CdSe-CdS / ZnS核-多壳量子点的近红外双光子吸收研究

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"这篇研究论文探讨了在近红外飞秒脉冲激光激发下，CdSe-CdS / ZnS核-多壳量子点（QDs）的双光子吸收（TPA）特性。在800纳米波长的激光激发下，不同尺寸的QD表现出显著的TPA能力，其TPA截面从1900到16060 GM不等，具有强烈的尺寸依赖性。此外，量子点的双光子激发荧光强度不仅与TPA容量有关，还与QD表面的不同涂层单分子层（MLs）钝化效果导致的量子产率提高有关。这些特性结合QDs的窄带发射光谱，使其成为多色双光子显微镜的理想探针。该研究发表于2009年的Laser Physics期刊第19卷第9期，ISSN号为1054-660X，由Pleiades Publishing, Ltd.出版。" 文章详细介绍了半导体量子点（QDs）的基本特性和应用，它们因为电子和空穴的三维量子限制而被认为是零维材料。QDs因其独特的光学和光电性质，被广泛应用于从基础物理到光电子设备、QD激光器、量子计算、生物标记和成像以及生物传感器等多个领域。特别是具有强光学非线性效应的QDs，可应用于多光子显微镜、光学限幅和稳定、全光开关等技术。本文着重关注的是QDs的光学非线性特性，特别是双光子吸收。与传统荧光染料相比，QDs具有更强的非线性效应。实验结果显示，QDs的TPA截面随着核心尺寸的增大而急剧增加，这表明尺寸对TPA的影响非常显著。此外，QD的双光子激发荧光强度与TPA容量以及QD表面通过不同MLs的钝化处理提升的量子产率都有关。这种钝化处理可以减少表面缺陷，从而提高量子效率，进一步增强QDs在双光子过程中的性能。由于这些优势，尤其是窄带发射光谱，使得CdSe-CdS / ZnS核-多壳量子点成为多色双光子显微镜的理想选择。双光子显微镜能够实现深组织成像，且对生物样品的损伤较小，因此QDs的这些特性对于生物科学和医学研究具有重大意义。这项工作为理解QDs的非线性光学行为提供了新的见解，并可能推动QDs在光学器件和生物成像技术中的应用发展。

ISSN 1054660X, Laser Physics, 2009, Vol. 19, No. 9, pp. 1886–1890.

Original Russian Text © Astro, Ltd., 2009.

1886

1. INRTODUCTION

Semiconductor quantum dots (QDs) are consid

ered as zerodimensional materials because of quan

tum confinement of electrons and holes in 3D, which

makes QDs exhibit unique optical and photoelectric

properties. Consequently, QDs as promising materials

have been studied from fundamental physics [1] to

various applications including optoelectronic devices

[2, 3], QDs lasers [4], quantum computation [5], bio

logical labels and imaging [6, 7], and biosensors [8].

Especially, QDs with strong optical nonlinear effect

can be applied for multiphoton microscopy [9], opti

cal limiting and stabilization [10], alloptical switch

ing devices [11], etc. Therefore, optical nonlinear

properties of semiconductor QDs have been attracted

much attention. Compared with traditional fluoro

phores, semiconductor QDs have stronger nonlinear

effect, which in combination with the advantages of

sizetunable emission spectra [12] with narrow band

width, high quantum yield, good photostability make

QDs appealing as fluorescence probes in multipho

ton microscopy.

It is well known that twophoton absorption (TPA)

of QDs is an important aspect in nonlinear effect,

which is subject to nanoparticle size distribution and

sizedependent effect. According to theoretical analy

sis, weak TPA effect results from relatively broad dis

tribution of nanoparticle size [13], which suggests

strong TPA effect of II–VI QDs due to the advantage

The article is published in the original.

of size tolerance. The TPA properties of QDs have

been researched using Zscan [14–17] or incorporat

ing twophotoninduced fluorescence methods [18].

Although some experimental results [14, 18] indicate

that TPA effect rapidly become smaller with decreas

ing size, there is an argument on TPA cross section

[17,18] whether the TPA should be smaller or larger

with size changes. Thus, experimental research on the

TPA properties of different QDs is very necessary for

understanding size effect of TPA.

In previous work, due to strong interband and

intraband optical coupling in QDs, we have observed

efficient and robust multiphoton fluorescence with

narrow band induced by femtosecond laser [19], indi

cating strong nonlinear effect in QDs. In this paper, we

concentrate further on the TPA and TPE fluorescence

properties of CdSecore CdS/ZnSmultishell QDs

under excitation of femtosecond pulses. Based on our

direct nonlinear transmission experimental results and

basic TPA principles, we obtained TPA coefficients

and calculated TPA cross sections of QDs. The size

dependent effect of TPA properties were preliminarily

discussed, the influence of different TPA capacity and

coated monolayers (MLs) on TPE fluorescence inten

sity were also analyzed.

2. EXPERIMENTAL

Three kinds of highquality colloidal coremulti

shell CdSe

−

CdS/ZnS QDs were produced with an

alternative “green” way [20, 21]. Table 1 illustrates the

LASER

SPECTROSCOPY

Efficient TwoPhoton Absorption of CdSeCdS/ZnS

CoreMultishell Quantum Dots under the Excitation

of NearInfrared Femtosecond Pulsed Laser

Y. Liu

, P. Chen

*, Z.H. Wang

, F. Bian

, L. Lin

, S.J. Chang

, and G.G. Mu

Institute of Modern Optics, Nankai University, Tianjin, 300071 China

The Key Laboratory of WeakLight Nonlinear Photonics (Ministry of Education), TEDA Applied Physics School,

Nankai University, Tianjin,300457 China

*email: chping@nankai.edu.cn

Received March 27, 2009; in final form, April 1, 2009

Abstract

—We report that twophoton absorption (TPA) properties of semiconductor CdSecore CdS/ZnS

multishell quantum dots (QDs) in toluene under excitation of femtosecond laser at 800 nm. The results show

efficient TPA process and large TPA cross section of three types of size QDs, which is 1900, 5710, and

16060 GM (1 GM = 10

–50

s photon

–1

), respectively. TPA cross section dramatically increases with

increased core size, showing a strong sizedependence effect. Furthermore, twophoton excitation (TPE)

fluorescence intensity not only depends on TPA capacity, but also relies on improved quantum yield resulting

from passivation of QD surface by different coated monolayers (MLs). These facts in combination with the

narrow fluorescence bandwidth make these QDs as promising probes for multicolor twophoton microscopy.

PACS numbers: 78.76.Hc

DOI:

10.1134/S1054660X09170113

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