Study On Three-dimensional Micro-fabrication Technique With Two-photon Absorption And Ultrafast Detection By Femtosecond Laser

Ultrahigh peak intensity can be achieved at low pulse energy with femtosecond laser, thus the interaction region of two photon absorption (TPA) was tightly confined in the focus, three dimensional(3D) micro-fabrication with high precision can be achieved by controlling the movement of the focus. With ultrashort pulse width, ultrafast dynamical processes in various subjects can be investigated in a femtosecond time scale.The principle of TPA was introduced, and TPA photo-polymerization model was founded based on radicals excited by laser in the resin. With the intensity function of Gaussian beam, the lateral and axial resolution was deduced. Femtosecond laser direct scanning 3D micro-fabrication system was set up; it consisted of laser system, microscope system, real-time detection system, 3D-movement system and so on. Corresponding controlling software has been developed with Visual Basic program language.TPA photopolymerization was achieved in ORMOCER resin, the resolution reached 0.7μm, less than the diffraction limitation. The dependence of line width with numerical aperture (NA), speed, and laser power was investigated. The line width was inversely proportional to the fabrication speed, but proportional to laser power and NA. The experiment results were simulated, beam waist of 0.413μm and TPA cross section of 2 × 10^-54cm⁴s was obtained. The reasons for beam waist broadening were analyzed and the factors which affected the fabrication resolution were discussed, at last the optimized fabrication parameter was determined.With TPA photo-polymerization,complex "CHINA" model with width of 0.72μm was made. Then typical optical micro-devices in micron scale, such as beam splitter, optical waveguide, array of micro-lens, and grating were manufactured. It was found that the fabrication quality of plane board was much better when the overlap rate is 50%, and micro-gear wheel with gear width of 5μm has also been fabricated.Primary investigation was carried out on photonic crystal(PhC), 3D PhC fiber with height of lOum and space of 5um was manufactured. At last, 3D photonic crystal of wood-pile structure with four layers was fabricated, the space between the layers and sticks were both 5um, the resolution of the stick was l.lum,it was the first PhC structure fabricated with TPA reported inland.These results proved that the microfabrication system of TPA can not only obtain the resolution down to micron level, but also realize real 3D microfabrication.Femtosecond laser pump probe experiment setup was established, and pump probe software was developed, the time resolution of setup reached 67fs. Dependence of transient reflectivity change on delayed time in GaAs was measured, it was made up of three processes: original scattering process of lOOfs, carriers-lattice thermal equilibrium of 2ps and recombination process of 500ps.The ultrafast relaxation of nonequilibrium carriers excited by femtosecond laser was discussed based on free carriers effect, lattice temperature effect and so on. The refraction change induced by free carriers and lattice temperature effect was respectively-7.33 X lO"4,*)^ X10"4.