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Studies of third order optical nonlinearity in iron (III) phthalocyanine μ-oxo dimers using picosecond four-wave mixing
1 July 1999
Optics Communications 165 Ž1999. 91–97 www.elsevier.comrlocateroptcom
Studies of third order optical nonlinearity in iron žIII / phthalocyanine m-oxo dimers using picosecond four-wave mixing Reji Philip a
a,1
, M. Ravikanth b, G. Ravindra Kumar
a,)
Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Bombay 400 005, India b Department of Chemistry, Kyoto UniÕersity, Kyoto, Japan Received 30 November 1998; received in revised form 4 May 1999; accepted 6 May 1999
Abstract We report on degenerate four-wave mixing studies of a set of new m-oxo dimeric iron ŽIII. phthalocyanine compounds excited by 532 nm picosecond pulses. The obtained figure of merit values are among the best reported so far in metallophthalocyanine systems. The excited state dynamics were probed from the time of evolution of the DFWM signal using two temporally coincident pump pulses and a delayed probe. We infer that there are at least two different timescales involved in the decay of the signal. q 1999 Published by Elsevier Science B.V. All rights reserved.
1. Introduction Study of the nonlinear optical properties of materials and the design and fabrication of devices based on them continue to be an exciting area of optoelectronics and photonics. In general, resonant nonlinearities are large in magnitude but slow in response, whereas nonresonant nonlinearities are smaller but much faster. Considerable effort is being invested in searching for molecules which combine maximum strength of nonlinearity with high speed w1–3x, and organic materials have been extensively investigated in this respect w4,5x. Among organic compounds, phthalocyanines and porphyrins are found to exhibit a variety of efficient second and third order nonlinear optical effects w6,7x. This large nonlinearity origi-
)
Corresponding author. Fax: q91-22-215-2110; e-mail: [email protected] 1 On leave from Sacred Heart College, Thevara, Cochin.
nates from their extensively delocalised two dimensional p-electron distribution. Harmonic generation w8,9x, wave mixing w10,11x, optical limiting and optical switching w12–16x under resonant and nonresonant conditions have been investigated in these molecules over a wide spectrum of laser frequencies. They form well-characterized complexes with a variety of metals, as a result of which metal-to-ligand and ligand-to-metal charge transfer states are woven into the electronic manifold, thereby enhancing the nonlinear susceptibility. Phthalocyanines ŽPc. can be easily derivatized through peripheral and axial positions, and they possess excellent chemical and thermal stability. The effect of the central metal atom w17x, peripheral substituents w18x and additional conjugation Žsuch as in naphthalocyanine. w19x on their x Ž3. values have been investigated. In addition, the role of the substituent metal, polymorphism, aggregation, and the size of the porphyrin ring on the nonlinearity have also been studied for some porphyrin molecules w20x.
0030-4018r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 Ž 9 9 . 0 0 2 3 1 - X
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R. Philip et al.r Optics Communications 165 (1999) 91–97
Various experimental techniques can be used for estimating the third order nonlinearity of a given sample, even though each technique addresses only a particular facet of the nonlinearity due to the frequency dispersion of x Ž3.. In general, the third order nonlinear optical susceptibilities x Ž3. Žy3 v ; v , v , v . and x Ž3. Žyv ; v , v ,y v . can be respectively characterized by the techniques of third harmonic generation ŽTHG. and degenerate four-wave mixing ŽDFWM.. THG has the advantage of probing the pure nonresonant electronic nonlinearity, but it does not provide information on its temporal evolution. On the other hand, DFWM measurements may include contributions from orientational as well as dynamic resonant nonlinearities Žexcited state populations, thermal effects, etc.. depending on the experimental conditions. x Ž3. Žyv ; v , v ,y v . is an important parameter for the design of devices utilizing optical switching and bistability, and it is possible to map out its temporal evolution using DFWM. Several metallophthalocyanines have been investigated by DFWM, most of them being monophthalocyanines. Among them, the largest second hyperpolarizability Ž²g :. value of 1 = 10y3 1 esu was found in PtPcŽCP.4 by Shirk et al. w21x. However, further measurements by them on several lanthanide bisŽphthalocyanines. and their anions showed that the ²g : values for metallo bisŽphthalocyanines. are about an order of magnitude larger than the typical metallophthalocyanines and a factor of 2 to 5 larger than PtPcŽCP.4 w22x. In this paper we report DFWM measurements of the magnitudes of x Ž3. Žyv ; v , v ,y v . and ²g : in a set of m-oxo dimers of iron ŽIII. phthalocyanines. The calculated values of the figure of merit F s x Ž3.ra Ž a is the linear absorption coefficient. show that these samples possess a large third order nonlinearity at 532 nm. Interestingly, the monomer shows the highest F value. We have also attempted to qualitatively correlate the observed temporal evolution of the nonlinearity to the excited state molecular dynamics.
