Fabrication Of Subwavelength Diffraction Grating By Two-photon Polymerization Based Direct Laser Writing

Subwavelength diffraction grating is a grating whose period is equivalent to the working wavelength of the grating,usually only zero or one order of diffracted light,with a wide spectrum,high reflection and other characteristics.It has a wide range of application such as:sensing,spectral control,power splitting,beam shaping,optical thermal absorption.However,the current subwavelength grating fabrication technology has problems such as expensive cost,and difficulty in large area preparation.To address these problems,we propose a two-photon polymerization-based femtosecond laser direct writing method for the fabrication of subwavelength diffraction gratings.With help of two-photon polymerization additive manufacturing technology,the photoresist region exposed to the femtosecond laser spot will solidify to form a grating through two-photon polymerization mechanism.The fabricated grating have high fineness and good uniformity,and its paramenters such as the grating period,line width,height and so on can be flexibly controlled by changing the displacement stage movement path and laser output power.Specific research includes:Based on the rigorous coupled wave analysis theory,a subwavelength planar diffraction grating model was established to simulate the effect of grating height,duty cycle,period and other parameters on diffraction efficiency;a 515 nm wavelength femtosecond laser direct writing processing system was built,and a spot maintenance system based on a 780 nm continuous laser was developed in the original optical path to adjust the relative position between the femtosecond laser spot and the substrate,so that the grating height was always kept constant,thus offsetting The subwavelength grating fabrication process was explored to study the effects of photoresist characteristics,femtosecond laser power,displacement table movement speed,laser focus spot position and other factors on grating preparation,and a subwavelength grating preparation process including substrate cleaning,gluing,debubbling,leveling,grating fabrication,development and other processes was explored.Based on the grating preparation process,a complete subwavelength planar diffraction grating with a size of20×20 mm,a grating period of 770.6 nm(1298 lines/mm),an average line width of 167.7 nm,and an average height of 604.7 nm was fabricated,and the grating homogeneity was tested.The main innovations of this dissertation are: Using the oil-immersion two-photon polymerization laser direct writing processing method,the objective lens is immersed in the photoresist,and the high refractive index photoresist replaces the air in the dry mirror method to obtain a larger numerical aperture,thus increasing the spatial resolution of the system.By introducing another band of 780 nm continuous laser in the optical path of the femtosecond laser direct writing,a spot maintenance system has been developed that can precisely control the position of the focused spot of the femtosecond laser relative to the grating substrate,which can offset the displacement deviation caused by the shrinkage of the mechanical parts of the platform due to temperature fluctuations and also increase the tolerance of the flatness of the machined surface.