APPLIED PHYSICS LETTERS 88, 042101 共2006兲

Enhancement of photoluminescence from Yb and Er co-doped Al2O3 films
by an asymmetric metal cavity
Nobuto Managaki, Minoru Fujii,a兲 Toshihiro Nakamura, Yoshiko Usui, and Shinji Hayashi
Department of Electrical and Electronics Engineering, Faculty of Engineering, Kobe University,
Rokkodai, Nada, Kobe 657-8501, Japan

共Received 24 June 2005; accepted 29 November 2005; published online 23 January 2006兲
The effects of Au thin layers on luminescence properties of Er and Yb co-doped Al2O3 films werestudied. The 1.54 ␮m emission of Er3+ excited by the energy transfer from Yb3+ is found to bestrongly enhanced by placing a Au layer on top of the films. Photoluminescence excitation spectraof the 1.54 ␮m PL of Er3+ and time transients of the 980 nm emission of Yb3+ revealed that PL
enhancement arises from two different mechanisms. The first one is the enhancement of local
electric fields accompanied by the excitation of surface plasmons of rough Au surfaces. The otherone is the enhancement of energy transfer rate from Yb3+ to Er3+ caused by the modification of thelocal photonic mode density at the position of the Yb–Er coupled system. © 2006 American
Institute of Physics. 关DOI: 10.1063/1.2166685兴
The emission property of an emitter can be significantly
modified if it is placed in a microcavity or in the vicinity ofmetal surface.1–3 The simplest explanation of the phenomenon is that the emitter is regarded as an electromagnetic
dipole oscillator, interacting with the reflected field of theemitter’s dipole radiation.4 If the reflected field interacts withthe emitter in phase, the radiative decay rate is enhanced,
while out of phase, it decreases. As a result, the radiative rateoscillates with the distance between the emitter and the surface. The other explanation of the phenomenon is that the
local photonic mode density 共PMD兲 is changed in the vicinity of metal surface, and this results in the modification of
the radiative rate, because it is proportional to PMD.4 The
modification of PMD is considered to affect the rate of
Förster energy transfer as well,5–7 although the mechanism is
not very clear.8 The control or the enhancement of Förster
energy transfer is of great importance for a lot of fields inwhich sensitized excitation of optically forbidden transitionsof an accepter by the energy transfer from a donor isrequired.
One of the examples of the Förster energy transfer issensitized excitation of Er by the energy transfer from Yb. Er
has trivalent ionic states 共Er3+兲 in solid matrices, and shows
atomic-like sharp luminescence at around 1.54 ␮m due tothe intra-4f shell transition.9–11 Since the wavelength corresponds to the minimum loss band of silica optical fibers, theEr-doped silica fiber amplifier 共EDFA兲 is now a standard
device for long-distance optical telecommunication. The
intra-4f shell transition is parity forbidden for dipole transition. The radiative rate as well as the absorption cross section
are thus very small. To overcome the small absorption crosssection and realize sufficient amplification, very long fibers
are used in EDFA. Another approach to overcome this problem is to simultaneously dope photosensitizers in glasses andto excite Er3+ indirectly by the energy transfer from the sensitizers. A typical photosensitizer for Er3+ is Yb3+.12–16 In theinset of Fig. 1共a兲, the process of sensitized excitation of Er3+
by Yb3+ is shown schematically. Yb3+ has only one exciteda兲

Author to whom correspondence should be addressed; electronic mail:fujii@eedept.kobe-u.ac.jp