ŽFePc. 2 O-Ž1. and ŽFePc. 2 O-Ž2., respectively, were synthesised by the procedure of Ercolani et al. w23x. The compounds wŽ t-butyl. 4 FePcx 2 O and wŽ nC 6 H 13 O.4 FePcx 2 O were synthesised following a procedure reported recently w24x. All samples were characterized by spectroscopic methods. Fig. 1 shows the structure of these molecules. The most intense visible absorption in metal phthalocyanines is due to the Q band, which corresponds to the lowest allowed p–pU transition in the phthalocyanine ring. In the present case, wŽ t-butyl.4 FePcx 2 O and wŽ n-C 6 H 13 O.4 FePcx 2 O are highly soluble in dichloromethane, and their solutions show this absorption band peaking around 700 nm as seen in Fig. 2Ža.. On the other hand, the FePc monomer and the m-oxo dimers are sparingly soluble in dichloromethane and introduce additional scattering losses to an incoming beam, significantly changing their transmission characteristics. Hence their apparent absorption spectra ŽFig. 2Žb.. show a marked difference from the typical phthalocyanine spectrum. Because of this feature, the same sample solutions used for absorption cross section measurements were used for DFWM experiments also in the case of
2. Experimental details IronŽII. phthalocyanine ŽFePc. was purchased from Aldrich. The bent m-oxoŽ1. and linear m-oxoŽ2. dimers of Žm-oxo.bisŽphthalocyaninato.ironŽIII.,
Fig. 1. Structure of the investigated iron phthalocyanines.
R. Philip et al.r Optics Communications 165 (1999) 91–97
93
of one of the pump beams was picked off and measured by a photodiode to monitor the input energy. The pump-probe energy ratio was adjusted to be approximately 6 and w was approximately 5 0 . The spacing of the grating created by the forward pump and probe was calculated to be 5.6 mm. All the beams were s-polarized. The DFWM signal generated in the sample solution was separated from the probe beam by a beam splitter and measured by a second photodiode. The photodiode signals were averaged over a number of laser shots and displayed by a Tektronix TDS320 digital oscilloscope. The maximum phase conjugate reflectivity achieved in these experiments was 1 = 10y2 . To monitor the temporal decay of excitation, the relative energies of the beams were modified. As shown in Fig. 3Žb., in this case the two forward beams were made equally intense Žthey become the pumps., while the intensity of the backward beam
Fig. 2. Ža. Optical absorption spectra of wŽ t-butyl.4 FePcx 2 O and wŽ n-C 6 H 13 O.4 FePcx 2 O in dichloromethane; Žb. optical absorption spectra of FePc, ŽFePc. 2 O-Ž1. and ŽFePc. 2 O-Ž2. in dichloromethane Žsee Section 2..
these three compounds. The finite absorption displayed by all the above samples at 532 nm is responsible for a slow component in the temporal evolution of the nonlinearity, as will be discussed later. The DFWM experiment uses 35 ps pulses at 532 nm from the second harmonic output of a hybridmodelocked Nd-YAG laser operating at a repetition rate of 10 Hz. The laser pulse energy at the sample was varied by a half-wave plate and polarizer combination. Samples were taken in a 1-mm thick glass cuvette, with concentrations chosen around 10y4 molrl to give a linear optical transmission of 70– 80% at 532 nm. We used the standard backward geometry for x Ž3. measurement, consisting of two strong, equal energy counter-propagating pump beams and a weak probe beam incident at a small angle w to one of the pumps w25x. The corresponding wave vectors are shown in Fig. 3Ža.. A small portion
Fig. 3. Ža. Beam geometry for x Ž3. measurement; k 1 , k 2 and k 3 are temporally coincident and k 4 is the DFWM signal beam, phase conjugate to k 3 ; Žb. beam geometry for measuring time evolution of the DFWM signal, k 1 and k 3 are temporally coincident while k 2 is delayed in time.
R. Philip et al.r Optics Communications 165 (1999) 91–97
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Žwhich becomes the probe. was limited to approximately 20% of the pumps. The DFWM signal was then measured as a function of the time delay of the probe with respect to the coincident pump beams. The solvent dichloromethane did not give any signal under both configurations of the experiment. 3. Results and discussion
Table 1 Absorption cross section, second hyperpolarizability and the figure of merit values for the samples investigated Molecule
s 0 Žcm2 . at 532 nm
FePc ŽFePc. 2 O-Ž1. ŽFePc. 2 O-Ž2. wŽ t-butyl.4 FePcx 2 O wŽ n-C 6 H 13 O.4 FePcx 2 O
0.5=10y1 7 2.7=10y31 2.4=10y1 7 6.4=10y31 5.1=10y17 10.0=10y31 2.8=10y17 3.0=10y31 4.9=10y17 13.0=10y31
²g : Žesu.
F s x Ž3. r a Žesu cm. 6.9=10y13 3.5=10y13 2.6=10y13 1.4=10y13 3.6=10y13
3.1. Calculation of the nonlinearity parameters Variation of the DFWM signal as a function of the pump intensity is shown in Fig. 4. The signal is proportional to the cubic power of the input intensity as given by the equation w25x: IŽ v . a
ž
2
v 2 ´ 0 cn
2
/
< x Ž3. < 2 l 2 I03 Ž v . ,
Ž 1.
where I Ž v . is the DFWM signal intensity, I0 Ž v . is
the pump intensity, l is the interaction length, and n is the refractive index of the medium. The solid curves in the figures are cubic fits to the experimental data. x (3) can be calculated from the equation:
x
Ž3.
