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Materials Sciences and Applications, 2011, 2, 299-306 doi:10.4236/msa.2011.25039 Published Online May 2011 (http://www.SciRP.org/journal/msa) Copyright © 2011 SciRes. MSA 299 Ultrafast Nonlinear Optical and Optical Limiting Properties of Phthalocyanine Thin Films Studied Using Z-Scan Soma Venugopal Rao, Puliparambil Thilakan Anusha, Turaga Shuvan Prashant, Debasis Swain, Surya P. Tewari Advanced Centre of Research in High Energy Materials (ACRHEM) University of Hyderabad, Hyderabad, India. Email: svrsp@uohyd.ernet.in, soma_venu@yahoo.com Received January 20th, 2011; revised March 10th, 2011; accepted March 20th, 2011. ABSTRACT We have investigated the nonlinear optical properties, optical limiting thresholds, and figures of merits for five different phthalocyanine thin films, achieved through doping in PMMA polymer, using the Z-scan technique at 800 nm with 2 ps laser pulse excitation. From the open-aperture Z-scan data we derived that these molecules exhibit strong two photon absorption (2PA) with the nonlinear absorption coefficients in the range of 15 - 200 cm/GW. We have also estimated the sign and magnitude of real part of third order nonlinearity through the closed aperture data. Preliminary femtose- cond pump- probe data suggests that the lifetimes of excited states are in the sub-100 ps regime for all the molecules in film form. Our studies provide concrete evidence that these phthalocyanines are prospective candidates for mul- ti-photon imaging and optical limiting applications. Keywords: Z-Scan, Phthalocyanines, Thin Films, Picosecond, Figures of Merit 1. Introduction Novel moieties with strong two-photon (2PA) and three- photon absorption (3PA) crossections/coefficients are attractive for potential applications in the fields of pho- tonics and bio-medicine [1-4]. Studies on a variety of molecules with strong 2PA/3PA have been established to be relevant in fluorescence spectroscopy, 3D imaging, and lithography because of high spatial resolution achie- ved through intensity dependent processes [3-7]. Addi- tionally, they can be used for optical data storage and optical limiting purposes. Several materials such as por- phyrins and phthalocyanines that possess such properties have been investigated extensively in recent times by several groups, including ours [8-24]. Especially, phtha- locyanines have been examined by chemists, material scientists, and physicists alike since these are ubiquitous materials in which an alteration in molecular configura- tion allows the engineering of both linear and nonlinear optical (NLO) properties for specific applications. These organic polymeric systems contain conjugated π electron structure and show large optical nonlinearities combined with nonlinear response time in the femtosecond time- scales [7,8]. Due to the large, ultrafast third-order non- linearities and the ease with which one can derive several new compounds through peripheral and axial substitu- tions, these molecules have generated tremendous inter- est in nonlinear optics [10-24]. The third order NLO properties of phthalocyanines are closely dependent on the central metal ion and other modifications to the core and peripheral substitution. However, phthalocyanines are hardly soluble in organic solvents and do not crys- tallize easily in matrices. Moreover, the molecule aggre- gation of phthalocyanines greatly influences the third order nonlinearity, χ(3). To circumvent these problems modification of phthalocyanines such as the introduction of peripheral substitutions or attaching them to polymer chains have been carried out. The latter approach has the advantage that a high concentration of nonlinear chro- mophores can be incorporated into the polymer system without crystallization, phase separation, or formation of concentration gradients. Several earlier reports [10-24] on the optical and NLO properties have been in various forms of thin films. We have recently reported the NLO studies of novel phtha- locyanines using continuous wave (cw), nanosecond (ns), Ultrafast Nonlinear Optical and Optical Limiting Properties of Phthalocyanine Thin Films Studied Using Z-Scan Copyright © 2011 SciRes. MSA 300 picosecond (ps), and femtosecond (fs) laser pulses [25- 33]. Our recent efforts have been towards understanding the nonlinearities in all the time domains, albeit in solu- tion form. We have also studied the optical limiting per- formance of these molecules in thin films form (doped in PMMA) and demonstrated strong limiting properties in the cw regime. Our comprehensive studies strongly sug- gest that these are potential molecules with strong nonli- nearities and figures of merit in each time domain. In this paper we report two-photon absorption (2PA) and the nonlinear refraction studies of phthalocyanine thin films achieved using ps pulses. The standard Z-scan technique was utilized to investigate the third order NLO properties of the film. The knowledge of ultrashort pulse nonlinear refractive index aids in deciding materials appropriate for optical switching applications. Herein we present the results of our investigations on the NLO properties of five films of phthalocyanines studied using 2 ps pulses. 