Study On Nonlinear Absorption Coefficient Relative To The Spatio-temporal Profiles Of Pump Laser

The study dealing with the Multiple-photon absorption properties of materials is one of the hotspots in Optoelectronics field. It belongs to a part of nonlinear optical areas. Induced by ultra-short pulse laser, generally, the nonlinear absorption coefficient (NAC) of media can be obtained by measuring the nonlinear energy transmission through the nonlinear medium. Usually, the nonlinear energy transmission depends on not only the absorption properties of measured materials, but also the pulse profile of pump laser. Since it is a hard technical task to give a precise description of the spatiao-temporal profiles of pump laser, some pulse model has to be assumed in the NAC measurements. As a result, for a set of experimental data of energy transmission, the different values of nonlinear absorption coefficient will be obtained based on different pulse models. According to the current formula, the nonlinear energy transmission is expressed, based on the some pulse models, in terms of time-integral expression. The further analytical solution can not be obtained based on this expression. This difficulty would make it uneasy for one to evaluate the NAC deviation from the pump laser pulse profiles. In other word, only by way of a lot of numerical integral resorting to a computer, can the NAC deviation resulting from the applications of different pulse profiles be analyzed and discussed.In this dissertation, the spatio-temporal profile influences of pump laser on NAC of media are mainly studied theoretically and numerically. We attempt to propose an approach to resolve the predicament mentioned above. Two characteristic parameters are introduced to describe, respectively, the pump laser pulse influences on NAC of the measured material. The approximation of our approach is also estimated quantitatively by way of numerical simulation. The theoretical analyses of NAC deviations resulting from the different pulse profiles is elucidated and discussed by employing the two factors. Our mainly academic achievement is following.1,Based on the nonlinear absorption equation, two factors (g_2PA and g_3PA) are introduced by using the temporal averaged pulse to describe the influences on two photon and three photon absorption coefficients of the measured medium respectively. A new expression of nonlinear energy transmission for difference pulse models is further derived and obtained. Compared with the traditional theory, the relative differences of 2-photon and 3-photon absorption coefficients obtained based on our method are less than 5.77% and 13.4%, respectively, in the typical range of nonlinear transmissions≥0.85. Our results suggest that the g factors may become useful in dealing with the influence of NAC deviation resulting from the pump laser pulse.2,In applying the g factor to Z-scan method, we obtained a new expression of nonlinear energy transmission. Compared with NAC based on the usual way dealing with the pulse profile in Z-scan, our numerical results show that the relative differences of 2-photon and 3-photon absorption coefficients obtained based on our method are less than 2.56% and 8.7%, respectively, in the typical 0.9¹.0 ranges of the nonlinear energy transmission for some typical ultra-short pulse models, such as sech², Lorentz and asymmetric sech² pulses. Therefore, the two factors may become potential candidates of the parameters eliminating the pulse profile influence on NAC in nonlinear absorption measurements of materials.3,We propose a new treatment in taking account of the influence on NAC from the transversely spatial profiles of pump laser, in which a factor (f-factor) is also introduced. The result of the numerical simulation shows that, in the range of nonlinear transmissions≥0.9, the relative differences of 2-photon and 3-photon absorption coefficients obtained based on f factor are no more than 3.02% and 13.4%.Our results suggest that the two factors (g and f factors) may become potential candidates of the parameters in deal with the spatio-temporal profile influence on NAC in nonlinear absorption measurements of materials.