Study Of The Principle Of Reflection Of Ultrashort X-ray Pulses Based On Multilayer Structures

The ultrashort X-ray pulses could be generated by High Harmonic Generation（HHG）via gas or solid media and Free Electron Laser（FEL）,and the techniques are becoming mature nowadays.In particular,as a new generation of light source,FEL can deliver ultra-short X-ray pulses with ultra-high brightness,which opened up new opportunities in many scientific fields,especially in the field of ultrafast dynamics.X-ray multilayer optical elements,as artificial nano periodically arranged photonic crystal structures,allow X-ray beams to improve the reflective efficiency through Bragg diffraction at the interface of multilayer materials.Multilayer optics is also a key component of the ultrafast experiments,being used for pulse reflection,transmission and focusing,in the meantime for constructing and shaping ultrashort pulses with well-defined characteristics.The X-ray pulses in femtosecond(fs=10^-15s)and attosecond(as=10^-18s)time scale generated by FEL present new opportunities and challenges for multilayer optics:The FEL pulses have the ultrafast time structures,so for their reflection on multilayer structures,the pulse dispersion effect and the mutual influence to multilayer due to the pulse must be taken into account.Therefore,it is important to study the reflection of FEL ultrashort pulses in X-ray wavelengths in multilayer structures.In order to study the relevant physical problems,the following works have been carried out in this dissertation.Firstly,we discuss the design and optimization of multilayer structures according to the fundamental optical principles and the material properties.A framework for the simulation of X-ray multilayer structures is constructed using the Parratts rigorous iterative algorithm,which incorporates procedures including multilayer optimization and reflectivity calculation.In particular,the reflectance calculation procedure is optimized to take account of the interlayer roughness and the interlayer material gradient diffusion structure which typically occurs during the manufacturing process of multilayer structures.Next,the investigation of the reflection of SASE FEL pulses on multilayer structures,especially for the tender X-ray pulses,in the photon energy of 1-3 ke V,was carried out,where SASE FEL pulses consisting multiple individual single spikes were concerned in the linear optical range using Fourier analysis to optimize the optical components.We investigated the change in the time and frequency domains to characterize the SASE FEL pulses after reflection from the designed Cr/B₄C multilayers and confirmed the reliability of the multilayers for reflecting SASE pulses.Finally,the reflection of hard X-ray ultrashort pulses,particularly single attosecond pulses,on multilayers,are investigated.We first introduced single-peaked Gaussian incident pulses,and by means of the Fourier transform and the inverse Fourier transform of the spectrum after reflection from a W/B₄C periodic multilayer film,we obtained the pulse structures and phase profiles of the ultrashort pulses reflected from the multilayer film.According to Fourier transform method and fundamental properties of Gaussian pulse and multilayers,we have developed a linear chirp approximation model to evaluate the reflected pulses.This model allows us to quantify the pulse length and chirp of the reflected pulses within a certain range,and thus demonstrating the linear reflection performance of the multilayers.By comparing the results of the inverse Fourier transform and the linear chirp approximation model,we are able to gain a preliminary understanding of the fundamental characteristics of the reflected ultrashort pulses from multilayers and the mutual influence applied to the multilayers due to the X-ray pulses.To summary,this dissertation research starts from the design of multilayers,using monochromatic X-rays for multilayer optimization,and implementing Parratt’s algorithm to build up a theoretical framework to calculate X-ray reflectivity from multilayer structures,and a comprehensive analysis of the performance of the multilayers is carried out,to exam the structural parameters of different periodic multilayer.Based on that,the reflection of ultra-short pulses from multilayer membrane structures is numerically simulated to gain insight into the interaction between X-ray pulses and multilayer structural materials,which could potentially provide theoretical references for applying multilayer membrane optical elements to the development and construction of FEL beamlines and end-stations.