XXIX ENFMC
- Annals of Optics
2006
The two-photon absorption cross-section spectrum of MEH-PPV polymer
determined by white-light continuum Z-scan technique
D. S. Corrêa, S. L. Oliveira,* L. De Boni, L. Misoguti, S. C. Zilio, C. R. Mendonça
Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970
São Carlos, SP, Brazil
*
[email protected]
Abstract
We have used the white-light continuum (WLC) Z-scan technique to determine the degenerate twophoton absorption (2PA) cross-section spectrum of poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene)
MEH-PPV. The results obtained show good agreement with those employing a single wavelength source,
although much faster because of the wavelength multiplexing introduced by the use of a broadband source.
Besides, the improved spectral resolution of the WLC Z-scan allowed the observation of a 2PA peak around 675
nm, a subtle feature in the nonlinear spectrum that could be fitted with a model based on sum-over-states model.
Introduction
The two-photon absorption (2PA) mechanism has been exploited in several studies leading to promising
applications, such as two-photon-pumped lasing,[1] optical limiting,[2] fluorescence excitation microscopy and
imaging,[3] three-dimensional optical data storage,[4] lithographic micro-fabrication,[5] and photodynamic
therapy.[6] Such 2PA process has interesting technological characteristics, such as improved spatial resolution
owing its quadratic dependence on the excitation irradiance and negligible linear absorption at the pumping
wavelength, with the resulting increase of penetration depth.
Although optical nonlinearities have been studied in several materials in the last years, only recently has
the nonlinear absorption spectrum over a wide spectral range begun to be determined.[7,8] Within this context, the
Z-scan[9] technique has been shown to be a useful technique for the nonlinear optical characterization of
materials performed at discrete wavelengths, using the available tunable sources (lasers and optical parametric
amplifiers). However, such measurements have proven to be very time-consuming if one wants to achieve a
reasonable resolution. In order to overcome this drawback, some methods have been proposed to measure the
wavelength-multiplexed nonlinear spectrum, most of them using WLC source. Recently, we have also reported a
single beam open-aperture Z-scan technique using a WLC beam and a spectrometer as detector for the direct
measurement of nonlinear absorption spectra.[10]
In the present work we report on the degenerate 2PA cross-section ( ) spectrum determination for
poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) in chloroform solution, obtained
through the WLC Z-scan technique recently proposed.[10] MEH-PPV is a π-conjugated polymer that has attracted
considerable interest due to its appealing optical and electrical properties. As the WLC Z-scan technique reported
allows only the nonlinear absorption spectrum determination in terms of the transmittance change ( ∆T ), this
work also presents a methodology, based on the relationship between ∆T spectrum and WLC irradiance, able of
providing the 2PA cross-section spectrum of nonlinear materials.
Experimental Setup
The nonlinear spectrum of MEH-PPV was determined through the open-aperture Z-scan technique[9],
which is a simple and sensitive method that allows the 2PA coefficient (β) determination. In nonresonant
conditions, the absorptive Z-scan signature presents a decrease in the transmittance around the focal position,
which can be described by time integrating the transmitted power to give the normalized energy transmittance,
assuming a temporally Gaussian pulse:
T (z) =
1
∞
[
]
ln 1+ q0 ( z ,0)e −τ dτ
π q0 ( z ,0) −∞
2
(1)
where q 0 ( z , t ) = β I 0 (t ) L (1 + z 2 / z 02 )−1 , L is the sample thickness, z0 is the Rayleigh length, z is the sample
position and I0 is the pulse irradiance at the focus (z=0). Thus, once the Z-scan measurement is performed, the
coefficient can be found unambiguously by fitting Eq. (1) to the experimental data. Besides, it can also be
obtained from the measurement of Tz = 0 as a function of laser irradiance. The 2PA cross-section, , is related to
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by means of the expression δ = hνβ / N , where N is the number of molecules per cm3 and h is the excitation
photon energy.
