| The arrival of ?big data' era has unprecedentedly increased people's demands on the speeds of data processing and data transfer,and the capacity of data storage.The integrated electric circuit technology using electron as information carrier can no longer fulfil people's requirement,which will be replaced by the development of integrated optical circuit technology that uses photons or photoelectrons as information carrier.The bottleneck in the development of integrated optical circuit technology is to prepare and integrate micro/nanostructures of various optical functions in all kinds of materials,such as semiconductors and glass,precisely as well as efficiently.Thanks to the rapid development of laser technology,the invention of femtosecond laser offers a powerful tool to break through that bottleneck.Due to its unique advantages of ultrashort pulse duration and ultrahigh peak intensity,femtosecond laser pulses can realize the fabrication of various three dimensional micro/nanostructures inside the bulk of transparent materials with precision beyond optical diffraction limit through nonlinear processes.In the recent decade,the investigation of femtosecond laser induced micro/nanostructures in transparent materials and their applications in optical waveguide,microfluidic devices,three dimensional optical devices and high capacity optical data storage and so forth has become an international research hotspot.In this thesis,the formation mechanism and applications of femtosecond laser induced nanogratings and other polarization dependent microstructures in various transparent materials were systematically investigated.Starting from laser parameters,material properties and other experimental parameters,we systematically investigated the formation process and features of self-organized nanogratings induced by femtosecond lasers in transparent materials.Moreover,we demonstrated various applications of such structures in optics.The main research content and several innovative results are summarized as follows:(1)Form birefringence was induced in fused silica by a low repetition rate(1 kHz)near infrared femtosecond laser,which was confirmed originated from the formation of nanogratings in the irradiation region by scanning electron microscope(SEM)observation.The formation of nanogratings and evolution of birefringence under different repetition rate,as well as the effect of other experimental parameters including pulse energy,scanning speed and laser polarization azimuth were systematically investigated.We found that the formation of nanogratings is as a result of multi pulses accumulation,which could be divided into three stages.The mutual orientation of laser polarization and scanning direction has a notable influence on the formation of nanogratings.We suggest that it is due to different energy absorption efficiencies as a result of anisotropic reflection at the interface of laser modified region and glass matrix for different polarization.Furthermore,we demonstrated that a planner structure consist of birefringent lines inscribed by femtosecond laser can function as a light attenuator.The attenuation efficiency of such a structure with respect to wavelength and temperature was further studied,and we found that this attenuator can resist a temperature as high as 900? and that proper heat treatment can improve the attenuation efficiency.We propose that this is because most microcracks were healed and stress was relaxed during heating treatment,which improved the regularity of the structure and thus reduced random scattering.(2)Utilizing femtosecond laser,self-organized nanogratings were induced in quartz crystal for the first time,and conditions suitable for nanogratings formation were investigated.Features of induced nanogratings in quartz crystals were compared with that in fused silica.Under certain experimental conditions,we observed formation of a dual-period nanograting structure in both the horizontal plane and the vertical plane,and we found that the dual-period in vertical plane is associated with laser polarization direction.We suggest that the formation of dual-period structure might be associated with the excitation of some wave by laser in crystals.Furthermore,we discussed the reason why the characterization of induced birefringence in quartz crystal was difficult.(3)Utilizing a femtosecond laser,we induced self-organized nanogratings in pure germania glass for the first time,to our best knowledge,which proved that formation of nanogratings is not restricted by glass species.The formation process and optical features of nanogratings induced by different repetition rate femtosecond pulses were investigated.We observed that parameter window for nanograting formation sharply reduces and birefringence intensity steeply drops with increasing repetition rate,which was ascribed to the effect of heat accumulation.The formation threshold of nanogratings in germania glass was explored by experiment,which is found to be much lower than that in fused silica.We propose that the bandgap difference of these two glasses may affect the nonlinear process of interaction between glass and femtosecond pulses and lead to a different modification threshold.The effects of various parameters such as laser pulse energy,pulse density and laser polarization direction were also investigated.We found that the period of nanogratings and induced birefringence intensity can be adjusted within a range by controlling the angle between laser polarization direction and scanning direction.Moreover,at certain conditions,we observed an opposite polarization dependence of birefringence intensity as compared to fused silica.The possible reason of this phenomenon was discussed from the aspect of glass structure.Based on the birefringence property of nanogratings,we fabricated various optical devices including polarization diffraction gratings,polarization converters and holograms in germania glass and demonstrated their optical applications.(4)Utilizing a high repletion rate near-infrared femtosecond laser,we observed an anomalous polarization dependent process in an isotropic aluminosilicate glass induced by long time stationary irradiation.Two distinctive types of polarization dependent microstructures were induced at different irradiation stages.At early stage(a few seconds),a dumbbell-shaped structure elongated perpendicularly to the laser polarization formed at the top of the modified region,which was later erased by further irradiation.The distribution of ions in laser modified region was analysed by Electron-Probe Microanalysis(EPMA),and the nature of the dumbbell-shaped structure is regarded as the aggregation of defects that are associated with ions migration.The precipitation of silicon nanocrystals occurred at the bottom of laser modified region as irradiation time increased,which was confirmed by Raman spectrum.At later stage(above 30 s),bubbles filled with O2 formed by the irradiation,which were distributed along the laser polarization at a distance far beyond the radius of the laser beam.Based on a simple modeling of light reflection on boundaries,a thermal accumulation process was proposed to explain the formation and evolution of the dumbbell-shaped microstructure.The possible factors responsible for polarization dependent distribution of bubbles are discussed,which needs further systematic investigations.The results may be helpful in the development of femtosecond laser microprocessing for various applications. |
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