Spectroscopic evaluation a new and novel Nd3+/Yb3+ co-doped CdO-V2O5 glass system for 1u202fμm laser application

Article abstract of DOI:10.1016/j.jallcom.2018.10.370

Structural and spectral evaluations of Nd³⁺ and Nd³⁺/Yb³⁺ doped CdO-V₂O₅ glasses based on X-Ray diffraction, Raman spectroscopy, absorption, excitation and emission spectra and decay time profiles, are performed. X-Ray diffraction patterns and Raman spectroscopy revealed that all samples are amorphous (glassy phase) up to 4 mol% of Yb³⁺, without segregation of any crystalline phase. The optical band gap values determined from the absorption spectra, assuming direct optical band gap, were in the range of 2.13–2.07 eV, depending on the Yb³⁺ concentration. The emission spectra of the Nd³⁺ and Nd³⁺/Yb³⁺ doped glasses upon Nd³⁺ excitation at 585 nm, displayed the NIR Nd³⁺ feature emissions centered at 880, 1060 and 1342 nm, and Yb³⁺ emission at 977 nm, being the latter one achieved at expenses of energy transfer from Nd³⁺. According to the shortening of the Nd³⁺ emission decay profiles such process is not radiative with efficiency in the range of 0.181–0.521, depending on the Yb³⁺ content. The 880 nm Nd³⁺ decay fitting using the Inokuti-Hirayama model considering, energy migration among donors, suggested that an electric dipole-dipole interaction might mediate the energy transfer process, within Nd³⁺-Yb³⁺ clusters.

Full text of DOI:10.1016/j.jallcom.2018.10.370

Journal of Alloys and Compounds 777 (2019) 886e893
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Spectroscopic evaluation a new and novel Nd3þ/Yb3þ co-doped CdO-
V2O5 glass system for 1 mm laser application
a
a
a
,
1
b
c
~
ꢀ
O. Soriano-Romero , R. Lozada-Morales , U. Caldino , A. Mendez-Blas , C. Falcony ,
d
a
e, *
ꢀ
E. Alvarez , M. Palomino-Ovando , A.N. Meza-Rocha
Benemerita Universidad Autonoma de Puebla, Postgrado en Física Aplicada, Facultad de Ciencias Físico-Matematicas, Av. 14 sur y Av. San Claudio, Col. San
a
ꢀ
ꢀ
ꢀ
Manuel, Puebla, Mexico
b
ꢀ
ꢀ
Instituto de Física, Benemerita Universidad Autonoma de Puebla, Apdo. Postal J -48, 72570, Puebla, Pue., Mexico
c
ꢀ
ꢀ
Centro de Investigacion y de Estudios Avanzados del IPN, Departamento de Física, Av. IPN 2508, Col. San Pedro Zacatenco, 07360, Ciudad de Mexico,
Mexico
d Departamento de Física, Universidad de Sonora (UNISON), Boulevard Luis Encinas y Rosales s/n, Hermosillo, Sonora, 83000, Mexico
e
ꢀ
ꢀ
ꢀ
CONACYT- Benemerita Universidad Autonoma de Puebla, Postgrado en Física Aplicada, Facultad de Ciencias Físico-Matematicas, Av. 14 sur y Av. San
Claudio, Col. San Manuel, Puebla, Mexico
a r t i c l e i n f o
a b s t r a c t
Structural and spectral evaluations of Nd3þ and Nd3þ/Yb3þ doped CdO-V2O5 glasses based on X-Ray
diffraction, Raman spectroscopy, absorption, excitation and emission spectra and decay time profiles, are
performed. X-Ray diffraction patterns and Raman spectroscopy revealed that all samples are amorphous
(glassy phase) up to 4 mol% of Yb3þ, without segregation of any crystalline phase. The optical band gap
values determined from the absorption spectra, assuming direct optical band gap, were in the range of
2.13e2.07 eV, depending on the Yb3þ concentration. The emission spectra of the Nd3þ and Nd3þ/Yb3þ
doped glasses upon Nd3þ excitation at 585 nm, displayed the NIR Nd3þ feature emissions centered at
880, 1060 and 1342 nm, and Yb3þ emission at 977 nm, being the latter one achieved at expenses of
energy transfer from Nd3þ. According to the shortening of the Nd3þ emission decay profiles such process
is not radiative with efficiency in the range of 0.181e0.521, depending on the Yb3þ content. The 880 nm
Nd3þ decay fitting using the Inokuti-Hirayama model considering, energy migration among donors,
suggested that an electric dipole-dipole interaction might mediate the energy transfer process, within
Nd3þ-Yb3þ clusters.
