Method For Probing Properties Of Nonlinear Optical Crystal

Nonlinear optical material is one of the important materials of optoelectronic information,which is closely related to laser,optoelectronics and optical communication.It has also been the trend of accelerating development in modulation switch,telematics and information processing.With the improvement of nonlinear optical crystal performance and the development of new crystals,the optical properties of these crystals are also particularly important and basic.For the test method of the performance of nonlinear optical crystal is also constantly updated and improved.This paper also studied several measurement methods,the specific contents include:1.This paper summarizes the theory and measurement principle of traditional Z-scan technique,and analyzes the advantages and disadvantages of the traditional Z-scan technique.And we found that if the position of the sample can be fixed,then the properties of the crystal with the external conditions such as the external electric field,temperature or pressure can be measured.Accordingly,we proposed focus-tunable Z-scan technique by using a series of lens with different focal length,i.e.changing the relative position z of the sample by changing the position of the focus to achieve Z-scan.According to the theoretical derivation and the preliminary experimental measurement data,it is found that the radius of the waist of each focal point and the radius of the spot on the far-field screen are changed due to the change of the focal length.The two factors affect the experimental data and caused a very big impact.For these two factors,we take the following two points to modify:Firstly,according to relationship between the focal length and the waist radius at focus,the measured experimental data should be modified;Secondly,the aperture should also be changed with the viable focal length based on the relationship between the focal length and the radius of the light spot on the far-field screen to ensure that the aperture transmittance constant.According to the modified measurement data,the Z-scan curve is obtained and the third-order nonlinear refractive index coefficient and the third-order nonlinear absorption coefficient of the crystal are calculated.The experimental results show that the measured curve and the calculated data are basically the same as those obtained by the traditional Z-scan technique.Consistent,thus proving that this method is feasible,and also providing a reference for the above mentioned potential research.2.The interaction between the magnetic field and the light in the medium extensively and deeply is discussed.The effects of the magnetophotorefractive effect and the Faraday Effect and the Kerr effect of the magneto-optical effect are introduced.In combination with these two effects,We conjecture that if the polarized light in a certain direction is incident on a medium in the direction of the magnetic field parallel to the direction of the optical axis of the medium,the effect of the magnetic field on the nonlinear optical medium will cause the polarization of the light to be deflected.According to this conjecture,the experimental optical path is designed to observe by using the Mach-Zehnder interference optical path to form the interference grating.Since this deflection angle is very small,we can only observed the small difference between the two gratings when adding the magnetic field.And it is not obvious,the stripe movement is too small,it is not easy to observe by eyes.In the study of the experimental optical path,a reflector is replaced with a wedge prism in the Mach-Zehnder interference optical path.The front and back surfaces of the wedge prism are used to reflect the presence of a strong and weak interferometric grating.There is a slight difference between the two gratings,so that the formation of moire fringes which would further enlarge the movement of the interference fringes.On the next experimental observation we did observe the movement of moire fringes,thus the conjecture was be verified.3.The dispersion relation of Kramers-Kronig is discussed,and the relationship between refractive index and absorption coefficient(or extinction coefficient)is discussed based on this relation.We first measured the transmittance spectra of different doping concentrations and different optical axes,and calculated the corresponding absorption coefficient curves according to Lambert-Beer's law.According to the Kramers-Kronig dispersion relation,the corresponding refractive index curve and extinction coefficient curve are calculated and compared.From the calculation results,the calculated optical constants of the media are consistent with those obtained with other measuring instruments or measurement techniques.