APPLIED PHYSICS LETTERS 92, 132105 共2008兲

Enhancement of upconversion luminescence of Er doped Al2O3 films
by Ag island films
Takeho Aisaka, Minoru Fujii,a兲 and Shinji Hayashi
Department of Electrical and Electronics Engineering, Faculty of Engineering, Kobe University, Rokkodai,Nada, Kobe 657-8501, Japan

共Received 5 October 2007; accepted 20 February 2008; published online 1 April 2008兲
Strong enhancement of upconversion photoluminescence 共PL兲 of Er3+ in the visible range wasachieved by placing it near rough Ag island films. In order to understand the mechanism of theenhancement, PL was excited by 978 and 488 nm light and the enhancement factors were obtainedin a wide spectral range 共520 nm to 1.6 ␮m兲. The emission and excitation wavelength dependenceof the enhancement factors revealed that both the incident electric fields and the radiative decay rateof Er3+ are enhanced due to the excitation of surface plasmons supported by Ag islands. It was foundthat the upconversion PL is more sensitive to the enhancement of the incident electric field than thedownconversion PL because of the multiphoton absorption process. The present result indicates thatthe upconversion PL obtains more benefit from the metal-enhanced fluorescence technique than thedownconversion PL. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2896303兴
In recent years, upconversion luminescence of rare-earth
doped materials has attracted much interest in their capability of converting infrared radiation into visible light. One of
the well studied systems is Er-doped glasses, which show
green and red photoluminescence 共PL兲 under excitation bynear-infrared 共NIR兲 light. Since low-cost and high-power
NIR laser diodes are available for the excitation source, Erdoped glasses are of great interest for applications such as
lasers,1 three-dimensional displays,2 and biological fluorescence imaging.3–6 However, the photon absorption cross section of Er3+ is very small due to the dipole-forbidden natureof the intra-4f transitions and, thus, the intensity of the upconversion PL is relatively low. In order to overcome thesmall absorption cross section, Yb3+ is usually doped simultaneously as a photosensitizer.
In this work, we will show another approach to enhance
the upconversion luminescence of Er3+. It has been demonstrated for different kinds of materials that the luminescenceintensity of emitters is strongly enhanced when they areplaced near metal nanostructures 共metal-enhanced
fluorescence兲.7–11 The enhancement is caused by the excitation of surface plasmons 共SPs兲 supported by metal nanostructures. The enhancement mechanism can be classified
into two categories, i.e., the enhancement of an electric fieldof incident light and the enhancement of the radiative decayrate. In this work, we apply the same strategy to the upconversion PL of Er3+. A similar approach was reported for Erdoped heavy-metal oxide glasses containing silver nanoparticles. However, the degree of the enhancement was not verylarge and the role of SP excitation on the PL enhancement
was not clarified.12
One of the purposes of this work is to achieve a large
enhancement of upconversion PL of Er3+ by placing it on Ag
island films with different morphologies. Another purpose isto clarify the mechanism of the PL enhancement. To achievethis, we measure upconversion and downconverson PL excited at 978 nm and downconversion PL excited at 488 nm,and obtain PL enhancement factors in a wide wavelengtha兲Author to whom correspondence should be addressed. Electronic mail:fujii@eedept.kobe-u.ac.jp.

range. By comparing the wavelength dependence of the enhancement factors with that of SP absorption spectra of Ag
islands and by comparing the enhancement factors of upconversion and downconversion PL, the relative contributions of
two enhancement processes are discussed.
Samples studied are Er doped Al2O3 films about 130 nmin thickness deposited on fused quartz substrates. Al2O3 andEr2O3 sputtering targets were simultaneously sputtered in Ar
gas of 2.7 Pa using a rf magnetron sputtering apparatus. Thefilms were then annealed in N2 gas atmosphere at 900 ° C for
30 min. The Er concentration was fixed to about 4.2 at. %. In
this Er concentration range, cooperative upconversion plays
a major role for the visible PL.13 After the annealing, Ag thinfilms approximately 7, 10, 13, and 16 nm in thicknesses weredeposited by vacuum evaporation. The deposition rate monitored by a quartz crystal microbalance was 0.02 nm/ s. Afterthe deposition, the samples were annealed at 300 ° C for 1 h.
Figure 1 shows atomic force microscope 共AFM兲 共SeikoInstruments Inc. NPX200兲 images of sample surface. Thethicknesses of Ag films are changed from 7 to 16 nm. We
can see that the size of Ag particles and the distance between

FIG. 1. 共Color online兲 AFM images of Ag island films deposited on Erdoped Al2O3 thin films. The mass thicknesses are 共a兲 7, 共b兲 10, 共c兲 13, and共d兲 16 nm. The average diameters are 共a兲 104, 共b兲 147, 共c兲 219, and 共d兲
313 nm and the average heights are 共a兲 16, 共b兲 28, 共c兲 52, and 共d兲 91 nm.

