Pure red visible emission via three-photon excitation
of colloidal Na
3
ZrF
7
:Er nanoparticles
using a telecom-band laser
Shuai Ye (叶 帅)
1,2
, Guangsheng Wang (王广盛)
1
, Maozhen Xiong (熊茂珍)
1
,
Jun Song (宋 军)
1,
*, Junle Qu (屈军乐)
1
, and Weixin Xie (谢维信)
2
1
Key Lab of Optoelectronic Devices and Systems of Ministry of Education/Guangdong Province,
College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China
2
College of Information Engineering, Shenzhen University, Shenzhen 518060, China
*Corresponding author: songjun@szu.edu.cn
Received September 23, 2016; accepted November 11, 2016; posted online December 14, 2016
We provide the first demonstration of pure red emission in the visible light region via three-photon excitation in
monodisperse Na
3
ZrF
7
:Er nanoparticles (NPs) by using a laser operating in the telecommunication band. NPs of
∼22 nm in diameter are synthesized at 260°C by the thermal decomposition method. The experimental results
reveal that the Na
3
ZrF
7
:Er NPs exhibit pure red emission in the visible region under 1480 nm laser excitation,
and the emission intensity is significantly influenced by the Er
3þ
ion concentration. The decay times of the
4
S
3∕2
→
4
F
15∕2
and
4
F
9∕2
→
4
F
15∕2
transitions of the Er
3þ
ions at 540 and 655 nm, respectively, are reduced
by increasing the Er
3þ
ion concentration in the Na
3
ZrF
7
:Er NPs. The suppressed emission intensity result from
the defect-related quenching effect: when trivalent Er
3þ
ions replac tetravalent Zr
4þ
ions, extra Na
þ
ions and F
−
vacancies are formed to re-balance the charge in the Na
3
ZrF
7
matrix. The emission color of the Na
3
ZrF
7
:Er NPs
is related to the cross relaxation between Er
3þ
ions. These results provide an important step toward more ef-
fective biological imaging and photodynamic therapy by minimizing the scattering of the excitation light and
increasing the penetration depth.
OCIS codes: 160.2540, 160.4760.
doi: 10.3788/COL201715.011601.
The multiphoton effect based on two-photon or three-
photon technology is receiving increasing attention due
to its comprehensive applications in biomedicine, photo-
voltaics, optical telecommunications, etc.
[1–10]
. Similar to
other nonlinear optical technologies, the common multi-
photon effect has very low efficiency to upconvert the
energy of excited photons, as the intermediate levels are
virtual
[11]
. In comparison, lantha nide-doped upconversion
nanoparticles (UCNPs) can effectively convert low-energy
photons (always infrared light) into ultraviolet, visible,
or near-infrared (NIR) photons, owing to their ladder-
like system of energy levels
[12]
. Until now, owing to their
high efficiency, the most frequently used UCNPs have been
hexagonal β-NaYF
4
nanoparticles (NPs) doped with
Yb
3þ
∕RE
3þ
(RE ¼ Er, Tm, Ho)
[13,14]
. Upon laser excitation
at 980 nm, these UCNPs emit visible light by a two-photon
process.
For biomedical applications, it is often important for the
excitation light to penetrate far into the sample;
however, the penetration depth is limited by Rayleigh
scattering, owing to the effect on the beam quality. Since
Rayleigh scattering scales as λ
−4
, long-wavelength multi-
photon excitation seems prom ising for biomedical applica-
tions, e.g., in vivo biological imaging and photo dynamics
therapy (PDT) of deep tumors. It has been shown that an
optimum wavelength window, owing to scattering and
absorption in tissue, lies close to the telecom band
[15]
.
Recently, Er
3þ
ions have been considered to be a promising
choice, owing to their strong absorption at ∼1500 nm,
which corresponds to the energy transfer from the
4
I
13∕2
energy level to the
4
I
15∕2
energy level
[16]
. LiYF
4
:Er
3þ
NPs
were first reported by Chen et al.
[17]
to exhibit multicolor
emission under 1490 nm laser excitation by three-photon
upconversion (UC) process. Subsequently, NaYF
4
:Er
3þ
UCNPs excited at 1550 or 1523 nm were reported
[18,19]
, and
it was found that the emission intensity could be largely
enhanced by the inert NaYF
4
shell
[20]
. These UCNPs irra-
diated almost multicolor light containing ∼540 nm (green),
∼660 nm (red), and ∼800∕980 nm NIR wavelengths. How-
ever, red emission is widely considered to be the most prom-
ising candidate for deep tissues, owing to its penetration
capabilities
[21,22]
. Additionally, the most frequently used
photosensitizer in PDT applications strongly absorbs in
the red light region
[23]
.
Many studies have investigated UCNPs with pure red
emission under a 980 nm laser excitation by a two-photon
UC process
[24–29]
, but no studies were published on NPs
upconverting from the telecom band to red light by a
three-photon process. In this work, Er
3þ
-doped Na
3
ZrF
7
NPs were synthesized by following a procedure that had
been previously used
[30]
. Upon 1480 nm laser excitation,
these UCNPs irradiated pure red light centered at
660 nm in the visible light region. The effect of the Er
3þ
ion concentration on the emission intensity of Er
3þ
-doped
COL 15(1), 011601(2017) CHINESE OPTICS LETTERS January 10, 2017
1671-7694/2016/011601(4) 011601-1 © 2016 Chinese Optics Letters
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