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OPTICS LETTERS / Vol. 37, No. 24 / December 15, 2012
Blue emissions in Dy3+ doped Y4Al2O9 crystals
for temperature sensing
Zuzanna Boruc,1,* Marcin Kaczkan,1 Bartosz Fetlinski,1 Sebastian Turczynski,2 and Michal Malinowski1
Institute of Microelectronics and Optoelectronics, Koszykowa 75, Warsaw 00-662, Poland
Institute of Electronic Materials Technology, Wolczynska 133, Warsaw 01-919, Poland
*Corresponding author: [email protected]
Received October 24, 2012; revised November 19, 2012; accepted November 19, 2012;
posted November 20, 2012 (Doc. ID 178579); published December 13, 2012
Temperature dependent emission spectra and decay times of trivalent dysprosium (Dy3 ) activated Y4 Al2 O9 (YAM)
crystals have been studied for the first time (to our knowledge). The ratio of emission lines intensity can be used in
temperature measurements, as it is not dependent on the variability of absolute intensity. The Boltzmann model was
applied for modeling the temperature variation of the 4 I15∕2 and 4 F9∕2 states emissions relative intensities 455 and
481 nm, respectively. The calculated approximation gives highest sensor sensitivity of about 3 Г— 10в€’3 В°Cв€’1 for the
600°C–800°C range, which allows for an expectation of usefulness of Dy3 :YAM in high-temperature luminescence
thermometry. Also, the measured decay times are suitable for temperature sensing. В© 2012 Optical Society of
OCIS codes: 160.2540, 160.5690, 280.0280, 280.6780.
Dysprosium doped crystals have been quite well-known
as temperature sensors for over two decades [1–3] especially by the fluorescence intensity ratio (FIR) method.
However, not many papers about combinations of dysprosium ion with various hosts can be found in the field of
phosphor thermometry. The FIR method is based on the
variation of two emission lines and therefore it is desirable for those lines to be quite well thermally coupled.
Such coupling allows the Boltzmann distribution to be
applied for approximation of the ratio’s temperature dependence. Another method of using fluorescence features
for temperature measurements is the decay time method.
This method has a more complicated measurement process, but it often gives higher sensor sensitivities than
the FIR method.
In this work, we investigated thoroughly the temperature influence on the emissions of 0.3% at. Dy3 :Y4 Al2 O9
(Dy:YAM) polycrystal in the form of rod 2–3 mm in diameter and 8 mm (initially few centimeter) long obtained
by the micro-pulling-down method. Y4 Al2 O9 (YAM), with
the formation of monoclinic and space group P21∕c, is
one of four crystalline phases in Y2 O3 :Al2 O3 maintaining
ratio 2∶1 (Y4 Al2 O9 ). The Y atoms are coordinated to
either six or seven oxygen atoms [4]. Other phases in
the system are cubic garnet YAG (Y3 Al5 O12 ), orthorhombic perovskite YAP (YAlO3 ), and metastable hexagonal
structure phase YAH (YAlO3 ), with the same stoichiometry as YAP, observed during the synthesis by soft chemistry methods.
Dy:YAM samples were grown in the Institute of Electronic Materials Technology in Warsaw. The spectroscopic
characteristics of Dy:YAM at room and low temperature
have already been investigated [5]. YAM undergoes phase
transition at about 1377В°C [6], which results in material
cracking during standard Czochralski growth method.
Therefore, the micro-pulling-down method [7] was used
to obtain the samples. As the cross-relaxation (CR) process was unfavorable for our study, only a 0.3% at. doped
sample was investigated.
Emission measurements were performed using Digikrom 480 grating monochromator manufactured by CVI
Laser Corporation followed by a photomultiplier tube
and SR400 gated photon counter manufactured by Stanford Research Systems. The excitation line was 355 nm
obtained by the third harmonic of pulsed Continuum Surelite Nd:YAG laser (10 ns pulse-width, repetition rate
10 Hz). Fluorescence dynamics profiles were recorded
using SR-430 multichannel analyzer controlled by a PC.
The Dy3 energy levels structure together with excitation spectra for Dy:YAM is presented in [5]. There are
two closely lying levels: 4 F9∕2 and above it 4 I15∕2 with the
smallest energy gap between them of about 600 cmв€’1 at
10 K. At room temperature emission bands originating
from the considered levels are nearly overlapping making
the thermal excitation of the 4 I15∕2 level more probable.
Although the maximum phonon energy of 813 cmв€’1
(obtained from Raman spectra measured in Institute of
Electronic Materials Technology) is quite high, the possibility of 4 F9∕2 emission quenching by multiphonon
relaxation is very low because the energy gap between
F9∕2 level and the nearest lower lying level is about
7 Г— 103 cmв€’1 , which is equal to eight phonons.