state in the 4f shell at 980 nm, which exactly coincides withthe excitation energy of Er3+ to the second excited state.
Since absorption cross section of Yb3+ is much larger thanthat of Er3+, in the co-doped system, excitation light at
980 nm is absorbed mainly by Yb3+, and then the electronic
excitation is transferred to the second excited state of Er3+ by
the Förster mechanism.
The purpose of this work is to obtain enhanced 1.54 ␮memission from Er3+ for Er and Yb co-doped glasses by controlling local electric fields and PMD by placing a thin metal
layer on top. The samples studied are Al2O3 films containing
Er and Yb 共Er: Yb: Al2O3兲. We will demonstrate that by placing a Au layer, PL intensity is strongly enhanced, and discuss
the mechanism from photoluminescence excitation 共PLE兲
spectra of the 1.54 ␮m PL of Er3+ and time transients of the980 nm PL of Yb3+.
Samples were prepared by a co-sputtering method.17,18
Al2O3, Er2O3, and Yb2O3 were simultaneously sputterdeposited on quartz substrates in Ar gas. The thickness offilms was fixed to be about 130 nm for all the samples. To
activate Er, deposited films were annealed in air for 30 min
at 900 ° C. In this work, we fixed Yb concentration to about
0.06 at. % and changed Er concentration from0 to 0.14 at. %. After the annealing, Au thin films 共25 or
67 nm兲 were deposited on the Er: Yb: Al2O3 films. Atomic
force microscope observations revealed that rms roughnessesof 0.7 and 1.1 nm exist on the surfaces of the 25 and 67 nm
thick Au films, respectively. To take PL and PLE spectra, andtime transients of PL, an optical parametric oscillator
pumped by the third harmonic of a Nd:YAG laser 共pulse
width 5 ns, repetition frequency 20 Hz兲 was used as an excitation source, and a liquid nitrogen cooled Ge photodiodeor near infrared photomultiplier 共Hamamatsu, R5509-72兲 forthe detection of near-infrared emission from Er3+ and Yb3+.
The
excitation
wavelengthwaschangedfrom
450 to 1050 nm.
The PLE spectra detected at 1.54 ␮m for the
Er: Yb: Al2O3 and Er: Al2O3 samples are shown in Fig. 1共a兲.
Several peaks corresponding to electronic excitation of Er3+
can be seen. By doping Yb, the PLE peak at 980 nm grows
significantly and a broad band appears in the range between

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042101-2

Managaki et al.

FIG. 1. 共a兲 PLE spectra detected at 1.54 ␮m for Er: Yb: Al2O3 and
Er: Al2O3 samples. Er concentration is fixed to 0.14 at. %. Inset: schematic
illustration of energy level diagrams of Yb3+ and Er3+. 共b兲 PL decay curves
detected at 975 nm for the samples with different Er concentrations. The
lifetimes estimated at the 1 / e of the intensity are 792, 298, 162, and 139 ␮sfor the Er concentrations of 0, 0.03, 0.07 and 0.14 at. %, respectively. Inset:
energy transfer rate estimated from the shortening of the lifetime at 975 nm.

850 and 1000 nm. Yb3+ has an absorption peak at 980 nm
and a broad absorption band in the range from
850 to 1000 nm.19 Therefore, the enhancement of the excitation cross section in this wavelength range is due to indirect
excitation of Er3+ to the second excited state by the energy
transfer from Yb3+. From the intensity ratio of Er: Al2O3 and
Er: Yb: Al2O3 samples at 980 nm excitation, Er3+ in
Er: Yb: Al2O3 samples is excited mainly via Yb3+.
The direct evidence of energy transfer from Yb3+ to Er3+
is obtained from the time transient of PL from Yb3+ at
980 nm. Figure 1共b兲 shows the PL decay curves at 975 nm
for the samples containing 0.06 at. % of Yb and different
amounts of Er. PL is excited at 900 nm, where Er3+ does nothave any states and cannot be excited directly. Without Er
doping, the decay curve is nearly a single exponential function and the decay time is 792 ␮s. By Er doping, the decay
curve deviates significantly from a single exponential function and the lifetime becomes shorter, which evidences nonradiative energy transfer from Yb3+ to Er3+. In a simplified
model, where the shortening of lifetime is caused by theenergy transfer to Er3+, the relaxation rate of Yb3+ can be
expressed as WYb,Er = WYb + WET, where WYb,Er and WYb are

Appl. Phys. Lett. 88, 042101 共2006兲

FIG. 2. 共a兲 PLE spectra detected at 1.54 ␮m for Er: Yb: Al2O3 samples withand without a Au layer 共25.0 or 67 nm兲 on top. Er and Yb concentrations are
0.14 and 0.06 at. %, respectively. Inset: enhancement factor of PL at
1.54 ␮m as a function of Au thickness. The excitation energy is 975 nm. 共b兲
PL decay curves detected at 975 nm for Er: Yb: Al2O3 and Yb: Al2O3
samples. The curves with and without a Au layer 共67 nm兲 are compared.

