Nanosecond, Picosecond, and Femtosecond Nonlinear Optical Properties of a Zinc Phthalocyanine studied using Z-scan and DFWM techniques.

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1 anosecond, Picosecond, and Femtosecond onlinear Optical Properties of a Zinc Phthalocyanine studied using Z-scan and DFWM techniques. B. M. Krishna Mariserla, a D. arayana Rao, a, * R.S.S. Kumar, a L. Giribabu, b S. Venugopal Rao c,# a School of Physics, University of Hyderabad, Hyderabad 546, Andhra Pradesh, India b anomaterials Laboratory, Inorganic & Physical Chemistry Division, IIC, Hyderabad 57, India c Advanced Centre of Research on High Energy Materials (ACRHEM, University of Hyderabad, Hyderabad 546, Andhra Pradesh, India *dnrsp@uohyd.ernet.in, # svrsp@uohyd.ernet.in Abstract: We studied the third-order nonlinear optical properties of Zinc Octa carboxy phthalocyanine with 8 nm femtosecond (fs and picosecond (ps, 53 nm nanosecond (ns pluses using Z-Scan and degenerate four wave mixing (DFWM techniques. onlinear absorption behavior in the nanosecond, picosecond, and femtosecond domains was studied in detail. We observed three-photon absorption with femtosecond and picosecond laser excitation whereas strong reverse saturable absorption was observed with nanosecond excitation. We have evaluated the sign and magnitude of the third-order nonlinearity. We evaluated and compared the magnitudes of χ (3 in fs, ps, and ns regimes. We recorded large off-resonant second hyperpolarizabilities (γ, with estimated values of esu, with ultrafast nonlinear optical response in the femtosecond domain using DFWM technique. 1. IRODUCIO ew molecules with high two-photon (PA and three-photon absorption (3PA cross-sections and/or ultrafast nonlinearities find potential applications in photonics and biomedical applications [5-7]. Phthalocyanines and their derivatives possess interesting nonlinear optical properties and find extensive applications in optical devices such as optical limiters and all-optical switches [1-4]. Recent studies suggest phthalocyanines possess strong two-photon absorption [PA] cross-sections. In organic materials 3PA typically occurs at longer wavelengths in the near infrared region (IR introducing advantages including minimization of the scattered light losses and reduction of undesirable linear absorption. Such materials will have a broad impact in biology and medicine through three-photon induced photodynamic therapy (PD in cancer treatment. In recent times novel materials including organic fluorophores like halogenated fluorine molecules, polydiaectylenes, semiconductor nanoparticles have been investigated for their 3PA properties using femtosecond (fsec and picosecond pulses in the IR spectral regions [8-1]. However, we understood that there are intermittent reports on organic molecules exhibiting 3PA in the significant wavelength region of nm corresponding to the output of commercially available femtosecond i:sapphire source routinely used by the researchers for biological applications. One such report measured two-photon absorption (PA spectra of a number of symmetrically substituted polydiaectylenes in the excitation wavelength region from λ ex = 8 to 16 nm [8]. Significant studies [11, 1] on application of phthalocyanines in PD have motivated us further to identify materials, especially phthalocyanines, with absorption in the UV region along with transmission in the IR range possessing strong multi-photon absorption. Zinc octacarboxy phthalocyanine (ZnOCPc is octasubstituted and is isomerically pure. Zn Figure 1 Molecular structure of Zinc Octacarboxy Phthalocyanine (ZnOCPc Strong nonlinearities in organic molecules usually arise from highly delocalized π-electron system. Phthalocyanines, e.g. figure 1, with their extensive two-dimensional π-electron system, fulfill this requirement and have been studied intensively as LO materials. hey exhibit other additional advantages, namely, exceptional stability, versatility, intense absorption in the red region of visible spectrum. Phthalocyanines are versatile because they offer enormous structural flexibility with capacity of