The wide spectral dependence of values was determined by performing Z-scan measurements using an
intense, broadband, WLC beam instead of a single wavelength source, which further details can be found in ref
[10]
. The WLC generation is achieved from 150 fs laser pulses at 775 nm delivered by a commercial Ti:sapphire
chirped pulse amplified system (CPA-2001 from Clark-MXR Inc.), operating at 1 kHz repetition rate. The WLC
from 550 to 720 nm is produced by focusing the pump light with a f = 11 cm lens into a 4 cm path length quartz
cell containing distilled water. The spectra are acquired for each z position as the sample is scanned along the zdirection and then normalized to the one obtained far from the focal plane. The energy at a specific wavelength
of the WLC was determined by considering the continuum as made up of a group of nearly bandwidth-limited
pulses centered at various wavelengths. From the total energy and spectral distribution of the WLC pulse, the
energy inside each bandwidth can be estimated.
Results and Discussion
2
0.6
0.4
1
0.2
0
∆T (z=0)
Linear absorption (arb. units)
Linear and nonlinear optical measurements were performed in MEH-PPV/chloroform solution (1x10-4
and 8.3x10-3mol/L in repeating unit, respectively) placed in a 2 mm path length quartz cuvette. The average
number of repeating units in the macromolecule is about 200. Figure 2 (solid line) exhibits the linear absorption
spectrum for MEH-PPV/solution, with its characteristic π-π* transition around 490 nm.
The typical transmittance change at z = 0 ( Tz=0) spectrum measured using the WLC Z-scan technique
is shown in Fig. 2 (solid line). It exhibits a strong 2PA process near to the linear absorption band. In the resonant
region an evident increase in the transmission of the WLC beam can be observed, which is related to the
saturable absorption effect. For comparison purposes, the ( Tz=0) spectrum obtained from the single wavelength
Z-scan technique using an OPA source is presented in Fig. 2 (open circles).
0.0
-0.2
300
400
500
600
700
800
900 1000
Wavelength (nm)
Fig. 2: Linear (solid line) and degenerate nonlinear absorption spectra of MEH-PPV/chloroform solution. The
solid line and open circles were obtained with WLC and discrete Z-scan technique, respectively.
In order to obtain the 2PA cross-section spectrum from the WLC Tz=0 spectrum, we have initially
chosen an arbitrary wavelength, in the present case 700 nm, and analyzed the respective Z-scan curve, depicted
in Fig. 3a. Through the previous determination of the energy at this wavelength and the theoretical fitting of the
data by means of the Eq. (1) (solid line in Fig. 3a) we can determine the beam waist size at the focus (w0 = 15
µm), the irradiance and the 2PA cross-section as = 1640 GM at 700 nm. In order to improve the reliability of
this scaling point we performed WLC Z-scan measurements for MEH-PPV at different pumping powers (not
shown). By means of the spectra we plot the Tz=0 at 700 nm as a function of pump irradiance, as shown in Fig.
3b. From the fitting of this curve using Eq. (1) with z = 0, we achieved a value equal to 1430 GM at 700 nm.
Although this value is similar to the one obtained by fitting a single Z-scan signature (Fig. 3a), as it takes into
account several 2PA curves carried out at distinct irradiances, it corresponds to a more accurate , which should
increase the scaling robustness.
- Annals of Optics
(a)
1.0
Normalized Transmittance (z=0)
Normalized Transmittance
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0.7
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-0.6 -0.3 0.0 0.3 0.6
2006
(b)
1.0
0.9
0.8
0.7
0 5 10 15 20 25
2
Irradiance (GW/cm )
z (cm)
Fig. 3: (a) The open-aperture Z-scan signature for MEH-PPV/chloroform solution at 700 nm; (b) Normalized
transmittance for an open-aperture Z-scan at z=0 as a function of irradiance at 700 nm. The lines represent the
theoretical fitting to the experimental data.
Figure 4 shows the degenerate 2PA cross-section spectrum for MEH-PPV obtained from scaling the
Tz=0 spectrum using the value at 700 nm obtained with the procedure formerly described. The figure also
exhibits, for comparison, the 2PA cross-section spectrum obtained with the single wavelength Z-scan using an
OPA source. The results obtained from the two light sources are in fairly good agreement, indicating the validity
of the approach based on WLC Z-scan technique. One clear advantage of this new procedure is that it is much
faster than the conventional Z-scan technique, due to the wavelength multiplexing introduced by the use of a
broadband source and a portable spectrometer.
