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Nonlinear Optical Spectroscopy of Cadmium Chalcogenide Nanocrystals J.T. Seo*,+, Q. Yang*, S. Creekmore*, J. Mangana*, J. Anderson*, C. Pompey*, D. Temple*, X. Peng**, J. L. Qu**, W. Yu**, A. Wang**, S. Jung†, H. Ruh†, A. Mott††, and M. Namkung‡ *Department of Physics, Hampton University, Hampton, VA 23668 **Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, AR 72701 † Korea Research Institute of Standards and Science, Daejeon, South Korea, 305-600 †† US Army Research Laboratory, 2800 Powder Mill Road, Adelphi, MD 20783 ‡ NES Branch, NASA Langley Research Center, Hampton, VA 23681 + e-mail: [email protected]

ABSTRACT We report on the nonlinear optical properties of cadmium telluride (CdTe) semiconductor colloidal quantum dots. Transmission electron microscopy measurements revealed that the size of CdTe nanocrystal quantum dots, dependent on the growth reaction time, was ~2-10 nm or near the exciton Bohr radius. The strong blue-shifts of the CdTe, CdSe and CdS nanocrystal absorption spectra and the atomic-like discrete energy states of exciton indicate an exciton quantum confinement. These are completely different optical properties from the bulk crystals. The energy transition for exciton absorption was assigned as h1-
e+, h2-
e+, h1+
e-, and h2+
e- for the 1st, 2nd, 3rd and 4th exciton absorption peaks. Z-scan and I-scan nonlinear spectroscopy revealed that the CdTe nanocrystal quantum dot in toluene (~8 x 10-5 mol/L) has the negative nonlinearity (self-defocusing) with ~ -1 x 10-13 m2/W and a high nonlinear figure of merit of ~200. For the optical power self-limiting experiment, the CdTe nanocrystal was almost opaque above ~0.8 MW/cm2 at the position of z~6.9 cm.

INTRODUCTION The high nonlinear figure of merit and large nonlinear refraction of chalcogenide semiconductor nanocrystals are of great interest for nonlinear optical applications. Ideal optical properties of nonlinear materials for nonlinear optical applications are an ultrafast optical response, a high nonlinear figure of merit (FOM = γ/βλ) and a wide dynamic range of laser wavelength (λ). The nonlinear optical applications include optical power self-limiting for homeland security of human-eye and sensor protection against potential laser terror threats, and agile laser threats on the battle field, optical switching, and pulse shaping in OPO/OPG. During the last decade, considerable theoretical and experimental efforts have been made in the fundamental aspects of quantum confinement effects and nonlinear properties of nanoscale materials. The use of semiconductor nanocrystals for optical power self-limiters is, however, still at the very beginning because matrix, temperature, size and shape dependent on nonlinear optical properties were not fully understood yet. A preliminary measurement and analysis of nonlinear optical properties of CdTe nanocrystal quantum dots in toluene matrix exhibits the strong optical power self-limiting effects mainly because of the large nonlinear refraction and high FOM.

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