measured decay rates of 975 nm emission of Yb3+ for
Er: Yb: Al2O3 and Yb: Al2O3 samples, respectively, and WET
is the energy transfer rate. We estimated the decay rates byfitting the decay curves by a stretched exponential functionand extracted the energy transfer rate. The estimated energy
transfer rates are plotted in the inset of Fig. 1共b兲. Since WET
is proportional to the Yb–Er interaction coefficient and to Yb
and Er concentrations, it becomes larger with increasing Er
concentration.
It is worth noting that although Er3+ is not directly excited by 900 nm light, it can be excited by the energy transfer from Yb3+, and the radiative transition from the second
excited to the ground states of Er3+ can in principle contribute to the 980 nm PL. To estimate whether the contributionof Er3+ is negligibly small or not, we measured the decay
curves of the PL at 975 nm for the samples not containing
Yb by exciting Er3+ by 520 nm light. The measured lifetime
was shorter than 20 ␮s. This time is much shorter than thoseobtained in co-doped samples shown in Fig. 2. Furthermore,
the intensities were much smaller. Therefore, the contribution of Er3+ to the 980 nm emission is negligibly small, andthe observed shortening of the lifetime by Er doping is due toenergy transfer to Er3+.
Figure 2共a兲 shows PLE spectra detected at 1.54 ␮m for
Er: Yb: Al2O3 samples with two different thicknesses of Au

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042101-3

Appl. Phys. Lett. 88, 042101 共2006兲

Managaki et al.

layers on top. Er and Yb concentrations are fixed to be 0.14
and 0.06 at. %, respectively. In the excitation wavelength
range between 450 and 680 nm, where Er is excited by direct
photon absorption, no significant change is seen on the spectra. On the other hand, strong enhancement of the intensity isobserved when the samples are excited by longer-wavelength
light. In the inset of Fig. 2共a兲, PL intensity at 1.54 ␮m excited by 975 nm light for Er: Yb: Al2O3 samples with various
Er concentrations are plotted as a function of the thickness of
the Au layer. The intensity is normalized to that of thesamples without Au layers. The vertical axis thus represents
the “enhancement factor.” We can see strong enhancement
for all the samples. No significant difference in the enhancement factor is seen between the Au thicknesses of 25 and67 nm.
One of the possible mechanisms for the enhancement of
the 1.54 ␮m PL when excited at around 980 nm is that local
electric field intensity of the incident light is enhanced due tothe roughness on the Au layer; that is, surface plasmons
propagating on the surface 共or at the interface兲 are perturbed
by the roughness and the perturbation allows the direct coupling of the plasmons with the incident light. The energy ofthe surface plasmon resonance depends strongly on the condition of the metal surface. In the present samples, we measured transmittance and reflectance of the Au films, and estimated the ratio of photons absorbed by the films. We foundthat the absorption due to surface plasmon resonances starts
from about 850 nm for both the 25 and 67 nm Au films.
Therefore, the observed strong PL enhancement when excited at around 980 nm and the lack of it when excited at 520
and 650 nm can be qualitatively explained by different degree of local field enhancement depending on the excitation
wavelengths.
Another possible mechanism of the PL enhancement
when excited at 980 nm is that the rate of energy transferfrom Yb3+ to Er3+ is enhanced by the existence of the Au
layer. In Fig. 2共b兲, the PL decay curves of Yb3+ detected at
980 nm for the samples with and without Au layers 共67 nm兲
are compared. For the sample not containing Er, we can see
the shortening of the lifetime. The shortening can be explained by two mechanisms.20 The first one is the enhancement of radiative rate due to that of PMD. The other one isresonant coupling of excited Yb3+ with the surface plasmons
and also the excitation of intra- and interband transitions of
the Au layer. These couplings result in nonradiative energy
transfer from Yb3+ to the Au layer, and thus the luminescence is quenched and the lifetime is shortened.
The shortening of lifetime can also be seen for Er andYb co-doped samples. By the procedure mentioned earlier,
we estimated the energy transfer rates for all series ofsamples. In Fig. 3, the energy transfer rates divided by thoseof the samples without Au layers are plotted as a function of
the Au film thickness. We can see 50% to 80% increase of
the energy transfer rate. The enhancement is considered to be
due to that of PMD by placing the Au layer nearby the
Yb–Er coupled systems. However, in order to discuss theeffects more in detail and quantitatively, the dependence ofthe rate as a function of the distance between the active andAu layers should be studied. These experiments are now underway in our laboratory. In Fig. 3, the energy transfer rates
do not depend strongly on Au thickness. It is of great interest
to study much thinner island films, because the interaction