Article history:
Received 12 September 2018
Received in revised form
25 October 2018
Accepted 29 October 2018
Available online 7 November 2018
Keywords:
CdO-V2O5 glasses
Nd3þ and Yb3þ co-doping
Energy transfer
1 mm laser application
© 2018 Elsevier B.V. All rights reserved.
1. Introduction
lanthanides ions. As strategy, the incorporation of alkaline and
suitable modifier oxides has shown to increase the glass trans-
For many years, vanadate-based glasses have been mainly
considered for applications such as switching, memory, charge
storage, among others [1,2], due to their semiconducting proper-
ties. Such characteristics have been attributed to the 3d1 electron
movement in states with different valences [1,3]. The electronic
transitions promoted by the 3d or 3s unpaired electrons jumping
parency [4,5], which could make vanadate-based glasses attractive
for optical applications. In this direction, it has been reported in the
binary CdO-V2O5 system that variations of the CdO/V2O5 mol%
starting ratio can change the phase from crystalline to amorphous
and modulate the optical band gap from 1.97 to 2.84 eV for 0/100
and 96/4 mol%, respectively [5]. The formation of a glassy (amor-
phous) phase and increment of band gap could turn the CdO-V2O5
binary system, in the region of high content of CdO, into interesting
for applications such as near infrared (NIR) solid-state lasers, op-
tical amplification, optical communications and so on. The glassy
nature of the CdO-V2O5 system in the region of high content of CdO
(96 mol%), is associated with the high polarizability of Cd2þ, which
allows it to play the role either network modifier or glass forming
[6]. Moreover, very low contents of V2O5 (4e5 mol%) might reduce
between vanadium ions with different valences (V3þ, V4þ and V5þ
)
reduce drastically the glass transparency [4]. This behavior repre-
sents a drawback for optical applications when they are doped with
* Corresponding author.
E-mail address: anehemiasme@conacyt.mx (A.N. Meza-Rocha).
On sabbatical leave from Departamento de Física, Universidad Autonoma
1
ꢀ
ꢀ
ꢀ
Metropolitana-Iztapalapa, P.O. Box 55-534, 09340 Mexico D.F., Mexico.
https://doi.org/10.1016/j.jallcom.2018.10.370
0925-8388/© 2018 Elsevier B.V. All rights reserved.

O. Soriano-Romero et al. / Journal of Alloys and Compounds 777 (2019) 886e893
887
or inhibit the segregation of lanthanide orthovanadate (LnVO4)
temperature.
crystalline phases [7]. Such fact enables incorporation of high
amounts of lanthanide ions without creating undesirable phases
that could act as non-radiative recombination centers.
3. Results and discussion
Among lanthanides, Yb3þ possesses emissions below the optical
band gap exhibited by CdO-V2O5 glasses in the region of high
content of CdO. Such ion is attractive for NIR laser applications,
since it possesses only two electronic energy levels. The simplicity
of levels avoids undesirable up-conversion processes and mini-
mizes concentration quenching effects [8,9], which might allow the
incorporation of high amounts of Yb3þ into the CdO-V2O5 glass
host. Moreover, the Nd3þ co-doping offers a versatile option to
Fig. 1 displays X-Diffraction patterns for all glasses under
studying. They present a broad band centered at around 2
q
¼ 31ꢀ,
which indicates short range order, typical of a glassy structure. It
can be noticed that, within the resolution of the diffractometer, the
low concentration of V2O5 (5 %mol) inhibits the segregation of
crystalline phases, such as neodymium and/or ytterbium
orthovanadates.
