Pureredvisibleemissionviathree-photonexcitationofcolloidalNa3ZrF7:Ernanoparticlesusingatelecom-bandlaser资源-CSDN文库

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Pure red visible emission via three-photon excitation

of colloidal Na

ZrF

:Er nanoparticles

using a telecom-band laser

Shuai Ye (叶帅)

1,2

, Guangsheng Wang (王广盛)

, Maozhen Xiong (熊茂珍)

Jun Song (宋军)

*, Junle Qu (屈军乐)

, and Weixin Xie (谢维信)

Key Lab of Optoelectronic Devices and Systems of Ministry of Education/Guangdong Province,

College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China

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

ZrF

: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

ZrF

: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

3∕2

→

15∕2

and

9∕2

→

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

ZrF

: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

ions and F

−

vacancies are formed to re-balance the charge in the Na

ZrF

matrix. The emission color of the Na

ZrF

: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

nanoparticles (NPs) doped with

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

13∕2

energy level to the

15∕2

energy level

[16]

. LiYF

: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

: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

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

ZrF

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

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