Studies On The Third-order Optical Nonlinearities Of A Series Of Hydroxylphenyl Porphyrin

Porphyrin compound is a special kind of large πelectron system withtwo-dimentsional conjugated molecular structure aromatic organiccompound, and there exist many natural porphyrin and metalloporphyrincompounds in the nature. Since natural porphyrin compounds possess someparticular physiological functions for the metabolism process, people havea special interests in porphyrin compounds. So it is attractive to use thesynthesized porphyrin compounds to substitute the natural ones and studytheir characteristics and new application. Specially, porphyrin complexesare used as a organic polymer material in nonlinear optics filed, due to theirgood optical properties, for instance, fast response time, large third-ordernonlinear optical susceptibility, easy to modulate its optical and thermalproperties by designing different molecular structure, people have a wideinterests in synthesizing and studying new porphyrin compounds.In this thesis, we study third-order nonlinear optical properties of aseries of porphyrin derivative compounds by using Z-scan technique. Bymeasuring nonlinear refractive index and anti-saturated absorptioncoefficient, we get third-order nonlinear optical susceptibility, and analyzeits physical origin and the effects which molecular structure have onthird-order nonlinear optical properties of the materials.Since the coming of laser, with the development of nonlinear optics,there are a lot of techniques to measure nonlinear optical coefficient of amedium, for instance, interferometric measurements,degenerate four wavemixing,two wave coupling,ellipse rotation,beam distortion,optical Kerreffect,third harmonic generation and z-scan technique, and so on. All thesemethods are viewed to measure directly or indirectly nonlinearsusceptibility or refractive of the medium. Although so many methods canbe used, there is almost no a single method to determine completely thewhole nonlinear processes, in other words, when the wavelength, the pulsewidth and the intensity of the incident laser are fixed in the experiment, anyone of all the methods only provides a little information on the medium.Because of its simplicity, sensitivity and accuracy, Z-scan technique hasbecome a standard tool for determining nonlinear refractive index andnonlinear absorption coefficient of various materials. In our experiments, we use single beam Z-scan setup, and put a beamsplitter in order to separate one beam laser light into two beams for thesame intensity. One of them is used to probe nonlinear refractive of thematerial, and the other is for nonlinear absorption. The light source used isa Q-switched Nd:YAG laser, its wavelength, pulse width and repletionfrequency are 532nm, 8ns and 1Hz, respectively. The laser light is focusedby a lens with 30cm focus length, the radius of beam waist of Gauss laserlight after the lens is 40μm. In the experiment, the thickness of the cellcontaining measured sample solution is 2mm, the linear transmittance ofthe aperture S is 0.1, and the peak power on the focus position of the lens is0.3GW/cm2. When we move the sample from –z to +z in the experiment,the detectors D1 and D2 record the experimental curve on the transmittanceas a function of the sample position z, i.e. it is T(z)-z scan curve. We can get the variation between peak and valley from thenormalized Z-scan measured curve, and use the formule?Τp ≈0.406(1?S)0.25 αL ?v ??0 , and ??0 = (2πλ)γI0(1?e? ) α, we cancalculate the nonlinear refractive index of the sample γ(m2/w). We can alsocalculate nonlinear absorption coefficient βby fitting the formulaT(z) = ∑[( ∞? q0(z)]m to the open-aperture Z-scan curve. Finally, by using / 2 m=0 m +1)3the relation terms χR (3)= 2cε0n0γand χI 2 (3)= (λcε0n0 /2π)β, we can 2get the third-order nonlinear susceptibility of the sample. From the obtained experimental curves, there is an obvious valleyclose to the focus position of the incident laser for open-aperture Z-scancase, and the transmittance decrease with the increase of the intensity of theincident light, this indicates that there is a strong absorption. From thepeak-volley feature of close-aperture transmittance curve, we can see thatthe intensity of transmitted light in the far light field increases duringmoving the sample in the –z direction and decreases for moving along the+z direction, it shows that the sign of the nonlinear transmittance of thesample is negative, the sample is a defocusing medium. Since there is anonlinear absorption in material, we think the normalized transmittance forclose-aperture case depends on both nonlinear refractive and nonlinearabsorption effects of the material, so the peak and valley in the measuredcurves are asymmetrical. Because of the presence of nonlinear absorptionvalley, the strong nonlinear absorption of the material reduces its nonlineartransmittance, this makes the peak suppressed and the valley go up in themeasured curve. If the measured Z-scan curve for close-aperture case isdivided by that for open-aperture case, we can get a Z-scan curve aboutpure nonlinear refractive effect of the sample, basically removing theaffection of nonlinear absorption of the material, and can see peak andvolley are symmetrically in pure nonlinear refractive curve. From the obtained experimental results, this series of porphyrinderivative compounds have large nonlinear absorption and self-defocusingproperties. If physical mechanism is concerned, we think strong nonlinearabsorption of the material is attributed to anti-saturated absorption ofporphyrin molecular, and polarization of π-bonding in porphyrin molecule,which belongs to photorefractive phenomena originated from the change ofelectron cloudy distribution, results in large nonlinear refractive index ofthe material. The effects of molecular structures on the third-order nonlinear optical propertieswere discussed. Experimental results show that the different subsistent influencesobviously the nonlinear refraction of tetraphenyl porphyrin. But the differences of thenonlinear absorption for all the samples are small. From the numbers of thehydroxyl, it is seen that sample 1 have the largest value of the nonlinearrefraction. Because the hydroxyl is a donating electron group, it interactswith the plane πelectron bond of the porphyrin; this increases the density ofelectron cloud and the electric polarity. So value of the nonlinear refractionfor sample 1 increased observably compared to H2TPP. To analyze the effects from different position of hydroxyl, we comparethe sample 2 with the sample 3. Both of them include two hydroxyls but therelative positions of hydroxyls are different. It is seen that the nonlinearrefraction index of trans-form (sample 2) is larger than that of cis-form(sample 3). Because the repulsion between the hydroxyls in thetrans-porphyrin is smaller than that of the cis-porphyrin, so the electroncloud density of the sample 2 is larger than that of the sample 3. The opticalnonlinearity of the porphyrin is proportional to the density of electron cloud,so the results of our experiments are in agreement with the theory. In the thesis, we can also analyze the effects of different substitutegroups on nonlinear optical properties of the materials. The obtained resultsindicate that different substitute groups mostly affect the nonlineartransmittance of the materials, almost have no effect on its nonlinearabsorption. From the number of substitute group, we can see the increase fornonlinear transmittance of the material is not much for such a sample onlycontaining one hydroxyl group, it is attributed to hydroxyl group beingpushing electron group, whose reaction with in-planeπ-bond of porphyrinincreases the polarization and the nonlinear refractive effect. If consideringdifferent position for substitute group, we can see the probability for thepresence of trans-structure molecular is bigger than that of cis-structure onewith two hydroxyl groups in material, the reason is that the repulsive forcebetween hydroxyl groups in trans-structure molecular is smaller than that incis-structure one, so the negative effect which trans-structure molecular hason the polarization of in-planeπ-bonding of porphyrin is small, this is whythe obtained nonlinear refractive transmittance in the trans-structuremolecular is bigger than that in cis-structure molecular. We obtain experimentally the nonlinear susceptibility coefficients oftwo porphyrin derivative compounds and analyze simply their physicalmechanism. We think that an increase of nonlinear refractive coefficient formethybrom porphyrin aether is attributed to the pushing electron effect ofsubstitute group, for hydroxyphenyl porphyrin terpolymer it comes from...