The Third-Order Optical Nonlinearities Of The Water Soluble CdTe Quantum Dots Investigated By Z-Scan

The Z -scan technique is a very important and sensitive method for characteriZing the optical nonlinearities, and can measure the magnitudes and signs of the real (Re[χ⁽³⁾]) and imaginary (Im[χ⁽³⁾]) parts of third-order susceptibility (χ⁽³⁾) simultaneously. Lots of investigators pay their attention to the application of water soluble CdTe quantum dots in medicine because of the character of water soluble and other optical characters of this material. In order to find out more detailed optical characters of water soluble CdTe quantum dots, we first study the third order nonlinearities of this material using femtosecond Z -scan technique.First, we introduced the theory of the Z -scan and review the history of this technique. Then, from the UV visible adsorption spectrum of the water soluble quantum dots, we find the absorption band edge of the water soluble CdTe quantum dots moves to the short wavelength comparing with the bulk CdTe. We attribute it to quantum effects, such as size effect of quantum, confinement effect of quantum and so on.The third-order susceptibility of the water soluble CdTe quantum dots was investigated by femtosecond Z-scan technique. From the experimental data, we got the two photon absorption coefficient and nonlinear refractive index. The nonlinear refractive index is positive, indicating the character of self focus. From the intensity relation between photoluminescence and input beam, we find they satify the second power relation under the intensity of the input beam less than 600GW/cm². Because the intensity of input beam in our Z -scan experient is smaller than 600GW/cm², so we get the conclusion that the nonlinear absorption of the sample is two photon absorption (TPA). We analyze the mechanism of the...