Enhanced emission of Tm3+:3F4?→?3H6 transition by backward energy transfer from Yb3+ in Y2O3 for mid-infrared applications

Article abstract of DOI:10.1016/j.jallcom.2017.06.088

Tm³⁺ doped luminescence materials have attracted great attention owing to their potential for achieving 2 μm laser. Here, we report that the 2 μm emission intensity of Tm³⁺:³F₄ → ³H₆ transition can be enhanced by as large as 1.8 times through introducing Yb³⁺ into Tm³⁺ doped Y₂O₃ upon 782 nm excitation, where the population of Tm³⁺:³F₄ level is increased by backward energy transfer from Yb³⁺ following the forward energy transfer from the upper level Tm³⁺:³H₄ to an intermediate level Yb³⁺:²F_5/2. In addition, the efficiencies of Yb³⁺ to Tm³⁺ backward energy transfer are determined based on the analysis of emission spectra and fluorescence time profiles. It is found that the Yb³⁺ to Tm³⁺ backward energy transfer upon Tm³⁺ excitation at 782 nm is more efficient than the Yb³⁺ to Tm³⁺ energy transfer upon Yb³⁺ direct excitation at 980 nm, which is explained by the preferential excitation of Yb³⁺ with a nearby Tm³⁺ in the forward energy transfer from Tm³⁺ to Yb³⁺ upon Tm³⁺ excitation. Our results demonstrate that codoping Yb³⁺ into Tm³⁺ activated materials offers a promising approach to obtain efficient 2 μm laser, and the efficient backward energy transfer may play a key role in other rare earth ions doped luminescence materials.

Full text of DOI:10.1016/j.jallcom.2017.06.088

Accepted Manuscript
3+ 3
3
Enhanced emission of Tm : F → H transition by backward energy transfer from
4
6
3+
Yb in Y O for mid-infrared applications
2
3
Dan Wu, Wenge Xiao, Xia Zhang, Zhendong Hao, Guo-Hui Pan, Liangliang Zhang,
Yongshi Luo, Jiahua Zhang
PII:
S0925-8388(17)32076-5
10.1016/j.jallcom.2017.06.088
JALCOM 42157
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 7 March 2017
Revised Date: 28 May 2017
Accepted Date: 7 June 2017
Please cite this article as: D. Wu, W. Xiao, X. Zhang, Z. Hao, G.-H. Pan, L. Zhang, Y. Luo, J. Zhang,
3+ 3
3
3+
Enhanced emission of Tm : F → H transition by backward energy transfer from Yb in Y O for
4
6
2 3
mid-infrared applications, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.06.088.
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ACCEPTED MANUSCRIPT
3
+ 3
3
Enhanced emission of Tm : F → H transition by backward
4
6
3
+
energy transfer from Yb in Y O for mid-infrared applications
2
3
1
,2
1,2
1
1
1
Dan Wu , Wenge Xiao Xia Zhang , Zhendong Hao , Guo-Hui Pan , Liangliang
1
1
*1
Zhang , Yongshi Luo , Jiahua Zhang
1
State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine
Mechanics and Physics, Chinese Academy of Sciences, 3888 Eastern South Lake Road,
Changchun 130033, China
2
University of Chinese Academy of Sciences, Beijing, 100049, China
E-mail: zhangjh@ciomp.ac.cn
3
+
3+
Keywords: 2 µm emission; Tm -Yb ; Backward energy transfer
ABSTRACT
3+
Tm doped luminescence materials have attracted great attention owing to their potential for
3
+ 3
3
achieving 2 µm laser. Here, we report that the 2 µm emission intensity of Tm : F → H transition
4
6
3
+
3+
can be enhanced by as large as 1.8 times through introducing Yb into Tm doped Y O upon
2
3
3
+ 3
7
82 nm excitation, where the population of Tm : F level is increased by backward energy
4
3+
3+ 3
transfer from Yb following the forward energy transfer from the upper level Tm : H to an
4
3
+ 2
3+
3+
intermediate level Yb : F . In addition, the efficiencies of Yb to Tm backward energy
5
/2
transfer are determined based on the analysis of emission spectra and fluorescence time profiles. It
3
+
3+
3+
is found that the Yb to Tm backward energy transfer upon Tm excitation at 782 nm is more
3
+
3+
3+
efficient than the Yb to Tm energy transfer upon Yb direct excitation at 980 nm, which is
3
+
3+
explained by the preferential excitation of Yb with a nearby Tm in the forward energy transfer
3
+
3+
3+
3+
3+
from Tm to Yb upon Tm excitation. Our results demonstrate that codoping Yb into Tm
activated materials offers a promising approach to obtain efficient 2 µm laser, and the efficient
backward energy transfer may play a key role in other rare earth ions doped luminescence
materials.
1