Ž3. s xref
Ž IrI03 . Ž IrI03 . ref
1r2
n n ref
al
=
ya l
Ž1ye
. ey a l r2
,
2
l ref l
Ž 2.
where the subscript ‘ref’ refers to the corresponding quantities measured for the standard reference CS 2 under identical conditions, a is the absorption coefy1 3 ficient, and xr(3) esu e f is taken to be 4.0 = 10 w21,22x. The figure of merit F is hence calculated. F is a measure of the nonlinear response that can be achieved for a given absorption loss, and is useful in comparing nonlinear materials in regions of absorption. The second hyperpolarizability values can be obtained from the relation:
x Ž3. ²g : s
Fig. 4. DFWM signal versus input pump energy for: Ža. ŽFePc. 2 OŽ2.; and Žb. wŽ n-C 6 H 13 O.4 FePcx 2 O. Solid curves are cubic fits to the experimental data.
L4 N
,
Ž 3.
where L s Ž n 2 q 2.r3 is the local field correction factor and N is the number density of the solute molecules in solution. Table 1 lists the calculated nonlinearity parameters. In Table 2, the corresponding values reported by Shirk et al. w22x for a set of scandium, yttrium and lanthanide bisŽphthalocyanines. at 1.06 mm are given. The linear absorption cross sections at the respective excitation wavelengths are shown in both tables. It can be seen that the present samples possess very good F values.
R. Philip et al.r Optics Communications 165 (1999) 91–97 Table 2 Second hyperpolarizability and the figure of merit values for the bisŽphthalocyanines., reproduced from Shirk et al. w22x Molecule
s 0 Žcm2 . at 1.06 mm
²g : Žesu.
F s x Ž3. r a Žesu cm.
ScPc 2 LuPc 2 YbPc 2 YPc 2 GdPc 2 EuPc 2 NdPc 2
2.8=10y1 7 1.2=10y17 1.4=10y17 0.4=10y17 0.4=10y17 0.4=10y17 0.5=10y17
4.8=10y31 3.4=10y31 4.1=10y31 2.6=10y31 2.2=10y31 2.2=10y31 1.5=10y31
0.6=10y13 1.0=10y13 1.0=10y13 2.0=10y13 2.0=10y13 2.0=10y13 1.0=10y13
We have calculated the absorption cross sections from the other two quantites.
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basket handle porphyrins under similar conditions have revealed two closely spaced sharp peaks, indicating that the sequential decay of the excited singlet and triplet states could be resolved w26x. Intersystem crossing lifetimes as low as 10 ps could be estimated in that case. The absence of a similar structure in the present case suggests that the net lifetime of all the excited states falls well within the laser pulse duration, such that the resonant nonlinearity itself is comparatively faster in these samples. However, subsequent solute thermalization results in the formation of a thermal grating in the medium. The build up time of the thermal grating is estimated
3.2. Temporal eÕolution of the nonlinearity The temporal evolution of the DFWM signal portrays the build-up and decay of the real Žresonant. and virtual Žnonresonant. excited states of the molecule. Fig. 5 shows the typical evolution of the DFWM signal as a function of the probe delay in the samples. It can be seen that the signal has two parts. The first part rises sharply, peaks at zero delay and drops with the same decay characteristics as that of the excitation pulse. It can be assumed that the fast nonresonant part of the nonlinearity is fully included in this sharp peak, even though the temporal resolution is apparently limited by the laser pulse width Ža similar study on CS 2 which is known to have a fast, nonresonant nonlinearity in the visible region also reproduced the same temporal profile.. At higher time delays the signal again starts to build up and increases throughout the delay times studied Ž1.4 ns.. To gain an insight into the dynamics involved, consider an energy level scheme generally applicable to the molecules investigated. Laser excitation of the molecule from the ground singlet state places it in a higher singlet state. Since the paramagnetic metal ion quenches fluorescence almost completely, a rapid decay to the lowest triplet state follows. The excited state lifetimes are very short, in some cases falling in the sub-100 ps domain w14x. Subsequent relaxation to the ground state has a comparable lifetime, although it can be larger in some cases. Whether the consecutive decays of these states can be resolved or not depends on their respective lifetimes and the excitation pulse width. In fact, previous experiments on
Fig. 5. Variation of the DFWM signal with positive time delay t of the backward beam, for Ža. ŽFePc. 2 O-Ž1.; and Žb. wŽ nC 6 H 13 O.4 FePcx 2 O. Circles: co-polarized pump beams, triangles: cross-polarized pump beams.