The temperature dependences of 4 I15∕2 and 4 F9∕2 emission lines were measured and are presented in Fig. 1. The
plots were normalized to unity at 4 F9∕2 emission line
(481 nm). The significant increase of 4 I15∕2 level emission
Fig. 1. (Color online) Temperature variability of the emission
spectra. The spectra were normalized to unity for 4 F9∕2
emission intensity.
В© 2012 Optical Society of America
December 15, 2012 / Vol. 37, No. 24 / OPTICS LETTERS
Table 1. Boltzmann Based Approximations of
Experimental Data
intensity with growing temperature could be observed,
which is the evidence of 4 I15∕2 level population thermal
growth. The measure of energy delivered to the system
by temperature is kT value and the probability of the
system being in state with energy E is equal to exp
(−ΔE∕kT). This means that for kT equal to ΔE there is a
probability of 0.37 that the ion will be in the higher state.
The kT value changes from 200 cmв€’1 for room temperature to 680 cmв€’1 for 700В°C. As the excitation line of energy
is over 28 × 103 cm−1 , the 4 I15∕2 and 4 F9∕2 levels can be excited after the phonon relaxation from the higher levels
and the distribution of their populations in thermal equilibrium would be directly dependent on the kT value.
Thus, the thermal coupling of the discussed pair of
levels is predicted to follow Boltzmann distribution:
N A ∕N B C⋅ exp −E∕kT , where N A , N B are the populations of considered energy levels and C is constant,
which depends on the radiative transition rates and
the degeneration of these levels. The ratio R of intensity
values at 455 nm (line A, 21978 cmв€’1 ) and 481 nm (line B,
20790 cmв€’1 ) was plotted against the inverted temperature. The Arrhenius plot of the obtained data is presented
in Fig. 2, which compares experimental data with theoretical values. The experimental plot is slightly nonlinear
and it maintained this tendency also when using integrated intensities (the comparison was made to confirm
the reliability of the obtained dependencies). The slope is
approximately equal to the theoretical value (1188 cmв€’1 )
only in the middle part. However, by dividing the plot
into two parts (below and over 300В°C) it can be approximated quite well by two linear functions shown in Fig. 2
and listed in Table 1.
The differences in slopes corresponding to О”E values
for lower (<300В°C) and higher temperatures (>300В°C)
may come from the quenching processes that decrease
emission intensity. For comparison, the results obtained
for YAG [8] arrange linearly on Arrhenius plot, as the О”E
value is constant. The difference between YAG and YAM
matrices may come from the crystal lattice character.
Since YAG has cubic formation, there is only one site that
could be occupied by Y ions (substituted then by Re3
ions), whereas in YAM there are four possible sites making emission variation with temperature more complicated and therefore harder to predict. In fact, three
sites should be considered. As two sites in YAM are very
similar [9] and could be hard to distinguish. Some of our
past low temperature measurements (at 10 K) seem to
confirm this idea because even 0.3% doped samples of
YAM characterized, by little nonexponentiality, and
whose source could be in different lattice sites. Also, low
temperature emission spectra of 4 F9∕2 state reveal three
different sites of Y ion present in this host [5].
Another issue that cannot be ignored is growth of the
kT value with temperature, which results in the activation of new CR channels. Many possible CR channels
leading to de-excitation of 4 F9∕2 level in Dy:YAG were
proposed in [10]. The CR process results in a faster
decrease of emission intensity than it could be expected,
e.g., from the multiphonon relaxation.
Taking into account the probability of the observational error occurrence, the dependence based on theoretical О”E value obtained from the energy difference
between chosen emission lines have also been calculated
and is presented in the lower part of Table 1. The respective sensor sensitivity is shown in Fig. 3.
The resulting sensor sensitivity is expressed by the
derivative: dR∕dT R⋅ ΔE∕kT2 and results for all temperature ranges are shown in Fig. 3. It can be seen that
the sensitivity is highest for 600°C–800°C range and
equals more than 3 Г— 10в€’3 В°Cв€’1 . When the theoretical О”E
value is applied in the Boltzmann equation, the maximum
sensitivity is obtained for 600В°C, however it should be
remembered that this dependence does not agree entirely
with measured values. In [3] results obtained for Dy:Al2 O3
are presented for temperatures up to 450В°C. Although,
we cannot refer much about those results because of a
narrow temperature range, it can be seen that sensor sensitivities for temperatures below 300В°C are in good agreement with our results, as well as the obtained О”E values
(0.8 Г— 10в€’3 В°Cв€’1 , 986 cmв€’1 in [3] and about 1.5 Г— 10в€’3 В°Cв€’1 ,
1000 cmв€’1 in this work, see Figs. 2 and 3).