ACCEPTED MANUSCRIPT
1
. Introduction
Recently, great attention has been paid to the infrared lasers. Particularly, 2 µm lasers have
received intense research for their potential applications including medicine, remote sensing and
military technologies [1-7]. Among various rare earth (RE) ions who possess abundant levels,
3+
3+
Ho and Tm are the most probable activators to generate 2 µm laser due to the transitions of
3
+ 5
5
3+ 3
3
3+
Ho : I → I and Tm : F → H emissions [8-12]. The laser operation of Ho is limited because
7
8
4
6
3+
there is no available upper level in Ho to be excited from the ground level with the commercial
3
+
8
00 or 980 nm laser diodes. Therefore, the desirable 2 µm emission of Ho can be obtained only
3
+
when the sensitizer Yb is codoped to provide efficient absorption at 980 nm [13-15]. As for
3
+
Tm , it has a reasonable absorption around 800 nm to be directly pumped by the available laser
diodes and the broad emission about 300 nm allowing wide wavelength tuning [17, 18].
For mid-infrared laser, an ideal host should possess low phonon energy to reduce multiphonon
relaxation and thus increase the quantum efficiency. However, since resonant pumping cannot
provide full population inversion and good signal-to-noise ratio, an upper level instead of the
emitting level is pumped; therefore, populating the emitting level from the upper level becomes
difficult owing to the slow multiphonon relaxation and the small branching ratio [19-21].
3
+
3+
3+
Accordingly, several RE ions such as Pr , Eu and Ce , have been used as the codopants to
3
+
3+
regulate the population [22-25]. For example, codoping Ce into Er activated luminescence
4
4
augments the population of the I level at the expense of that of the I level due to the energy
1
3/2
11/2
3
+ 4
4
3+ 2
2
transfer from Er ( I – I ) to Ce ( F – F ), resulting in an enhancement of the 1550 nm
11/2
13/2
5/2
7/2
3
+
emission to realize laser operation pumped by 980 nm [26, 27]. The incorporation of Ce can also
3
+ 5
5
enhance the emission of Ho : I → I to obtain 2 µm laser [28, 29]. Besides, Özen et al. [30-32]
7
8
3
+
3+
studied the blue upconversion emission of Tm under red light excitation at 683 nm in Tm and
3+
Yb codoped fluorophosphate glasses. They observed a considerable enhancement of the blue
3
+
3
upconversion emission in the presence of Yb due to the increased population of F from which
4
1
the blue level G is reached by absorbing a red photon.
4
3
+
The sesquioxide Y O is a suitable matrix for RE activated solid state lasers because of their
2
3
excellent properties like good chemical stability, low thermal expansion and high thermal
conductivity [33-35]. Moreover, it can be made into highly transparent ceramic with large sizes as
well as good transparency even in the infrared region, which has been applied to obtain high
power solid state lasers [36-38]. In addition, Y O possesses relatively low phonon energy
2
3
-
1
3+ 3
3
(
phonon cutoff ≈ 591 cm ) [39], which is beneficial to the 2 µm emission of Tm : F → H but
4 6
3
also results in a relative low feeding to F level upon 782 nm excitation. Generally, the population
4
3
3
3
3
3
of F level can be doubled due to the cross relaxation energy transfer ( H , H )→( F , F ) when
4
4
6
4
4
3+
the Tm concentration is high enough [17].
3
+
3+
In the present work, we introduce Yb into Tm doped Y O to enhance the 2 µm emission
2
3
3
+ 3
3
3
intensity of Tm : F → H transition upon 782 nm excitation, in which the population of F level
4
6
4
3
+
3+
3+
is increased by the effect of Tm -Yb -Tm forward-backward energy transfer (FBET), i.e. the
3
+ 3
3+ 2
forward energy transfer (FET) from the upper level Tm : H to an intermediate level Yb : F
4
5/2
3
+
3+
and the subsequent backward energy transfer (BET) from Yb to Tm for populating the lower
3
+ 3
3+
3+
level of Tm : F . Furthermore, the BET efficiencies from Yb to Tm in the FBET process are
4
calculated based on the analysis of emission spectra and fluorescence time profiles. The results are
3
+
also compared with the energy transfer efficiencies upon Yb direct excitation at 980 nm,
3
+
3+
3+
indicating that the Yb to Tm backward energy transfer in FBET is more efficient than the Yb
2

