Investigation Of Mechanism And Technology Of Multi-photon Ultra-diffraction Nanofabrication

With the rapid development of laser and nano technology,the process technology of micro-nano devices is booming in recent years,which includes photolithography,electron beam lithography,focused-ion-beam lithography,nanoimprint lithography and multiphoton photopolymerization.Multiphoton photopolymerization with femtosecond laser has the virtues of low cost,high resolution,high precision and could be used for any complex 3D structures.It has been applied to fabricate functional structures and devices such as optical devices,microelectronics devices,microelectromechanical systems,photonic crystals and metamaterials.Multiphoton photo-polymerization usually uses commercial photoresist,which is sensitive to ultraviolet light and has small multi-photon absorption cross section.Thus higher laser power or more exposure time are required for the polymerization of photoresist due to its light triggered active molecules with low activity.This is not only detrimental for the resolution,but also easier to exceed the damage threshold of the material and further leads to defects in the micro/nanostructures.Therefore,studying the influencing factors of multiphoton absorption cross section is of great significance for the application of multiphoton photopolymerization by reducing the laser power density and improving the resolution.Based on a through understanding of the mechanism of multiphoton absorption,this thesis studies the effect of multiphoton absorption coefficient and absorption cross section on polymerization threshold.Materials with large two-photon absorption cross section have low threshold for two-photon photopolymerization,which means small processing laser power.Thus,the action area of the femtosecond laser can be reduced and the resolution of the multiphoton photopolymerization can be improved.The thesis includes the following aspects:1)The influence factors of non-degenerated two-photon absorption coefficient and two-photon absorption cross-section were illustrated in theory.The nonlinear dynamic processes in ZnS including non-degenerated two-photon absorption(ND-2PA),degenerated-three-photon absorption(D-3PA)and multiphoton absorption-induced free carrier absorption(FCA)were studied using pump-probe system.In addition,the effect of photon energy and molecular structure of one-arm and two-arn carbazole salts on non-degenerated two-photon absorption cross-sections(ND-2PC)were studied.2)Two photon polymerization was tested under different laser wavelengths and the result showed that with the increase of photon energy,the absorption cross section increased and the threshold of polymerization decreased.For the two-beam photopolymerization,when the power of 400 nm laser and 800 nm laser were both less than their own threshold values,the polymer lines could be obtained by combining the two beams together.3)Multilayer sub-microstructures with tunable morphology were obtained by the interference assisted two-photon photopolymerization on silicon substrate.The distribution of light intensity at the focal area was simulated and the modulation mechanism was explained.The variation rules of the dependence of line width on laser power and focal depth were studied.There was a good agreement between experiment and theory.In addition,structure grating was made using this multilayer structures and its reflection spectrum was observed.