Fig. 2. (Color online) Arrhenius plot of the intensity ratio
temperature dependence.
Fig. 3. (Color online) Temperature dependence of the
predicted sensor sensitivity.
23В°C < T < 300В°C
300В°C < T < 700В°C
ln R
1.5 − 1438.7∕
T 273
R 4.5⋅ exp −1438.7∕
T 273
23В°C < T < 700В°C
ln R
2.0 − 1709.4∕ T
R 7.4⋅ exp −1709.4∕ T
2.4 − 1937.6∕
T 273
11.0⋅ exp −1937.6∕
T 273
OPTICS LETTERS / Vol. 37, No. 24 / December 15, 2012
Fig. 4. (Color online) Temperature dependence of the decay
times. The inset on the right shows the same dependence in the
logarithmic scale, and the inset on the left shows the obtained
sensor sensitivity.
In [3] the authors measured the О”E value also for higher concentrations (1% and 2%) and the О”E values were
growing. It is clear that it should be attributed to the
growing probability of energy transfer processes (like
the CR process), which is consistent with our explanation of two ranges with different О”E values caused by
activation of new CR channels.
It has to be noted that we considered emission intensities as corresponding directly to the populations of
the levels, that is I A ∕I B N A ∕N B . However, this assumption does not always have to be true. The CR process
is an example of the exception to this rule. It can
significantly decrease the intensity of emission (not
whole population would be transferred to the lower level
by radiative transition) while N parameters refer to the
populations resulting from thermal equilibrium. At the
same time the population of the second considered level
would stay unchanged due to lack of any resonance
Yet another source of difference between theoretical
and experimental value of О”E is the simplification in presented model assuming that there are only two electronic
states (two discrete values of energy), which are considered as corresponding to 4 F9∕2 and 4 I15∕2 levels.
The decay times were measured for the same temperature range. The effective decay time values increased
from 486 Ојs for room temperature to 662 Ојs for 700В°C.
This is quite unique in rare earth doped materials, as
so-called temperature quenching was expected to be
observed. However, observed tendency is similar to
the reported for Dy:YAB [11]. The authors note that there
seems to be a specific behavior for dysprosium doped
materials and explain the phenomenon by significantly
different values of the rates of transitions from Stark
components of 4 F9∕2 level.
The experimental results together with obtained approximations and sensor sensitivity are presented in
Fig. 4. It can be seen that decay times of Dy:YAM could
be useful for temperature sensing for temperatures below 300В°C, as the resulting sensor sensitivity is at least
two orders of magnitude higher than that obtained for
the FIR method. Above 300В°C the sensitivity decreases
fast with growing temperature, the П„ T plot flattens.
However, the very fast decrease of the decay times
(resulting in high sensitivity) is observed for much higher
temperatures, e.g., for Dy:YAG above 1100В°C) [12].
Nevertheless, FIR method is much easier in application
as no absolute values resulting from measurements are
needed, making FIR method less prone to observational
error and thus potentially more precise and reliable.
In conclusion, there are four key points to present,
(1) there are two parts of temperature ranges with different activation energies (1000 cmв€’1 for T < 300В°C and
1347 cmв€’1 for T > 300В°C), which could be caused by
multisite effects and CR process; (2) increasing temperature causes activation of new CR channels due to rich
energy level structure; (3) the sensor sensitivity for
FIR method is highest for 600°C–800°C range and equals
more than 3 Г— 10в€’3 В°Cв€’1 ; and (4) the decay time temperature characteristics can be used as temperature sensor
only for temperatures below 300В°C and above 1000В°C.
At the end of this work, the issue of the tradeoff between sensor sensitivity in FIR method and energy gap
to be overcome by ion excitation needs to be mentioned.
On the one hand, the higher energy gap is preferable due
to larger resulting sensor sensitivity (steeper slope on
Arrhenius plot), but on the other hand the larger the energy gap, the smaller the possibility of transferring electrons to a higher level. Taking both issues into account, it
should be remembered that the main objective of the
future research is not high sensor sensitivity, but the best
combination of energy level structure and crystal lattice
interaction, as it determines final efficiency of the sensor.
The dysprosium doped materials have already been
studied in some papers as they seem to be quite matched
to the requirements posed.
This work was supported by the Polish Ministry of
Science and Higher Education Grant No. N N515 081537.
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