ACCEPTED MANUSCRIPT
3
+
3+
3+
to Tm energy transfer upon Yb direct excitation at 980 nm. A preferential excitation of Yb
3
+
3+
3+
with a nearby Tm in the FET process is proposed to explain the efficient Yb to Tm backward
energy transfer.
2
. Experimental
2
.1 Sample preparation
The samples series of (Y(1-x-y)Tm Yb ) O (x=0-0.1, y=0.0005-0.01) powders were synthesized via
y
x 2
3
the normal firing precursor method, which is considered to be more favorable to obtain uniform
and highly crystallized samples than the traditional solid-state reaction. Typically, the appropriate
amounts of Y O (5N), Tm O (5N) and Yb O (6N) were first dissolved in dilute nitric acid
2
3
2
3
2
3
(
G.R.), respectively, to get the Y(NO ) , Tm(NO ) and Yb(NO ) solutions. Tm(NO ) , Yb(NO )
3 3 3 3 3 3 3 3 3 3
and Y(NO ) solutions were mixed stoichiometrically and stirred vigorously for 30 minutes at
3
3
room temperature, and then dried at 95ꢀ for 24 hours in a drying oven, followed by being
preheated at 700ꢀ for 3 hours to obtain the precursor. After grinding, the precursor was put into
an alumina crucible and sintered at 1600ꢀfor 5 h in air.
2
.2 Measurements and Characterization
The crystal structure of samples were characterized by X-ray diffraction (XRD) (Bruker D8 Focus
diffractometer with Cu Kα radiation λ = 1.54056 Å). The emission spectra were measured using
FLS920 spectrometer (Edinburgh Instruments, U.K.) in the wavelength range from 900 to 1700
3
+ 3
3+ 2
nm. A 782 nm and a 980 nm laser diodes were used to pump Tm : H level and Yb : F level,
4
5/2
2
respectively. The output excitation power density of 782 nm laser diodes was fixed at 35 mW/mm
2
while that of 980 nm laser diodes was fixed at 5 mW/mm . In fluorescence lifetime measurements,
an optical parametric oscillator (OPO) was used as an excitation source and the signal was
detected by a Tektronix digital oscilloscope (TDS 3052). All the measurements were carried out at
room temperature.
3
. Results and discussion
3
+
3+
As shown in Fig. 1, the XRD patterns of Tm and/or Yb doped Y O matches well with the
2
3
JCPDS standard patterns of Y O (PDF #41-1105), respectively. No impurity phase can be
2
3
detected, indicating the formation of pure phase.
3
+
3+
Fig. 1 The XRD patterns of Tm and/or Yb doped Y O , the standard pattern of Y O (PDF #41-1105) is also
2
3
2
3
presented for comparison.
3

ACCEPTED MANUSCRIPT
3+ 3
Fig. 2a and b Emission spectra of (Y0.9995-xTm0.0005Yb ) O (x=0.005-0.1) upon excitation to Tm : H level at 782
x 2
3
4
3
3
nm. The intensity of H → F emission in each spectrum is normalized in Fig.2b.
4
4
3
+
Fig. 2a and 2b show the emission spectra of (Y0.9995-xTm0.0005Yb ) O with a fixed Tm
x 2
3
3
+
3+ 3
concentration at 0.05 mol% and various Yb concentrations x from 0 to 0.1 upon Tm : H
4
3
+ 3
3
excitation at 782 nm, respectively. In Fig. 2b, the emission intensity of Tm : F → H transition in
4
6
3
+
Tm singly doped sample is scaled to that in codoped sample. The emission spectra of the
codoped samples consist of three emission bands around 900 -1200 nm, 1350-1580 nm and
3
+ 2
2
3+ 3
3
3
3
1
580-1700 nm, which are assigned to Yb : F → F transition, Tm : H → F and F → H
5/2 7/2 4 4 4 6
transitions, respectively. It should be noted that the peak around 1630 nm is only a part of
3
+ 3
3
Tm : F → H emission (the complete emission is in the range of 1600-2200 nm) because the
4
6
3
+
InGaAs detector used has a cutoff wavelength of 1650 nm. The appearance of Yb emission in
4

