New Fabrication Methods To Overcome The Diffraction Limit Using Temporally/Spatially Shaped Femtosecond Laser Based On Electrons Dynamics Control

Laser micro/nano fabrication technology has become one of the most important methods for high-end manufacturing due to its wide processing range,various processing forms,and no need for contact.It plays an important role in such fields as defense,aviation,information,microelectronics,and new energy.However,due to the limitation of the optical diffraction limit,the structural size limit of laser manufacturing was limited to the wavelength level,which greatly affects the wide application of laser micro/nano fabrication technology.How to break the limit of diffraction limit and improve the precision of laser processing has become one of the important challenges in the field of laser micro/nano fabrication.The traditional method of breaking the diffraction limit has some drawbacks,such as limited applicable materials,low processing efficiency,poor processing quality,and complex three-dimensional processing.It is urgent to develop new processing methods to overcome the above problems.The invention of the femtosecond laser has brought new possibilities for the processing of breakthrough diffraction limits.Because of its characteristics such as ultrafast(up to 10^-15 s in duration)and super-strong(up to 10¹⁵ W/cm² in peak power density),femtosecond laser processing shows strong nonlinear and non-equilibrium effects,which is inherently diffrent with long pulsed lasers.In femtosecond laser processing,the absorption of photon energy is mainly accomplished by electrons as carriers.The state of electrons also determines the properties of transient localized materials and affects the subsequent phase transition process.Therefore,our group proposed an ultrafast laser with temporal/spatial shaping to control the electronic dynamics of the machining process and achieve new manufacturing goals.The size of the electron excitation region will also determine the size of the final processing structure.If we can control the localized electron dynamics at the nanometer scale,we can achieve laser processing that breaks through the diffraction limit.Based on the above scientific ideas,this paper explored a new method of sub-diffraction-limit processing based on electronic dynamics control.The main work of the dissertation includes:（1）Exploring the main methods of nanoscale localized electronic dynamic control,and presenting the specific mechanism of breaking the size limit of the processing;（2）Designing and constructing an experimental device for femtosecond laser shaping to achieve effective control of electron dynamics（3）Realizing nanoscale localized electronic dynamics control（including electron density,temperature,distribution,etc.）through temporally/spatially shaping femtosecond laser,and then controlling the physical/chemical properties of materials and the phase transition process of materials,thereby realizing sub-diffraction-limit processing;（4）Exploring the special characteristics of the processed structures and its potential applications.The main innovations achieved in this paper are as follows:1.A new temporal shaping device based on multi-film-system was proposed and implemented,which can flexibly realized multi-parameter light field control,such as time domain intensity,delay between pulses,polarization,and so on.Compared with traditional shaping methods,the device’s volume,delay range,stability,damage power,and cost are all greatly optimized.With an integrated design,the device’s volume can be reduced to less than 0.5 dm³,and the delay can be adjusted from femtosecond(10^-15 s)to picosecond(10^-12s)or even nanosecond(10^-9 s).The damage power is up to 0.5 W/cm² and the cost is greatly reduced.The device greatly improves the control of the electronic state and has great potential for industrialization.2.Temporally shaped femtosecond laser pulses were designed and used to control the electrons excitation process.By using the pre-pulse to affect the subsequent pulses,the electrons-excitation area was modulated such that an ablation pit with a minimum processing radius of 75 nm was realized on the surface of the fused silica,which was more than 6 times higher than the unshaped processing method.The process is simulated based on the plasma model,and theoretical results are consistent with experimental ones.3.The far-field spatially shaped femtosecond laser pulses were used to control the spatial distribution of electrons,which in turn controls the phase transitions of materials and the micro-zone transfer of materials,ultimately enabling the fabrication of nanostructures.Based on the method,the width of the processed metal nanowires can reach 1/14 of the wavelength,and the conductivity can reach up to 1/4 of the bulk material,which greatly increases the conductivity of the nanowires and improves the applications of microelectronic systems and other fields.In addition,based on this method,patterned nanostructures are obtained on the surface of silicon.By combining with interference methods,the processing efficiency of nanostructures was greatly improved,and the single pulse processing area can reach 1600μm² or more,which greatly promotes the practicality of nanostructures application.4.A method combining the near-field spatial shaping method with the non-diffraction beam is proposed.The dimensionality reduction of 3D surface periodic micro/nano structures is successfully achieved.The 2D translation stage can be used to realize the micro/nano structure preparation on the 3D surface.And without the need to measure the three-dimensional structure in advance,greatly reducing the processing difficulty of three-dimensional surface.This method has successfully achieved the three-dimensional grating structure and the processing of three-dimensional superhydrophobic surface.The ultra-high-sensitivity detection substrate was successfully prepared using the processed micro-nano hierarchical surface structure,and a solution with a concentration as low as10^-14 M/L can be measured,which was more than 57 times higher than the conventional surface enhanced Raman detection substrate.