2 hosting ~7 different elements in the central cavity. Major drawback of these molecules is majority of them are insoluble in common solvents. Huge variety of substituent s that can be attached to the phthalocyanine core, at the peripheral and nonperipheral positions, these chemical variations can change the electronic structure of Phthalocyanines and they allow the fine tuning of the nonlinear response and has established to improve solubility. Figure shows the linear absorption spectrum of ZnOCPc. (3 with n being the order or absorption process, I is the peak intensity, Z is the sample position, z = πω /λ is the Rayleigh range; ω is the beam waist at the focal point (Z =, λ is the laser wavelength; effective path lengths in the sample of length L for PA, 3PA / is given as L eff = [1- exp (-α L]/α and L eff = [1- exp (-α L]/α, respectively. onlinear refractive index of the material can be estimated by closed aperture of Z-Scan technique. he closed aperture data was fitted with theoretical equation CA 4 ( z / z [1 ( z / z ][9 ( z / z 1 ] (4 Figure Linear absorption spectrum of ZnOCPc We studied nonlinear optical properties of ZnOCPc at 8 nm and 53 nm which are non-resonant, clear from the absorption spectrum in figure.. Results and Discussion.1 heoretical considerations Assuming a spatial and temporal Gaussian profile for laser pulses and using the open aperture Z-scan theory for multi-photon absorption (MPA given by Sheik Bahae et al. (13,14 we have (1 Where α n is the effective MPA coefficient (n = for PA; n = 3 for 3PA, and so on; and I is the input irradiance. If we retain only the PA term and ignore all other terms, we have an analytical expression for OA Z-scan for merely two-photon absorbers. Similarly retaining the 3PA term and ignoring the other terms provides us with an analytical expression for OA scans for only threephoton absorbers. OA( npa OA(PA OA(3PA I n1 ( n 1 n eff 3/ n 1 n (1 z / z I L L 1 eff 3/ (1 z / z / 3I Leff 1 3/ 3 (1 z / z ( where Δφ was the phase change of the laser beam by reason of the nonlinear refraction. Δφ value is estimated from the theoretical fitting for experimental data. hird order nonlinear refractive index n calculated from the equations n ( cm W 1 I L eff (5 heoretical fitting for the experimental data done by using equations and 3 to obtained the nonlinear absorption coefficients, n was estimated from the equations 4 and 5 for ZnOCPc in fs, ps, and ns regimes.. hree photon absorption Z-scan studies with 8 nm, 1 fs pulses. Figure 3 shows representative open aperture scan for ZnOCPc recorded at 8 nm using ~1 fsec pulses with an input irradiance of ~389 GW/cm. he three photon absorption coefficient estimated from the theoretical fitting is ~ cm 3 /W. Figure 3 Z-scan Open aperture curve of ZnOCPc with 8 nm, 1 fs and 1 KHz pulses. Inset shows the closed aperture scan. Inset of figure 3 shows the closed aperture scan.

3 Open circles represents experimental data while the solid line represents theoretical fit with three-photon absorption. We observed strong reverse saturable absorption (RSA kind of behavior in the intensity range of GW/cm. Obtained experimental data was fitted using equation and 3 and we found the best fit was obtained with the transmission equation for three-photon absorption (3PA.he solid line in the figure represents the theoretical fit with equation 3. It is evident that 3PA is the dominant mechanism for the observed RSA kind of behavior. o verify the presence of 3PA in the OA data we carried out the least square fitting test and obtained a value of χ ~.5 for ZnOCPc. Owing to large peak intensities at the focal point with fs laser excitation we can expect either PA or 3PA as the possible nonlinear absorption mechanism. Further, due to presence of large number of absorption bands in the excited state there is a possibility of resonance enhancement for these processes. In order to distinguish the multiphoton process contributing to the present data we performed intensity dependent absorption studies in the OA configuration. apparent that ZnOCPc shows Positive nonlinearity as indicated by the valley-peak structure. he closed aperture data, CA, was fitted to the standard equation 4 for closed aperture transmittance [13]. he magnitude of the nonlinear refractive index n evaluated was ~ cm /W for ZnOCPc. Estimated χ (3 from n was ~ esu.3 hree photon absorption Z-scan studies with 8 nm, 5 ps pulses Figure 5 shows the representative open aperture scan data of ZnOCPc obtained with 8 nm, 5 ps pluses. We observed reverse saturable absorption (RSA for input intensity 1 GW/cm. Figure 5 Open aperture Z-scan curve for ZnOCPc with 8 nm, 5 ps pulses. Inset shows the closed aperture scan. Open circles represents experimental data while the solid line represents theoretical fit with three-photon absorption. he magnitude of the nonlinear refractive index n evaluated was ~ cm /W for ZnOCPc. Estimated χ (3 from n is ~ esu Figure 4 Plot showing the α3 for ZnOCPc obtained with 8 nm pulses. We observed that for ZnoCPc, as is evident in figure 4, we find that α 3 remained constant with increasing intensities. his clearly indicates that the nonlinear absorption process involved is certainly 3PA. Interestingly, within these range of intensities, the samples remained stable after long exposure to the laser irradiation. However, beyond the intensities of 389 GW/cm we noted that the sample started degrading. Inset of figure 3 illustrates the typical closed aperture Z-scan curve obtained for ZnOCPc with a peak intensity of ~83 GW/cm. hese curves represent normalized data obtained after division of closed aperture data with the open aperture data to eliminate the contribution of nonlinear absorption. he curves were obtained at low peak intensities to avoid contributions to the nonlinearity that are not electronic in origin. It is.4 wo photon absorption Z-scan studies with 53 nm, 6 ns pulses Figure 6 shows representative open aperture scans of ZnOCPc with 53 nm, 6 nsec pulses. he effective nonlinear absorption coefficient estimated from the theoretical fitting was ~43.1cm/GW. We observed reverse saturable absorption (RSA in these molecules for input intensities in the range of 1 79 MW/cm.

