup was 200 ps. PL lifetime decays were acquired from a
1.2
1.1
1.0
0.9
–1.0
1.4
1.2
1.0
0.8
QE (
V
G
)/QE
0
–1.0
0.0
0.5
1.0
–0.5
–0.5
0.0
0.5
1.0
V
G
(V
)
V
G
(V
)
rad
(
V
G
)/
0
rad
a
b
Fig. 5
Active control of quantum yield of quantum dots (QDs) embedded in
a gated plasmonic heterostructure.
a
Radiative decay rate (
Γ
rad
) of InP QDs
(normalized to radiative decay rate at zero bias) embedded in the TiN/
SiO
2
/Ag plasmonic heterostructure as a function of gate voltage (within
the photoluminescence (PL) intensity measurement error, which is the
standard deviation of the PL intensity measured at the same excitation spot
of the sample at zero bias). Under positive bias
Γ
rad
shows a relative
increase of 15% while under negative bias
Γ
rad
shows a relative decrease of
11%.
b
Dynamically tunable quantum yield of QDs (normalized to quantum
yield at zero bias). When gate voltage
V
G
is increased from 0 to +1 V, we
observe a 35% relative increase of quantum yield. As gate voltage
V
G
varies
from 0 to
−
1 V, we observe a relative quantum yield decrease of 21%
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particular region
<
5
μ
m in diameter that was illuminated with an iris. During
lifetime measurements, a 600
–
650 nm band-pass
fi
lter was used. When measuring
the QD lifetime, we illuminated the same region of the sample that was used for PL
measurements. This enabled direct comparison of the QD lifetime and PL intensity
data sets and further analysis of the radiative and non-radiative recombination rate.
Even though we measured slight differences in the emission spectrum depending
on the part of the sample that was illuminated, we obtained reproducible data when
the position of the illumination spot was
fi
xed. We also veri
fi
ed that the shape of a
PL intensity spectrum does not depend on pump
fi
eld intensity (Supplementary
Fig.
16
). In addition, the voltage bias was applied to the heterostructure by a source
meter (Keithley 2400 source-meter).
Electromagnetic simulations
. Numerical electromagnetic simulations were per-
formed using a
fi
nite difference time domain method. When modeling the per-
formance of our structure under applied bias, we assumed that the complex
refractive index of a 1 nm-thick TiN layer at the interface with SiO
2
is modulated
by the applied bias (Supplementary Figs.
6
,
9
and
11
).
Quantum yield measurements
. To measure the quantum yield of our InP QDs,
we used a standard
fl
orescence dye (Rhodamine 6G) with a known quantum yield
of 95% (Supplementary Fig.
12
). When calculating radiative emission decay rate,
we took into account that our InP QDs were embedded in the SiO
2
layer of the
ENZ-TiN/SiO
2
/Ag heterostructure. Since the QDs were embedded in the ENZ-
TiN/SiO
2
/Ag heterostructure, a portion of the laser excitation (
λ
=
375 nm) was
absorbed in the top TiN layer, which affected the excitation intensity of the QDs.
To estimate the effect of the possible variation of the excitation intensity, we
measured the absorbance of the ENZ-TiN/SiO
2
/Ag plasmonic heterostructure at an
excitation wavelength of
λ
=
375 nm under an applied bias. As seen in Supple-
mentary Figs.
13
and
14
, the absorbance stayed almost constant for negative biases
and showed a slight decrease for positive biases. Since absorption primarily
occurred in the TiN layer, high absorbance resulted in the reduced excitation
intensity of the QDs. Taking this into account, the bias dependent radiative
emission decay rate (
Γ
rad
) given by the following formula
26
,
30
:
Γ
rad
V
G
ðÞ
=
Γ
0
rad
¼
I
PL
V
G
ðÞ
=
I
0
PL
1
A
laser
V
G
ðÞ
ðÞ
=
1
A
0
laser
:
ð
1
Þ
Here
Γ
0
rad
the radiative emission decay rate under zero bias,
I
0
PL
the peak PL
intensity under zero bias,
I
PL
(
V
G
) the bias-dependent PL intensity,
A
0
laser
the
absorbance in the ENZ-TiN/SiO
2
/Ag heterostructure at an excitation wavelength of
λ
=
375 nm at zero bias, and
A
laser
(
V
G
) the bias dependent absorbance at
λ
=
375 nm.
V
G
denotes the gate voltage applied between the Ag and TiN layers in the
TiN/SiO
2
/Ag heterostructure. We would like to emphasize that absorbance in the
TiN/SiO
2
/Ag heterostructure primarily occurred in TiN layer. To calculate the
bias-dependent quantum yield of our QDs embedded in TiN/SiO
2
/Ag hetero-
structure, we used Eq. (
1
) and took into account that quantum yield of the emitter
η
was de
fi
ned as a ratio of radiative and total decay rates
η
=
Γ
rad
/
Γ
tot
.
Data availability
. The data sets analyzed during this study are available from the
corresponding author on reasonable request.
Received: 13 April 2017 Accepted: 20 October 2017
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Acknowledgements
This work was supported by Samsung Electronics, the Air Force Of
fi
ce of Scienti
fi
c
Research under grant number FA9550-16-1-0019 (device-related work), and the
Department of Energy under grant number DE-FG02-07ER46405 (transparent
conductor-related work). We also acknowledge use of facilities supported by the Kavli
Nanoscience Institute (KNI) and the Joint Center for Arti
fi
cial Photosynthesis (JCAP) at
Caltech. Y.-J.L. acknowledges the support from Ministry of Science and Technology,
Taiwan (Grant numbers: 104-2917-I-564-057). The authors would like to thank Wei-
Hsiang Lin, Anya Mitskovets, and Panos Patsalas for useful discussions. The authors
gratefully acknowledge Erin Burkett from the Hixon Writing Center at Caltech for
providing feedback and guidance on writing during the revision process. The authors
also deeply appreciate help in the form of the close reading of the manuscript and review
responses by Kelly Mauser, Dagny Fleischman, Rebecca Glaudell, Haley Bauser, Cora
Went, Phil Jahelka, and Michael Kelzenberg.
Author contributions
Y.-J.L., R.S. and H.A.A. proposed the original idea. Y.-J.L. performed all experiments,
calculations, and data analysis. R.S. proposed the theoretical model and performed cal-
culations. R.P. and W.-H.C. helped with the optical setup. K.T., G.K.S., W.-H.C., and R.S.
performed ellipsometry measurements and analyzed the ellipsometry data. A.R.D. helped
in discussion. Y.-J.L., R.S., and H.A.A. wrote the paper, and all authors discussed and
revised the
fi
nal manuscript.
Additional information
Supplementary Information
accompanies this paper at doi:
10.1038/s41467-017-01870-0
.
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7
Competing interests:
The authors declare no competing
fi
nancial interests.
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