Research On The Fabrication And Mechanism Of Geometric Phase Components By Femtosecond Laser Micro-nano Machining

The micro-nano structure optics based on the artificial regulation of the optical phase,polarization,wavelength and other physical dimensions constitutes the bedrock of the contemporary information optoelectronic technology.As the main instrument of optical integration,the micro-nano structure optics becomes one of the important ways to break through the prospective bottleneck of the microelectronic technology.Among these ways,the geometric phase for micro-optical components that accurately controls the polarization,phase,amplitude and other properties of the optical field at the subwavelength scale through periodically arranging nanoscale unit structures greatly simplifies the integrated design of optical systems relying on the optical control capability of high compactness and performance.In holographic imaging,polarization optics and other fields,it has gained widespread attention and is hailed as a revolutionary breakthrough in optics.However,for reasons based on similarity,the geometric phase optical components require higher processing resolution.Although electron beam lithography(EBL)and focused ion beam(FIB)can provide high processing resolution,the simpler,cheaper and more flexible processing technologies guarantee that the geometric phase optics can be applied more extensively.Among those technologies,the femtosecond laser micro-nano machining technology has become an important tool for high precision micro-nano machining because of its advantages such as no material selectivity,high flexibility and high precision,and it is also one of the reliable choices for fabricating geometric optical devices.Focused on the accuracy,surface roughness and internal crystal device preparation problems in fabricating geometric phase components by femtosecond laser micro-nano machining,this paper conducts a systematic and in-depth study from the outside to the inside to improve the precision of micro-nano structure preparation of geometric phase devices on the gold film surface.The auxiliary process of thermal annealing technology is used to reduce the nano-roughness formed during the interaction between laser and gold film,prompting the geometric figure to be more regular and complete and realizing the high precision preparation of micro-nano structure on the gold film surface.For the preparation of geometric phase devices suitable for the high temperature or power circumstances,we initiatively studied the structure formation of the femtosecond laser-induced birefringence inside the sapphire;summarized the aforesaid physical mechanism and optimized physical properties.Through preparing nano-grating with the cyclicality of approximately 300 nm,the visible wavelength Pancharatnam-Berry lens and Q-plate components are implemented,obtaining good optical performance.To further understand the formation mechanism of laser-induced birefringence,the relationship between structure birefringence and stress birefringence during femtosecond laser processing in the sapphire crystal is studied by thermal annealing technique,and a multilayer geometric phase focusing lens is fabricated.The specific researches of this paper are as follows:(1)Fabrication of efficient geometric phase devices for near-threshold femtosecond laser.Relying on controlling the laser pulse energy slightly higher than the laser action threshold,single-point exposure time(scanning speed)and other parameters,the high-resolution gold film surface can be fulfilled.In the experiment,we obtained the nano-grating with cyclicity of 300 nm and a groove width of 170 nm in 50 seconds by direct ablation at 5 mm/s.Whereafter,the nanoparticles produced in the femtosecond laser direct writing process are removed by thermal annealing technique,making the surface of the fabricated nano-grating to be regular and smooth so that the surface quality is good and the surface roughness is greatly reduced.By applying this technology,we fabricated polarized diffraction grating,metasurface lenses,Q-plate and "M" hologram,and verified the optical properties at the wavelength of 808 nm,proving the reliability of our technology.(2)The formation mechanism of nano-grating in sapphire and the preparation of geometric phase components.The influence of femtosecond laser processing parameters,such as pulse energy and pulse number,on the formation of nano-grating is systematically studied.The formation process of nano-grating in the sapphire crystal is revealed.It is found that the micropores(amorphous region)will be formed firstly in the center,then the nano-gaps at the upper and lower ends,and finally the nano-gratings with the increase of pulse number.By applying this mechanism,nano-gratings with the cyclicality of 150 ~ 300 nm,the non-polarized refractive index of 1~2×10-3and birefringence of 6 × 10-4 are prepared.The Pancharatnam-Berry(PB)phase lens and Q-plate are prepared.In device preparation,it is found that the stress generated around the nano-grating structure and the edge of the device has a significant influence on its optical performance.(3)The relationship between stress birefringence and structural birefringence in sapphire.The stress birefringence affects the structural birefringence,resulting in the decrease of design polarization and performance.Therefore,we initiatively studied the influence of temperature and time on the stress generated by internal defects in sapphire crystal,and found that annealing at 1000?for 6 hours could keep the phase delay of nano-grating unchanged,and eliminate the internal stress of crystal,basically eliminating the influence of stress birefringence on the performance of structural birefringence.We fabricated a high-efficiency embedded multilayer(PB)lens by femtosecond laser direct writing combined with thermal annealing.