Four-photon upconversion induced by infrared diode laser excitation in rare-earth-ion-doped Y2O3 nanocrystals

Article abstract of DOI:10.1016/j.cplett.2007.09.078

Room-temperature blue and violet upconversion (UC) emissions of Er³⁺ at 409 nm (²H_9/2 → ⁴I_15/2) and 390 nm (⁴G_11/2 → ⁴I_15/2) were observed in Y₂O₃:Yb³⁺-Er³⁺ nanocrystals by diode laser excitation of 980 nm. At low Yb³⁺ concentration both wavelengths result from three-photon UC processes. Unexpectedly, a four-photon and a decreased three-photon UC processes were observed at high Yb³⁺ concentration for the violet and the blue emissions, respectively. The four-photon process supposably results from the cross relaxation ⁴D_7/2 + ⁴I_15/2 → ⁴G_11/2 + ⁴I_13/2 of Er³⁺ ions with the ⁴D_7/2 state populated by the 2²F_5/2(Yb) + ⁴F_9/2(Er) → 2²F_7/2(Yb) + ⁴D_7/2(Er) process, whereas the decreased photon process arises from the larger UC rate at high Yb³⁺ concentration.

Full text of DOI:10.1016/j.cplett.2007.09.078

Available online at www.sciencedirect.com
Chemical Physics Letters 448 (2007) 127–131
www.elsevier.com/locate/cplett
Four-photon upconversion induced by infrared diode laser excitation
in rare-earth-ion-doped Y2O3 nanocrystals
a
a
a,
a
a
G.Y. Chen , Y. Liu , Z.G. Zhang *, B. Aghahadi , G. Somesfalean ,
b
b
Q. Sun , F.P. Wang
a
Department of Physics, Harbin Institute of Technology, 150001 Harbin, People’s Republic of China
Department of Applied Chemistry, Harbin Institute of Technology, 150001 Harbin, People’s Republic of China
b
Received 5 April 2007; in final form 26 September 2007
Available online 29 September 2007
Abstract
4
4
Room-temperature blue and violet upconversion (UC) emissions of Er3+ at 409 nm (2H9/2 ! I15/2) and 390 nm (4G11/2 ! I15/2) were
observed in Y2O3:Yb3+–Er3+ nanocrystals by diode laser excitation of 980 nm. At low Yb3+ concentration both wavelengths result from
three-photon UC processes. Unexpectedly, a four-photon and a decreased three-photon UC processes were observed at high Yb3+ con-
centration for the violet and the blue emissions, respectively. The four-photon process supposably results from the cross relaxation
4
4D7/2 + 4I15/2 ! G11/2 + 4I13/2 of Er3+ ions with the 4D7/2 state populated by the 22F5/2(Yb) + 4F9/2(Er) ! 22F7/2(Yb) + 4D7/2(Er) pro-
cess, whereas the decreased photon process arises from the larger UC rate at high Yb3+ concentration.
Ó 2007 Elsevier B.V. All rights reserved.
1. Introduction
IR-induced four-photon UC laser in the visible range [7].
However, the first step to develop UC–UV lasers, i.e., high
multi-photon UC–UV radiation induced by IR excitation,
has not been fully characterized [3–5]. Recently, we
reported on the optical characterization of IR-induced
UV generation by a five- and six-photon UC process in
Y2O3:Yb3+–Tm3+ nanocrystals [2]. In the present work,
we report on high multi-photon UC–UV emission induced
by IR diode laser excitation in Y2O3:Yb3+–Er3+
nanocrystals.
