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Enhancing Eu3+ magnetic dipole emission by resonant plasmonic nanostructures

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Enhancing Eu3+ magnetic dipole emission by resonant plasmonic nanostructures

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We demonstrate the enhancement of magnetic dipole spontaneous emission from Eu3+ ions by an engineered plasmonic nanostructure that controls the electromagnetic environment of the emitter. Using an optical microscope setup, an enhancement in the intensity of the Eu3+ magnetic dipole emission was observed for emitters located in close vicinity to a gold nanohole array designed to support plasmonic resonances overlapping with the emission spectrum of the ions.
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Enhancing Eu3+ magnetic dipole emission by resonant
plasmonic nanostructures
Rabia Hussain,1Sergey S. Kruk,2Carl E. Bonner,1Mikhail A. Noginov,1Isabelle Staude,2
Yuri S. Kivshar,2Natalia Noginova,1,3 and Dragomir N. Neshev2,*
1Center for Materials Research, Norfolk State University, Norfolk, Virginia 23504, USA
2Nonlinear Physics Centre, Research School of Physics and Engineering, Australian National University, Canberra ACT 2601, Australia
3e-mail: nnoginova@nsu.edu
*Corresponding author: Dragomir.Neshev@anu.edu.au
Received December 17, 2014; revised February 26, 2015; accepted March 9, 2015;
posted March 13, 2015 (Doc. ID 230842); published April 3, 2015
We demonstrate the enhancement of magnetic dipole spontaneous emission from Eu3ions by an engineered plas-
monic nanostructure that controls the electromagnetic environment of the emitter. Using an optical microscope
setup, an enhancement in the intensity of the Eu3magnetic dipole emission was observed for emitters located
in close vicinity to a gold nanohole array designed to support plasmonic resonances overlapping with the emission
spectrum of the ions. © 2015 Optical Society of America
OCIS codes: (300.2140) Emission; (310.6628) Subwavelength structures, nanostructures; (240.6680) Surface
plasmons.
http://dx.doi.org/10.1364/OL.40.001659
The pioneering work of Purcell [1] has played a pivotal
role in our understanding of the nature of spontaneous
emission processes. It has shown that such processes
are not only an intrinsic property of the emitter, but
can also be strongly modified by resonant coupling to
the electromagnetic environment. This has opened en-
tirely new possibilities for the enhancement of emission
from even weak sources. Nevertheless, while Purcell de-
veloped his work for the enhancement of the emission
from a microwave magnetic dipole, the implementation
of this concept in optics was mostly carried out using
electric dipole emitters. These studies have created a
huge arsenal of techniques for the control of the electro-
magnetic environment and, more specifically, the en-
hancement of the electric field near an emitter.
However, the control of emission from optical magnetic
dipoles remains largely unexplored.
Magnetic dipole transitions at optical frequencies are
usually weak; however, the possibility to enhance the
emission rates of such weak emitters has promised to
open many applications, including display technologies,
fluorescent bioprobes, and quantum light sources. Such
possibilities have recently triggered renewed attention
for the control of magnetic dipole emitters by photonic
nanostructures [28]. However, most works to date have
only focused on the use of simple nonresonant structures
such as metaldielectric stacks [26,917] and there are
no experiments on the manipulation of magnetic dipole
emission by engineered resonant nanostructures. The lat-
ter therefore represents an important milestone in the de-
velopment of bright magnetic emitters. Here we present,
for the first time to our knowledge, the enhancement of
magnetic dipole emission by a resonant nanostructure
with magnetic response. In particular, we use a thin layer
of Eu3ions, which have closely adjacent magnetic and
electric dipole emission lines, and place it over an array
of nanoholes exhibiting plasmonic resonances in the
Eu3emission range. We show an enhancement of the
magnetic dipole with respect to the electric dipole emis-
sion due to the presence of the nanohole array.