Fig. 2 shows the Raman spectra for all glasses. All spectra consist
of two broad bands located in the 280e440 and 710-1050 cmꢁ1
ranges. Such bands are respectively attributed to the broadening of
the 260, 312 and 352, and 820, 848 and 875 cmꢁ1 vibrational
modes, characteristics of the crystalline Cd2V2O7 structure [5,11].
The vibrational band broadening has been related to rupture of V-
O-V bridges of the [V2O7]4ꢁ units, which induces an amorphization
effect, as consequence of the incorporation of excessive amounts of
Cd2þ [5].
achieve excitation of Yb3þ by energy transfer process. This process
4
is frequently arisen through excitation of the neodymium G5/2
þ
4
2G7/2 and F5/2
þ
2H9/2 energy levels. After excitation, these levels
non-radiatively relax to the 4F3/2 one, from which the Nd3þ / Yb3þ
energy transfer takes place, as consequence of the almost resonant
2
energy difference with the ytterbium F5/2 energy level. Thus, the
Nd3þ/Yb3þ co-doped glasses become interesting for laser applica-
tions at around 1 mm [10].
Therefore, based on the potential applications of Nd3þ/Yb3þ co-
doped glasses and the suitable optical characteristics of the CdO-
V2O5 glass system in the region of high CdO content (95 mol%), in
this work an evaluation of the structural and spectroscopic prop-
erties of Nd3þ and xYb3þ doped CdO-V2O5 glasses is performed.
Fig. 3 (a) shows the absorption spectra for glasses co-doped with
different amounts of Yb3þ. The spectrum of the singly Nd3þ doped
CdO-V2O5 glass displays bands located at 517, 528, 585, 681, 750,
806 and 880 nm, which are respectively associated with Nd3þ ab-
4
sorption transitions of the G9/2
,
2K13/2
þ
4G7/2
,
4G5/2 þ2G7/2
, ,
4F9/2
4F7/2 þ 4S3/2, 4F5/2 þ2H9/2 and 4F3/2 states. As Yb3þ is incorporated, a
band located at 976 nm gradually grows due to the Yb3þ 2F7/
:
2. Experimental details
2 / 2F5/2 absorption transition.
Fig. 3 (b) displays plots of the square of the absorption times the
The glass samples were synthetized by the conventional melt-
quenching method, which consisted in heating the precursors up
to the melting temperature at 1000 ꢀC in high alumina crucibles for
one hour. After that, the melting was poured onto a stainless-steel
plate and pressed, to achieve disc-looking glass samples. The host
composition in all cases was 95-5 mol% of CdO (99%, Sigma-Aldrich)
and V2O5 (99%, Sigma-Aldrich), respectively. The Nd3þ and Yb3þ co-
doping was achieved by using nitrates. The Nd3þ doping was fixed
at 2 mol%, whereas the Yb3þ one was varied from 1 to 4 mol% in
steps of 1 mol%, regarding the host composition in all cases. The
tags of the synthesized samples were associated with the nominal
lanthanide rate (Nd3þ/Yb3þ), used in the fabrication of each sample.
So, hereafter the samples will be referred as indicated in Table 1.
The molar composition and ion concentration (ions/cm3) of each
glass are portrayed in Table 1 as well.
The structure was analyzed by recording X-Ray diffraction pat-
terns in a Bruker D8 Discover diffractometer. The vibrational modes
were obtained by Raman spectroscopy in a Horiba LabRam HR
spectrometer by using a 633 nm laser. The optical absorption
spectra were registered in a UVeViseNIR Cary 5000 spectrometer.
The excitation and emission spectra and decay time profile mea-
surements were performed in a FLS 1000 Edinburgh spectrometer.
The NIR photoluminescence was recorded by using an InGaAs pin
diode, whereas the decay profiles were recorded by using a PMT-
980 detector. All measurements were carried out at room
Fig. 1. X-Ray diffraction patterns for the glass host and glasses doped with 2 mol% of
Nd3þ and co-doped with different amounts of Yb3þ
.
Table 1
Molar composition of the glasses under studying and Nd3þ and Yb3þ concentrations.