ACCEPTED MANUSCRIPT
3
+ 3
3+ 2
the codoped samples indicates the existence of energy transfer from Tm : H level to Yb : F
4
5/2
3
+ 3
3
level. It is clear that the absolute intensity of Tm : F → H emission in the codoped sample has a
4
6
3
+
remarkable enhancement with increasing x when compared with the Tm singly doped sample,
and it increases by as high as 1.8 times at x=0.1(see Fig. 2a). This enhancement directly reflects
3
+
3+
the effect of the FBET processes, as shown in Fig. 3, which has been reported in Er -Yb system
as well [40]. To evaluate the efficiency of FBET, the efficiencies of both FET and BET should be
known first.
3
+
3+
3+ 3
Fig. 3 Energy level diagrams and FBET process in Tm and Yb codoped crystal upon excitation to Tm : H4
level at 782 nm.
3+ 3
Fig. 4 Fluorescent decay curves of Tm : H level in (Y0.9995-xTm0.0005Yb ) O (x=0.005-0.1) upon excitation to
4
x 2
3
3+ 3
Tm : F level at 683 nm.
3
The energy transfer efficiency can be calculated by
τ
η
= 1−
(1)
τ0
where τ and τ denote the excited state lifetimes of donor in the absence and in the presence of
0
3
+ 3
3+ 3
acceptor, respectively. To obtain Tm : H lifetimes, the fluorescence decay curves of Tm : H
4
4
3
+ 3
3
level are measured by monitoring the Tm : H ꢁ H emission at 811 nm under the excitation of
4
6
5

ACCEPTED MANUSCRIPT
3+ 3
3+
3+
Tm : F at 683 nm, as shown in Fig. 4. Using Eq. (1), the efficiencies of FET from Tm to Yb
3
ηTm-Yb are calculated and listed in Table 1. It is exhibited that η
reaches 88% in Y O for
2 3
Tm-Yb
3
+
3+
3+
codoping with 10 mol% Yb . In order to obtain the efficiency of BET from Yb to Tm , ηYb-Tm,
3
+ 3
it is necessary to study the dynamical processes for populating Tm : F level in the absence and
4
3
+
3+ 3
in the presence of FBET. In the absence of FBET, for Tm singly doped sample, the Tm : F
4
3
level is populated by H level via intrinsic relaxation including multi-phonon relaxation (MPR)
4
3
3
3
via H level and the radiative transitions to H or directly to F levels. In other words, except the
5
5
4
3
3
radiative transition to the ground state, all the rest of the population of H level feeds the F level
4
4
3
+
3+
in the Tm singly doped sample. Note the cross relaxation (CR) process between two Tm ions,
3
3
3
3
3
3+
described as ( H , H )→( F , F ) [12], is neglected for populating the F level since Tm
4
6
4
4
4
concentration is fixed at as low as 0.05 mol% in this work. θ is defined as the ratio of the rate of
3
3
3
3+
intrinsic relaxation of H → F to the total rate of intrinsic decay of H in Tm singly doped
4
4
4
3
+
sample and thus it is reasonable to suppose θ is independent of Yb concentration. In the presence
3
+ 3
3
of FBET for the codoped samples, the origin of Tm : F → H emission can be divided into two
4
6
3
3
3
parts upon H excitation. One is contributed by the intrinsic relaxation of H → F responsible
4
4
4
3+
for Tm singly doped sample (under the black line), denoted as I . The other is contributed by
0
3
+
FBET responsible for the emission enhancement due to the effect of the codopant Yb (shaded
3
3
area), denoted as ꢀI. The portion of H population to populate the F level via FBET process is
4
4
3
then expressed as ηTm-YbηYb-Tm and the H population without undergoing FET is 1-η
of
Tm-Yb
4
3
which θ(1-ηTm-Yb) populates the F level through intrinsic relaxation. Therefore, we have
4
∆
I
ηTm−YbηYb−Tm
=
(2)
I0
θ(1 − ηTm−Yb)
where ꢀI/I can be obtained directly from the integral area under the emission spectra in Fig. 2b, θ
0
is calculated to be 0.31 for Y O (the calculation details are provided in Supporting Information).
2
3
Using Eq. (2), ηYb-Tm in Y O is calculated and listed in Table 1. One can find η can reach 88%
Yb-Tm
2
3
3
+
3+
in Y O when codoped with 10 mol% Yb although the concentration of Tm is as low as 0.05 %.
2
3
Accordingly, such highly efficient BET process makes the efficiency of FBET reach up to 77% in
3
+
3+
3+
Y O for codoping with 10 mol%Yb . With regard to the efficient BET from Yb to Tm , it is
2
3
3
+
3+
suggested that Tm prefers transferring energy to a nearby Yb in the forward process of FBET,
3
+
3+
thereby leading to the energy transfer from such excited Yb back to Tm efficiently, whereas the
3
+
3+
direct excitation of Yb at 980 nm gives equal probability to all Yb ions of which only a small
3
+
3+
fraction has a nearby Tm for a low concentration of Tm in this work.
3+ 3
Table 1 The lifetimes of Tm : H level, and the efficiencies of
4
FET, BET and FBET in (Y0.9995-xTm0.0005Yb ) O (x=0.005-0.1).
x 2
3
3
+ 3
x
Tm : H (µs) ηTm-Yb ηYb-Tm
ηFBET
4
λEX=683 nm
λEM=812 nm
458
0
0
0
.005
.01
397.7
343.3
0.13
0.25
0.82
0.74
0.11
0.19
6