The control of the magnetic environment at the nano-
scale has experienced great progress in the past decade
with the development of metamaterials possessing
unique optical properties and desired electromagnetic re-
sponses [18]. In particular, with the recent development
of magnetically resonant metallic [19] and high-index
dielectric nanoparticles [20] at optical frequencies, the
engineering of the magnetic environment promises im-
portant future developments. In fact, one of the simplest
nanostructures for the control of the magnetic field is
represented by a nanohole in a metal film. Such nano-
holes have been proven to act as efficient probes of
the magnetic field [21] and have been used for near-field
magnetic field detection [2225]. Therefore, nanoholes
represent an attractive option for achieving a resonantly
enhanced magnetic response.
In our experiments, we employ rare-earth metal ions,
such as Eu3ions, having both electric and magnetic di-
pole transitions in their spontaneous emission spectra
[2,4,12]. The spectroscopic properties of Eu3lumines-
cence, here in EuTTA3L18[an alkylated europium
complex, tris(α-thenoyltrifluoroacetone)(1-octadecyl-
2(-2-pyridyl) benzimidazole) europium(III)], are summa-
rized in Fig. 1. Figure 1(a) depicts the energy-level
diagram for the absorption and emission transitions be-
tween the 5D0and 7Fnenergy states of Eu3. Figures 1(b)
and 1(c) show, respectively, the excitation and emission
spectra of the EuTTA3L18complex. The strongest ex-
citation band occurs in the ultraviolet range with maxi-
mum absorption at around 330 nm. Importantly, the
Eu3emission line at λ590 nm is associated primarily
with a magnetic dipole transition 5D07F1, whereas the
strongest line in the emission spectrum at λ611 nm
corresponds to an electric dipole transition 5D07F2
[26]. The effects of the local optical environment on
the electric and magnetic dipole emission have been
studied theoretically and experimentally in the flat geom-
etry [24,1013]. Variation of the relaxation rate and the
interplay of the relative intensities of the electric and
April 15, 2015 / Vol. 40, No. 8 / OPTICS LETTERS 1659
0146-9592/15/081659-04$15.00/0 © 2015 Optical Society of America
magnetic dipole transitions have been observed as a
function of the distance between the Eu3emitters and
a plane mirror [3,13]. A reduction of the electric dipole
emission and an enhancement of the magnetic dipole
emission for emitters placed in the vicinity of an ideal
electric mirror (perfect conductor) were predicted due
to the boundary conditions for the electric and magnetic
components of the optical fields [2,10]. However, the
emission of Eu3in the vicinity of nanostructured reso-
nant surfaces, especially those exhibiting artificial mag-
netism, has never been experimentally studied before.
To show how such metasurfaces can influence the
emission from electric and magnetic dipoles, we design
and fabricate a plasmonic nanohole array that supports a
resonance in the spectral range of the Eu3magnetic di-
pole transition. A sketch of our experimental geometry is
shown in Fig. 2(a). The samples (40 μ40 μm) are fab-
ricated by focused-ion beam milling of a 150 nm thick
gold film deposited on a glass substrate. Nanoholes with
sizes 200 nm × 280 nm were arranged into a square array
with 400 nm lattice constant. A scanning electron micro-
graph (SEM) image of the fabricated sample is shown in
Fig. 2(b). The fabricated nanohole array is optically char-
acterized using a home-built transmittance setup and an
Ocean Optics visible spectrometer. The measured spec-
tra (red solid lines) for x- and y-polarized incident light
are displayed in Figs. 2(d) and 2(e), respectively. A plas-
monic resonance can be observed for both polarizations
around 600 nm wavelength. Importantly, this resonance
overlaps with both transition lines of Eu3(5D07F1;2),
as indicated by the green and red solid vertical lines in
Figs. 2(d) and 2(e). Note that the subsequent deposition
of the Eu3thin film only negligibly red-shifts the spec-
tral transmission spectrum of the nanohole array, as con-
firmed by numerical simulations using the commercial
software package CST Microwave Studio. In the simula-
tions, we use a semi-infinite glass substrate with a refrac-
tive index of n1.5and for the permittivity of gold we
use data from Ref. [27]. The calculated spectra [black
solid lines in Figs. 2(d) and 2(e)] are in good agreement
with the experimental results and well reproduce the
resonances at 600 nm wavelength.