2. Experimental Details Figure 1 depicts the schematic of the experimental set up used in the present study. Z-scan measurements were performed using an amplified Ti:sapphire laser system (LEGEND, Coherent) delivering nearly transform-limited pulses of ~2 ps with a repetition rate of 1 kHz at 800 nm. The pulse duration was confirmed using an intensity autocorrelation experiment at 800 nm using a BBO crys- tal in non-collinear geometry. The amplifier was seeded with ~15 fs (55 - 60 nm FWHM) pulses from an oscilla- tor (MICRA, Coherent, 1 W average power, 80 MHz repetition rate, 800 nm central wavelength). Laser pulses with typically 2 - 5 μJ energy were used for the experi- ments. The beam was focused using 200 mm focal length lens into the sample. The beam waist (ωo) estimated was 20 ± 2 μm with a Raleigh range of 1.6 ± 0.3 mm. The transmittance changes of the sample placed on the trans- lation stage (Newport, ILS250PP), which was controlled by Newport ESP 300 Motion Controller, were measured with a sensitive power meter in the far-field. An aperture was placed in front of detector for closed aperture scans. The power meter and the translational stage were inter- faced to the computer. We established that the pulse en- ergies remained low to avoid contribution from higher order nonlinearities. The experiments were repeated more than once and the best data were used for obtaining the nonlinear optical coefficients from the best fits. These mo- lecules are highly soluble in chloroform in which com- mercially available PMMA (poly methyl methacrylate) was dissolved and the resulting solution was processed into thin films. Such films, upon curing, produced cross linked polymeric system in which phthalocyanine deriva- tive is covalently linked. Typically the concentration of phthalocyanines in polymer was ~1% by weight (of PMMA) though we could put in a maximum of ~5% by weight. PMMA concentration was ~10% by weight in solution. 3. Results and Discussion The IUPAC nomenclature for the compounds is shown in Table 1. The complete details of synthesis of these mole- Figure 1. Experimental set up used for open aperture and closed aperture Z-scans. NDF stands for neutral density filters and PM stands for power meter. Current Distortion Evaluation in Traction 4Q Constant Switching Frequency Converters Copyright © 2011 SciRes. MSA 301 Table 1. Nomenclature of compounds used in the present study. Sample Nomenclature PC1 2,3,9,10,16,17,23,24-octakis-(heptyloxy)phthalocyanine PC2 2,3,9,10,16,17,23,24-octakis-(heptyloxy)phthalocyanine Zinc(II) PC3 (2-(3-(Butane-1,4-dioic acid)-9(10), 16(17), 23(24)-tri tert-butyl phthalocyanine Zinc(II) PC4 2(3),9(10),16(17),23(24) tetra tert-butyl phthalocyanine PC5 2(3), 9(10),16(17),23(24) tetra tert-butyl Zinc phthalocyanine cules, their molecular structure, and their detailed absor- ption spectra are reported in our earlier publications [28-33]. All the molecules were purified prior to thin film fabrication achieved through spin coating. The mo- lecules were dissolved in a sonicated crystal clear solu- tion of PMMA in chloroform. The solution was then transferred to clean microscopic slides and spin coated to achieve films of various thicknesses. The typical thick- ness of the films measured using a profilometer were in the 8 - 12 μm range. The absorption coefficients of all the films were estimated to be in the range of 104 m−1 for all the films. Linear absorption spectra of typical PC2 and PC3 thin films are shown in Figure 2. The slight broa- dening and splitting observed in Q band (arising from electronic transitions) and broadening of Soret band in the thin film absorption spectra, compared to solutions, could be ascribed to the intermolecular interaction and molecular distortion/deformation [34,35]. In addition there is also the possibility of aggregation in solid state thereby slightly modifying the energy level configuration. Figures 3(a) to 3(e) show the open aperture data (open stars) of phthalocyanine thin films PC1 to PC5 recorded at 800 nm with 2 ps pulses and input peak intensities in the range of 200 GW/cm2. The presence of valley in normalized transmittance in open aperture (OA) scans indicates strong reverse saturation absorption (RSA) at these peak intensities. Inset of Figure 3(a) shows the typical closed aperture data for PC1 film. The peak fol- lowed by valley in the normalized transmittance data clearly suggests that the sample possesses negative type of nonlinearity and self-defocusing behavior. Similar behavior for n2 was observed for other thin films also. The n2 values obtained from the fits to data