ACCEPTED MANUSCRIPT
0
0
0
.02
.05
.1
359.5
126.7
57
0.43
0.72
0.88
0.79
0.90
0.88
0.34
0.65
0.77
3
+
3
3
To prove the above point, we further compared the intensity of Tm : F → H emission
4
6
3
+
3+
3+ 3
contributed by BET from Yb to Tm in FBET upon Tm : H excitation at 782 nm with that
4
3
+ 2
upon Yb : F
direct excitation at 980 nm. Fig. 5 shows the emission spectra of
/2
5
3
+
(
Y0.9995-xTm0.0005Yb ) O (x=0.005-0.1) upon 980 nm excitation (solid). The spectra of Yb
x 2 3
3
+ 3
3
emission as well as the incremental of Tm : F → H emission due to the effect of FBET process
4
6
upon 782 nm excitation obtained by spectral subtraction from Fig. 2b are also presented (dotted)
3
+
for comparison, in which, for clarity, the intensity of the Yb emission upon 980 nm excitation is
3
+
scaled to that upon 782 nm excitation. Upon 980 nm excitation, all the Yb ions are equally
3
+
3+
excited, and then, the energy transfer from Yb to Tm is a random process dominated by energy
3
+ 3
3
diffusion. One can readily find that the Tm : F → H emission for 980 nm excitation is always
4
6
3
+
weaker than that upon 782 nm excitation, especially for low doping level of Yb . In both cases,
3
+ 3
3
3+
3+
since the Tm : F → H emission is entirely due to the energy transfer from Yb to Tm , the
4
6
3
3
weaker F → H emission for 980 nm excitation means lower transfer efficiency. If the intensity
4
6
3
+ 3
3
ratio of the Tm : F → H emission upon 782 nm excitation (dotted ) to that upon 980 nm
4
6
3
+
3+
excitation (solid) is denoted by n, and the efficiency of energy transfer from Yb to Tm for 980
nm excitation is denoted by η'Yb-Tm , then we have
ηYb−Tm /(1− ηYb−Tm
)
)
n
=
(3)
η
′
/(1− η
′
Yb−Tm Yb−Tm
3
+
where η'Yb-Tm can be calculated using Eq.(1) with the fluorescence lifetimes τ and τ of Yb in the
0
3+
3+
absence and in the presence of Tm , respectively. To obtain the lifetimes of Yb , the fluorescence
3
+ 2
3+
decay curves of Yb : F level in Yb singly doped and codoped samples are upon 980 nm
5
/2
3
+
excitation, as shown in Fig. 6 (a) and (b). The obtained fluorescence lifetimes of Yb , η'
and
Yb-Tm
ηYb-Tm calculated by Eq. (3) are listed in Table 2. Comparing Table 2 with Table 1, one can
conclude that the values of ηYb-Tm calculated by Eq. (2) and that by Eq. (3) are consistent with
each other. One may notice from Fig. 5 and Table 2 that the n value decreases from 13.9 to 2.9
3+
with Yb concentration increasing from x=0 to x=0.1. The above decrease indicates that the
3
+
3+
energy diffusion among Yb with the increase of Yb concentration is unavoidable and it
3
+
3+
weakens the selectivity between Yb and Tm in the backward energy transfer process. The
3
+
increased energy diffusion goes against the effect of preferential excitation of Yb that followed
3
+
by transferring to a nearby Tm . Therefore, the backward energy transfer process is more
effective in the lowly doping samples.
7