Sample
CdO (mol%)
V2O5 (mol%)
Nd (%mol)
Yb (%mol)
Nd concentration (ions/cm3, ꢂ 1021
)
Yb concentration (ions/cm3, ꢂ 1021
)
CdO-V2O5 (Host)
2Nd/0Yb
2Nd/1Yb
2Nd/2Yb
2Nd/3Yb
95
95
95
95
95
95
95
5
5
5
5
5
5
5
0
2
2
2
2
2
0
0
0
1
2
3
4
2
e
e
1.07
1.07
1.07
1.07
1.07
e
e
0.62
1.28
1.79
2.30
1.28
2Nd/4Yb
0Nd/2Yb

888
O. Soriano-Romero et al. / Journal of Alloys and Compounds 777 (2019) 886e893
Fig. 2. Typical Raman spectra recorded upon 633 nm laser excitation, for the host and
glasses doped with 2 mol% of Nd3þ and co-doped with different amounts of Yb3þ
.
phonon energy, (ahn n) for CdO-V2O5
)2, versus the photon energy (h
glasses with different amounts of Yb3þ, assuming direct transitions.
Band gap (Eg) values can be determined by extrapolating the ab-
sorption edge to the x-axis (hn) through linear fit. So that, it is found
that the Eg values are slightly reduced from 2.13 to 2.07 eV for 0 and
4 mol% of Yb3þ, respectively. Such behavior could be associated
with the increment of non-bridging oxygen, as Yb3þ is incorporated
[12].
Fig. 4 (a) displays the excitation spectra of Nd3þ singly doped
(2Nd/0Yb) and Nd3þ/Yb3þ co-doped (2Nd/2Yb) glasses, by moni-
toring the Nd3þ 4F3/2 / 4I13/2 emission at 1342 nm, wherein Yb3þ
:
does not emit. Both spectra display the well-known neodymium
excitation bands located at 513, 529, 586, 684, 748, 805 and 880 nm,
observed in the absorption spectrum (Fig. 3 (a)). It can be noticed
that the addition of Yb3þ 2 mol% reduces the Nd3þ excitation in-
tensity, which could be indicating energy transfer from Nd3þ to
Yb3þ. To clarify this assumption, the excitation spectra of Yb3þ
singly doped (0Nd/2Yb) and Nd3þ/Yb3þ (2Nd/2Yb) co-doped
Fig. 3. (a) Absorption spectra and (b) plots of the square of the absorption times the
phonon energy (ahn)
2 versus the photon energy for glasses doped with 2 mol% of Nd3þ
:
glasses are recorded, sensing the Yb3þ 2F5/2 / 2F7/2 isolated
and co-doped with different amounts of Yb3þ. The dot line represents the best linear
fitting to obtain the optical band gap by extrapolating the absorption edge with the x-
axis.
emission at 977 nm (Fig. 4(b)). The excitation spectrum of the Yb3þ
singly doped glass does not exhibit any excitation band, whereas
that of the Nd3þ/Yb3þ co-doped glass displays the feature Nd3þ
excitation bands observed in the Nd3þ excitation spectrum
(Fig. 4(a)). This fact evidences that the Yb3þ emission at 977 nm is
content higher than 2 mol%, could be associated with back Nd3þ
)
Yb3 energy transfer or Yb3þ-Yb3þ cross relaxation, due to the
interatomic distance reduction.
achieved at expenses of energy transfer from Nd3þ
.