Researches of compact all-solid-state lasers in the spec-
tral range from ultraviolet (UV) to green are of great inter-
est due to their applications in areas as diverse as optical
data storage, color displays, and environmental monitoring
[1–3]. There is a challenge to develop an optically pumped
UV laser based on the Stokes law, since there are no pump-
ing diode lasers available [3]. Hence, scientists are urged to
employ multi-photon frequency upconversion (UC) tech-
niques to develop short-wavelength lasers, due to the ample
availability of high-power infrared (IR) and visible diode
lasers together with a large variety of equivalent energy-
level spacing in rare-earth-ions [3–5]. IR-induced multi-
photon visible UC radiations have been widely investigated
in recent years, which has resulted in the development of,
e.g., a room-temperature continuous-wave (cw) two-pho-
ton green UC laser with an output of 0.5 W [6]; and an
Several aspects of IR-induced high multi-photon UC
processes in rare-earth-ion-doped materials deserve consid-
eration. Primarily, traditional high multi-photon processes,
e.g., high harmonic generation, only occurs when ultra-
short IR lasers of high-power density, e.g., 109 W/cmꢀ2
,
are used to ablate materials [8]. In contrast, high multi-
photon UC can easily be induced by cw IR diode laser exci-
tation of low power density, e.g., 102 W/cmꢀ2 [2]. The large
discrepancy in power density may indicate the possibility of
using the oscillations of intra-4f electrons in trivalent rare-
earth-ions to greatly favor certain laser output in nonlinear
materials. Secondly, IR-induced high multi-photon pro-
*
Corresponding author.
E-mail address: zhangzhiguo@hit.edu.cn (Z.G. Zhang).
0009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2007.09.078

128
G.Y. Chen et al. / Chemical Physics Letters 448 (2007) 127–131
cesses can probe the nature and the modality in which high-
level states can be populated to induce the radiation, which
constitutes the first step in the development of UC–UV
lasers. Especially, the coupling dynamics between low-
and high-level transitions can be clearly displayed, which
can be used to design UC spectra for, e.g., multicolor bio-
logical imaging [9,10], or to understand some basic phe-
nomenon in laser engineering [11]. Thirdly, from the
practical point of view, these studies can additionally lead
to the discovery of new UV laser materials. Since high-
power and low-cost pumping IR lasers are commercially
available, the costs of such UV lasers may be greatly
reduced.
fraction (XRD) patterns recorded on a Rigaku D/max-cB
diffractometer show that the crystal structure is cubic. The
morphology of the prepared powders was characterized by
transmission electron microscopy (TEM) using a HIT-
ACHI H-8100 microscope. Room-temperature UC fluores-
cence measurements were performed by irradiating the
disks via a variable-power 980 nm diode laser with a max-
imum output of 500 mW (Hi-Tech Optoelectronics Co.,
Ltd, Beijing), and detecting the emission by a monochro-
mator (Zolix Instruments Co., Ltd, Beijing) with a spectral
resolution of 3 nm equipped with a photomultiplier tube
(Hamamatsu CR131).
Rare-earth-ion-doped Y2O3 sensitized by Yb3+ ions, i.e.,
3. Results and discussion
the systems Er3+–Yb3+, Tm3+–Yb3+, and Ho3+–Yb3+
,
have been widely investigated in the literature for efficient
visible UC generation [12–14]. The reasons are: firstly,
the Y2O3 crystal have intriguing physical and chemical
properties, e.g., its transparency range is about 0.23–8 lm
and its phonon energy is relatively low (maximum phonon
energy 600 cmꢀ1) [2,15]; secondly, the Yb3+ ion is known
to be a good sensitizer for trivalent rare-earth-ions, e.g.,
Yb3+–Er3+ is one of the most efficient UC systems [3]. It
is also well-known that the intra-4f electronic transitions
of the Er3+ ion cover a wide electromagnetic range from
IR to vacuum-UV [16,17]. Combining these merits, the
Yb3+–Er3+-doped Y2O3 is investigated as an ideal system
for generation of efficient IR-induced UC–UV radiation.
In this Letter, we report on the first observation of a
four-photon UC process in Yb3+–Er3+-doped Y2O3
nanocrystals.
Fig. 1 displays a TEM micrograph of the Y2O3 nano-
crystals doped with Er3+ ions of 1 mol% and with Yb3+
ions of 10 mol%. As evident in the figure, these powders
are aggregated, which have a nearly spherical shape and
an average diameter of about 40 nm.
Fig. 2 is the recorded UC luminescence of the nanocrys-
tals and the bulk Y2O3 doped with Er3+ 1 mol% and Yb3+
10 mol% under diode laser excitation of 980 nm. The two
diagrams are normalized to the highest peak. As can be
seen from Fig. 2, two bands generated by intra-4f electron
transitions in Er3+ ions and centered at 390 and 409 nm are
4
observed, which correspond to the transitions G11/2
!
