Study On Third-Order Nonlinear Optical Properties Of Precious Metal/Indium Tin Oxide Multilayers

The key to transform continuous laser into pulsed laser is the technology of locking film and Q switching.Both passive film locking and passive Q modulation are realized by saturable absorption.The third-order nonlinear optical properties of saturated absorber materials will directly affect the performance of laser.Because different materials have different responses to different wavelength lasers,different materials should be selected to act as saturable absorbers for different wavelength lasers,which undoubtedly increases the pressure of production and scientific research.It is found that free carrier materials have a huge nonlinear enhancement phenomenon in the near zero region（ENZ）,which provides us with a new way to select saturable absorbers.We can choose a material with excellent nonlinear optical properties as the basis and adjust its ENZ wavelength to match the incident laser of different wavelength.The ENZ wavelength can be regulated by changing the preparation conditions,but its regulation range is limited and cannot meet the actual production needs.The dielectric constant theory of multilayer film provides a simple method to adjust the ENZ wavelength.It can combine the noble metal with the ENZ wavelength in the ultraviolet band and the material with the ENZ wavelength in the infrared region.The ENZ wavelength of composite material can be adjusted by adjusting the proportion of noble metal and ENZ medium,and then the nonlinear optical response of multilayer film can be adjusted.The specific research content of this paper is as follows:1、Indium tin oxide（ITO）is selected as the basic material in this paper.ITO has good conductivity and high transmittance at visible wavelengths,and previous studies have shown that ITO film has a huge nonlinear enhancement effect at its ENZ wavelength.However,the ENZ wavelength of ITO film is not suitable for the laser with low incident wavelength in the infrared region.2、Using the dielectric constant theory of multilayer filmε_eff=ε_af_a+ε_bf_b,the regulation of ENZ wavelength of noble metal Ag inserted into ITO with different thickness was calculated numerically,and it was found that Ag nanolayer embedding could indeed play a role in regulating the wavelength of ITO near zero region.ITO（40 nm）/[Ag（x nm）/ITO（40 nm）]₄multilayers were prepared by magnetron sputtering.The ENZ wavelength of the multilayer film is measured and calculated.When x=4,the ENZ wavelength of the multilayer is 1.32μm,and it is found that the third-order nonlinear optical response of the sample embedded in Ag nanolayer is greatly improved.Under the condition of incident light with wavelength of 1.064μm,Its third-order nonlinear absorption coefficient and refractive index reach-1.21×10^-7 m/W and9.06×10^-14 m²/W,which are 2.4 times and 3.5 times of pure ITO,respectively.The enhanced third-order nonlinear optical response of ITO films at ENZ wavelength is described.3、Because Ag is easy to oxidize,it cannot exist stably in ITO for a long time,resulting in third-order nonlinear optical property instability of multilayer films.ITO（40 nm）/[Au（x nm）/ITO（40 nm）]4 multilayers were prepared by replacing Ag element with more stable Au.The experimental results show that the ENZ wavelength of the multilayer films shifts blue continuously with the increase of Au layer thickness,which is basically consistent with the theoretical calculation trend.When the Au layer thickness is 4 nm,the ENZ wavelength reaches0.94μm.Meanwhile,at x=4 nm,the nonlinear absorption coefficient and nonlinear refractive index reach-1.78×10^-7 m/W and 1.12×10^-13 m2/W,which are 3.5 times and 4.4 times of pure ITO,respectively.4、Annealing method is used to improve the crystallinity of the multilayer film,and further improve the third-order nonlinear optical response of the multilayer film.The nonlinear absorption coefficient and refractive index are 1.5 times of that before annealing.Due to the more stable chemical properties of Au,the stability of the multilayer films prepared using Au is also enhanced.The results show that ITO/Au multilayer is a promising candidate material for all optical devices.The results show that it is a feasible method to regulate the ENZ wavelength of dielectric materials and the third-order nonlinear optical properties of multilayers by the precious metal.The nonlinear absorption coefficient and nonlinear refractive index of ITO/Au multilayer structure are greatly improved compared with pure ITO,and the ENZ wavelength can be adjusted in a wide band.The composite material with wide band adjustable and large nonlinear absorption coefficient can be used as an excellent saturable absorber to realize Q-switched and mode-locked laser,and the large nonlinear refractive index can be used as a candidate material for all-optical switching and optical bistable.Therefore,ITO/Au multilayer structure has great application prospect and potential value in blow-point field.