4
e′′
3
δ (10 GM)
e′
3
e
2
g
1
550
600
650
700
750
800
850
Wavelength (nm)
Fig. 4: Degenerate two-photon absorption cross-section spectra obtained from WLC (solid line), discrete Z-scan
techniques (open circles) and theoretical model (dashed line). The inset exhibits the four-energy-level diagram
employed to describe the 2PA spectrum.
The 2PA spectrum, depicted in Fig. 4, shows a general trend of increasing the 2PA cross-section as the
wavelength moves toward the absorption band. Such behavior is related to the resonance enhancement of the
nonlinearity as the one-photon transition is approached. Furthermore, a 2PA peak around 675 nm can clearly be
seen through the WLC Z-scan data (solid line). Although such a peak can also be noted with the OPA data (open
circles), its presence could only be assured when the WLC Z-scan was employed due to the good spectral
resolution of this method. In this way, the WLC Z-scan technique allowed us to observe a new feature in the
2PA MEH-PPV spectrum, besides confirming the general trend reported previously.
In order to further understand this result, we can use a model where the 2PA cross-section dependence
on the excitation laser frequency, ν, is obtained theoretically from SOS model,[11] considering the four-energylevel diagram showed in the inset of Fig. 4. The δ (ν ) values can be written as:
δ (ν ) ∝
ν2
A1
A2
+
2
2
2
2
(ν eg −ν ) + Γeg (ν e′g − 2ν ) + Γe′g (ν e′′g − 2ν )2 + Γe2′′g
(2)
where νmn and Γmn represents, respectively, the transition energy and damping constant of the n→m transition.
In this equation, A1 = µ e´e 2 µ eg 2 Γe´ g and A2 = µ e′′e 2 µ eg 2 Γe′′g .
According to Eq. (2), the 2PA cross-section is determined by the 2PA lineshape functions (terms inside
XXIX ENFMC
- Annals of Optics
2006
the brackets) for two-photon transitions to the bands e′ (solid arrow in the inset of Fig. 4) and e ′′ (dashed
arrow in the inset of Fig. 4). As the laser frequency approaches the one-photon transition (intermediate state e ),
the 2PA tensor term, outside the brackets, increases its contribution resulting in the one-photon resonance
enhancement.
The dashed line in Fig. 4 represents the theoretical fitting obtained by using Eq. (2) with ν eg , ν e´ g and
ν e′′g around 490, 340 and 200 nm, respectively. As can be seen from the fitted curves in Fig. 4, there is a good
agreement between the experimental results and the model given by Eq. (2). For the damping constant we have
used Γeg ≈ 3000 cm-1, Γe´ g ≈ 900 cm-1 and Γe′′g ≈ 10000 cm-1 in accordance with the linewidths for the
corresponding transitions. The ν eg and Γeg values were taken from the π→π* absorption band, which is the onephoton allowed transition. As it is known, a transition that is one-photon forbidden cannot be two-photon
allowed for symmetric molecules since the parity of the electronic states of the molecule must be either even or
odd. Because MEH-PPV is a π-conjugated polymer with C2h symmetry, the electronic states identified from our
study are distinct from those observed approximately at 340 and 200 nm [12] in the linear absorption spectrum.
This assumption is in agreement with that reported by Frolov and co-workers [13] for the polymer poly(2,5dioctyloxy-p-phenylene vinylene) whose molecular structure is similar to the MEH-PPV, where the 2PA band
around 340 nm was attributed to the transition to an electronic state different from that they observed in the
linear absorption spectrum.
Conclusions
In summary, we have used the WLC Z-scan method and a scaling procedure to obtain the degenerate
2PA cross-section spectrum for MEH-PPV/chloroform solution from 590 nm up to 720 nm. The spectrum
achieved from this approach exhibits good agreement with that obtained employing the traditional single
wavelength source, although much faster and with a better spectral resolution. The 2PA cross-section spectrum
of MEH-PPV achieved from the WLC Z-scan method displays a slight 2PA peak around 675 nm and resonant
enhancement of the 2PA cross-section, which could be described by means of a sum over states model. The
achieved results point out the WLC Z-scan method as a powerful technique for the measurement of 2PA crosssection spectra of organic polymers with prospects in photonic applications.
Acknowledgement
Financial support from Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), Conselho Nacional
de Desenvolvimento Científico e Tecnológico (CNPq) and Coordenação de Aperfeiçoamento de Pessoal de
Nível Superior (CAPES) is gratefully acknowledged.
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