An Investigation On Ultrafast Laser Induced Self-organized Nanograting Structures In Transparent Media

The self-organized nanograting structure induced by ultrafast laser in transparent media has broad application prospects in diffractive optics,fiber optics,and nonlinear optics,etc.,which has aroused widespread attention from scholars all over the world.However,the ultrafast laser-induced nanograting structure has only been observed in a very limited number of transparent media.The existing mechanism varies from material to material and can not explain various new phenomena observed in recent years.There is still no universal theory to explain the generation and regulation of nanogratings,and the materials suitable for the preparation of nanogratings have not yet been fully explored.In this paper,the research focuses on ultrafast laser-induced nanogratings in transparent media.Through experimental observation and theoretical modeling,the formation mechanism and control methods of nanogratings are revealed.A set of universal nanograting formation theories are proposed and a series of media suitable for nanograting formation is established,which promotes the development of ultrafast processing technology.The main research results of this paper are as follows:(1)The formation of ultrafast laser-induced nanogratings in the La₂O₃-Ta₂O₅-Nb₂O₅ glass system was studied.Experiments show that the formation of nanogratings is highly dependent on the laser-induced thermal accumulation and the concentration of the nucleating agent(Ta₂O₅).Further observations show that this nanograting is a periodically arranged glass-crystal periodic structure that exhibits a near-infrared light attenuation property.The absorption spectrum shows that it has a very significant photonic bandgap in the 980-2000 nm,which indicates it is a brand-new type of nanograting structure.(2)The three-dimensional morphology of the nanograting was studied,and an unprecedented periodic crystal array was discovered.The top view of the structure appears as arc-shaped stripes(XY plane).The side view(XZ plane)of the structure appears as gradual oblique fringes,and the cross-section appears as periodic fringes along the laser propagation direction(YZ plane).This indicates it is a holographic structure produced by a clearly defined interference field.On this basis,a single scattering center model is proposed,and the focal area interference field distribution under this model is calculated.Experiments show that the calculated interference field is almost perfectly consistent with the actual structure.According to this model,a variety of new properties of nanogratings are predicted and methods for manipulating nanogratings are developed,which have been verified by experiments.Under the guidance of the theoretical model,we proposed a self-organized phase transition lithography technology and prepared an all-inorganic photonic crystal structure,which possesses an adjustable photonic bandgap in the near-infrared band.The waveguide made of this photonic crystal exhibits excellent light-guiding performance at a bending angle of 90°.By comparing the formation of nanogratings in a variety of transparent media,the universality of the theoretical model is verified.(3)The dynamic process of the nanograting formation was studied.Experiments show that the formation of nanograting relies on ultrafast laser-induced crystallite seeds.The crystallite seeds can provide a special auxiliary effect that significantly reduces the pulse energy,pulse density,and peak energy threshold for the nanograting formation.Through the analysis of the electron paramagnetic resonance spectroscopy,it is confirmed that the crystallite seeds contain a large amount of hole-center defects,which can be easily re-excited by the subsequent pulses.On the other hand,the crystallites also provide suitable nucleation positions for the nanograting formation.Driven by the auxiliary effect,the nanograting can be induced with extremely high efficiency,the number of pulses required is reduced by tens of thousands of times compared with static irradiation,the scanning speed can reach 6 mm/s,and the energy threshold is only 20%of static irradiation.On this basis,an efficient rewritable optical data encryption and storage based on nanograting was demonstrated.