The emission spectra of glasses co-doped with different
amounts of Yb3þ upon neodymium 4I9/2 / 4G5/2 þ2G7/2 excitation
at 585 nm, are shown in Fig. 5 (a). The spectrum of the Nd3þ singly
doped glass (also shown in Fig. 5(a)) presents emissions bands
centered at 880, 1060 and 1341 nm, which are associated with
To infer the nature of the Nd3þ / Yb3þ energy transfer process,
the decay of the isolated Nd3þ emission at 880 nm was recorded for
glasses co-doped with Nd3þ 2 mol% and different amounts of Yb3þ
(Fig. 6(a)). In the Nd3þ singly doped glass, the Nd3þ emission decay
is non-exponential. The decays remain non-exponential and
become gradually faster, as the Yb3þ concentration increases. This
fact suggests that Nd3þ / Yb3þ energy transfer is non-radiative
[13]. Because of the non-exponential time evolution, the lifetime
was considered as the average one by using the following
relationship:
4
4
neodymium transitions from the F3/2 state to the I9/2,
4I11/2 and
4I13/2 ones, respectively. As Yb3þ is incorporated, a band in the
937e1043 nm range attributed to the ytterbium 2F5/2 / 2F7/2
transition emerges, reaching its optimum intensity at 2 mol% of
Yb3þ (Fig. 5 (a) and (b)). For Yb3þ concentration higher than 2 mol%,
the Yb3þ emission is reduced. These behaviors are followed by a
gradual reduction of the neodymium 880 nm emission, as illus-
trated in Fig. 5 (b). The gradual reduction of the Nd3þ emission as
the Yb3þ content increases, indicate that Nd3þ / Yb3þ energy
ð
∞
tIðtÞdt
0
ð
t
¼
:
(1)
∞
4
transfer takes places from the neodymium F3/2 energy level. The
IðtÞdt
attenuation of the 977 nm emission, illustrated in Fig. 5 (b) for Yb3þ
0

O. Soriano-Romero et al. / Journal of Alloys and Compounds 777 (2019) 886e893
889
Fig. 5. (a) Emission spectra of glasses doped with 2 mol% of Nd3þ and co-doped with
different amounts of Yb3þ, and (b) Nd3þ and Yb3þ emission intensity at 880 and
977 nm, respectively, as a function of the Yb3þ concentration.
Fig. 4. Excitation spectra for (a) 2Nd/0Yb and 2Nd/2Yb glasses, and (b) 0Nd/2Yb and
2Nd/2Yb glasses, monitoring respectively the Nd3þ and Yb3þ emissions at 1342 and
977 nm.
reduction of the Nd3þ / Yb3þ energy transfer efficiency [17].
To analyze the type of dominant electrostatic mechanism in the
Nd3þ / Yb3þ energy transfer a detailed analysis was carried out by
applying the Dexter's energy transfer expression for multipolar
interaction and Reisfeld's approximation. Thus, the energy transfer
mechanism of multipolar interaction can be obtained through the
following relation [18,19]:
Thus, the intrinsic average lifetime resulted to be 71
m
s (2Nd/
0Yb). This value is gradually reduced up to 34
m
s, as the Yb3þ con-
tent increases (Fig. 6 (b)). The Nd3þ intrinsic average lifetime is
shorter than those obtained using Eq. (1) in lithium lead alumino
borate (97
lurite (75
m
s), gadolinium calcium silica borate (270
ms), and tel-
m
s) glasses, doped with 2 mol% of Nd3þ [14e16]. From the
average lifetimes, energy transfer efficiencies are obtained in the
0.181e0.521 range (Fig. 6(b)), according to the following equation:
hNd
hNdþYb
fCn 3
;
(3)
=
tNdꢁYb
tNd
hET ¼ 1 ꢁ
;
(2)
where hNd and hNdþYb are the luminescence quantum efficiencies of
the Nd3þ donor in absence and presence of the Yb3þ activator,
respectively, C is the sum of the Nd3þ and Yb3þ contents, and n ¼ 6,
where tNd and tNdꢁYb are the Nd3þ average lifetimes in absence and
presence of Yb3þ respectively. The Yb3þ emission intensity
reduction for concentrations of Yb3þ higher than 2 mol%, despite
8
and 10 for electric dipole-dipole, dipole-quadrupole and
,
quadrupole-quadrupole interactions, respectively. The hNd
ratio can be approximated to the luminescence intensity ratio
Nd=INdþYb, where INd and INdþYb represent the emission intensities
of Nd3þ in absence and presence of Yb3þ, respectively, so that:
=
hNdþYb
the increment of the energy transfer efficiency, suggest that Yb3þ
-
Yb3þ cross relaxation dominates over the Nd3þ ) Yb3þ back en-
I
ergy transfer, since the back-energy transfer process might lead to a

890
O. Soriano-Romero et al. / Journal of Alloys and Compounds 777 (2019) 886e893
Fig. 6. (a) Decay time profiles at 880 nm upon pulsed excitation at 585 nm and (b)
average lifetime and energy transfer efficiency as function of the Yb3þ content.
