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3D Modulaton of Self-organized Nano-gratings in Fused Silica Induced by Femtosecond Laser

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Tutor: DaiZuo
School: Shanghai University
Course: Optics
Keywords: Laser material processing,Subwavelength,Birefringence,Femtosecond laser
CLC: TN249
Type: Master's thesis
Year:  2013
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Femtosecond laser pulses with ultrashort duration time and ultrahigh peakintensity, which traditional CW laser and long-pulse laser cannot reach, is a noveltool in the field of micro-processing, such as the fabrication of microfluidic channels,waveguides, and optical data storage. Depending on the incident pulse energy,femtosecond laser can induce three types of micro-structure in fused silica.Especially in2003, a new type of structure-nano-gratings was found, that has manycharacteristics such as the typical periodicity, erasing and rewriting. It has shownsome potential applications in many fields. Up to date, nano-gratings has become ahot spot, especially, the study on its physical mechanism and the influence factors inthe formation phase will help us a better understanding of the nano-gratings and useit to gain an advantage.In this thesis, the femtosecond pulse laser with the repetition rate of250KHzwas focused into the fused silica. The variation of incident pulse energy, scanningdirection, the polarization plane azimuth and scanning speed were used to study theinfluence on the nano-gratings through methods of hydrofluoric acid solutionetching, optical microscope and scanning electron microscope. A modulation tonano-gratings model of polarization plane azimuth was proposed. The contentsinclude the following aspects:The reversed directions were used to induce the nano-gratings. We found thatthe intensity of the birefringence signals is different in each group. This directionaldependence effect was mainly due to a finite tilt of intensity front possessed by theincident pulses which has an effect on the formation of nano-gratings. Thisphenomenon depends on the correlation between the scanning direction and thetilted intensity front.By the rotation of the polarization plane azimuth, we also found an interestingphenomenon. Along the light propagation direction, nano-gratings are rotated. We attribute this phenomenon to that the existence of an electric field component in thedirection of the light propagation has an effect on the nano-gratings formed in thelongitudinal direction. Therefore, by changing the polarization plane azimuth, wecan control the rotation of nano-gratings in the three-dimensional space. We believethat the anisotropy caused by the presence of the pulse front tilt can be applied inmany areas.A wide range of experimental parameters were used to study the influence onthe nano-gratings. We get an optimal parameter window for the formation ofnano-gratings (within the parameters of our experiments). This can provide areference for our follow-up research, to get better quality nano-gratings.
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