As the next step, we deposit a thin film of
EuTTA3L18onto the sample using the Langmuir
Blodgett technique [5,28]. A light microscope image of
the resulting sample is shown in Fig. 2(c). Chloroform
solutions of EuTTA3L18and polystyrene were mixed
in a proportion of 1:5. A 30 μl drop of the mixture solution
was spread on water surface. After the evaporation of
chloroform, a thin polymeric film was formed on the
water surface. By immersing the sample from the top,
a film with a uniform thickness of approximately 30
40 nm (measured by a profilometer) is transferred onto
the sample surface with large area coverage of approx-
imately 80 mm2.
After the EuTTA3L18thin-film deposition, we
analyzed the emission from Eu3electric and magnetic
250 300 350 400 450
0.0
0.5
1.0
Eu3+ excitation spectrum
Wavelength, nm
550 600 650 700
0.0
0.5
1.0
5
D
0
-
7
F
4
5
D
0
-
7
F
3
5
D
0
-
7
F
2
5
D
0
-
7
F
1
5D0 - 7F0
Eu3+ emission spectrum
Wavelength, nm
(a) (b)
(c)
Fig. 1. Spectroscopic properties of EuTTA3L18.
(a) Energy-level diagram showing absorption and spontaneous
emission transitions between the 5D0and 7Fnenergy states of
Eu3; (b) excitation spectrum of EuTTA3L18; and (c) emis-
sion spectrum recorded at 330 nm excitation.
(a) (b)
(c)
z
x
y
glass
substrate
gold
3+
Eu layer
1 µm
20 µm
100
75
50
25
0
Transmittance, %
Experiment
Theory
500 700 900
(d) (e)
EE
Wavelength, nm
500 700 900
Fig. 2. (a) Sample geometry: a gold film with rectangular nano-
holes supporting plasmonic resonances around 600 nm for both
orthogonal polarizations is covered by a thin layer (red area)
containing Eu3ions. (b) A SEM image of the nanohole array
and (c) an optical microscope image of the sample with the de-
posited Eu3film. (d), (e) Experimentally measured (red solid
line) and numerically calculated (black solid line) linear trans-
mittance spectra of the sample for (d) xand (e) ypolarizations.
1660 OPTICS LETTERS / Vol. 40, No. 8 / April 15, 2015
dipole transitions in the presence of the plasmonic nano-
structure. We first perform photoluminescence micros-
copy using a Zeiss Imager Z2 microscope equipped
with an Axiocam camera, which allows simultaneous
observation of the emission from Eu3deposited on dif-
ferent areas of the sample. Eu3is excited with a HeCd
laser at λ325 nm wavelength via a multimode step-
index optical fiber (NA 0.22, core diameter 200 μm).
Interferometric narrow-band transmission filters (10 nm
bandwidth) centered at 610 and 590 nm are inserted in
the recording channel in order to separately collect the
emission signals corresponding to the electric and mag-
netic dipole transitions.
A white-light microscope image of the area of interest
is shown in Fig. 3(a). We chose the field of view such that
it contains the nanohole array (marked by yellow dashed
arrows) as well as two different reference areas: the gold
film and a region of bare glass substrate obtained by
scratching away the gold film in the vicinity of the array
(dark stripe on the right). The distinct interactions of
these reference areas with the electric and magnetic
dipole emitters have been investigated in detail as a
function of the gold film thickness [24,1013]. It was
concluded that for both electric and magnetic dipole
transitions the emission from Eu3ions on top of a gold
film with thickness exceeding 70 nm is brighter as
compared with the emission on top of the bare glass
substrate.
