4C06

Nonresonant optical properties of CdTe
nanoparticles detected by nonlinear techniques
Lingyun PAN a , Kenj KAMADA b , Atsushi ISHIKAWA a ,Junichi NAKAYAMA a , Naoto TAMAI a
a:Kwansei Gakuin University;b:Natinoal Institute of Advanced Industrial Science

Introduction
Recently, much attention has been paid for nonlinear optical properties ofsemiconductor nanoparticles because of their potential applications [1]. In the present
study, we have examined optical properties of CdTe nanoparticles by resonant and nonresonant time-resolved spectroscopy and laser trapping with near IR picosecond pulselaser. Unexpected anisotropy property of CdTe nanoparticles was detected with theseexperiments. TPA (two-photon absorption) induced fluorescence was used to confirm thelaser trapping of nanoparticles.
Experiments
The CdTe nanoparticles (diameter 3.1~4.9nm) in aqueous solution with aconcentration of ~10-6M were used as samples. For laser trapping experiments, D2O wasused as a solvent. The sample was excited with SHG (420nm) of fundamental of fsTi:sapphire laser and SHG (524nm) of fundamental of ps Nd:YLF laser (~4ps,80MHz).
Polarization direction of excitation laser was adjusted by λ/2 plate to perpendicular one,and emission polarization (I║ and I┴) was selected by film polarizer. Luminescence decays
were measured by picosecond single-photon timing spectroscopy with MCP-PMT. In laser
trapping experiments, the fundamental of Nd:YLF laser was introduced into an inverted
microscope with an oil immersion objective lens (60×, NA 1.25). TPA induced
luminescence from trapped particles was detected with a CCD through the same objectivelens. TPA cross-sections of CdTe nanoparticles were measured by a z-scan method asdescribed previously [2].
Results and discussion
TPA cross sections of CdTe (~nm) was estimated to be ~104GM (GM=10-50 cm4 s
photon-1 particle-1). Polarization dependence of TPA induced luminescence intensity wasdetected in laser trapping experiments as shown in Fig. 1. The anisotropy behavior ofCdTe nanoparticles is clearly detected although the value is smaller than that for typicaldye molecule, rhodaming B. The steady state anisotropy, r, is 0.059, 0.085 for 4.2nm and4.9nm diameter nanoparticles, respectively. This unexpected anisotropy for sphere CdTe
nanoparticles is confirmed by time-resolved anisotropy decay experiments. A typical datais shown in Fig. 2. Two-photon anisotropy at the initial time, r(0), is 0.16 for 4.2nm
diameter CdTe nanoparticles. One photon anisotropy decay measurement shows similarresults. This is the first observation for anisotropy decay in semiconductor nanoparticles
in our knowledge. This can be analyzed by the assumption that individual nanocrystals
have a 2D fluorescence transition dipole, a so-called bright plane and dark axis [3,4]. The
anisotropy decay time will be discussed in terms of volume and the interaction of CdTe
nanoparticles with surrounding medium.
Reference[1] Gianpiero Banfi, Vittorio Degiorgio, Advances in Physics, 47,447(1998)[2] Kenji Kamada, Koji Ohta, Yoichiro Iwase and Koichi Kondo,Chemical Physics
Letters, 302,615(1999)[3]S.A. Empedocles, R. Neuhauser， M. G. Bawendi, Nature,199,126(1999)[4]Ha, T., Laurence, T. A., Chemla, D. S., Weiss, S., J. Phys. Chem. B, 103,90
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F i g . 1 T PA i n d u c e d l u m i n e s c e n c e

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