Third-order Optical Nonlinearity And Laser Damage Of Sulfide Glasses

Chalcogenide glass is an amorphous material based on chalcogen elements（S/Se/Te）.Compared with fluoride and oxide glasses,chalcogenide glasses have some unique properties in the mid-infrared spectral region,such as high nonlinear refractive index and wide infrared transmittance range,which make them good materials for the preparation of special mid-infrared devices.In recent years,one of the research hotspots in the field of materials science and nonlinear optics is to use chalcogenide glass as the nonlinear medium to generate high-power and broadband mid-infrared supercontinuum（SC）.The main feature of the mid-infrared SC light source is that it covers most of the chemical functional group spectral area,biomolecular fingerprint spectral area and two atmospheric windows of 3-5μm and 8-12μm,and has high brightness,wide spectral r ange and good spatial coherence.Therefore,it has great application prospects in molecular spectroscopy,chemical detection,biomedical imaging,photoelectric countermeasures and so on.The laser damage threshold（LDT）and the third-order nonlinear coefficient of chalcogenide glasses are two important parameters for the generation of high-power and broadband mid-infrared SC.Among them,the high laser damage threshold makes the material withstand higher power pumping,and ultimately higher power output could be obtained;while the large third-order nonlinear coefficient is benefit to obtaining wider SC.Based on this,a series of chalcogenide glasses were prepared in Ge-As-S and Ge-Sb-S systems.The third-order nonlinear refractive index,multiphoton absorption coefficient and femtosecond laser damage characteristics of the glass at different wavelengths were systematically studied.Based on the previous experimental results and the later theoretical design results,chalcogenide glass fiber w ith core/cladding structure was successfully fabricated,and its application in high-power mid-infrared SC generation was studied.The obtained main results are as follows（1）The nonlinear properties of Ge-As-S and Ge-Sb-S glasses in the mid-infrared were systematically studied.The third-order nonlinear coefficients and multiphoton absorption coefficients of Ge-As-S and Ge-Sb-S glasses at 1.55μm,2.4μm,3μm and 3.6μm were measured by femtosecond pulse laser.The results show that the third-order nonlinear refractive index n₂ and linear refractive index n₀ follow the semi-empirical Miller law;with the increase of wavelength,the n₂exhibits nonlinear attenuation;within the measurement band,a multiphoton absorption with 2 to 7 photons was observed.（2）The laser damage thresholds of Ge-As-S and Ge-Sb-S glasses were systematically studied.The LDT of Ge-As-S and Ge-Sb-S glasses at 1.55μm,2.4μm,3μm and 3.6μm were measured by femtosecond pulsed laser.It is found that LDT has the maximum value at the stoichiometric composition.This is associated that the glass with stoichiometric composition has the highest average bonding energy.The larger the wavelength of the radiation light,the higher the LDT of the glass.This is consistent with that the glass has small multiphoton absorption coefficient at longer wavelength.（3）The Ge₂₅As₁₀S₆₅/Ge₂₆As₈S₆₆fiber with core diameter of 15μm and Ge₂₅Sb₁₀S₆₅/Ge₂₅Sb₈S₆₇ fiber with core diameter of 10μm were prepared by the rod in tube method.The supercontinuum output of 2.5-7.5μm and 3-8μm were obtained by pumping the above fibers with 4.8μm and 170 fs（100 k Hz）laser pulses,respectively.The experimental results show th at Ge₂₅As₁₀S₆₅/Ge₂₆As₈S₆₆fiber and Ge₂₅Sb₁₀S₆₅/Ge₂₅Sb₈S₆₇ fiber are promising nonlinear media for generating high brightness broadband mid-infrared SC.In addition,Ge-Sb-S glass system does not contain toxic As element,which is harmless to the environment and human body.Therefore,the Ge-Sb-S glass is an environment friendly mid-infrared nonlinear material.