The recorded photoluminescence microscopy images
for the 610 and 590 nm detection wavelengths for the
same area as in Fig. 3(a) are displayed in Figs. 3(b)
and 3(c). The image in Fig. 3(b) is recorded with the
610 nm filter and corresponds to the electric dipole
transition. Here, a similar enhancement as for the metal-
lic mirror is observed, such that the array cannot be dis-
tinguished from the surrounding gold film. In Fig. 3(c),
which was recorded with the 590 nm filter at the wave-
length of the magnetic dipole transition, in contrast, the
array is seen as a bright square on the darker gold film
background. We note that for emitters with high quantum
yield, as in our case [5], the observed fluorescence en-
hancement can underestimate the actual radiative decay
rate enhancement. In accordance with previous studies
[12], we also find that the bare glass area looks darker
than the gold film for both electric and magnetic dipole
emission. This clearly indicates that the enhancement in
magnetic transition observed in the vicinity of the nano-
hole array cannot merely be explained by the partial
absence of gold and is a result of the resonant response
of the nanoholes.
Next, we measure the emission spectra from the three
different areas (nanohole array, gold film, and bare glass
substrate). The emission spectra were recorded using
an Ocean Optics PC2000 fiber optic spectrometer with
a 400 μm diameter fiber optic probe. An objective lens
10 magnification, 0.25 NA) was used to obtain a
magnified image of the sample region of interest and
project it onto the optical fiber probe. Figure 4(a) shows
the three spectra, each normalized to the intensity of
the dominant spectral line 5D07F2around 610 nm.
The normalization allows assessing the changes in the
relative emission intensities of the spectral lines. The
Fig. 3. (a) Image of the sample area of interest illuminated
with white light. The dark square highlighted by the yellow
dashed arrows is the nanohole array, while the dark stripe to
the right is a region where the gold film has been removed
by scratching. (b), (c) A photoluminescence microscope image
[same area as in (a)] of Eu3at (b) λ610 nm (electric dipole
transition) and (c) λ590 nm (magnetic dipole transition).
The excitation wavelength is λ325 nm.
580 590 600 610 620 630
0
1
2
Emission
Enhancement
Wavelength, nm
Photoluminescence
Normalized
(a)
(b)
0.0
0.5
1.0
gold
glass
array
7F2
5D0
7F1
5D0
Fig. 4. (a) Emission spectra of the EuTTA3L18film on top
of the nanohole array (red solid line), the 150 nm thick gold film
(green solid line), and the bare glass substrate (blue solid line).
The spectra are normalized to the maximum of the electric
dipole emission at 611 nm. (b) Emission enhancement, defined
as the ratio of the normalized emission spectrum of Eu3over
the nanohole array and the emission from Eu3over the plane
gold surface, above the noise level.
April 15, 2015 / Vol. 40, No. 8 / OPTICS LETTERS 1661
normalized spectra of Eu3emission on top of gold and
on top of the glass look identical, whereas in the spec-
trum of Eu3on top of a nanohole array, the magnetic
line 5D07F1becomes stronger. To emphasize the
changes in the relative intensities of the Eu3lines, we
plot in Fig. 4(b) the emission enhancement defined as
the ratio of the spectra measured on top of the nanohole
array and on top of gold. We observe over 50% enhance-
ment of the magnetic line 5D07F1emission as
compared with the dominant electric line 5D07F2
emission.
In conclusion, an enhancement of magnetic dipole
transition in the spontaneous emission spectrum of
Eu3is observed in a system of a gold nanohole array
featuring plasmonic resonances in the visible spectral
range, designed to overlap with the magnetic and electric
dipole transitions of Eu3. These findings present the
first step toward using engineered plasmonic or dielec-
tric nanostructures to obtain bright magnetic dipole emit-
ters at optical frequencies.
We acknowledge the support by the National Science
Foundation (NSF) PREM grant no. DMR 1205457, NSF
IGERT grant no. DGE 0966188, the Army Research
Office (ARO) grant W911NF-14-1-0639, the Australian
National Fabrication Facility, and the Australian
Research Council.
