Vector Tightly Focused Light Field And Its Application In Laser Micromachining

Femtosecond laser direct writing technology is a kind of laser fabrication technology which makes use of femtosecond laser to expose and develop the photoresist on the surface of substrate to obtain the required structure.It can achieve micro-nano level machining in size,and can realize the preparation of high-precision three-dimensional structure by computer control of high-precision displacement table,vibrating mirror and other mechanical devices.The energy density of the center of the femtosecond laser focal spot is very high,which can act on the photoresist to produce the two-photon polymerization(TPP)reaction based on two-photon absorption to achieve the ultra-diffraction limit of micro-nano processing.However,people have certain requirements for the speed and efficiency of laser processing in practical applications.For example,large-scale processing of array structures requires a lot of time,which becomes a major obstacle in practical applications.Therefore,people began to use various schemes to transform the incident light,so that a beam of incident light is divided into multiple beams,so that the processing efficiency can be multiplied.With the rapid development of micro-nano machining and the popularity of spatial light modulator,the use of phase liquid crystal spatial light modulator(SLM)to regulate the light field has been paid more and more attention because of its high flexibility and wide application range.By designing different phase diagrams by computer,the incident femtosecond laser beam can be modulated into dot array spot,graphic light field,special spot and so on,which can meet different processing requirements and improve processing efficiency.The main contents of this paper are as follows:1.A method for parallel fabrication of grating with multiple focal spots is proposed,and phase-only distribution patterns of multiple focal spots are designed and generated by using vector iterative optimization algorithm.The incident light can be modulated into a required multifocal array by using a spatial light modulator and a phase-only hologram.1 × 11 multifocal spots and 5 × 5 multifocal spots for example,11-channel and 25-channel laser parallel processing has been realized based on femtosecond laser processing system and spatial light modulator.The grating with nano-scale width can be produced simultaneously with 11 line.And the microlens can be produced simultaneously with 25 spots.The experimental results verify the vector iterative optimization algorithm can be used to produce multifocal spots with high uniformity,which provides a method for improving the processing efficiency of femtosecond laser fabrication.2.In order to realize the simultaneous control of multi-focus dynamic micro-nano laser machining,different trajectories of multiple focal points were designed,and a group of pure phase distribution maps were obtained based on the complex amplitude coding method.The spatial light modulator was used to dynamically load the pure phase distribution maps,so as to achieve the purpose of adjusting the incident light and realize the simultaneous movement of multiple focal points.The femtosecond processing system built by the author is used to control the movement of multiple focal points along different trajectories by the method of simultaneous movement of multiple focal points.The quality of the machined structure can be optimized by adjusting the moving speed of the laser spot by controlling the speed of machining points and the spatial light modulator dynamic loading phase diagram.By using this machining method,different structures have been processed successfully.3.According to the Richards-wolf vector diffraction integral theory,the single integral expression form of the electric field intensity and magnetic field intensity of a cylindrical vector beam with any polarization order is derived.Based on this integral expression,the intensity distribution of light field and magnetic field in different order is simulated numerically,and the intensity distribution of 3D under tight focusing field is given.It is found that the spatial distribution and weight intensity of radial and azimuth polarization components can be changed by changing the order,so that different complex distribution of electric field components can be generated.Further,based on the distribution of light field and magnetic field,the gradient force is given,in femtosecond vector beam force distribution,the distribution of different order dint is given,by simulating the study found that with arbitrary polarization distribution vector beam can produce with multiple the stability of the optical trap force potential.Vector light field is expected to be of special application value in femtosecond laser micro-nano machining due to its special tight focusing property.