FIG. 3. Enhancement factor of the energy transfer rates as a function of Au
film thickness for the samples with different Er concentrations.

between light and surface plasmons in islands is much different from that in continuous films.
In conclusion, we demonstrated that the 1.54 ␮m emission from Er and Yb co-doped Al2O3 films is strongly enhanced by just placing a thin Au layer on top when they are
excited by near-infrared light. The enhancement is caused by
the strong local electric fields accompanied by the excitationof surface plasmons in a rough metal surface. In addition, we
demonstrated the enhancement of the energy transfer rate,
which is probably due to the enhancement of PMD in thepositions of Yb–Er coupled systems.
This work was supported by a Grant-in-Aid Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
G. W. Ford and W. H. Weber, Phys. Rep. 113, 197 共1984兲.
R. M. Amos and W. L. Barnes, Phys. Rev. B 55, 7249 共1997兲.
3
P. T. Worthing, R. M. Amos, and W. L. Barnes, Phys. Rev. A 59, 865
共1999兲.
4
W. L. Barnes, J. Mod. Opt. 45, 661 共1998兲.
5
M. Hopmeier, W. Guss, M. Deussen, E. O. Göbel, and R. F. Mahrt, Phys.
Rev. Lett. 82, 4118 共1999兲.
6
P. Andrew and W. L. Barnes, Science 290, 785 共2000兲.
7
R. L. Hartman and P. T. Leung, Phys. Rev. B 64, 193308 共2001兲.
8
M. J. A. de Dood, J. Knoester, A. Tip, and A. Polman, Phys. Rev. B 71,
115102 共2005兲.
9
A. Polman and F. C. J. M. van Veggel, J. Opt. Soc. Am. B 21, 871 共2004兲.
10
A. Biswas, G. S. Maciel, R. Kapoor, C. S. Friend, and P. N. Prasad, Appl.
Phys. Lett. 82, 2389 共2003兲.
11
A. Polman, J. Appl. Phys. 82, 1 共1997兲.
12
L. D. da Vila, L. Gomes, L. V. G. Tarelho, S. J. L. Ribeiro, and Y.
Messadeq, J. Appl. Phys. 93, 3873 共2003兲.
13
W. H. Wong, E. Y. B. Pun, and K. S. Chan, Appl. Phys. Lett. 84, 176
共2004兲.
14
J. Zhou, F. Moshary, B. M. Gross, M. F. Arend, and S. A. Ahmed, J. Appl.
Phys. 96, 237 共2004兲.
15
C. Strohhöfer and A. Polman, Opt. Mater. 共Amsterdam, Neth.兲 21, 705
共2003兲.
16
A. Suarez-Garcia, R. Sarna, M. Jiménez de Castro, C. N. Afonso, and I.
Vickridge, Appl. Phys. Lett. 84, 2151 共2004兲.
17
M. Fujii, M. Yoshida, S. Kanzawa, S. Hayashi, and K. Yamamoto, Appl.
Phys. Lett. 71, 1198 共1997兲.
18
M. Fujii, M. Yoshida, S. Hayashi, and K. Yamamoto, J. Appl. Phys. 84,4525 共1998兲.
19
C. Lester, A. Bjarklev, T. Rasmussen, and P. G. Dinesen, J. Lightwave
Technol. 13, 740 共1995兲.
20
J. Kalkman, L. Kuiper, and A. Polman, Appl. Phys. Lett. 86, 041113
共2005兲.
1
2

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