Size dependent optical properties of the CdSe-CdS core-shell quantum dots in the strong confinement regime

We have investigated the third-order optical nonlinearity in CdSe and CdSe-CdS core-shell quantum dots with particle sizes in the range 4.4–5.2 nm by the z-scan technique. Optical absorption and fluorescence properties show quantum confinement effect and the fluorescence peak is shifted when the diameter of the core-shell quantum dots becomes large. Their nonlinear absorption and nonlinear refraction were observed with nanosecond laser radiation of 532-nm wavelength. At an excitation irradiance of 392 MW/cm2, the CdSe-CdS core-shell quantum dots exhibit a reverse saturation and a negative nonlinear refractive index indicating a clear nonlinear behavior. The introduction of CdS shell around CdSe core gives large third-order nonlinearity and third-order optical susceptibility decreases with increasing particle core size in the case of core-shell quantum dots within the range of our investigations. The third-order susceptibility of these quantum dots vary from 3.39 × 10−11 to 5.77 × 10−11 esu. The optical limiting threshold of the quantum dots varies in the range from 0.298 to 0.345 GW/cm2 and this therefore makes CdSe-CdS core-shell quantum dots a promising candidate for reverse-saturable absorption based devices at high laser intensities such as optical limiters.