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R. Philip et al.r Optics Communications 165 (1999) 91–97
to be 2.6 ns in our samples, given the grating spacing of 5.6 mm and the speed of sound in dichloromethane of 1090 mrs at room temperature w27x. The second part of the signal can thus be inferred to originate from this thermal grating. To verify this point, we repeated the experiment with cross-polarized pump beams, where a thermal grating is not expected. As shown in Fig. 5, the DFWM signal was virtually unaffected in the first part while it disappeared completely in the second part, thereby confirming the importance of thermal dynamics in the medium. We further observed that the DFWM signal behaves in an interesting manner when the delay of the probe is made negative, i.e., when the probe is allowed to reach the sample ahead of the pump beams. With a 35 ps pulse, the signal would normally be expected to trace a Gaussian in this time regime, reproducing the laser pulse shape, since the room temperature dephasing times ŽT2 . in liquids are known to be less than a few picoseconds. We verified this in the case of CS 2 , where the signal reproduced the temporal profile of the pulse. However, the signal decay rate in the samples investigated was found to be slower in comparison, as shown in Fig. 6. One possible explanation of this phenomenon would be scattering from a residual diminishing thermal grating that follows each ‘previous’ pump pulse. However, if this were the case, the signal would be expected to increase rather than decrease at higher negative delays. Furthermore, typical thermal grating decay times are in the order of microseconds, as
shown below. The decay time can be estimated from the Rayleigh lifetime parameter, R w28x: Rs
r 0 Cp Kq 2
Ž 4.
where r 0 denotes the equilibrium solvent density, Cp represents the specific heat at constant pressure, K is the thermal conductivity, and q s 2 k Sin wr2 is the grating wave vector with k s Ž nrc . v . Using parameter values appropriate to dichloromethane at room temperature Ž r 0 s 1.32 = 10 3 kg my3, Cp s 1.19 = 10 3 J kgy1 Ky1 , K s 1.1 = 10y1 W my1 Ky1 and n s 1.42. gives a value of R s 6.7 ms. In the present case, successive laser pulses were separated by 100 ms, giving enough time for thermal grating to vanish. Alternative explanations therefore have to be sought: one possibility is to visualize the scenario in terms of coherent excitation as follows. The backward beam k 2 excites the molecules in the interaction region and, before the phase memory is completely lost, the forward beam k 3 interacts with the medium to form a transient grating. The coincident beam k 1 is diffracted by this grating and produces the signal. This explanation assumes the existence of a T2 that is higher than conventionally found in such systems, and more experiments are necessary to explore this aspect further.
4. Conclusions We studied the third order optical nonlinearity of a set of m-oxo dimeric iron ŽIII. pthalocyanines by DFWM. The calculated figures of merit are found to be some of the best reported in metallophthalocyanine systems. Temporal evolution studies of the nonlinearity show that the molecular dynamics is quite fast, so that measurements are limited by the excitation pulse width. It is expected that femtosecond excitation can resolve the evolution of the nonlinearity into finer detail. Experiments in this direction are in progress.
Acknowledgements Fig. 6. Variation of the DFWM signal with negative time delay t of the backward beam. Triangles: wŽ t-butyl.4 FePcx 2 O, filled circles: CS 2 .
The authors wish to thank Sudeep Banerjee for useful discussions. RP is grateful to the management
R. Philip et al.r Optics Communications 165 (1999) 91–97
of Sacred Heart College, Thevara, Cochin, for grant of a study leave. References w1x H.S. Nalwa, Appl. Organomet. Chem. 5 Ž1991. 3493. w2x H.S. Nalwa, Adv. Mater. 5 Ž1993. 341. w3x S.R. Marder, J.E. Sohn, G.D. Stucky ŽEds.., Materials for Nonlinear Optics - Chemical Perspectives, ACS Symposium Series 455, American Chemical Society, Washington, DC, 1991. w4x K.K. Sharma, K.D. Rao, G.R. Kumar, Opt. Quant. Electron. 26 Ž1994. 1. w5x J.-L. Bredas, C. Adant, P. Tackx, A. Persoons, B.M. Pierce, Chem. Rev. 94 Ž1994. 243. w6x H.S. Nalwa, S. Miyata ŽEds.., Nonlinear Optics of Organic Molecules and Polymers, Chap. 11, CRC Press, Boca Raton, FL, 1997. w7x H.S. Nalwa, J.S. Shirk, in: C.C. Leznoff, A.B.P. Lever ŽEds.., Pthalocyanines: Properties and Applications, vol. 4, VCH, New York, 1996, p. 79. w8x M.A. Diaz-Garcia, I. Ledoux, F. Fernandez-Lazaro, A. Sastre, T. Torres, F. Agullo Lopez, J. Zyss, J. Phys. Chem. 98 Ž1994. 4495. w9x Z.Z. Ho, C.Y. Ju, W.M. Hetherington III, J. Appl. Phys. 62 Ž1987. 716. w10x R.A. Norwood, J.R. Sounik, Appl. Phys. Lett. 60 Ž1992. 295. w11x D.V.G.L.N. Rao, F.J. Aranda, J.F. Roach, D.E. Remy, Appl. Phys. Lett. 58 Ž1991. 1241. w12x S. Guha, K. Kang, P. Porter, J.F. Roach, D.E. Remy, F.J. Aranda, D.V.G.L.N. Rao, Opt. Lett. 17 Ž1992. 264. w13x C. Li, L. Zhang, M. Yang, H. Wang, Y. Wang, Phys. Rev. A 49 Ž1994. 1149.