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92, 132105-1© 2008 American Institute of Physics
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132105-2

Aisaka, Fujii, and Hayashi

FIG. 2. 共Color online兲 PL spectra of Er-doped Al2O3 thin films with andwithout Ag island films on top excited at 共a兲 978 and 共b兲 488 nm. The mass
thicknesses of Ag island films are 7, 10, 13, and 16 nm. Four main peaks
correspond to transitions from 2H11/2, 4S3/2, 4F9/2, and 4I13/2 states to 4I15/2
state of Er3+. Spectra below and above 800 nm are normalized by the intensity of 4S3/2 and 4I13/2 peaks, respectively, of the sample without a Ag island
film. In 共a兲, spectra below 800 nm for the samples without a Ag island film
and with 7-nm-thick Ag film and spectra above 800 nm for all the samples
are multiplied by 5.

them increase as the mass thickness increases. The average
diameters are 共a兲 104, 共b兲 147, 共c兲 219, and 共d兲 313 nm andthe average heights are 共a兲 16, 共b兲 28, 共c兲 52, and 共d兲 91 nm.
For these samples, optical transmittance 共T兲 and reflectance
共R兲 were measured in the 400– 1600 nm range using a UVvisible-NIR scanning spectrometer 共Shimadzu UV-3101兲 andthe absorptance, which is defined as 1-T-R, was obtained.
For the PL measurements, samples were excited by
978 nm light from a cw Ti:sapphire laser or a 488 nm line ofan Ar ion laser. PL excitation was made from the substrateside about 45° from the normal to the surface and PL to thenormal to the surface was collected from the same side. PLspectra were recorded using a single grating monochromatorequipped with a charge coupled device for visible PL or withan InGaAs diode array for NIR PL. All the measurements
were performed at room temperature.
Figures 2共a兲 and 2共b兲 show PL spectra excited at 978 and488 nm, respectively. Three peaks in the visible range areassigned to the transitions from the 2H11/2, 4S3/2, and 4F9/2
states to the ground state 共 4I15/2兲, respectively, and a peak in
the NIR range to the 4I13/2 to 4I15/2 transition. The three peaksin the visible range in Fig. 2共a兲 are upconversion PL and theother peaks are downconversion PL. The spectra below
800 nm are normalized to the intensity of the 4S3/2 peak of
the sample without Ag islands and those above 800 nm tothat of the 4I13/2 peak. Figure 2共a兲 clearly demonstrates that
PL intensities are strongly enhanced when Ag island films
exist and the degree of the enhancements depends on thethickness of the films. Furthermore, the enhancement ismuch larger for the upconversion PL in the visible range thanfor the downconversion PL in the NIR range.
Similar enhancement of PL intensities are observed
when the samples are excited at 488 nm 关Fig. 2共b兲兴. How-Appl. Phys. Lett. 92, 132105 共2008兲

FIG. 3. 共Color online兲 Enhancement factors obtained by dividing PL spectraof samples with Ag islands by those without Ag islands 共symbols兲 共left axis兲.
Excitation wavelengthes are 共a兲 978 and 共b兲 488 nm. For comparison, absorptance 共1-transmittance-reflectance兲 spectra are also shown 共right axis兲.
Note that the same absortance spectra are shown in 共a兲 and 共b兲.

ever, the enhancement in the visible range is much smaller
than that in Fig. 2共a兲. At this excitation wavelength, broad
background PL, apparently differing from Er3+ PL, appears.
The broad PL is considered to arise from oxides on the surface of Ag islands or emission of SPs supported by Ag islands. In order to extract the information of Er3+ PL, we
subtracted PL spectra of Ag island films deposited on fuzedquartz substrates from those of the samples. The spectra
shown in Fig. 2共b兲 are those after the subtraction.
To discuss the degree of PL enhancement in more detail,wavelength dependence of enhancement factors are obtainedby dividing PL spectra of the samples with Ag islands by
those without Ag islands. The divided spectra for 978 and488 nm excitation are shown in Figs. 3共a兲 and 3共b兲, respectively, by symbols 共left axis兲. Note that the data of the divided spectra are limited in specific wavelength ranges because of discrete PL bands of Er3+. The absorptance spectra
are also shown in the same figure 共right axis兲 to discuss thePL enhancement mechanism. Broad absorption bands due toSP resonance of Ag islands can be seen. The SP resonanceshifts to longer wavelengths and broadens as the Ag island
films become thick, i.e., the Ag particles become large, due
mainly to the retardation effect.14,15
In Fig. 3共a兲, we can see that the upconversion PL isstrongly enhanced at around 520 nm, except for the sample
with the thinnest Ag film. The maximum enhancement factor
reaches 220 for the sample with a 13-nm-thick Ag film.
Compared to this, the enhancement factors of downconversion PL at 1.54 ␮m are very limited 共at maximum 9兲. Thislarge difference of the enhancement factors between visibleand NIR PL cannot be seen when the spectra are excited at
488 nm 关Fig. 3共b兲兴. In this case, the maximum enhancement
factor in the visible range is 25, while that in the NIR range
is 8.
In general, metal-enhanced fluorescence arises from theenhancement of an electric field of incident light and also
from that of the radiative decay rate. The distinction of these

