J. Phys. Chem. B 2004, 108, 5221-5224
5221
Nonlinear Absorption Spectrum in MEH-PPV/Chloroform Solution: A Competition between Two-Photon and Saturated Absorption Processes L. De Boni, A. A. Andrade, D. S. Correˆ a, D. T. Balogh, S. C. Zilio, L. Misoguti, and C. R. Mendonc¸ a* Departamento de Fı´sica e Cieˆ ncia dos Materiais, Instituto de Fı´sica de Sa˜ o Carlos, Caixa Postal 369, 13560-970 Sa˜ o Carlos, SP, Brazil ReceiVed: December 16, 2003
Knowledge about the nonlinear absorption spectra of materials used in photonic devices is of paramount importance in determining their optimum operation wavelengths. It is well-know that resonances enhance the nonlinear response of a material, but in this case, slow excited states processes, propagation losses, and photodegradation can take place, which are known to be deleterious for photonic applications. With this concern, we have studied the nonlinear degenerate spectrum of poly(MEH-PPV) using femtosecond laser pulses from 460 to 1000 nm. Such a wide band spectrum has never been investigated in previous works reported in the literature. We were able to distinguish between a strong two-photon absorption (2PA) mechanism at nonresonant conditions and a saturated absorption (SA) that overcomes the 2PA process as we approach the absorptive region. The results were fitted with rate equations obtained from an energy level model.
Introduction Among conjugated polymers, the family of poly(phenylenevinylene) (PPV) has attracted considerable attention owing to its interesting optical and electrical properties.1,2 PPV-based polymers are very promising for photonic applications due to the combination of their excellent film-forming properties with interesting light-emitting ability. Concerning nonlinear optics, PPV derivatives were shown to exhibit strong two-photon processes. Recent works have studied a two-photon absorption followed by emission, which opens prospects for their use for up-conversion lasing.3,4 Even though high optical nonlinearities (absorption and refraction) have been reported for several polymers, including PPV,5-7 most of them were measured with a single wavelength. From the application point of view, the knowledge of both linear and nonlinear absorption is important because a device must operate in a wavelength presenting high nonlinear response and low propagation losses. Besides, the optimization of nonlinear optical properties can only be done if the nonlinearity mechanism is fully understood. Again, this task can only be accomplished if a database with nonlinear optical dispersion over a wide spectral range is obtained for different materials. In a 2PA process, which usually predominates at the nonresonant region, two photons of longer wavelength are simultaneously absorbed by a chromophore that would normally be excited by a single photon with a shorter wavelength. Besides the ability to create excited states with photons of half the required excitation energy, the I 2 dependence of 2PA allows chromophores excitation with a high degree of spatial selectivity. Therefore, the 2PA process has attracted great interest in different fields of application, such as chemistry, optical data storage,8,9 two-photon fluorescence imaging,10-14 and twophoton photodynamic cancer therapy.15,16 On the other hand, at the resonant condition, nonlinear processes associated with excited state population, such as saturable absorption (SA), can * Address correspondence to this author.
take place. The SA process has been extensively studied for use in optical switching, short pulse generation, etc., applications that require strong and fast SA processes. In this way, there is considerable interest in the design, synthesis, and characterization of new organic chromophores and polymers with potentially large absorptive nonlinear optical properties (2PA and SA). Moreover, nonlinear optical properties of organic materials may be optimized, at least in principle, by molecular engineering methods to match the application requirements. Certainly, mechanical properties and processability can be improved as well. In this context, this work reports on the degenerate nonlinear absorption spectrum for MEH-PPV/chloroform solution, obtained through the femtosecond Z-scan technique, where a competition between two-photon and saturated absorption processes was observed. Such competition between nonlinear mechanisms is rarely explored in the literature and can lead to misinterpretation of the nonlinear effect. This kind of investigation in MEH-PPV, an important material for photonic applications, and a difference in the 2PA maximum are features never observed in previous investigations. Experimental Section Nonlinear optical measurements were carried out with the Z-scan technique, which is a simple and sensitive method to determine nonlinear optical properties.17,18 Our open aperture Z-scan experiments employed laser pulses from an optical parametric amplifier (TOPAS, from Light Conversion) pumped by 150 fs pulses at 775 nm delivered by a Ti:sapphire chirped pulse amplified system (CPA-2001, from Clark-MXR Inc.) operating at a 1 kHz repetition rate. Pulse energies used here were limited to 0.01 µJ to avoid photodegradation of the polymer. Linear and nonlinear optical measurements were performed in MEH-PPV/chloroform solution (10-4 mol/L in repeating unit). The average number of repeating units in the polymeric macromolecule is about 200 (MW ) 5.0 × 104, D ) 1.6).
10.1021/jp0378891 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/03/2004
5222 J. Phys. Chem. B, Vol. 108, No. 17, 2004
De Boni et al.
Figure 3. Three-energy-level diagram used to model the nonlinear optical processes.
Figure 1. Open aperture Z-scan signature for MEH-PPV chloroform solution at (a) 600, (b) 555, and (c) 570 nm. The inset shows the molecular structure of MEH-PPV.
Figure 2. Linear and degenerate nonlinear absorption spectra for MEH-PPV.