4 Figure 6 Open Z-scan of ZnOCPc with 53 nm, 6 ns and 1 Hz pulses. Inset shows the closed aperture scan. hese molecules exhibited strong nonlinear absorption behavior in nanosecond regime. It is well established that nonlinear absorption in such materials due to ns pulses has contributions from both excited singlet and/or triplet states apart from two-photon absorption depending on the excitation wavelength. A comprehensive five-level modeling along with the accurate knowledge of the excited state life times is necessary to pin-point the exact contribution of each of these processes. However, for 53 nm excitation we can approximate the nonlinear absorption to an effective process and evaluate the nonlinear coefficient. he role of instantaneous two-photon absorption in the present case is negligible due to the excitation wavelength of 53 nm, which is far from two-photon resonance. he data obtained with ns pulses was fitted using equation. he best fit produced an effective nonlinear absorption coefficient (α of 43.1 cm/gw for ZnOCPc measured with a peak intensity of W/cm. Inset figure. 6 shows the typical closed aperture Z-scan curve obtained for ZnOCPc with a peak intensity of ~15 MW/cm. he magnitude of nonlinear refractive index n evaluated was ~ cm /W. he real and imaginary parts of third order nonlinearity for ZnOCPc was evaluated. Re[χ (3 ] was estimated to be ~ esu and Im[χ (3 ] to be ~ esu for ZnOCPc..5 hird order nonlinear optical measurements by DFWM technique. he DFWM set-up used was in box-car geometry [15]. In brief, the fundamental beam was divided into three nearly equal intensity beams in such a way that the three form three corners of a square box and are focused into the nonlinear medium (sample both spatially and temporally. he resultant DFWM signal that comes as the fourth corner of the box generated as a result of the phasematched interaction k 4 = k 3 -k +k 1 of the three incident beams. he sample under consideration is taken in the form of solution filled in a 1-mm glass cuvette. Care was taken to reduce the contribution of the cuvvette towards the overall DFWM signal by choosing suitable focusing conditions. By maintaining same polarization for the three incident beams we estimated χ ( A half-wave plate was introduced in the path of beam- to have its polarization perpendicular to that of beam-1 and beam-3 so that we could estimate χ (3 11. he transient DFWM profiles for the sample were obtained by delaying the beam-3 with respect to the other two incident beams. By performing the nonlinear transmission experiments on the sample, the input powers for the three input pulses were chosen such that effect of nonlinear absorption can be neglected and hence the obtained DFWM signal contains purely instantaneous nonlinear response of the sample. he obtained χ (3 data is hence purely real in nature without any contribution of imaginary components due to multi-photon absorption. Also the choice of low input powers allows us to neglect the contribution of higher order nonlinearities. Since the sample shows no absorption at 8 nm, we expect χ (3 and γ values are non resonant. Figure 8 DFWM temporal profile for ZnOCPc in deionized water (3 1-4 M as a function of beam 3 delay. We ascertained that origin of DFWM does not have contribution from any two-photon absorption through intensity dependent measurements. he temporal response for molecule ZnOCPc is shown in figure 7. he nonlinear responses are very fast and are of the order of 1 fs or faster. χ (3 value was measured to be esu. he second order hyperpolarizability (γ obtained from the χ (3 was esu. REFERECES [1] G. de la orre et al. Chem. Rev. 14, (4. [] R.S.S. Kumar, et al. Chem. Phys. Lett. 447, 74 (7. [3] S.V. Rao, et al. Opt. Mater (9. [4] S.V. Rao, et al. J. Appl. Phys. 15, 5319 (9;. orres et al. Chem. Rev. 14, 373 (4. [5] D.M. Friedrich et al. J. Chem. Phys. 75, 358 (1981. [6] F.E. Hernandez et al. Chem. Phys. Lett. 391, (4 [7] S. Maiti et al. Science. 75, 53 (1997. [8] S. Polyaov et al. Phys. Rev. B 69, 11541(4. [9].C. Lin, et al. J. Mater. Chem., 16, 49(6. [1] I. Cohanoschi et al. Chem. Phys. Lett. 43, 133 (6. [11] C.G. Claessenset al. Proc. SPIE, 583, 379

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