for all the films were in the range of (2 - 15) × 10−13 cm2/W. In or- der to extract the information of nonlinear refraction alone, the experimental closed aperture Z-scan data was divided by the open aperture data. This greatly eliminat- ed the influence of nonlinear absorption on the non- li- near refraction data. Figure 2. UV-visible absorption spectra of PC2 (top, red color) and PC3 (bottom, green color) doped in PMMA. CA and OA data for all the films were fitted using fol- lowing equations [36]: 0 22 00 4 1 91 CA z z T zz zz (1) 2 0 0 1 ,1In1 ,0ed π,0 TzSq z qz (2) 0 22 2 00 1 ,1 π,0 lnln 1exp2,0expd TzSpz ppz (3) With q0 = βLeffI0, 12 2 030 2eff pLI , β is the 2PA coefficient and α3 is the 3PA coefficient, I00 is the peak intensity, Z is the sample position, 2 0 π 0 z is the Ray- leigh range; ω0 is the beam waist at the focal point (Z = 0), λ is the laser wavelength; effective path lengths in the sample of length L for 2PA, 3PA is given as 00 2 00 1e 1e , . 2 L L eff eff LL From all the fits to experimental data it is evident that 2PA is the dominant mechanism for the observed RSA kind of behavior. 2PA absorption coefficients estimated from the fits were in the range of 15 - 200 cm/GW. 3PA fits are also shown in the figures for comparison and un- doubtedly the fits are off from the experimental data. All the samples were checked for presence of any nonlinear absorption losses due to scattering. The transmitted light was devoid of any scattering in the far-field indicating Current Distortion Evaluation in Traction 4Q Constant Switching Frequency Converters Copyright © 2011 SciRes. MSA 302 Figure 3. OA Z-scan curves of PC1 to PC5 recorded with peak intensity of 200 GW/cm2. Open stars represent the experi- mental data while the solid lines are fits. Red (dotted) and blue (solid) lines represent theoretical fit for 3PA and 2PA, respec- tively. Inset of (a) shows CA scan recoded at same intensities. that 2PA indeed was the main mechanism contributing to nonlinear absorption. The values of nonlinear coeffici- ents presented here are accurate within 15% and the er- rors arise due to uncertainties in the estimation of spot size at focus and peak intensities, fitting procedures, ca- libration of neutral density filters etc. The χ(3) values, ~10−10 e.s.u., obtained from our films are comparable to one of the highest reported nonlinearity for Aluminum phthalocyanine films [21]. The best n2 value was obtained for PC3 (1.44 × 10−16 m 2/W) is also comparable to the values obtained in self-assembled multilayer films con- taining tetrasulfonated iron phthalocyanine [20]. Howev- er, in their case due to the multi-layer nature of films the π-electron molecular orbits of macrocycles in the aggre- gates probably coupled with neighbors so strongly enough to enlarge the conjugation system, thus, changing the electronic structure of the phthalocyanine molecule and improving the third-order optical response. Ma et al. [16] reported χ(3) values of 10−10 e.s.u. for their tita- nylphthalocyanine films which is again comparable to the values obtained in present study. Furthermore we observed strong 2PA in our molecules which find poten- tial applications in bio-imaging and good optical limiting thresholds. Whilst the solution studies of these molecules, excited with fs pulses exhibited, strong 3PA in alkyl phthalocya- nines (PC4 and PC5) [29], an amalgamation of excited state absorption, 2PA, and 3PA in alkoxy phthalocya- nines (PC1 and PC2) [28], and saturable absorption be- havior with ps pulses in the asymmetric phthalocyanine (PC3) [31,32] all the films studies obviously pointed to the presence of strong 2PA. This could be explained with the changes in absorption spectra, and thereby the energy level configuration, corresponding to solutions and thin films combined with the aspect of peak intensities used for these studies. With the broadening of Soret band, there is a possibility of direct two-photon state existence for the molecule when excited with 800 nm photon in the bulk unlike in solutions. PC3 in solution depicted SA (due to small absorption at 800 nm) when excited with lower peak intensities but immediately switched to RSA with increasing peak intensities [31,32]. PC1 and PC2 also demonstrated similar behavior of RSA with increas- ing peak intensities. The nonlinear absorption in such molecules is indeed complex phenomena involving sev- eral excited state mechanisms such as 2PA/3PA induced excited state absorption, pure 2PA/3PA, single photon induced excited state absorption etc. [37-41]. Further detailed wavelength and intensity dependent studies are in progress to elucidate the complete nonlinear absorp- Ultrafast Nonlinear Optical and Optical Limiting Properties of Phthalocyanine Thin Films Studied Using Z-Scan Copyright © 2011 SciRes. MSA 303 tion behavior in these materials. We have assessed the merit factors W and T for all the thin films studied and defined