4I15/2 and H9/2 ! I15/2, respectively [12]. There are some
differences in the UC luminescence intensity of Y2O3 nano-
crystals doped with Er3+ 1 mol% and Yb3+ 2 mol%, and
with Er3+ 1 mol% and Yb3+ 10 mol%, respectively, but
the spectral shapes and positions are the same. To investi-
gate the fundamental UC mechanisms of the two samples,
the pumping power dependence of the fluorescent radia-
tions was further investigated, and was shown in Figs. 3
and 4 for nanocrystals Y2O3:Er3+ 1 mol%, Yb3+ 2 mol%
and for nanocrystals Y2O3:Er3+ 1 mol%, Yb3+ 10 mol%,
respectively. For an unsaturated UC process, the number
of photons which are required to populate the upper emit-
ting state, can be obtained by the following relation [5],
2
4
2. Experimental procedure
Y2O3 powders doped with 1 mol% Er3+ ions, and 2 or
10 mol% Yb3+ ions were synthesized following the complex
precursor method described briefly as follows [2,9]: yttrium
oxide (Y2O3, 99.99%), erbium oxide (Er2O3, 99.99%), and
ytterbium oxide (Yb2O3, 99.99%) were dissolved in nitric
acid. All chemicals were purchased from Beijing Chemical
Corporation and were used as received. After the solution
was dried, the corresponding nitrates were obtained. The
yttrium nitrate, erbium nitrate, and ytterbium nitrate with
corresponding mole ratio of cations were then completely
dissolved in deionized water using a constant rate of stir-
ring. Subsequently, citric acid was added into the solution
with a mole ratio of (Y + Er + Yb) to citric acid of 1:4.
After complete dissolution, the pH of the solution was
adjusted to 6.0 by addition of ammonium hydroxide. The
resulting solution was dried at 120 °C for 24 h until it
was transformed into a black bulk, which was further cal-
cined at 800 °C for 2 h. Then the powders were pressed to
form smooth and flat disks for spectroscopic analysis. The
bulk Y2O3 doped with 1 mol% Er3+ and 10 mol% Yb3+
ions was obtained by sintering the corresponding disk in
a tubular furnace at 1400 °C for 24 h in air. The X-ray dif-
If / Pn;
Fig. 1. TEM micrograph of Y2O3 nanocrystals doped with Er3+ 1 mol%
and Yb3+ 10 mol%.

G.Y. Chen et al. / Chemical Physics Letters 448 (2007) 127–131
129
where If is the fluorescent intensity, P is the pump
laser power, and n is the number of the laser photons
required.
The observed fluorescent bands at 564 and 662 nm result
2
4
from the well-known transitions H11/2/4S3/2 ! I15/2 and
4F9/2 ! I15/2 of the Er3+ ion, respectively [12,18]. As illus-
4
trated in Fig. 3, the n values for the two bands are 2.08 and
1.84, respectively, which are consistent with the reported
values in the literature [3,12,18]. The n values for the 390
and 409 nm bands are 2.45 and 2.51, respectively, which
indicates that three-photon processes are involved to pop-
4
2
ulate the G11/2(Er) state and the H9/2(Er) state [12,18].
Unexpectedly, n = 3.66 and 2.32 are observed in Fig. 4
for the 390 and 409 nm bands, respectively. The value
n = 3.66 for the 390 nm band indicates that a four-photon
process is involved to populate the 4G11/2(Er) state in Y2O3
nanocrystals doped with Er3+ 1 mol% and Yb3+ 10 mol%.
The discrepancy of multi-photon process for the same
4G11/2(Er) state indicates that another mechanism involv-
Fig. 2. Measured UC luminescence of the nanocrystals and the bulk Y2O3
doped with Er3+ 1 mol% and Yb3+ 10 mol% in the wavelength range of
300–500 nm.
