Measurement Of Third-Order Nonlinear Properties Of Semiconductor CdSe Nanoparticles Using Z-Scan Method

Semiconductor nanoparticles whose radii are smaller than the bulkexciton Bohr radius constitute a class of materials intermediate betweenmolecular and bulk forms of matter. Quantum confinement of both theelectron and hole in all three dimensions leads to an increase in theeffective band gap of the material with decreasing particles size.Consequently, both the optical absorption and emission of nanoparticlesshift to the blue(higher energies) as the size of the particle gets smaller.Although nanoparticles have not yet completed their evolution into bulksolids, structural studies indicate that they retain the bulk particle structureand lattice parameter. Recent advances in the synthesis of highlymonodisperse nanoparticles have paved the way for numerousspectroscopic studies assigning the nanoparticles electronic states andmapping out their evolution as a function of size.In recent years, the third-order nonlinearity of various types ofsemiconductor nanoparticles have been investigated because of potentialapplications in optoelectronic and photonic devices. Using picosecond andfemtosecond pulses, Someone measured the ultrafast nonlinear index,two-photon absorption and free carrier refraction coefficients of glassesdoped with Semiconductors.This article is based on the above disscusion. In order to allow thereader to develop a basic understanding of nonlinearity, a brief overviewof third-order nonlinearity is introduced. Then we review the history ofTi:Sapphire laser system. Several important experiment techniques such asthree-wave mixing, four-wave mixing and Z-Scan are discussed.Z-Scan technique has become a useful technique in the field ofnonlinear optics and material science since it is presented by Sheik-Bahae in 1989.It is simple and sensitive for measuring nonlinear refraction and absorption.We describe the principle, characteristics and application of Z-Scantechnology, and analyze the single wavelength Z-Scan measurements. Asimple yet highly sensitive single beam experimental technique for thedetermination of both the sign and magnitude of nonlinear refraction indexis presented.The nonlinear refraction index of Semiconductor CdSe nanoparticleshave been studied using Z-Scan at room temperature with Ti:Sapphirefemtosecond laser. The samples, in the form of nanoparticles in toluene,were placed in a cuvette with a thickness L=1 mm for the liquid. Thesample is moved along the z direction of a focused Gaussian beam. Anaperture was placed before the Z-Scan detector, and the beam passingthrough it was monitored as a function of the position z of the sample. Themeasurement system was detected by standard sample CS2.The resultant plot of transmittance through an aperture in the far fieldyields a dispersion-shaped curve from which n2 is easily calculated. Theresearch result showed that the value of nonlinear refraction index in theCdSe nanoparticles is about ( )?1 .703 ± 0.077 × 10 ?18 m 2Wat the wavelengthof 788 nm . Compared with the value of pure organic solution10 ?2 1 ~10 ?2 0 m 2W. We found the third-order nonlinear refraction index forthe semiconductor CdSe nanoparticles is increased.The measurement of the nonlinear refractive index is an importantmethod for the study of the nonlinear optical quality and investigation ofoptical inherent damages in crystal.