Studies On The Third-order Optical Nonlinearities And Optical Limiting Of Novel Porphyrin Dimers

Porphyrin compound is a special kind of large n electron system with two-dimensional conjugated molecular structure aromatic organic compound, and there exist many natural porphyrin and metalloporphyrin compound in the nature. Since natural porphyrin compounds possess some particular physiological function for the metabolism process, it is attractive to use the synthesized porphyrin compounds to substitute the natural ones and study their characteristics and new application.Specially, porphyrin complexes are used as a organic polymer material in nonlinear optics filed,due to their good optical properties, for instance, fast response time, large third-order nonlinear optical susceptibility, and easy to modulate their optical and thermal properties by desiging different molecular structure, people are very interested in synthesizing and studing new porphyrin componds.In this thesis, we studied third-order nonlinear optical properties of a series of porphyrin dimer compounds by using Z-scan technique. Through measuring nonlinear refractive index and anti-saturated absorption coefficient, we obtained a series third-order nonlinear optical susceptibility data, analyzed its physical origin and the effects of molecular structure on third-order nonlinear optical properties of the materials. In the meantime, we studied optical limiting properties of the porphyrin dimer compounds, and analyzed its physical origin.Since the coming of laser, with the development of nonlinear optics, there are a lot of techniques to measure nonlinear optical coefficient of medium. All these methods are used to directly or indirectly measure nonlinear susceptibility or refractive of the medium. Although so many methods can be used, there is almost no a single method to determine completely the whole nonlinear properties, in other words, when the wavelength, the pulse width and the intensity of the incident laser are fixed in the experiment, one method measurement can only provide a little information about the medium. Because of its simplicity, sensitivity and accuracy, Z-scan technique has become a standard tool for determining nonlinear refractive index and nonlinear absorption coefficient of various materials.In our experiments, we use single beam Z-scan setup, and put a beam splitter in order to split one beam laser light into two beams for the same intensity. One of them is used to probe nonlinear refractive of the material, another is for nonlinear absorption. The light source used is a Q-switched Nd:YAG laser of continnum company, its wavelength, pulse width and repetition frequence are 532nm, 4.5ns and 1Hz, respectively. The laser light is focused by a lens with 30cm focus length, the radius of beam waist of Gauss laser light after the lens is 26μm. In the experiment, the thickness of the cell containing measured sample solution is 2mm, the linear transmittance of the aperture S is 0.13, andthe peak power on the focus position of the lens is 0.3GW /cm². When wemove the sample from -Z to +Z in the experiment, the detectors D₁ and D₂record the transmittance curve as a function of the sample position Z, i.e. it is T(z)-z scan curve.From the normalized Z-scan curves, we can get the value different betweenpeak and valley. Use the formulaeâ–³T_P-V≈0.406(1 - S)^0.25|â–³(?)₀| andâ–³(?)₀=(2Ï€/λ)γI₀(1-e^-αL/α, we can calculate the nonlinear refractive index of thesampleγ(m² / w). We can also calculate nonlinear absorption coefficientβby fitting the formula T(z) =sum from m=0 to∞[-q₀(z)]^m/(m+1)^3/2 to the open-aperture Z-scan curve.Finally, using following relationshipsχ_R³=2cε₀n₀²Î³,χ₁³=(λcε₀n₀²/2Ï€)β.We can drive the third-order nonlinear susceptibility of the sample.There is an obvious valley close to the focus position of the incident laser for open-aperture Z-scan case, and the transmittance decrease with the increase of the intensity of the incident light, this indicates that there is a strong absorption. From the Peak-volley feature of close-aperture transmittance curve, one can see that the intensity of transmitted light in the far light field increase during moving the sample in the -Z direction and decreases for moving along the +Z direction, that shows that the sign of the nonlinear transmittance of the sample is negative, and the sample is a defocusing medium. Since there is a nonlinear absorption in material, we think the normalized transmittance for close-aperture case including both nonlinear refractive and nonlinear absorption effects of the material, so the peak and valley in the measured curves are asymmetrical. If the measured Z-scan curve for close-aperture case is divided by that for open-aperture case, we can get a Z-scan curve about pure nonlinear refractive effect of the sample, basically removing the affection of nonlinear absorption of the material, and peak and volley are symmetrically in pure nonlinear refractive curve.From the obtained experimental results, we found that this series of porphyrin dimer have large nonlinear absorption and self-defocusing properties. We think the physical mechanism of strong nonlinear absorption of the material is reverse saturated absorption of porphyrin molecular, and polarization ofÏ€-bonding in porphyrin molecule, which belongs to photorefractive phenomena originated from the change of electron cloudy distribution, results in larger nonlinear refractive index of the material.The effects of molecular structures on the third-order nonlinear optical properties were discussed. Experiment results show that the different bridge-groups influence both the nonlinear refraction and the nonlinear absorption of porphyrin dimer. And the numbers of bridge-groups as well as the structure of linking between bridge-groups and monomer also can influence both the nonlinear refraction and the nonlinear absorption.Finally, we studied the optical-limiting effect of two different samples(b and c), and analyzed the physical original of optical-limiting effects. we make the conclusion that for the incidence of impulse physical original of optical-limiting effects of our two samples is the reverse saturated absorption. Because the trend of reverse saturated absorption coefficient is b > c, the trend of optical-limiting effect is that b is better than c ;for the incidence of continuous laser light, physical original of optical-limiting effects of our two samples is the nonlinear refraction coming from heating effect, Because the trend of nonlinear refraction coefficient is b > c, the trend of optical-limiting effect is that b is also better than c.