IR-UV Femtosecond Laser-induced High-spatial-frequency Periodic Surface Structures On FTO Film And The Birefringence Effect

In the past 20 years,laser-induced periodic surface structures(LIPSS)on metal,semiconductor,medium and other materials have been induced by femtosecond laser,which has become an effective method to control the surface properties of materials.And it has been applied in information storage,polarizer,surface structure color,light absorption and luminescence enhancement.In this paper,the High Spatial Frequency LIPSS(HSFL)with birefringence effect was efficiently prepared by using the infrared,visible and ultraviolet femtosecond laser on the doped tin oxide(FTO)film,and the birefringence effect was systematically studied.The main research achievements are as follows:1.The periodic nanostructures were efficiently prepared on FTO thin films with thickness of 650 nm fabricated by femtosecond laser with a central wavelength of 1030nm.The morphology,duty cycle and birefringence effect of LIPSS on FTO thin films were systematically studied by changing the laser fluence and scanning speed.Under the irradiation of 1030 nm laser with scanning speed of 0.01 mm/s and laser fluence of89 m J/cm~2,the difference of refractive index of FTO film is 0.2,the thickness of the HSFL layer was measured to be approximately 499 nm,and the retardance can reach96 nm.Using 1030 nm femtosecond laser direct writing technology focused by cylindrical lens,large area HSFL was efficiently prepared on FTO thin films,which can be used as an optical attenuator with a tunable range of 71%-100%for 532 nm laser.2.In order to improve the retardance of FTO thin films,the morphology,duty cycle and birefringence effect of LIPSS on FTO thin films with thickness of 650 nm fabricated by the femtosecond laser with a central wavelength of 515 nm were investigated.The results show that when the scanning speed is 0.01 mm/s and the laser fluence is 23 m J/cm~2,the obtained HSFL period is 172 nm,the width of groove is 52nm,the difference of refractive index is 0.21,the thickness of the HSFL layer was measured to be approximately 650 nm,and the retardance is 135 nm.A large area HSFL is efficiently prepared by using a 515 nm femtosecond laser focused on a FTO film with a cylindrical lens,and the characteristics of a quarter wave plate at 532 nm laser are demonstrated.3.In order to further improve the retardance of FTO thin films,the morphology,duty cycle and retardance of LIPSS on FTO thin films with thickness of 650 nm fabricated by the femtosecond laser with a central wavelength of 343 nm were investigated.It was found that the difference of refractive index is 0.21,the thickness of the HSFL layer was measured to be approximately 230 nm which was much smaller than the thickness of HSFL layer under 515 nm and 1030 nm laser,and the maximum retardance could only reach 47 nm.For FTO film,by comparing the results of infrared,ultraviolet and visible laser processing,it is found that the transmission of 515 nm femtosecond laser is greater than the other two wavelengths,so it is easier to penetrate the material surface,the penetration depth is the largest,the thickness of HSFL layer is also the largest,so as to obtain the maximum retardance.The results of this paper provide a new processing method for information storage and fabrication of polarizer.