Direct demonstration of sensitization at 980nm optical
excitation in erbium-ytterbium silicates
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Vanhoutte, Michiel, Bing Wang, Zhiping Zhou, Jurgen Michel,
and Lionel C. Kimerling. “Direct demonstration of sensitization at
980nm optical excitation in erbium-ytterbium silicates.” In 7th
IEEE International Conference on Group IV Photonics, 308-310.
Institute of Electrical and Electronics Engineers, 2010. ©
Copyright 2010 IEEE
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http://dx.doi.org/10.1109/GROUP4.2010.5643345
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P2.29
17:15 – 19:00
Direct demonstration of sensitization at 980nm
optical excitation in erbium-ytterbium silicates
Michiel Vanhoutte*, Student Member, IEEE, Bing Wang**, Zhiping Zhou**,
Jürgen Michel, Member, IEEE and Lionel C. Kimerling, Member, IEEE
Microphotonics Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
Abstract — Sensitization of erbium by ytterbium in
ErxYb2-xSiO5 thin films at 980nm optical excitation is
demonstrated by means of comparison of the 1.54µm
photoluminescence intensities excited with 488nm and
980nm light. Additionally, it is shown that detrimental ErEr interactions such as concentration quenching increase
non-radiative decay rates at high erbium concentrations.
Dilution of erbium by ytterbium reduces these
interactions, leading to an increase of internal quantum
efficiency.
I. INTRODUCTION
Erbium-doped materials have important applications in
silicon photonics thanks to their strong light emission at
1.54µm. Nevertheless, the low solubility of erbium (typically
NEr < 1020cm-3) limits the achievable gain in erbium-doped
materials. Erbium compounds such as erbium oxide (Er2O3)
and erbium silicates (Er2SiO5 and Er2Si2O7) have been
proposed as a solution to this problem [1]-[3]. In these
material systems, erbium concentrations are of the order NEr =
1022cm-3. At these high erbium concentrations however, the
small Er-Er distances lead to increased interionic interactions
such as upconversion and concentration quenching. These
phenomena deplete the population of the excited erbium level
at 1.54µm and are hence detrimental for light emission.
Dilution of erbium by means of partial substitution with other
rare earth ions helps to decrease Er-Er interactions. This idea
has already been implemented in ErxY2-xO3 and ErxY2-xSiO5,
where erbium is partially substituted by yttrium [4]-[5].
Another strategy to achieve dilution in erbium compounds is
to use ytterbium as a dilutant instead of yttrium. The
successful fabrication of erbium-ytterbium monosilicates
(ErxYb2-xSiO5) and their light emission at 1.54um have
recently been demonstrated [6]. In addition to dilution of
erbium, ytterbium is an excellent candidate for sensitization of
erbium at 980nm optical pumping. With an absorption cross* Corresponding author (e-mail: mvhoutte@mit.edu)
** Current address: State Key Laboratory of Advanced
Optical
Communication
Systems
and
Networks,
Peking University, Beijing, 100871, China.
978-1-4244-6346-6/10/$26.00 ©2010 IEEE
section at 980nm of about an order of magnitude higher than
that for erbium and energy resonance of the 2F5/2  2F7/2
transition in Yb3+ with the 4I11/2  4I15/2 transition in Er3+,
ytterbium can absorb the pump energy and subsequently
transfer it to erbium [7].
In the present work, the effect of ytterbium as a sensitizer at
980nm is demonstrated by comparison of the
photoluminescence (PL) intensity excited at 488nm and
980nm, causing direct excitation of erbium and excitation
through ytterbium respectively. Secondly, through a study of
the decay rate of the 1.54µm PL signal, the effect of erbium
dilution with ytterbium is investigated.
II. EXPERIMENTAL
Erbium-ytterbium silicate thin films were deposited by
means of RF-magnetron co-sputtering of Er2O3, Yb2O3 and
SiO2 targets on crystalline p-type Si. The composition was
controlled by variation of the power applied to the different
targets during co-sputtering and was verified by energydispersive X-ray spectroscopy (EDS) and Rutherford
backscattering (RBS) measurements. The deposited films were
annealed in O2 for 1h at 1200ºC.
Room-temperature PL experiments were done with free
space excitation with 488nm light from an argon laser and
980nm diode laser light. The pump signal was modulated at
81Hz and detected with a liquid N2-cooled photomultiplier
tube connected to a lock-in amplifier.
III. RESULTS AND DISCUSSION
A. Sensitization of erbium by ytterbium at 980nm pumping
Figures 1a and 1b show the photoluminescence spectra of
erbium-ytterbium silicate thin films with different
compositions, excited with 488nm and 980nm light
respectively. It can be seen that the shape of the PL spectrum
for a given composition does not depend on the pump
wavelength. However, a very different dependence of PL
308
intensity on erbium-ytterbium composition is clearly observed
for the two different pump wavelengths.