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w14x W. Blau, H. Byrne, W.M. Dennis, J.M. Kelly, Opt. Commun. 56 Ž1985. 25. w15x A. Sevian, M. Ravikanth, G. Ravindra Kumar, Chem. Phys. Lett. 263 Ž1996. 241. w16x J.W. Perry, K. Mansour, S.R. Marder, K.J. Perry, D. Alvarez Jr., I. Choong, Opt. Lett. 19 Ž1994. 625. w17x J.S. Shirk, J.R. Lindle, F.J. Bartoli, Z.H. Kafafi, A.W. Snow, M.E. Boyle, Int. J. Opt. Phys. 1 Ž1992. 699. w18x H.S. Nalwa, A. Kakuta, A. Mukoh, Chem. Phys. Lett. 203 Ž1993. 109, and references therein. w19x H.S. Nalwa, A. Kakuta, A. Mukoh, Phys. Chem. 97 Ž1993. 1097. w20x M. Ravikanth, G. Ravindra Kumar, Curr. Sci. 68 Ž1995. 1010. w21x J.S. Shirk, J.R. Lindle, F.J. Bartoli, Z.H. Kafafi, A.W. Snow, in: S.R. Marder, J.E. Sohn, G.D. Stucky ŽEds.., Materials for Nonlinear Optics-Chemical Perspectives, ACS Symposium Series 455, American Chemical Society, Washington, DC, 1991. w22x J.S. Shirk, J.R. Lindle, F.J. Bartoli, M.E. Boyle, J. Phys. Chem. 96 Ž1992. 5847. w23x C. Ercolani, M. Gardini, F. Pennesi, G. Rossi, Inorg. Chem. 22 Ž1983. 2584. w24x M. Ravikanth, C. Achim, J.S. Tyhonas, E. Munck, J.S. Lindsey, J. Porphyrins Phthalocyanines 1 Ž1997. 385. w25x R.A. Fisher ŽEd.., Optical Phase Conjugation, Academic Press, New York, 1983. w26x G. Ravindra Kumar, M. Ravikanth, S. Banerjee, A. Sevian, Opt. Commun. 144 Ž1997. 245. w27x B.Ya. Zeldovich, N.F. Pilipetskii, V.V. Shkunov, in: Principles of Optical Phase Conjugation, Springer-Verlag, Berlin, 1985. w28x H.J. Hoffman, P.E. Perkins, IEEE J. Quant. Electron. 22 Ž1986. 563.
Optics Communications 165 Ž1999. 91–97 www.elsevier.comrlocateroptcom
Studies of third order optical nonlinearity in iron žIII / phthalocyanine m-oxo dimers using picosecond four-wave mixing Reji Philip a
a,1
, M. Ravikanth b, G. Ravindra Kumar
a,)
Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Bombay 400 005, India b Department of Chemistry, Kyoto UniÕersity, Kyoto, Japan Received 30 November 1998; received in revised form 4 May 1999; accepted 6 May 1999
Abstract We report on degenerate four-wave mixing studies of a set of new m-oxo dimeric iron ŽIII. phthalocyanine compounds excited by 532 nm picosecond pulses. The obtained figure of merit values are among the best reported so far in metallophthalocyanine systems. The excited state dynamics were probed from the time of evolution of the DFWM signal using two temporally coincident pump pulses and a delayed probe. We infer that there are at least two different timescales involved in the decay of the signal. q 1999 Published by Elsevier Science B.V. All rights reserved.
1. Introduction Study of the nonlinear optical properties of materials and the design and fabrication of devices based on them continue to be an exciting area of optoelectronics and photonics. In general, resonant nonlinearities are large in magnitude but slow in response, whereas nonresonant nonlinearities are smaller but much faster. Considerable effort is being invested in searching for molecules which combine maximum strength of nonlinearity with high speed w1–3x, and organic materials have been extensively investigated in this respect w4,5x. Among organic compounds, phthalocyanines and porphyrins are found to exhibit a variety of efficient second and third order nonlinear optical effects w6,7x. This large nonlinearity origi-
)
Corresponding author. Fax: q91-22-215-2110; e-mail: [email protected] 1 On leave from Sacred Heart College, Thevara, Cochin.
nates from their extensively delocalised two dimensional p-electron distribution. Harmonic generation w8,9x, wave mixing w10,11x, optical limiting and optical switching w12–16x under resonant and nonresonant conditions have been investigated in these molecules over a wide spectrum of laser frequencies. They form well-characterized complexes with a variety of metals, as a result of which metal-to-ligand and ligand-to-metal charge transfer states are woven into the electronic manifold, thereby enhancing the nonlinear susceptibility. Phthalocyanines ŽPc. can be easily derivatized through peripheral and axial positions, and they possess excellent chemical and thermal stability. The effect of the central metal atom w17x, peripheral substituents w18x and additional conjugation Žsuch as in naphthalocyanine. w19x on their x Ž3. values have been investigated. In addition, the role of the substituent metal, polymorphism, aggregation, and the size of the porphyrin ring on the nonlinearity have also been studied for some porphyrin molecules w20x.