INd
INdþYb
fCn=3
:
(4)
Fig. 7 shows the plots of the dependence of INd=INdþYb of Nd3þ on
(a) C6=3, (b) C8=3 and (c) C10=3. The R2 fitting values in the three
fitted lines are also shown in Fig. 7. A lineal relation is better fitted
with n ¼ 6 and n ¼ 8, indicating that the Nd3þ to Yb3þ energy
transfer mechanism could be through an electric dipole-dipole or
dipole-quadrupole interaction.
To decide if the dominant electrostatic mechanism involved in
the Nd3þ / Yb3þ energy transfer process is predominantly through
an electric dipole-dipole or dipole-quadrupole interaction, in the
Nd3þ/Yb3þ co-doped glasses, the neodymium 4F3/2 / 4I9/2 emis-
sion decays were analyzed with the Inokuti-Hirayama model
considering energy migration among donors [17]. Thus, the tem-
poral evolution of the Nd3þ emission intensity was fitted to the
following expression for electric multipolar interaction [9,17]:
ꢀ
ꢁ
t
t0
3
Fig. 7. Dependence of INd=INdþYb of Nd3þ on (a) C6=3, (b) C8=3 and (c) C10=3
.
s
IðtÞ ¼ I0exp
ꢁ
ꢁ
gst ꢁ wt
(5)
where I0 is the decay intensity at t ¼ 0, t0 is the intrinsic lifetime,
quadrupole interactions, respectively, is a constant related to the
direct Nd3þ / Yb3þ energy transfer, and w is the energy migration
parameter. In all cases, the best fittings are attained for S ¼ 6,
considered in this case as 71
m
s (average lifetime of the Nd3þ
emission in the singly doped glass), gs with S ¼ 6, 8 and 10 for
electric dipole-dipole, dipole-quadrupole and quadrupole-

O. Soriano-Romero et al. / Journal of Alloys and Compounds 777 (2019) 886e893
891
particularly for times longer than 120 ms, with R-square values
RC6
t0
higher than 0.998, as illustrated in Fig. 8 (a) for the 2Nd/4Yb glass.
From the Dexter's energy transfer expression for multipolar inter-
action and Reisfeld's approximation, and the Inokuti-Hirayama
fitting, it is inferred that an electric dipole-dipole interaction
might mediate the Nd3þ / Yb3þ energy transfer process more than
dipole-quadrupole one. The g6 and w parameter values increase
from 116 to 274 sꢁ1/2 and 2000 to 9000 sꢁ1, for co-doped glasses
with 1 and 4 mol% of Yb3þ, respectively (Fig. 8(b)). Such behavior
indicates a more efficient energy transfer and an increment of en-
ergy migration among donors, as the Yb3þ concentration increases,
which is favored by an inter-atomic distance reduction. The energy
transfer microparameter CDA is related with the g6 parameter by
the following equation [9]:
CDA
¼
(7)
where t0 is the intrinsic average lifetime (71
m
s). Thus, Rc values in
the 5.1e6.0 Å range are found, see Table 2. On the other hand, the
Nd3þ-Yb3þ distances assuming a random ion distribution [23], is
gradually reduced from 10.7 to 8.3 Å for glasses with 1 and 4 mol%
of Yb, respectively. Such values are larger than the Rc ones, which
suggests that the Nd3þ / Yb3þ energy transfer process takes place
in Nd3þ-Yb3þ clusters instead of ions randomly distributed.
The Rc interaction distance can also be determined applying the
Dexter theory. Assuming an electric dipole-dipole interaction
mechanism, Rc can be obtained from the following equation [24]:
!