At 488nm pumping, erbium ions are directly excited since
ytterbium has no atomic levels in this energy range. The only
role ytterbium can play in this case is to dilute the erbium
concentration. Figure 1a shows that the film with intermediate
erbium-ytterbium content Er0.5Yb1.5SiO5 emits the strongest
PL, with a an intensity of about 1.25x stronger than the PL for
composition Er0.2Yb1.8SiO5 and about 2.8x stronger than the
PL for composition Er1.0Yb1.0SiO5. At 980nm pumping, both
ytterbium and erbium can absorb the pump light and
sensitization by ytterbium can occur. Comparing the PL of the
same films pumped by 980nm light, Figure 1b shows that the
PL increases sharply with decreasing erbium and increasing
ytterbium content: the PL of Er0.2Yb1.8SiO5 is about 2.3x
stronger than the PL of Er0.5Yb1.5SiO5 and 28x stronger than
the PL of Er1.0Yb1.0SiO5.
(a) 488 optical pumping
This result can be explained by Equation 1 for the
photoluminescence intensity in the low-flux regime. In this
expression, σex is the effective erbium excitation cross-section
at the excitation wavelength, Φ is the pump flux, τrad is the
radiative lifetime and τ is the overall signal decay time.
(Eq.1)
The overall decay time τ follows from the rate equation of
the first excited erbium level given in Equation 2. This
equation includes effects such as radiative decay, nonradiative decay by phonon emission, upconversion and
parasitic effects such as concentration quenching. In the latter
effect, the excitation energy migrates between neighboring
erbium ions until a luminescence quenching center (e.g. an
OH-impurity) is reached and the excitation energy is lost. The
energy migration rate is proportional to the concentration of
quenching centers Nq and depends strongly on erbium
concentration NEr (see below) [8].
(Eq. 2)
In the studied low-flux regime, the effects of upconversion
can be neglected. The decay rate of the other effects may
depend on the refractive index and erbium environment (τr),
the phonon energy (τnr), the erbium concentration (CErEr, NEr)
and the fabrication conditions (quenching center concentration
Nq). However, for a given film, the overall decay rate and
hence the factor τ/τrad in Equation 1 should not depend on the
pump wavelength.
x5
(b) 980nm optical pumping
Fig. 1. Room temperature photoluminescence spectra around
1550nm of erbium-ytterbium silicate films with different
compositions at 488nm and 980nm optical pumping. All films
were annealed for 1h in O2 at 1200ºC.
For a film with a given erbium concentration NEr, it follows
from Equation 1 that the ratio of photoluminescence
intensities at 488nm pumping and 980nm pumping is given by
(Eq. 3)
Since the fluxes are independent of erbium concentration,
the only remaining explanation of the different dependence of
309
PL intensity on erbium concentration must be due to an
increasing effective excitation cross-section at 980nm with
increasing ytterbium concentration. This is in other words a
demonstration of sensitization by ytterbium at 980nm.
The dependence of rise and decay times on erbium
concentration demonstrates how dilution of the erbium
concentration with ytterbium can indeed mitigate parasitic
effects such as energy migration to luminescence centers and
thus increase the internal quantum efficiency of light emission.
B. Effect of erbium dilution by ytterbium
Figure 2 shows the rise and decay of the PL signal at
1540nm of the different silicate films on a semilog plot. Two
features of the lifetime curves draw attention. First of all, the
decay is singly exponential. Secondly, the rise and decay rates
increase significantly with erbium concentration.
IV. CONCLUSION
Based on the study of 1.54 µm photoluminescence intensity
and lifetimes of ErxYb2-xSiO5 thin films, we have shown that
the role of ytterbium in erbium-ytterbium silicates is twofold.
The difference in dependence on ytterbium concentration of
the PL intensity at 980nm and 488nm excitation can only be
explained by a strong dependence of the 980nm excitation
cross-section on ytterbium concentration. This is thus a
demonstration of sensitization of erbium through ytterbium at
980nm excitation. Secondly, the rise and decay rates of the
1.54µm signal are singly exponential and increase strongly
with increasing erbium concentration. This behavior is
explained by concentration quenching due to migration of the
excitation energy to luminescence quenching centers, which is
reduced by means of dilution of the erbium concentration by
ytterbium.
ACKNOWLEDGMENTS
Fig. 2. Semilog plot of rise and decay times of the PL signal at
1540nm (excited with 980nm light) for different erbiumytterbium concentrations. The rise and decay rates are
exponential and increase with erbium concentration.
This behavior can be explained by the rate equation of the
first excited erbium level given in Equation 2. In this equation,
concentration quenching due to energy migration to
luminescence quenching centers is the only effect that has a
strong erbium concentration dependence while still remaining
singly exponential. The coefficient of energy migration CErEr
describes dipole-dipole interactions between erbium ions and
scales with the distance between erbium ions as 1/d6. Since the
interionic distance scales with concentration as 1/NEr3, this
coefficient will increase quadratically with erbium
concentration NEr.
Though the upconversion coefficient C11 has a similar
quadratic dependence on the erbium concentration, this effect
becomes only important at higher pump fluxes and does not
cause an exponential decay. Upconversion can therefore not
explain the observed lifetime behavior.
This work was supported by the Si-based Laser Initiative of
the Multidisciplinary University Research Initiative (MURI)
sponsored by the Air Force Office of Scientific Research
(AFOSR).
Bing Wang acknowledges support from the Chinese
Scholarship Council during his stay at MIT.
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