Studies On Low Speed Z-scan Method By Theory And Experiment

Since the coming of laser, with the development of nonlinearoptics, there are a lot of techniques to measure nonlinear opticalcoefficient of a medium, for instance, interferometric measurements,degenerate four wave mixing,two wave coupling,ellipse rotation,beam distortion,optical Kerr effect,third harmonic generation and z-scantechnique, and so on. All these methods are viewed to measure directlyor indirectly nonlinear susceptibility or refractive of the medium.Although so many methods can be used, there is almost no a singlemethod to determine completely the whole nonlinear processes, inother words, when the wavelength, the pulse width and the intensity ofthe incident laser are fixed in the experiment, any one of all themethods only provides a little information on the medium. Because ofits simplicity, sensitivity and accuracy, Z-scan technique has become astandard tool for determining nonlinear refractive index and nonlinearabsorption coefficient of various materials. In our experiments, we use Low speed Z-scan setup, and thissetup is used to measure third-order nonlinear refractive of thin filmmaterials. This method could exhibit the nonlinear accumulation of thesample with the effect of the laser. The light source used is an Ar+continuum laser, its wavelength is 488nm. The peak power density ofthe laser is 95mW/cm2. The laser light is focused by a lens with 10cmfocus length. In the experiment, the film is made by mixing the samplewith PMMA in the chcl3 solution. The sample film is ruby. Thethickness of the film is 35um. The linear transmittance of the apertureS is 0.000893, and the peak power on the focus position of the lens is75mW/cm2. When we move the sample from –z to +z in theexperiment, the photoelectric detector record the experimental curveon the transmittance as a function of the sample position z, i.e. it isT(z)-z scan curve.We can get the variation between peak and valley from thenormalized Low speed Z-scan method measured curve, and use theformula ΔΤ p ?v≈0 .406(1?S)0.25Δ?0, ΔΦ 0 (t ) =kΔn0(t )L eff, and, we can calculate the nonlinearrefractive index of the sampleΔΤ p ?v ≈ 0.406(1 ? s )0.25× k × δnv × Leffδ nv.When the velocity is 10steps/s, we can find the curve of the smallaperture unitary rate of the permeation goes vale first then peak fromthe experimental curve. It means the light intensity become small atthe far field light axis when the sample move along -z direction andthe light intensity become big at the far field light axis when thesample move along z direction. It also shows that the sign of thenon-linear refractive index of the sample is positive, so the sample isself-focalized medium. Since the non-linear absorption exists, thecurve of the small aperture unitary rate of the permeation is result ofthe corporation of the effect of non-linear refraction and the effect ofnon-linear absorption. So the curve shows that the magnitude of thevale and that of the peak are asymmetry. The apex of the rate of thepermeation is restrained apparently, and the vale of it is boosted up. Itis because the strong non-linear absorption weakens the rate of thenon-linear permeation corresponding to the existence of the vale innon-linear absorption process. We could see that the vale and the apexboth in curve of the rate of the permeation become small from thecurves of Z scan at different rate when the velocity increase. It showsthat the accumulated effect of non-linear property of the sampledecreases gradually, and the rate of non-linear refraction also decreasesgradually when the velocity increases, that is because that theinteractional time of laser and the sample become shorter.From the experiment result, the speed of the sample graduallybecome higher, the velocity enlarge gradually from the 10 steps/s to300 steps/s, we can get the relation curve of the speed and thenonlinear refractive index of the sample. From the diagram we can see,with the enlarge of the speed, the nonlinear refractive index of thesample become higher gradually, while nearing the 300 steps/s becomedown suddenly, at 10 steps/s, the nonlinear refractive index attain asteady biggest value. It is obvious that there is a accumulate effect inthe interactional process between the light field and sample, and theaccumulate effect become higher when the time is longer. At the sametime we use another method named photo-induced birefringence tomeasure the nonlinear refractive index of the same sample, and obtainthe same result of nonlinear refractive index with the low speed Z-scan method. The results show that our experiment is correct.From the calculation result, we could see that the nonlinearrefraction curve obtained by photo-induced birefringence method, theδ n=4.91×10-3, when the interactional time of the laser and thesample is long enough. The nonlinear refractive index using the lowspeed Z-scan method is δ n=1.38×10-3,Comparing two methods, wecould see that the nonlinear refractive index of them have the samequantity. But the low speed Z-scan method has a more simpleexperiment process. That is what we expect.