Reversible Periodic Surface Structure Induced By Ultrashort-pulse Laser Based On Phase-change Modulation

With the rapid development of information,more stringent requirements are put forward for communication efficiency and information processing speed.Traditional integrated circuits have reached the development bottleneck.High efficiency and low loss micro-nano optical devices will become the core of the next generation technology.In order to adapt to multi-application scenarios,modulable optical devices have been gradually researched as the hotspot in this field.Due to the distinct dielectric constrants between crystalline and amorphous state,phase-change material,Ge₂Sb₂Te₅(GST),has been used as a reversible dielectric layer to control the oscillatory properties of periodic surface structures.However,traditional processing methods based on material removal fix surface periodic structure elements,which limit the modulation range of devices.A kind of new periodic surface structure processing and modulation method need to be designed and achieve multilevel modulation and reversible reconfiguration of the structure elements in the device.Laser with ultra-short pulse width has unique properties in the fields of precision processing and phase modulation of GST.In this paper,a reversible grating structure is induced on GST film surface by picosecond laser direct writing then the reversible reset of surface optical properties is achieved by controlling morphology and crystallinity.Firstly,the morphology characteristics(cycle length,direction,grain shape,etc.)and the evolution of structure properties(dispersion color effect,chemical active etching)of GST surface induced by ultrashort pulsed laser are systematically studied.It is found that four kinds of periodic surface structure can be induced by laser pulse energy enhancement:modified ripple structure,fully crystallized surface structure,ablated nanodots structure and ablated ripple structure.The morphology evolution is caused by the combination of material threshold effect and surface structure-induced energy redistribution.In-depth study of the modified ripple structure exhibit that the structure is composed of periodic arrangement of crystalline and amorphous stripes,which has highly uniform and consistent morphology.It is the key to achieve the reversible periodic surface structure.Combining the mechanism of SPPs directional scattering to the morphological characterization of ripple structure,it is revealed that the periodic direction of modified ripple structure induced by laser direct writing has intimate relation to the laser polarization.Direct writing of surface ripple structure with different structure orientation angles can be achieved by laser polarization control.Surface grating are formed by surface undulation of modified ripple structure,which disperses white light and therefore produces color effects on the film surface under specific observation conditions.The tone of the color is controlled by the orientation angle of the laser polarization-dependent ripple structure,and color saturation is controlled by the laser pulse energy.Surface multilevel color information can be recorded by this means of laser parameter modulation.Due to the non-linear and non-equilibrium characteristic of the interaction between ultrafast laser pulse and materials,the mechanism of the phase evolution needs to be explored in order to precisely control the periodic structure of the surface.Therefore,based on the temporal-spatial pump detection technology,the evolution of single-pulse ablation and multi-pulse crystallization are discussed respectively in combination with the transient phase dynamics and structure surface morphology.It is found that a unique quasi-amorphous state is formed during the multi-pulse induced crystallization process.The crystallized threshold is different between quasi-amorphous the crystalline GST,induced the crystallization threshold extension outward with the enhancement of pulses number.Based on the color effect of modified ripple structure,an application is designed that a multilevel graphics storage realize by color enhancement surface graphics display,which records and reads the information of two graphics in the same area.To reveal the reversible reconfiguration properties of modified ripple structures,three steps including surface structure erase,surface phase reset and ripple structure rewrite are designed.A pulse sequence perpendicular to the initial laser polarization direction is induced as the erasing light,the ripple structures with orthogonal structure attenuates the influence of the initial ripple orientation and constitute uniform crystallized structure.The surface resets to amorphous state by irradiating a single pulsed excimer laser.Further research demonstrates that reset amorphous surface can also induce modified ripple structure to achieve the reversible reconfiguration of the surface ripple structure.