780 CHINESE OPTICS LETTERS / Vol. 8, No. 8 / August 10, 2010
Spectroscopic characteristics of Yb
3+
:LiLa(WO
4
)
2
crystal
Xinyang Huang (###)
Institute of Research on the Functional Materials, Jiangxi University of Finance and Economy, Nanchang 330013, China
∗
E-mail: xyhuang0202@hotmail.com
Received April 15, 2010
The spectroscopic characterization and fluorescence dynamics of Yb
3+
:LiLa(WO
4
)
2
crystal are investi-
gated. The Yb
3+
:LiLa(WO
4
)
2
crystal exhibits a broad absorption and emission spectral bands, large ab-
sorption and emission cross sections, and moderate fluorescence lifetime. Blue light emission around 480 nm
is observed at 10 K and is demonstrated through cooperative upconversion from the deexcitation of excited
Yb
3+
-Yb
3+
pairs.
OCIS co des: 140.3613, 160.5690, 300.2140, 300.2530.
doi: 10.3788/COL20100808.0780.
Recently, with the development of high-power InGaAs
diode lasers, increasing attention has been paid to the re-
search on Yb
3+
-doped laser materials. This research was
stimulated because Yb
3+
demonstrates many advantages
over other rare earth ions. Yb
3+
only has two electronic
manifolds (i.e., ground-state manifold
2
F
7/2
and excited-
state manifold
2
F
5/2
) separated by approximately 10000
cm
−1
. In addition, Yb
3+
hardly suffers from cross-
relaxation, upconversion, concentration quenching, or
excited-state absorption because it lacks relevant high-
lying excited states. Its quantum efficiency is also very
high, greatly lightening thermal loads, and its long fluo-
rescence lifetime is beneficial for storing energy
[1,2]
. Due
to the presence of strong electron-phonon coupling, Yb
3+
ions have broad absorption and emission bands, offering
the possibility of near-infrared tenability and the gener-
ation of ultrashort pulses
[3−7]
.
Double tungstate crystals with the general formula
AB(WO
4
)
2
(A = Li, Na, or K, B = rare earth) from a
wide variety of inorganic compounds having monoclinic
and tetragonal symmetries have recently attracted grow-
ing interest. Their structure diversity provides these
crystals with excellent physical and chemical properties
as laser hosts, such as excellent chemical durability in air
atmosphere, large rare-earth ion admittance, and large
absorption and emission cross-sections of rare-earth ions
in its lattice
[3−11]
. Yb
3+
-doped double tungstate crystals
with sheelite-type local disorder structure exhibit rela-
tively broad bandwidths, promising to become rare earth
laser hosts for applications requiring tunability or ultra-
short pulses. For example, Yb
3+
laser operation has been
demonstrated in several crystal hosts: NaRe(WO
4
)
2
(Re
= Y, Gd, and Lu
[3−7]
). LiLa(WO
4
)
2
, a Li-based dou-
ble tungstate crystal, possesses no phase transition and
can be directly grown from the flux
[12−14]
. Moreover,
the Yb
3+
:LiLa(WO
4
)
2
crystal exhibits excellent ther-
mal and spectroscopic characteristics
[13,14]
. However,
to our knowledge, the polarized optical characteristics
and upconversion emission of Yb
3+
:LiLa(WO
4
)
2
crys-
tal have not been investigated yet, and this may limit
the study of the Yb
3+
:LiLa(WO
4
)
2
laser. Therefore,
this letter is mainly concerned with the polarized spec-
troscopic characterization and fluorescence dynamics of
Yb
3+
:LiLa(WO
4
)
2
crystal.
The Yb
3+
:LiLa(WO
4
)
2
crystal was grown by the
Czochralski method. The growth procedure was sim-
ilar to those described in Refs. [12,14]. The X-ray
diffraction (XRD) patterns were collected using a
D/max-rA diffractometer and employing CuK
α
radi-
ation (λ = 0.154056 nm) at room temperature. The
Yb
3+
concentration was measured by inductively cou-
pled plasma atomic emission spectrometry (ICP-AES).
The sample, with the dimension of 5 × 5 × 0.5 (mm),
was cut along the [001] orientation from the as-grown
Yb
3+
:LiLa(WO
4
)
2
crystal and polished for spectroscopic
measurement. According to the tetragonal characteristic
of lattices, the experimental spectra were labeled as σ or
π, defined in terms of the E-vector being perpendicular
and parallel to the optical axis c, respectively. Polar-
ized absorption spectra were obtained using a Perkin-
Elmer ultraviolet-visible-near infrared (UV-VIS-NIR)
spectrometer (Lamada-900) in the range of 800 – 1100
nm at room temperature. Emission spectra and fluores-
cence decay curves were recorded on Edinburgh Instru-
ments FLS920 spectrofluorimeter equipped with both
continuous (450 W) and pulsed xenon lamps. For low-
temperature measurements, the sample was mounted on
a closed cycle cryostat (4-350 K, DE202, Advanced Re-
search Systems).
The XRD pattern of as-grown Yb
3+
:LiLa(WO
4
)
2
crys-
tal at room temperature is shown in Fig. 1. The pattern
was analyzed using the Topas 2 program. Results ob-
tained show that the as-grown Yb
3+
:LiLa(WO
4
)
2
crys-
tal belongs to the tetragonal system with space group
I4(1)/a (C
6
4h
) and unit cell parameters: a
0
= 0.5232 nm
and c
0
= 1.1342 nm
[12]
.
Fig. 1. XRD pattern of as-grown Yb
3+
:LiLa(WO
4
)
2
crystal.
1671-7694/2010/080780-04
c
° 2010 Chinese Optics Letters