0030-4018r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 Ž 9 9 . 0 0 2 3 1 - X
92
R. Philip et al.r Optics Communications 165 (1999) 91–97
Various experimental techniques can be used for estimating the third order nonlinearity of a given sample, even though each technique addresses only a particular facet of the nonlinearity due to the frequency dispersion of x Ž3.. In general, the third order nonlinear optical susceptibilities x Ž3. Žy3 v ; v , v , v . and x Ž3. Žyv ; v , v ,y v . can be respectively characterized by the techniques of third harmonic generation ŽTHG. and degenerate four-wave mixing ŽDFWM.. THG has the advantage of probing the pure nonresonant electronic nonlinearity, but it does not provide information on its temporal evolution. On the other hand, DFWM measurements may include contributions from orientational as well as dynamic resonant nonlinearities Žexcited state populations, thermal effects, etc.. depending on the experimental conditions. x Ž3. Žyv ; v , v ,y v . is an important parameter for the design of devices utilizing optical switching and bistability, and it is possible to map out its temporal evolution using DFWM. Several metallophthalocyanines have been investigated by DFWM, most of them being monophthalocyanines. Among them, the largest second hyperpolarizability Ž²g :. value of 1 = 10y3 1 esu was found in PtPcŽCP.4 by Shirk et al. w21x. However, further measurements by them on several lanthanide bisŽphthalocyanines. and their anions showed that the ²g : values for metallo bisŽphthalocyanines. are about an order of magnitude larger than the typical metallophthalocyanines and a factor of 2 to 5 larger than PtPcŽCP.4 w22x. In this paper we report DFWM measurements of the magnitudes of x Ž3. Žyv ; v , v ,y v . and ²g : in a set of m-oxo dimers of iron ŽIII. phthalocyanines. The calculated values of the figure of merit F s x Ž3.ra Ž a is the linear absorption coefficient. show that these samples possess a large third order nonlinearity at 532 nm. Interestingly, the monomer shows the highest F value. We have also attempted to qualitatively correlate the observed temporal evolution of the nonlinearity to the excited state molecular dynamics.
ŽFePc. 2 O-Ž1. and ŽFePc. 2 O-Ž2., respectively, were synthesised by the procedure of Ercolani et al. w23x. The compounds wŽ t-butyl. 4 FePcx 2 O and wŽ nC 6 H 13 O.4 FePcx 2 O were synthesised following a procedure reported recently w24x. All samples were characterized by spectroscopic methods. Fig. 1 shows the structure of these molecules. The most intense visible absorption in metal phthalocyanines is due to the Q band, which corresponds to the lowest allowed p–pU transition in the phthalocyanine ring. In the present case, wŽ t-butyl.4 FePcx 2 O and wŽ n-C 6 H 13 O.4 FePcx 2 O are highly soluble in dichloromethane, and their solutions show this absorption band peaking around 700 nm as seen in Fig. 2Ža.. On the other hand, the FePc monomer and the m-oxo dimers are sparingly soluble in dichloromethane and introduce additional scattering losses to an incoming beam, significantly changing their transmission characteristics. Hence their apparent absorption spectra ŽFig. 2Žb.. show a marked difference from the typical phthalocyanine spectrum. Because of this feature, the same sample solutions used for absorption cross section measurements were used for DFWM experiments also in the case of
2. Experimental details IronŽII. phthalocyanine ŽFePc. was purchased from Aldrich. The bent m-oxoŽ1. and linear m-oxoŽ2. dimers of Žm-oxo.bisŽphthalocyaninato.ironŽIII.,
Fig. 1. Structure of the investigated iron phthalocyanines.
R. Philip et al.r Optics Communications 165 (1999) 91–97
93
of one of the pump beams was picked off and measured by a photodiode to monitor the input energy. The pump-probe energy ratio was adjusted to be approximately 6 and w was approximately 5 0 . The spacing of the grating created by the forward pump and probe was calculated to be 5.6 mm. All the beams were s-polarized. The DFWM signal generated in the sample solution was separated from the probe beam by a beam splitter and measured by a second photodiode. The photodiode signals were averaged over a number of laser shots and displayed by a Tektronix TDS320 digital oscilloscope. The maximum phase conjugate reflectivity achieved in these experiments was 1 = 10y2 . To monitor the temporal decay of excitation, the relative energies of the beams were modified. As shown in Fig. 3Žb., in this case the two forward beams were made equally intense Žthey become the pumps., while the intensity of the backward beam
Fig. 2. Ža. Optical absorption spectra of wŽ t-butyl.4 FePcx 2 O and wŽ n-C 6 H 13 O.4 FePcx 2 O in dichloromethane; Žb. optical absorption spectra of FePc, ŽFePc. 2 O-Ž1. and ŽFePc. 2 O-Ž2. in dichloromethane Žsee Section 2..
these three compounds. The finite absorption displayed by all the above samples at 532 nm is responsible for a slow component in the temporal evolution of the nonlinearity, as will be discussed later. The DFWM experiment uses 35 ps pulses at 532 nm from the second harmonic output of a hybridmodelocked Nd-YAG laser operating at a repetition rate of 10 Hz. The laser pulse energy at the sample was varied by a half-wave plate and polarizer combination. Samples were taken in a 1-mm thick glass cuvette, with concentrations chosen around 10y4 molrl to give a linear optical transmission of 70– 80% at 532 nm. We used the standard backward geometry for x Ž3. measurement, consisting of two strong, equal energy counter-propagating pump beams and a weak probe beam incident at a small angle w to one of the pumps w25x. The corresponding wave vectors are shown in Fig. 3Ža.. A small portion
Fig. 3. Ža. Beam geometry for x Ž3. measurement; k 1 , k 2 and k 3 are temporally coincident and k 4 is the DFWM signal beam, phase conjugate to k 3 ; Žb. beam geometry for measuring time evolution of the DFWM signal, k 1 and k 3 are temporally coincident while k 2 is delayed in time.