as 2 1 s at nI W (4) 2 1 2 n T (5) where λ is the wavelength and Isat is the light intensity at which the nonlinear refractive index saturates. Ideally W must be > 1 and T ought to be < 1 for practical device applications. In the present case W is certainly > 1 for all the films while T is also > 1. We strongly feel that these robust molecules are good for multi-photon absorption based applications due to the presence of strong 2PA. The nonlinear refractive index recorded was also high for these molecules compared to some the recently reported molecules of interest and furthermore the excitation wave- length was non-resonant. However, supplementary wave- length dependent studies will reveal the actual potency of these molecules for optical switching based applications which is strongly dependent on n2. Optical limiting is a phenomena observed when the transmission of a medium decreases with increasing in- put laser intensity (or fluence). An effective optical limi- ter will have low limiting threshold, high optical damage threshold and stability, fast response time, high linear transmittance throughout the sensor band width, optical clarity and robustness. One of the major mechanisms involved is reverse saturable absorption (RSA), usually observed with nanosecond pulse excitation. OL can be achieved through various nonlinear optical mechanisms such as multi-photon absorption (MPA), excited state absorption (ESA), free carrier absorption (FCA), self- focusing, self-defocusing, nonlinear scattering, photo-re- fraction etc. Coupling two or more of these mechanisms has also causes OL like self de-focusing along with MPA. In the case of Pc thin films, it is observed that sample exhibits strong two-photon absorption and therefore is responsible for the limiting reported here. Figure 4 shows a typical optical limiting curve depicting the sample transmission as a function of input laser fluence (i.e. energy per area). The input laser energy density was evaluated using the relation 2 32 4ln2 π in Ez E, where Ein is the input laser pulse energy and ω(z) is the beam radius (HW1/e2M). It is evident that under strong irradiance the curve departs from Beer’s law leading to optical power limiting in the thin film. The limiting thre- sholds evaluated from the data and fits were in the 0.1 - 0.6 mJ/cm2 range. Table 2 summarizes the nonlinear coefficients of all the films extracted from the present study. The high nonlinearity of PC3 compared to other phthalocyanines can be ascribed to the asymmetry in structure compared to other symmetrical phthalocyanines and is consistent with our earlier results [28-33]. Metal phthalocyanine films exhibited higher nonlinearities when compared to free base phthalocyanine films for both alkyl and alkoxy cases whereas alkoxy phthalocya- nine films displayed higher nonlinearity compare to alkyl phthalocyanine films. We are also investigating the response time of these Figure 4. Optical limiting cure recorded for PC3 in PMMA. Blue (solid) line is the fit for 2PA while red (dotted) line is the fit for 3PA. Table 2. Summary of the nonlinear coefficients of phthalocyanine thin films extracted from the present study. Sample n2 (m2/W) × 10−17 n2 (e.s.u) × 10−10 Re|χ(3)| (m2/V2) × 10−19 α2 (m/W) × 10−10 Im|χ(3)| (m2/V2) × 10−19 |χ(3)| (m2/V2) × 10−19 |χ(3)| (e.s.u) × 10−11 Figure of Merit W Figure of Merit T Limiting Threshold (J/cm2) PC1 4.30 1.53 5.06 6.00 4.50 6.78 4.86 3.7 11.2 0.164 PC2 6.94 2.47 8.17 18.0 13.5 15.8 11.3 7.3 20.7 0.137 PC3 14.4 5.12 17.0 20.0 15.0 22.6 16.2 2.6 11.1 0.112 PC4 2.18 0.78 2.57 6.00 4.50 5.18 3.71 2.3 22.0 0.131 PC5 2.30 0.82 2.71 1.50 1.13 2.93 2.10 3.1 5.2 0.550 Ultrafast Nonlinear Optical and Optical Limiting Properties of Phthalocyanine Thin Films Studied Using Z-Scan Copyright © 2011 SciRes. MSA 304 films using ps and fs pulse excitation. Our initial fs pump-probe studies offered a testimony for the ultrafast response time with the decay observed in the sub-100 ps time scale [42]. Several recent reports too predict the lifetimes in a number of phthalocyanine thin films to be in the ps time domain [43-46]. Our objective is to obtain a complete perception of how these molecules respond to cw, ns, ps, and fs pulse excitation in both solution and thin film form by evaluating the nonlinear coefficients, response times, and figures of merit to entirely categorize and utilize their potential in each of these time domains. 4. Conclusions We have investigated the nonlinear optical properties of five different phthalocyanine thin films at 800 nm with 2 ps pulses. All the samples were found to possess good n2 values when compared to some of the recently reported values for similar molecules. The sign of the nonlinearity was confirmed to be negative. Open aperture studies demonstrated strong two photon absorption to be respon- sible for the nonlinear absorption. 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