Nonlinear Optical Spectroscopy of Cadmium Chalcogenide Nanocrystals

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 h 1 →e+, h 2 →e+, h 1 +→e-, and h 2 +→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 × 10 -5 mol/L) has the negative nonlinearity (self-defocusing) with ∼ -1 × 10 -13 m 2 /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/cm 2 at the position of z∼6.9 cm.