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1662 OPTICS LETTERS / Vol. 40, No. 8 / April 15, 2015
... For instance, europium-doped polymers are among the most appealing for the visible spectrum owing to their easy fabrication, their transparency and, above all, the highly efficient room-temperature photoluminescence of Eu 3+ , whose ED and MD transitions are spectrally close and well-resolved around λ = 615 nm. In recent years, the fluorescence of Eu 3+ embedded in polymers or glasses has been studied in combination with various systems such as thin films, plasmonic nanostructures or dielectric metasurfaces, pointing out its ED and MD emissions to be adjustable depending on the type or geometry of the underlying material as well as by changing the tilt angle of the sample [31][32][33][34][35]. ...
... Five emission peaks are revealed in the spectra, which correspond to the Eu 3+ radiative transitions according to its multilevel diagram (see inset in Figure 2a). By using the Russel-Saunders notation ( 2S+1 L J ), the peak at 592 nm corresponds to the 5 D 0 → 7 F 1 transition and it has a magnetic dipole (MD) nature; instead the peaks at 580 nm, 615 nm, 651 nm and 699 nm have an electric dipole (ED) nature and they correspond to the transitions from the 5 D 0 excited state to the states 7 F 0 , 7 F 2 , 7 F 3 and 7 F 4 respectively [22,32]. By considering the ENZ properties of the metamaterials, the Eu 3+ transitions can fall in two different regimes (hyperbolic when ε < 0 or elliptical when ε > 0) depending on the underlying HM and the spectral position of λ ENZ (indicated by the vertical dashed lines in Figure 2a,b). ...
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... For instance, quantifying the ED and MD transitions around the 1.55 μm spectral line of trivalent erbium (Er 3+ ) could improve the design of optical amplifiers by reducing spontaneous emission noise (Digonnet, 2001;Taminiau et al., 2012). Coupling of lanthanide ions and QDs to various nanophotonic systems including bulk materials (DeLoach et al., 1993), planar structures (Taminiau et al., 2012;Karaveli y Zia, 2011), dielectric (Shi et al., 2012;Sanz-Paz et al., 2018) and plasmonic nanoantennas (Hussain et al., 2015;Feng et al., 2011), and metamaterials (Simovski et al., 2012;Poddubny et al., 2013) have been proposed. Here, advances in nanofabrication techniques, along with the increasing study of magnetic quantum emitters have stimulated the investigation of the magnetic side of spontaneous emission. ...
... Jusqu'à présent, le contrôle de la génération de photons uniques par émission spontanée a été principalement axé sur la recherche de la décroissance spontanée des transitions dipolaires électriques (ED), car la force des transitions ED dans des sources quantiques optiques typiques est supérieure que celle des transitions dipolaires (MD) magnétiques (Landau y Lifshitz, 1984). Cependant, certains émetteurs quantiques, tels que les ions de terres rares (Carnall et al., 1968;Judd, 1962;Ofelt, 1962) et al., 1993), des structures planaires (Taminiau et al., 2012;Karaveli y Zia, 2011),des nanoantennas diélectriques (Shi et al., 2012;Sanz-Paz et al., 2018) et plasmoniques (Hussain et al., 2015;Feng et al., 2011) et des métamatériaux a été proposé (Simovski et al., 2012;Poddubny et al., 2013 ...
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... [62][63][64][65] For instance, Eu 3+ is often used as a magnetic dipole because of its 5 D 0 → 7 F 1 MD transition, which falls in the visible range (≈610-620 nm). [66][67][68] The second approach comprises internal modification of metallic nanostructures. The generation of a magnetic field requires a proximal electric field to support the magnetic dipole and store magnetic energy. ...
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We report on an experimental technique to quantify the relative importance of electric and magnetic dipole luminescence from a single nanosource in structured environments. By attaching a $Eu^{3+}$-doped nanocrystal to a near-field scanning optical microscope tip, we map the branching ratios associated to two electric dipole and one magnetic dipole transitions in three dimensions on a gold stripe. The relative weight of the electric and magnetic radiative local density of states can be recovered quantitatively, based on a multilevel model. This paves the way towards the full electric and magnetic characterization of nanostructures for the control of luminescence at the single emitter level.
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