Ultrafast third-harmonic generation from nanostructured optical thin films and interfaces
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Optical harmonic generation from nanostructured thin films and interfaces was investigated experimentally and theoretically. Sample materials were large band gap optical semiconductors (AlN/GaN), rare earth oxides (NdAlO3), and noble metals (Ag). They were examined as solids, nanoparticles, and as hybrid nanocomposites. The goal of the project was to create and characterize high susceptibility, third-order (third-harmonic) materials that relied on nanostructure for an enhanced response. Laser ablation of a microparticle aerosol (LAMA) was used to produce these materials. Two routes to nanostructured materials were investigated. In the first method, a microparticle aerosol, composed of a small concentration of metal or semiconductor, and a larger amount of glass microparticles, was ablated by a focused excimer laser, and the resultant nanoparticle aerosol was supersonically deposited and sintered. In the second method, a monolayer of silver nanoparticles was deposited by LAMA, and this film was further processed by pulsed laser deposition (PLD) of either a passive glass or active matrix material. Better optical quality was found in the hybrid LAMA/PLD materials. Many optical properties were required for characterization: linear transmission and absorption spectroscopy of plasmon resonances, second-harmonic generation (SHG) for field-enhancement analysis, and fluorescence spectroscopy and fluorescence lifetime experiments provided preliminary data for third-harmonic generation studies. The third-harmonic generation experiments were performed using an ultrafast laser system, and modeling the ultrafast dynamics of harmonic generation showed that pulse breakup occurs in the third-harmonic field. Interfaces were found to produce the harmonics, through cooperative group-velocity and phase mismatching. This uniquely ultrafast effect allowed for z-scan measurements to be simplified and for focusing effects to be eliminated. Using frequency-domain interferometry allowed for the measurement of the absolute phase of a third-harmonic pulse, and for an accurate determination of the third-order susceptibility of AlN. Finally, enhancement of second- and third- harmonic generation in PLD-coated Ag nanoparticle films was found to depend both on the material microstructure and the fundamental laser intensity.