R. Philip et al.r Optics Communications 165 (1999) 91–97
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Žwhich becomes the probe. was limited to approximately 20% of the pumps. The DFWM signal was then measured as a function of the time delay of the probe with respect to the coincident pump beams. The solvent dichloromethane did not give any signal under both configurations of the experiment. 3. Results and discussion
Table 1 Absorption cross section, second hyperpolarizability and the figure of merit values for the samples investigated Molecule
s 0 Žcm2 . at 532 nm
FePc ŽFePc. 2 O-Ž1. ŽFePc. 2 O-Ž2. wŽ t-butyl.4 FePcx 2 O wŽ n-C 6 H 13 O.4 FePcx 2 O
0.5=10y1 7 2.7=10y31 2.4=10y1 7 6.4=10y31 5.1=10y17 10.0=10y31 2.8=10y17 3.0=10y31 4.9=10y17 13.0=10y31
²g : Žesu.
F s x Ž3. r a Žesu cm. 6.9=10y13 3.5=10y13 2.6=10y13 1.4=10y13 3.6=10y13
3.1. Calculation of the nonlinearity parameters Variation of the DFWM signal as a function of the pump intensity is shown in Fig. 4. The signal is proportional to the cubic power of the input intensity as given by the equation w25x: IŽ v . a
ž
2
v 2 ´ 0 cn
2
/
< x Ž3. < 2 l 2 I03 Ž v . ,
Ž 1.
where I Ž v . is the DFWM signal intensity, I0 Ž v . is
the pump intensity, l is the interaction length, and n is the refractive index of the medium. The solid curves in the figures are cubic fits to the experimental data. x (3) can be calculated from the equation:
x
Ž3.
Ž3. s xref
Ž IrI03 . Ž IrI03 . ref
1r2
n n ref
al
=
ya l
Ž1ye
. ey a l r2
,
2
l ref l
Ž 2.
where the subscript ‘ref’ refers to the corresponding quantities measured for the standard reference CS 2 under identical conditions, a is the absorption coefy1 3 ficient, and xr(3) esu e f is taken to be 4.0 = 10 w21,22x. The figure of merit F is hence calculated. F is a measure of the nonlinear response that can be achieved for a given absorption loss, and is useful in comparing nonlinear materials in regions of absorption. The second hyperpolarizability values can be obtained from the relation:
x Ž3. ²g : s
Fig. 4. DFWM signal versus input pump energy for: Ža. ŽFePc. 2 OŽ2.; and Žb. wŽ n-C 6 H 13 O.4 FePcx 2 O. Solid curves are cubic fits to the experimental data.
L4 N
,
Ž 3.
where L s Ž n 2 q 2.r3 is the local field correction factor and N is the number density of the solute molecules in solution. Table 1 lists the calculated nonlinearity parameters. In Table 2, the corresponding values reported by Shirk et al. w22x for a set of scandium, yttrium and lanthanide bisŽphthalocyanines. at 1.06 mm are given. The linear absorption cross sections at the respective excitation wavelengths are shown in both tables. It can be seen that the present samples possess very good F values.
R. Philip et al.r Optics Communications 165 (1999) 91–97 Table 2 Second hyperpolarizability and the figure of merit values for the bisŽphthalocyanines., reproduced from Shirk et al. w22x Molecule
s 0 Žcm2 . at 1.06 mm
²g : Žesu.
F s x Ž3. r a Žesu cm.
ScPc 2 LuPc 2 YbPc 2 YPc 2 GdPc 2 EuPc 2 NdPc 2
2.8=10y1 7 1.2=10y17 1.4=10y17 0.4=10y17 0.4=10y17 0.4=10y17 0.5=10y17
4.8=10y31 3.4=10y31 4.1=10y31 2.6=10y31 2.2=10y31 2.2=10y31 1.5=10y31
0.6=10y13 1.0=10y13 1.0=10y13 2.0=10y13 2.0=10y13 2.0=10y13 1.0=10y13
We have calculated the absorption cross sections from the other two quantites.