6
3ħ4c4Q
4p
n4
pffiffiffiffiffiffiffiffi
4
3
A U
;
(8)
g6
¼
p
3=2NA CDA
;
(6)
RC
¼
where NA is the acceptor (Yb3þ) concentration (ion/cm3). So that,
CDA values in the range of 6.30 ꢂ 10ꢁ40 to 2.55 ꢂ 10ꢁ40 cm6sꢁ1 are
found. Such values are close to those reported in fluoroindogallate
glasses (3.0 ꢂ 10ꢁ40-3.2 ꢂ 10ꢁ40 cm6sꢁ1) [20,21]. The critical
interaction distance (Rc) is related with the CDA parameter by the
following relationship [22]:
where ħ is the Dirac's constant, c is the light velocity, n is the
refractive index of the host glass and is the spectral overlap in-
U
tegral between the normalised line-shape functions of ytterbium
2F7/2 / 2F5/2 absorption and neodymium 4F3/2 / 4I9/2 emission
R
(U
¼
½FNdðEÞFYbðEÞ=E4ꢃdE) and QA is the acceptor integrated ab-
sorption coefficient defined by:
ð
1
NA
QA
¼
að
l
Þd
l
;
(9)
R
being að
l
Þd
l
the integrated optical absorption coefficient of the
ytterbium 2F7/2 / 2F5/2 absorption transition and NA the acceptor
(Yb3þ) concentration (ion/cm3). Fig. 9 shows the optical absorption
coefficient versus wavelength of the Yb3þ absorption transition for
each the Nd3þ/Yb3þ codoped glasses, from which were evaluated
R
their corresponding
aðlÞd
l
coefficients. Table 2 lists the
R
(
að
l
Þd
l
) and QA coefficients of the ytterbium 2F7/2 / 2F5/2 ab-
sorption transition for each the Nd3þ/Yb3þ co-doped glasses. A
value of 0.747 eV-5 was obtained for the
U
overlap integral, see inset
in
of Fig. 10. Using n ¼ 2.51 and the values estimated for QA and
U
Eq. (8), it is found Rc values (5.3e5.5 Å) similar to those obtained
using the Inokuti-Hirayama model, see Table 2.
The spectral overlap between the Nd3þ emission and Yb3þ ab-
sorption bands, displayed in Fig. 10, reveals that the Nd3þ / Yb3þ
energy transfer process can occur through the following channel
illustrated in Fig. 11:
ET1: 4F3=2½Nd3þꢃþ F7=2½Yb3þꢃ/4I9=2½Nd3þꢃþ F5=2½Yb3þꢃ
2
2
In addition, considering that the energy mismatch between the
maximum of the Nd3þ 4F3/2 / 4I11/2 emission (1060 nm) and that
:
of the Yb3þ 2F7/2 / 2F5/2 absorption (976 nm) is ~812 cmꢁ1, the
:
energy transfer could be also assisted by the absorption of a host
phonon (ET2), with energy in the 800-1000 cmꢁ1 range, see host
Table 2
R
Integrated optical absorption coefficients,
ð
aðl
Þd
l
; 10ꢁ6
Þ
and (ðQA;
ꢂ
2
10ꢁ23 eV ꢄ cm2Þ, of the ytterbium F7/2
/
Yb3þ codoped glasses, and critical interaction distance for Nd3þ to Yb3þ energy
2F5/2 absorption transition in the Nd3þ
/
transfer (RC, Å) estimated from the Dexter and Inokuti-Hirayama (IH) models.
R
Glass
QA
RC (Å)
aðlÞdl
Dexter model
IH model
Fig. 8. (a) Fitting to the Nd3þ 880 nm emission decay profile using the Inokuti-
Hirayama model. The lines solid (S ¼ 6), break (S ¼ 8) and dots (S ¼ 10) illustrate the
possible electrostatics mechanism involved in the energy transfer process. The inset
displays an amplification of the decay evolution and fittings. (b) Evolution of the g6 and
w parameters as a function of Yb3þ content.
2Nd/1Yb
2Nd/2Yb
2Nd/3Yb
2Nd/4Yb
2.75
5.46
7.73
3.6
3.4
3.5
3.9
5.4
5.3
5.4
5.5
5.8
5.1
4.9
5.1
11.25

892
O. Soriano-Romero et al. / Journal of Alloys and Compounds 777 (2019) 886e893
Fig. 11. Nd3þ and Yb3þ energy level diagram illustrating the Nd3þ and Yb3þ emissions,
and energy transfer and cross relaxations pathways.
