Research On Sub-Diffraction Limit Fslaser Direct Writing Technology

The development of basic science and cutting-edge technology in the direction of miniaturization has fully entered nano field.Nano-scale functional structures have special properties such as mechanics,optics and electronics,and have significant advantages and broad application prospects in the fields of micromechanics,micro optics,optoelectronics,and biomedicine.For the past dacades,ultrafast laser manufacturing technology,especially for femtosecond laser direct writing(Fs DLW),has become a flexible,efficient,precise and intelligent processing and manufacturing method for nanomaterials and nanostructures.While,the feature size and resolution of Fs-DLW are usually above hundreds of nanometers due to the optical diffraction limit,which limits the wide application in the field of nanomanufacturing.Many method such as Multi-photon polymerization(MPP),Diffraction elements for laser focusing,Laser-induced periodic surface structure(LIPSS)and Stimulated emission depletion direct laser writing(STED-DLW)have been used to break the diffraction limit.However,the resolution of MPP,diffractive elements lithography,and LIPSS is above 100 nm,and the ripples produced by LIPSS are always random.Laser-induced periodic surface structure,stimulated emission depletion fluorescence microscopy and direct laser writing was combined respectively,to carry out sub-diffraction limited femtosecond laser direct writing with resolution below 100 nm.Analysis and research on equipment,method,mechanism and process of Sub-diffraction direct laser writing were carried out,respectively.A sub-diffraction limited direct laser writing system was bulit,which compose of a laser sources and modulation subsystem,a beam transmission subsystem,a monitoring and imaging subsystem,a three-dimensional mobile platform and its control subsystem.One-beam and two-beam sub-diffraction direct laser writing experiments can be performed on this platform.The laser sources and modulation subsystem includes a Ti-sapphire femtosecond laser with wavelengths of 800 nm,a continuous laser with 532 nm wavelength,a chirped pulse compressor(CPC)and an electro-optical modulator(EOM).The beam transmission subsystem is mainly composed of a half wave plate,a beam amplifier,a series of lenses and mirrors.It is used to transmit the laser energy to the processing position accurately.Monitoring and imaging subsystem is used to monitor the laser focus position and nanostructure manufacturing process in real time.Three-dimensional mobile platform and its control subsystem are composed of a three-dimensional piezoelectric mobile platform and processing control software to achieve the laser focus position in XYZ three Real-time control of all directions.One-beam sub-diffraction limited direct laser writing experiment was carried out on the built system.The influence of main process parameters such as incident laser power and laser polarization direction on the surface morphology of nanostructures was explored and analyzed.Controllable fabrication of nanowires with width of 40 nm,nanogrooves with width of 60 nm and nanowire arrays with a period of 180 nm is realized with appropriate parameters.Forming principles of nanowires and nanogrooves was analysed.And combined with experimental results,the nanostructurs were found forming by the interference between incident laser and surface plasmons.Theoretical model of One-beam sub-diffraction-limited laser direct writing process was established,and forming mechanism of sub-diffraction-limited nanostructures is analyzed.Nonlinear propagation of ultrafast laser pulse in photoresist materialwas simulated and analyzed with Matlab software,surface free electron density and dielectric constant of the dielectric material were calculated.The electromagnetic field on SU-8 surface generated by the interference between incident laser and surface plasmons was simulated with obtained dielectric constant.The intensity and period of the interference fringes are determined.Temperature rise of the photoresist surface is simulated by the finite element method.Two-beam sub-diffraction limited direct laser writing experiment was carried out using the built sub-diffraction system with a rod-shaped effective focusing spot.Nanostructures with feature sizes far below the diffraction limit were prepared.Effects of laser process parameters(including photoinitiator type,excitation laser power and depletion laser power)on the width of nanowires was systematically studied.Single nanowire with 45 nm width is obtained,which is ?/18 of excitation wavelength and the minimum linewidth in pentaerythritol triacrylate(PETA)photoresist.A series of complex three-dimensional structures were prepared based on the developed sub-diffraction limit direct laser writing system.Compared with the traditional direct laser writing and laser-induced periodic surface ripple,our method has the capability to create far below the diffraction limit nanostructures with specific length and orientation.It also provides effective technical and theoretical support for the preparation of nano devices and functional structures,and promotes the resolution of laser direct writing to a smaller scale.