ACCEPTED MANUSCRIPT
3+ 2
Fig. 5 Emission spectra of (Y0.9995-xTm0.0005Yb ) O (x=0.005-0.1) upon 980 nm direct excitation to Yb : F level
x 2
3
5/2
3+ 3
(
solid). For comparison, the corresponding emission spectra upon excitation to Tm : H level at 782 nm (dotted)
4
3+
are also given. Note that the intensity of Yb emission upon 980 nm excitation is scaled to that upon 782 nm
excitation.
3+ 2
Fig. 6 Fluorescent decay curves of Yb : F level in (Y0.9995-xTm0.0005Yb ) O3 (x=0.005-0.1) (a) and in
5/2
x 2
(
Y1-xYb ) O (x=0.005-0.1) upon excitation at 972 nm.
x 2 3
3+ 2
Table 2 The lifetimes of Yb : F level, the ratio n, BET efficiency ηYb-Tm and the
5/2
normal energy transfer efficiency η'Yb-Tm in (Y0.9995-xTm0.0005Yb ) O (x=0.005-0.1).
x 2
3
x
τ0(µs)
τ(µs)
η'Yb-Tm
n
ηYb-Tm
λEX=972 nm
λEX=972 nm
λEM=1033 nm λEM=1033 nm
0
0
0
.005
.01
1240
1100
1030
886
747
616
0.29
0.23
0.40
13.9
5.5
0.85
0.62
0.67
.02
3.0
8

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0
0
.05
.1
810
440
308
188
0.62
0.57
2.4
2.9
0.80
0.80
4
. Conclusions
3
+ 3
3
In summary, the emission intensity of Tm : F → H transition is enhanced by 1.8 times through
4
6
3
+
3+
3
introducing Yb into Tm doped Y O upon 782 nm excitation, in which the population of F
4
2
3
3
+
level is increased by backward energy transfer from Yb following the forward energy transfer
3
+ 3
3+ 2
from the upper level Tm : H to an intermediate level Yb : F . The FBET process in
4
5/2
3
+
(Y0.9995-xTm0.0005Yb ) O is investigated in detail as a function of Yb concentration. The BET
x 2 3
3
+
efficiency is calculated to be as high as 88% for codoping with 10 mol% Yb . The FBET process
3+
is also compared with the normal energy transfer process upon direct excitation to Yb at 980 nm,
indicating that the BET is usually more efficient than the normal energy transfer, especially when
3
+
3+
the Yb concentration is relatively low, which is explained by the preferential excitation of Yb
3
+
3+
with a nearby Tm in the FET process. Our results demonstrate that Yb can act as an effective
3
3
3+
bridge between the upper and excitation level H and the lower and emitting level F of Tm to
4
4
obtain efficient 2 µm emission. Furthermore, the efficient backward energy transfer that is in favor
of the population of the lower energy level may also play a key role in other codoped
luminescence materials.
Acknowledgements
This work was partially supported by the National Key Research and Development Program of China
(
Grant No. 2016YFB0701003, 2016YFB0400605), National Natural Science Foundation of China (Grant
No. 61275055, 11274007, 51402284 and 11604330), Natural Science Foundation of Jilin province (Grant
No. 20140101169JC, 20150520022JH and 20160520171JH), and the prior sci-tech program of innovation
and entrepreneurship of oversea Chinese talent of Jilin province.
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µm emission of Tm : F → H transition is largely enhanced by codoping Yb .
4
6
3
+
The BET efficiency is as high as 88% for codoping with 10 mol% Yb .
The BET upon 782 nm excitation is more efficient than ET upon 980 nm excitation.