Research On Broadband Reflector For The Water Window Region Based On Stacked Multilayer Structure

Water Window Region attosecond pulses are widely used in physics,astronomy,and biology as an effective tool for observing and controlling electron dynamics and ultrafast processes in atoms,molecules,and condensed matter.Multilayer structure is an important optical component for the regulation of water window attosecond pulses due to its flexible spectral tuning capability and stable performance.In this paper,two methods are proposed to optimize the stacked multilayer structure.The first method is based on a two-parameter merit function combined with a genetic algorithm to optimize the thickness and the stack period number of the stacked multilayer structure to achieve a broadband reflection for the water window attosecond pulse.The second method is based on a linear chirp structure to optimize the thickness and the stack period number of stacked multilayer structure to compress and shape attosecond pulses.In addition,a reflective quarter-wave plate based on an aperiodic multilayer structure is proposed to realize the generation of circularly polarized water window attosecond pulses.Firstly,the stacked Cr/Sc multilayer broadband mirror designed by using a two-parameter merit function which based on root-mean-square deviation and mean reflectivity combined with a standard genetic algorithm achieves a mean reflectivity of about 4.90%in the photon energy range of 370-390 e V.The stacked multilayer mirror has better optimization efficiency,design robustness,and thickness tolerance than traditional aperiodic multilayer broadband mirrors.The optimized structure of the stacked multilayer broadband mirror can be used for effective reflection of water window attosecond pulses.Secondly,a stacked Cr/Sc multilayer mirror with linear chirp structure is proposed in this paper.Three chirped stacked multilayer mirrors are designed for the attosecond pulses with different dispersion conditions(2000 as~2,4000as~2,6000 as~2).In the photon energy range of 300-400 e V,the reflected attosecond pulses are all close to the Fourier transform limit,and the pulse widths are compressed from 210.06 as,465.48 as,and 766 as to 37.61 as,37.47as,and 37.14 as,respectively.The chirped stacked multilayer mirrors can be used to chirp compensation and Compression shaping for water windowed attosecond pulses.Finally,a Cr/Sc broadband reflective quarter-wave plate with an aperiodic multilayer structure designed based on a two-parameter evaluation function of circular polarization and reflectivity combined with a genetic algorithm is proposed,which achieves more than 95%circular polarization in the photon energy range of different bandwidths(385-395 e V,380-395 e V,375-395 e V).This broadband reflective quarter-wave plate can be used for the conversion of linearly polarized attosecond pulses to circularly polarized attosecond pulses.In this paper,a stacked Cr/Sc multilayer mirror and an aperiodic Cr/Sc multilayer mirror based on genetic algorithm optimization design and another stacked Cr/Sc multilayer mirror based on linear chirp structure are proposed to realize the performance control of the attosecond pulse in the water window region(such as broadband reflection,phase control,compression shaping,polarization state conversion,etc.).It provides a certain theoretical guidance for the future realization of the efficient manipulation of the attosecond pulse in the water window region.