Fig. 9. Ytterbium optical absorption coefficient versus wavelength of the Nd3þ/Yb3þ
co-doped glasses.
phonon-assisted Nd3þ ) Yb3þ back energy transfer, and in
consequence to a reduction in the net Nd3þ / Yb3þ energy transfer
efficiency. Therefore, the Yb3þ emission intensity reduction for
concentrations of Yb3þ higher than 2 mol%, could be related to cross
relaxation (CR) between the Yb3þ
:
2F5/2 and F7/2 levels (Fig. 11)
2
rather than back Nd3þ ) Yb3þ energy transfer.
4. Conclusions
Nd3þ and Nd3þ/Yb3þ doped CdO-V2O5 glasses, in the region of
high content of CdO, were synthetized by melt-quenching method.
X-Ray diffraction patterns and Raman spectroscopy revealed that
they remain amorphous without segregation of additional NdVO4
and/or YbVO4 crystalline phases, which might inhibit the formation
of non-radiative recombination centers. The incorporation of Yb3þ
leads to a gradual reduction of the optical band gap values from 2.13
to 2.07 eV. Such effect was attributed to the formation of non-
bridging oxygen. The NIR emission spectra of the Nd3þ/Yb3þ
doped CdO-V2O5 glasses, upon Nd3þ excitation at 585 nm, dis-
played Nd3þ emissions at 880, 1060 and 1341 nm, and Yb3þ emis-
sion at 977 nm. The Yb3þ emission was achieved by non-radiative
Fig. 10. Overlap spectral region between the Nd3þ emission and Yb3þ absorption
bands. The inset shows the FNdðEÞ and FYbðEÞ normalised line-shape functions used to
compute the
U value.
Raman spectrum Fig. 2:
ET2:4F3=2½Nd3þꢃþ F7=2½Yb3þꢃ/4I11=2½Nd3þꢃþ F5=2½Yb3þꢃ
2
2
þ hvphonon
ð
ꢅ 812 cmꢁ1Þ:
Fig. 12 illustrates the spectral overlap between the Yb3þ emis-
sion and Nd3þ absorption. It can be noticed that the spectral overlap
is despicable, which indicates non-resonance energy condition
between the donor (Yb3þ) and acceptor (Nd3þ). The vanishing
spectral overlap between donor (Yb3þ) emission and acceptor ab-
sorption (Nd3þ) leads to a negligible Yb3þ / Nd3þ back energy
transfer as it has been demonstrated experimentally in other
phosphors activated with Yb3þ/Nd3þ [17]. On the other hand, it has
also been demonstrated in other Nd3þ and Yb3þ co-doped phos-
phors [25] that laser pump induced thermal loading could lead to a
Fig. 12. Overlap spectral region between the Yb3þ emission and Nd3þ absorption.

O. Soriano-Romero et al. / Journal of Alloys and Compounds 777 (2019) 886e893
[6] J.S. Kumar, J.L. Kumari, M.S. Rao, S. Cole, Opt. Mater. 35 (2013) 1320.
893
energy transfer from Nd3þ. This fact was verified by the gradual
ꢀ
ꢀ
[7] Y.A. Gonzalez-Rivera, A.N. Meza-Rocha, L. Aquino-Meneses, S. Jimenez-San-
reduction of the Nd3þ emission intensity and shortening of the
Nd3þ emission decay times, as Yb3þ is incorporated. The energy
transfer process was predominantly mediated by an electric dipole-
dipole interaction, as revealed by the Dexter expression and fitting
to the Nd3þ emission decay profile using the Inokuti-Hirayama
model, considering energy migration among donors.
ꢀ
~
doval, E. Rubio-Rosas, U. Caldino, E. Alvarez, O. Zelaya-Angel, M. Toledo-Sol-
ano, R. Lozada-Morales, Ceram. Int. 42 (2016) 8425.
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^
Earths 35 (2017) 361.
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[12] P. Suthanthirakumar, P. Karthikeyan, P.K. Manimozhi, K. Marimuthu, J. Non
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ꢀ
A.N. Meza-Rocha and R. Lozada-Morales thank Catedras CON-
~
ACYT (Grant no. 2801). U. Caldino acknowledges the CONACyT
ꢀ
(Mexico) for his sabbatical year Grant No. 322721.
~
ꢀ
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