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basket handle porphyrins under similar conditions have revealed two closely spaced sharp peaks, indicating that the sequential decay of the excited singlet and triplet states could be resolved w26x. Intersystem crossing lifetimes as low as 10 ps could be estimated in that case. The absence of a similar structure in the present case suggests that the net lifetime of all the excited states falls well within the laser pulse duration, such that the resonant nonlinearity itself is comparatively faster in these samples. However, subsequent solute thermalization results in the formation of a thermal grating in the medium. The build up time of the thermal grating is estimated
3.2. Temporal eÕolution of the nonlinearity The temporal evolution of the DFWM signal portrays the build-up and decay of the real Žresonant. and virtual Žnonresonant. excited states of the molecule. Fig. 5 shows the typical evolution of the DFWM signal as a function of the probe delay in the samples. It can be seen that the signal has two parts. The first part rises sharply, peaks at zero delay and drops with the same decay characteristics as that of the excitation pulse. It can be assumed that the fast nonresonant part of the nonlinearity is fully included in this sharp peak, even though the temporal resolution is apparently limited by the laser pulse width Ža similar study on CS 2 which is known to have a fast, nonresonant nonlinearity in the visible region also reproduced the same temporal profile.. At higher time delays the signal again starts to build up and increases throughout the delay times studied Ž1.4 ns.. To gain an insight into the dynamics involved, consider an energy level scheme generally applicable to the molecules investigated. Laser excitation of the molecule from the ground singlet state places it in a higher singlet state. Since the paramagnetic metal ion quenches fluorescence almost completely, a rapid decay to the lowest triplet state follows. The excited state lifetimes are very short, in some cases falling in the sub-100 ps domain w14x. Subsequent relaxation to the ground state has a comparable lifetime, although it can be larger in some cases. Whether the consecutive decays of these states can be resolved or not depends on their respective lifetimes and the excitation pulse width. In fact, previous experiments on
Fig. 5. Variation of the DFWM signal with positive time delay t of the backward beam, for Ža. ŽFePc. 2 O-Ž1.; and Žb. wŽ nC 6 H 13 O.4 FePcx 2 O. Circles: co-polarized pump beams, triangles: cross-polarized pump beams.
96
R. Philip et al.r Optics Communications 165 (1999) 91–97
to be 2.6 ns in our samples, given the grating spacing of 5.6 mm and the speed of sound in dichloromethane of 1090 mrs at room temperature w27x. The second part of the signal can thus be inferred to originate from this thermal grating. To verify this point, we repeated the experiment with cross-polarized pump beams, where a thermal grating is not expected. As shown in Fig. 5, the DFWM signal was virtually unaffected in the first part while it disappeared completely in the second part, thereby confirming the importance of thermal dynamics in the medium. We further observed that the DFWM signal behaves in an interesting manner when the delay of the probe is made negative, i.e., when the probe is allowed to reach the sample ahead of the pump beams. With a 35 ps pulse, the signal would normally be expected to trace a Gaussian in this time regime, reproducing the laser pulse shape, since the room temperature dephasing times ŽT2 . in liquids are known to be less than a few picoseconds. We verified this in the case of CS 2 , where the signal reproduced the temporal profile of the pulse. However, the signal decay rate in the samples investigated was found to be slower in comparison, as shown in Fig. 6. One possible explanation of this phenomenon would be scattering from a residual diminishing thermal grating that follows each ‘previous’ pump pulse. However, if this were the case, the signal would be expected to increase rather than decrease at higher negative delays. Furthermore, typical thermal grating decay times are in the order of microseconds, as
shown below. The decay time can be estimated from the Rayleigh lifetime parameter, R w28x: Rs
r 0 Cp Kq 2
Ž 4.
where r 0 denotes the equilibrium solvent density, Cp represents the specific heat at constant pressure, K is the thermal conductivity, and q s 2 k Sin wr2 is the grating wave vector with k s Ž nrc . v . Using parameter values appropriate to dichloromethane at room temperature Ž r 0 s 1.32 = 10 3 kg my3, Cp s 1.19 = 10 3 J kgy1 Ky1 , K s 1.1 = 10y1 W my1 Ky1 and n s 1.42. gives a value of R s 6.7 ms. In the present case, successive laser pulses were separated by 100 ms, giving enough time for thermal grating to vanish. Alternative explanations therefore have to be sought: one possibility is to visualize the scenario in terms of coherent excitation as follows. The backward beam k 2 excites the molecules in the interaction region and, before the phase memory is completely lost, the forward beam k 3 interacts with the medium to form a transient grating. The coincident beam k 1 is diffracted by this grating and produces the signal. This explanation assumes the existence of a T2 that is higher than conventionally found in such systems, and more experiments are necessary to explore this aspect further.
4. Conclusions We studied the third order optical nonlinearity of a set of m-oxo dimeric iron ŽIII. pthalocyanines by DFWM. The calculated figures of merit are found to be some of the best reported in metallophthalocyanine systems. Temporal evolution studies of the nonlinearity show that the molecular dynamics is quite fast, so that measurements are limited by the excitation pulse width. It is expected that femtosecond excitation can resolve the evolution of the nonlinearity into finer detail. Experiments in this direction are in progress.
Acknowledgements Fig. 6. Variation of the DFWM signal with negative time delay t of the backward beam. Triangles: wŽ t-butyl.4 FePcx 2 O, filled circles: CS 2 .
The authors wish to thank Sudeep Banerjee for useful discussions. RP is grateful to the management
R. Philip et al.r Optics Communications 165 (1999) 91–97
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