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132105-3

Appl. Phys. Lett. 92, 132105 共2008兲

Aisaka, Fujii, and Hayashi

two contributions is usually not simple but rough estimationof relative contributions is possible from the data in Fig. 3.
We start from the simplest case in Fig. 3共b兲, where the excitation is made to the 4F7/2 state by one photon absorption. In
this case, the degree of the incident field enhancement isindependent of the emission wavelength. Therefore, the observed wavelength dependence of the enhancement factors
reflects that of the radiative decay rate. In the first approximation, the radiative decay rate is strongly enhanced at thewavelength where SPs are efficiently excited, i.e., the wavelength where the absorptance is large. In fact, we can see inFig. 3共b兲 that wavelength dependence of the enhancement
factors is quite similar to that of the absorptance of Ag
islands.
In Fig. 3共b兲, for the sample with a 7-nm-thick Ag film,
the absorptance in the NIR region 共around 1.54 ␮m兲 is 0.
This means that the small PL enhancement in this region
共factor of about 2.5兲 is purely due to the enhancement ofincident electric field. The maximum enhancement factor forthe sample is about 14 at 520 nm. Therefore, in this sample,
the PL enhancement due to the enhancement of electric fields
of incident light is about 2.5 and that due to the radiative
decay rate enhancement is about 5.6 at 520 nm, resulting in
the total enhancement factor of 14.
Similar discussion can be made for the same sample excited at 978 nm 关Fig. 3共a兲兴. In this case, the absorptance atthe excitation wavelength is also 0 and, thus, no enhancement of NIR PL is expected. In fact, the enhancement factorsof the sample around 1.54 ␮m are almost unity. In contrast,
upconversion PL of the same sample in the visible range isslightly enhanced and the enhancement factor depends on thewavelength. Since the incident electric field is not enhanced,
the observed PL enhancement is purely due to the enhancement of the radiative decay rate. We see that the radiative
decay rate enhancement at 520 nm is about 2.5. This value isabout a half of that obtained above for 488 nm excitation.
The reason for the disagreement is not clear but it probably
arises from the fact that the subtraction of the backgroundsignal caused by Ag islands at 488 nm excitation is not
perfect.
In Fig. 3共a兲, the enhancement factors in the visible range
for the samples with Ag films thicker than 10 nm are muchlarger than that for the sample with a 7-nm-thick Ag film.
The significant difference in the enhancement factors can be
attributed to whether the incident electric field is enhanced ornot. Samples except for that with a 7-nm-thick Ag film havestrong absorption at 978 nm due to the excitation of SPs in
Ag islands. This result strongly suggests that in upconversion
PL, the enhancement of incident electric fields play a significant role in the strong PL enhancement.
For cooperative upconversion of Er3+, multiphotons arerequired to obtain one visible photon and the emission intensity 共Iup兲 is proportional to the nth power of the excitation
nintensity 共Iexc兲, Iup ⬀ Iexc
, where n is the number of excitationphotons required for emission of one visible photon.16–18
Therefore, the upconversion PL intensity is proportional to
the 2nth power of the incident field intensity in contrast to
the square of that in the case of downconversion PL. Thismeans that upconversion PL can obtain more benefit fromthe strong electric fields of SPs than downconversion PL.
This results in significant differences of the enhancement
factors between the visible upconversion PL and the NIR

downconversion PL in Fig. 3共a兲 or between the visible upconversion PL in Fig. 3共a兲 and the visible downconversion
PL in Fig. 3共b兲.
As discussed above, in the first approximation, the wavelength dependence of the enhancement factors reflects that of
the radiative decay rate enhancement. However, this model is
not applicable to upconversion PL if the excitation processes
are different at different emission bands. The number of photons 共n兲 required for upconversion PL can be obtained fromthe slope of a log Iup versus log Iexc plot. For the presentsamples, n for 520 and 660 nm PL were 2.0 and 1.7, respectively. This indicates that the 520 nm PL enjoys the benefitof the incident field enhancement more than the 660 nm PL.
This results in large difference of the enhancement factors of
the upconversion PL between 520 and 660 nm. In fact, thedifferences in the enhancement factors between 520 and660 nm are larger for the upconversion PL 关Fig. 3共a兲兴 thanthe downconversion PL 关Fig. 3共b兲兴 for almost all thesamples.
In conclusion, we have succeeded in achieving strong
enhancement of upconversion PL of Er3+ by placing it near
Ag island films. The maximum enhancement factors of 220
was achieved for the 520 nm emission. The present results
indicate that both the incident electric fields and the radiative
decay rates are enhanced and the combination of them resultsin strong upconversion PL. We also demonstrated that inupconversion PL because of the multiphoton absorption process, the intensity is much more sensitive to the field intensity of incident light than that of downconversion PL. Therefore, by tuning the maximum of the SP absorption at
978 nm, much stronger upconversion PL is expected.
This work is supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
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