4
ing higher levels than the G11/2(Er) state occurs at Yb3+
ion concentration of 10 mol%. The decreased n value
2
resembles a two-photon process for populating the H9/
2(Er) state (Fig. 4). It is reasonable to ascribe this to the
enhancement of the UC rate at the 4F9/2 state together with
4
the reduction of the n value for the F9/2 state, caused by
the average reduction of the distance between the Yb3+
and the Er3+ ions and the increased ratio of Yb3+ ions
4
per Er3+ ion. The decreased n value for the F9/2 state
was demonstrated to originate from the enhancement of
4
the UC rate at the I11/2 state in ZrO2:Yb3+ + Er3+ nano-
4
crystals [10]. Similarly, the enhanced UC rate at the F9/2
state can lead the n value of the 2H9/2(Er) state to decrease,
4
even down to that of the F9/2(Er) state [19]. The same
argument can also be used to explain the values of
4
2
n = 2.45 and 2.51 for the G11/2(Er) and the H9/2(Er)
states for Y2O3 nanocrystals doped with Er3+ 1 mol%
and Yb3+ 2 mol%. Additionally, n = 2.59 with a large dis-
crepancy from n = 2.08 in Fig. 3 is observed in Fig. 4 for
the 564 nm band, which indicates that a three-photon pro-
Fig. 3. Pump power dependence of the fluorescent bands centered at 390,
409, 564, and 662 nm in Y2O3 powders doped with 1 mol% Er3+ and
2 mol% Yb3+. The diagram is in a double logarithmic scale.
2
cess is involved to populate the same H11/2/4S3/2(Er) state
in Y2O3 nanocrystals doped with Er3+ 1 mol% and Yb3+
10 mol%. This is in agreement with the formerly reported
behavior, where a three-photon process is involved to gen-
erate the green emission under large pumping power
[11,12,18]. Here, we report about a four-photon UC pro-
cess in Er3+-ion-doped materials, which, to our knowledge,
has never been reported before. We believe that further
investigation of multiphoton processes in novel material
structures by adjusting the Judd–Offelt parameters as in
Ref. [4] will eventually lead to stimulated UV radiation
with considerable light output induced by IR diode laser
excitation. Let us exemplify by the Y2O3 matrix whose
Judd–Offelt parameters for the Er3+ ions are X2 =
4.59 0.25, X4 = 1.21 0.21, and X6 = 0.48 0.33 [20].
As indicated in Ref. [6], under diode lasers pumping of
980 nm the two-photon process can possibly induce strong
Fig. 4. Pump power dependence of the fluorescent bands centered at 390,
409, 564, and 662 nm in Y2O3 powders doped with 1 mol% Er3+ and
10 mol% Yb3+. The diagram is in a double logarithmic scale.
4
population inversion between, e.g., the F9/2(Er) and

130
G.Y. Chen et al. / Chemical Physics Letters 448 (2007) 127–131
4
4I15/2(Er) states. Therefore, stimulated UV radiation at
390 nm becomes possible by greatly increasing the lifetime
which is then nonradiatively relaxed to the D7/2(Er)
state. Subsequently, the cross relaxation D7/2 + 4I15/2
!
4
4
of the F9/2(Er) state via reducing the X4 parameter – due
4G11/2 + 4I13/2 of Er3+ ions occurs and populates the
4G11/2(Er) state with the energy discrepancy of 1050 cmꢀ1
easily dissipated by the Y2O3 lattice (maximum phonon
energy of 600 cmꢀ1). Therefore, a four-photon process is
expected for the violet emission. However, n = 2.32 is
to [U(2)]2 = 0.0, [U(4)]2 = 0.5655, and [U(6)]2 = 0.4651 for
4
4
the strong F9/2 ! I15/2 transition [21].
Fig. 5 shows the energy-level diagrams of the Yb3+ and
Er3+ ions, as well as the proposed mechanism to produce
UV fluorescent radiations [16]. The ground state and the
excited state absorption of Er3+ ions are neglected, since
the Yb3+ ion has a much larger absorption cross section
and an efficient energy transfer to Er3+ ions under laser
excitation of 980 nm [10,12,18]. The reason for populating
2
obtained for the H9/2(Er) state, which consequently will
lead to a maximum value of n = 3.32 for the G11/2(Er)
state contradicting the experimental observation of
n = 3.66. Such a result indicates that another mechanism
4
4
is responsible for the population of the D7/2(Er) state.
