Journal of Alloys and Compounds 370 (2004) 94–98
Energy transfer and frequency upconversion in Yb
3+
, Er
3+
co-doped sodium–lead–germanate glasses
Zhongmin Yang
∗
, Shiqing Xu, Lili Hu, Zhonghong Jiang
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PR China
Received 16 June 2003; received in revised form 4 September 2003; accepted 5 September 2003
Abstract
Yb
3+
–Er
3+
co-doped Na
2
O–GeO
2
–PbO glass suitable for developing optical fiber lasers and amplifiers has been fabricated. The energy
transfer efficiency from Yb
3+
to Er
3+
was investigated in the glasses with different Yb
3+
–Er
3+
concentration ratios and the maximum energy
transfer efficiency was found to be 67% at the Yb
3+
–Er
3+
concentration ratio of 5:1. Subsequently, the studies of upconversion emissions in
the visible range were performed. An intense green together with a relatively weak red emission was observed under the excitation of 976 nm
diode laser. The quadratic dependence of the green emission on excitation power indicates that a two-photon absorption process occurs under
a 976 nm excitation. However, for the red emission the slope of log-log plot of integrated intensity versus pump power declines with increasing
Yb
3+
ions content, indicating it is not resulted from a biphotonic process.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Luminescence; Rare-earth; Glass
1. Introduction
Heavy metal oxide (HMO) glasses, which contain high
contents of PbO and Bi
2
O
3
, are now considered important
materials for certain technological applications such as new
lasing materials, upconversion, and planar optical waveg-
uides [1,2]. These applications arise from certain proper-
ties of this group of materials, for instance, high density,
high refractive index, good chemical durability, and excel-
lent transmission in the near infrared region (IR). Moreover,
the maximum phonon energy is much lower in these glasses
compared to silicate, borate or phosphate glasses contain-
ing lighter elements. The lower vibrational cut-off frequency
(typically in the IR) increases the quantum efficiency of lu-
minescence from the excited states of trivalent lanthanide
ions as dopants in these hosts [3,4].
HMO glasses mainly including tellurite, bismuthate and
germanates are desirable hosts for optically active ions. Of
these HMO materials, lead germanate glasses are of growing
interest because they combine higher mechanical strength,
higher chemical durability, and better thermal stability even
though the maximum phonon energy (810 cm
-1
) is slightly
∗
Corresponding author.
E-mail address: wuhanyangzm@yahoo.com (Z. Yang).
larger than that of tellurite glasses (750–780 cm
-1
) [5–8]. In
addition, the theoretical values of optical losses, calculated
from Rayleigh scattering and multiphonon absorption, are
0.02–0.05 dB/km at 2.89 m for GeO
2
[9]. The lower loss
of germante glasses in the IR makes them more promising
for practical use as long optical fibers. Furthermore, the
germanate glasses are photosensitive and Bragg gratings
can be achieved by UV-light exposure [10,11]. In this work,
we report the energy transfer efficiency and upconversion
properties in Yb
3+
–Er
3+
co-doped sodium–lead–germanate
glasses.
2. Experimental
2.1. Glass composition and preparation
The glasses used in this work have the following compo-
sitions in molar fraction: 0.1Na
2
O–0.6GeO
2
–0.3PbO–xYb
2
O
3
–0.002Er
2
O
3
with x = 0.2, 0.6, 1.0, 1.4, and 2.0%. They
were synthesized by a conventional melting and quenching
method which can be available everywhere [16]. The starting
materials are analytical grade chemicals of Na
2
CO
3
, Pb
3
O
4
(Pb
3
O
4
→ 3PbO + 1/2O
2
↑), Yb
2
O
3
and high purity GeO
2
with 99.999%. The Er
3+
-doping concentration in the glasses
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2003.09.009