Results and Discussion Figure 1 presents open aperture Z-scan signatures for three distinct wavelengths. For nonresonant excitation (λ ) 600 nm), the decrease in the normalized transmittance indicates a 2PA process, as seen in Figure 1a. When resonant excitation at 555 nm is employed (Figure 1b), an increase in the transmittance, characteristic of saturated absorption, is observed. This result indicates that excited state processes take place under resonant excitation. However, no changes in the normalized transmittance are observed at 570 nm (Figure 1c), because the 2PA and the SA effects cancel each other. The solid line in this figure represents the theoretical fitting obtained with a model including a 2PA effect17 and a resonant process described by a threeenergy-level diagram presented below. Thus, once an open aperture measurement is performed, the absorptive nonlinearity can be found unambiguously by fitting the experimental data. Similar Z-scan measurements were obtained for other wavelengths to determine the degenerated nonlinear absorption spectrum. The transmittance change (∆T) measured through the Z-scan experiment as a function of the wavelength (points) and the linear absorption spectrum (solid line) are plotted in Figure 2. As can be seen, the ∆T spectrum exhibits a strong SA process
in the resonant region (open circles) and a 2PA process (solid circles) away from the fundamental linear absorption band. To understand the results of 2PA and SA presented in Figures 1 and 2, we consider the three-energy-level diagram depicted in Figure 3. According to this model, when nonresonant excitation is employed (Figure 3a) molecules at the ground state S0 are promoted to an excited vibronic level |ν〉 of S1 by twophoton absorption. At this level molecules relax to the bottom of the vibronic band, S1 then decaying to the level S0 via spontaneous emission. By changing the excitation wavelength to resonant conditions, the one-photon absorption process dominates, as illustrated in Figure 3b. In this case, molecules are excited to the vibronic level |ν〉 of S1 and can either decay radiatively to the level S0 via stimulated emission or undergo a relaxation to the bottom of the vibronic band S1, thus radiatively decaying to level S0 via spontaneous emission. According to this model, when the excitation wavelength is tuned from the red (nonresonant) to the blue (resonant) excitation, the nonlinear optical absorption process changes from a 2PA to a dominant SA. For wavelengths close to the bottom of the absorption band, around 570 nm, there is a competition between the two mechanisms. The rate equations used to describe the number of molecules, niν, at each electronic state i and vibronic level ν are
n10 dn0 ) -W01n0 + W01n1ν + - W2PAn0 dt τ10
(1)
n1ν dn1ν ν ) W01n0 - W01n1 - W01 + W2PAn0 dt τrel
(2)
dn10 n1ν n10 ) dt τrel τ10
(3)
where W01 ) σ01 I(t)/(hν) and W2PA ) δ I2(t)/(2pω) are the transition rates for one- and two-photon process, respectively, with σ01 and δ being the ground sate and two-photon absorption cross sections. τrel and τ10 are respectively the intraband relaxation time and the lifetime of the first excited state. This set of equations was numerically solved, yielding the population dynamics, ni(t). The time evolution of the nonlinear absorption can be calculated according to
n0(t)δ 2 σ01 dI ) -[n0(t) - n1ν(t)] I I dz hν 2hν
(4)
where the first term describes the SA process due to the ground state depletion, which includes stimulated emission, and the second term accounts for the 2PA process. Depending on the
Study of Nonlinear Absorption Spectra
Figure 4. 2PA cross-section values experimentally determined and the SOS model.
excitation wavelength, one of these two terms will dominate. At resonant conditions the SA process overcomes the 2PA, while far from it, the 2PA process prevails. Of course in the region close to the absorption band there will be a competition between these two mechanisms. A similar description was used recently to study 2PA and saturated absorption in metalloporphyrindoped boric acid glasses.19 By fitting each of the Z-scan signatures obtained at the nonresonant condition, the 2PA cross section, δ, can be determined by using the repeating unit concentration. The solid line in Figure 1a displays one such fitting. The values obtained for the 2PA cross sections range from 1.4 × 10-46 cm4 s photon-1 (600 nm) to 6 × 10-47 cm4 s photon-1 (740 nm). Furthermore, these values are of the same order of magnitude as those reported in the literature for femtosecond pulses.6 To obtain the δ value using the chromophores concentration, we must use the average number of repeating units in the macromolecule given previously. Results in the spectral range from 650 to 800 nm, reported in ref 7 for a similar PPV derivative, presented a 2PA maximum around 820 nm. However, as seen in Figure 2 we covered a wider spectral range (4601000 nm) and found a peak around 600 nm, which corresponds to the contribution of both 2PA and SA. This indicates that the pure 2PA peak should be below 600 nm. We believe that this difference is probably related to the picosecond pulse duration employed in ref 7, which allows excited-state contribution to the nonlinear absorption, once the excited-state lifetime is in the picosecond time scale. In a recent publication, Chung et al.6 have also measured the degenerate two-photon absorption spectrum for MEH-PPV in a different spectral range (700900 nm), using two-photon excited fluorescence measurements. They observed two peaks in the 2PA intensity around 790 and 840 nm with a decreasing trend to the blue. However, as shown in the 2PA cross-section spectrum displayed in Figure 4, a different behavior, characterized by an increase in the nonlinear absorption cross section up to about 600 nm, was observed. This behavior is in agreement with the resonant denominator in the sum-over states (SOS) model,20-22 assuming that the π f π* transition (corresponding to the peak of the absorption band) gives the major contribution to the virtual intermediate state, as displayed by the solid line in Figure 4. The different trend observed by Chung et al.6 could be due to polymer photodegradation23-25 because their experiments were performed with a high repetition rate (84 MHz) and average power (500 mW). To minimize photodegradation, our measurements were
J. Phys. Chem. B, Vol. 108, No. 17, 2004 5223 carried out with a low repetition rate (1 kHz) and low average power (