4
4
the I13/2(Er) state and the F9/2(Er) state is discussed in
detail in another recent paper, see Ref. [10]. It should be
noted that the contribution to the blue and the violet UC
It is reasonable to ascribe this to a co-sensitizing effect
22F5/2(Yb) +4F9/2(Er)! 22F7/2(Yb) +4D7/2(Er), since high
Yb3+ ion doping can easily lead to clusters formation,
namely, Yb3+ ion-pairs in nanocrystals [22,23]. This result
can be clearly seen from the n-value analysis, i.e., that
n = 1.66 of the 662 nm band sensitized by Yb3+ ion-pairs
will lead to n = 3.66 of the 390 nm band, which corre-
sponds well to the experimental data (see Fig. 3). More-
over, this conclusion can be further verified by Fig. 2
where the 390 nm band shows a dramatic decrease for
the bulk Y2O3 as compared with the nanocrystals Y2O3
(doped with Er3+ 1 mol% and Yb3+ 10 mol%). As we
know, the annealing of nanocrystals Y2O3 at the high tem-
perature of 1400 °C for 24 h can lead the doping ions to
move into the lattice, thus dissociating the Yb3+ ion pairs.
Therefore, the decrease of the band at 390 nm can be
expected according to our proposed cosensitizing
mechanism.
4
radiations can mainly come from the F9/2(Er) state due
to the following two facts: firstly, the green emission at
564 nm is almost quenched, while the red emission at
662 nm is greatly enhanced at high Yb3+ ion concentration
of 10 mol% [18]; secondly, the green emission is a three-
photon process (n = 2.6 in our experiments), with the
2
2H11/2/4S3/2(Er) state populated by the higher H9/2(Er)
4
and G11/2(Er) states [11,12]. This precludes the possibility
2
4
that the population of the H9/2(Er) and G11/2(Er) states
come from higher levels populated by a UC process from
2
the H11/2/4S3/2(Er) state. The energy transfer from the
4
Yb3+ ion to the F9/2(Er) state enables the excitation of
2
4
the H9/2(Er) or G11/2(Er) states by phonon assistance.
4
2
The mechanism involved in the F9/2 ! H9/2 transition
requires 929 cmꢀ1 to be dissipated by phonons, while the
4
4
mechanism involved in the F9/2 ! G11/2 transition
requires 921 cmꢀ1 of energy from the lattice to bridge the
gap [16]. Fig. 2 displays that the preferred transition is to
4. Conclusions
2
the H9/2(Er) state. At the high Yb3+ ion concentration
2
of 10 mol%, the population in the H9/2(Er) state will
In summary, laser excitation of 980 nm induces violet
4
become larger and it leads to the higher level population.
and blue UC emissions at 409 (2H9/2 ! I15/2) and
2
2
4
Thus, the transition H9/2 ! D5/2 of Er3+ ion occurs
390 nm (4G11/2 ! I15/2) of Er3+ ions in Y2O3:Yb3+–Er3+
nanocrystals. Three-photon UC processes were observed
for both violet and blue radiations at low Yb3+ ion concen-
tration, while the violet emission displayed a four-photon
process, and the blue emission displayed a nearly two-pho-
ton process at high Yb3+ ion concentration. We believe
that the cross relaxation process 4D7/2 + 4I15/2
!
4G11/2 + 4I13/2 of the Er3+ ions accounts for the population
of the 4G11/2 state, whereas the enhanced UC rates result in
the decrease of the photon process for the blue emission at
high Yb3+ ion concentrations. Further, we demonstrated
that the cosensitizing process 22F5/2(Yb) + 4F9/2(Er) !
22F7/2(Yb) + 4D7/2(Er) is responsible for populating the
4D7/2(Er) state.
Acknowledgement
This work was supported by the 863 Hi-Tech Research
and Development Program of the People’s Republic of
China.
Fig. 5. Energy-level diagrams of the Yb3+ and Er3+ ions as well as the
proposed UC mechanism to produce the UV emission bands.

G.Y. Chen et al. / Chemical Physics Letters 448 (2007) 127–131
131
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