The third-order nonlinear optical materials can play key role to establish the all-optical devices which can utilize for ultra-high speed transmitting, processing and storing of mass information by controlling the optical signal only without any process of electronic modulations. Especially, the nano-size materials are of great interests for wide applications due to their unique and enhanced properties compared to the bulk materials which are mainly originated from the quantum confinement effect.
In this thesis, we synthesized different types of nanomaterials based on the semiconducting, carbon nanostructured-based, dielectric and metal nanoparticles; zinc oxide nanorods, single-walled carbon nanotubes, anodized alumina oxide nano-arrays, and Au:SiO2 nanocomposites, respectively. The enhanced third-order (second-order) nonlinear optical properties of such nanostructures were studied by single-beam z-scan and optical Kerr gate technique employing ultrafast laser sources at resonant and non-resonant wavelengths and enhanced nonlinearities were investigated.
For analyzing the optical nonlinearities, the linear optical properties such as linear refractive index and absorption coefficient were examined by measuring the linear transmission and reflection properties. And the structural morphologies of these materials were processed by scanning electron microscopy, transmission electron microscopy and x-ray diffraction to understand the structural contribution to the nonlinear optical properties of these nanomaterials.
The ZnO nanorods as semiconducting material in nano scale were synthesized with different rod diameters ranging from 50 to 240 nm using low-temperature wet-chemical method, showing highly-ordered structures analyzed by XRD spectra. The nonlinear refractive indices and nonlinear absorption coefficients of these nanorods were estimated to be 0.92？e10-10 to 3.11？e10-10 cm2/W and 2.91？e10-6 to 5.61？e10-6 cm/W, respectively, which correspond to 2~6 and 5~10 times enhanced values of that compared to ZnO thin films.
The arc-discharge made single-walled carbon nanotube (SWCNT) were synthesized as suspension and polymer composite. The distributions of SWCNTs were varied with different sample thickness and concentration to optimize the optical nonlinearity of SWCNTs. The magnitude of third-order nonlinear susceptibilities and nonlinear response of SWCNTs suspension and composites were explored by optical Kerr gate (OKG) technique. Both phase of SWCNTs exhibited ultrafast nonlinear responses comparable to or less than 100 fs, which are equal or faster than that of the incident pulse. The second-order hyperpolarizabilities of SWCNT suspension were estimated to be 1.37？e10-32 to 3.87？e10-32 esu and the third-order nonlinear susceptibilities of SWCNT/PMMA composite 2.19？e10-10 to 1.33？e10-9 esu, respectively. Moreover, it was observed that the screen effect caused by bundling of SWCNTs can decrease the nonlinear optical properties. This leads to the decrement of OKE signals at the specific concentration.
The nanoporous anodized aluminum oxide (AAO) array structures were fabricated using two-step process. Their morphologies, quantum confined band structures and linear optical properties were observed by scanning electron microscopy, photoluminescence and linear transmission and reflection measurements. The third-order nonlinear susceptibilities of the nanoporous AAOs represented a value in the order of 10-10 esu, 4 orders of magnitude larger values than that of the bulk dielectric materials, in which the optical nonlinearities could be varied by controlling the pore size and shape.
The Au:SiO2 nanocomposite films with different Au particle sizes were prepared by alternating sputtering method. The linear refractive indices and absorption coefficients were deduced by analyzing the linear transmission and reflection data, whereas the Au particle size was determined using transmission electron microscopy. The third-order nonlinear susceptibilities of all samples have the order of 10-10 to 10-9 esu at the off-resonant region, which values are much higher than the theoretical prediction based on the Maxwell-Garnett model. Invariant Imbedding method was adapted to explain the contribution of interparticle interactions between the Au particles and showed good agreement with the experimental results. The particle size dependencies of third-order nonlinear susceptibilities were also observed due to the electron mean free path change of the Au particles.