Anisotropic Organic Materials - American Chemical Society

Chapter 1

Characterization of NLO-Materials for Photonic Application S. Grossmann, T . Weyrauch, S. Saal, and W . Haase

Downloaded by FREIE UNIV BERLIN on June 30, 2015 | http://pubs.acs.org Publication Date: November 2, 2001 | doi: 10.1021/bk-2001-0798.ch001

Institute of Physical Chemistry, Darmstadt University of Technology, 64287 Darmstadt, Germany

The use o f electroabsorption ( E A ) spectroscopy and pyroelectric investigations by the laser intensity modulation method ( L I M M ) for the characterization o f nonlinear optical ( N L O ) polymer systems is demonstrated. EA spectroscopy may be used to evaluate basic parameters o f NLO chromophores and their polar order. The influence o f the rotational mobility o f the chromophores on the EA spectra and the application o f this effect to characterize the relaxation o f the chromophores is shown. EA spectroscopy may be applied to determine in situ the electric field in the electrooptic active layer o f multi layer polymer stacks. Pyroelectric LIMM measurements are applied to characterize the polarization profile in double layer polymer systems.

Introduction Second order nonlinear optical ( N L O ) polymers are promising materials for a large variety o f integrated optical devices such as frequency converters, switches and electro-optic ( E O ) modulators (2,2). A lot o f physical properties are responsible for the working efficiency of photonic elements. The important properties may be assigned to three major tasks: (i) Optimization o f molecular properties, (ii) improvement of the polar orientation, which must be induced to the material, and its thermal and temporal stability, (Hi) integration o f materials into device structures. In order to accomplish tasks (i) and (ii) several experimental methods have been used and developed during the last 15 years (2). E . g. electric field induced second harmonic generation ( E F I S H ) and Hyper-Rayleigh scattering ( H R S ) are suitable for the characterization o f molecular nonlinear optical susceptibilities. Besides N L O methods such as second harmonic generation ( S H G ) and Pockels effect, a series o f methods sensitive to polar properties have been applied for the study o f the poling process and the poling stability. Here piezoelectricity and pyroelectricity as well as thermally stimulated depolarization techniques should be mentioned. The

2

© 2002 A m e r i c a n C h e m i c a l Society

In Anisotropic Organic Materials; Glaser, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.


3 experimental methods for the work on task (Hi) depend sensitively on the specific device and its requirement on the materials, e. g. the possibility to form 3-dimensional waveguide structures. Another point is the behavior o f polymeric sandwich structures in electric field, a problem which is relevant for the poling procedure and during operation o f different devices such as modulators based on Mach-Zehnder interferometers or polymeric light emitting diodes. A method not mentioned in this paper until now is the electroabsorption ( E A ) or Stark spectroscopy, i . e. the measurement of absorbance changes of N L O chromophores in an electric field. One aim o f this paper is to show the various possibilities o f this multifunctional technique in order to get information with regard to all three tasks (i-iii) with comparable low experimental expenditure. It w i l l be shown, that molecular parameters, polar order parameters and the study o f voltage distributions in multilayer polymer films all are possible with one experimental setup only. The study o f multilayer films is one aspect o f the more general task of characterizing polarization profiles in poled polymers. Therefore the laser induced pressure pulse ( L I P P ) method or the laser intensity modulation method ( L I M M ) may be used. W e used L I M M because the method is definitely non-destructive and the experimental setup (using a lock-in technique similar to E A spectroscopy with high signal-to-noise ratio) is much simpler in comparison to the L I P P method. However, the evaluation o f polarization profiles has some peculiarities, which w i l l be discussed within this paper.

Electroabsorption spectroscopy Background Orientationally Under

Fixed

the

Chromophores

assumption

o f an

orientationally

fixed

chromophore

molecule

characterized by two energy levels (the energy o f the electronic ground state and the lowest excited state) the electroabsorption ( i . e. the change o f the absorbance band under the action o f an external electric field) is due to the Stark effect. The absorption band

A(y)

is shifted according to the differences

o f dipole moments

Αμ and

polarizabilities Δ α in the ground and the excited state. Experimentally one usually measures the effective value o f the change in absorbance

M (co) eff

at fixed wave

number ν under application o f an ac electric field Ε ,β(ω) with frequency ω by lock-in (

technique. The quadratic effect (effective value o f change in absorbance measured at the frequency 2 ω ) is given by KA9 ro \ « W Mir(2ώ) = —^= S £

ff

Δ

α

2

2

- â(A ν) (Αμ) „ â (A/v) —ν „ ' + y . r — ^ 10AV âv

m

κ

h

c

d

v

2

(1) 1


4 for the case o f an isotropic or weak polar sample (3). The quadratic effect may also be measured at the frequency ω i f a dc field EQ is applied simultaneously. Thus Δα

~ â(A/v)

he

âv

2

+

2

(Αμ)

2

10/i c

~â (A/v) 2

2

âv

(2)

this may be referred to as the quasilinear effect. If the sample has a polar average orientation o f the relevant chromophore the linear Stark effect 2

:cosffsin 0>


< sin" Θ >

ΑμΕ^{ω) ~â{Alv) âv

he

(3)

w i l l also be measured at the modulation frequency a. Thus the measurable signal is in general a superposition o f the linear and the quasilinear effect. However they are distinguishable due to the different spectral behavior o f the linear effect in comparison to the quasilinear effect. Orientationally Mobile Chromophores If the N L O chromophore in the sample has an electric dipole moment and is free to reorient, the application o f the modulation voltage in the electroabsorption experiment w i l l vary the orientation o f this molecule and thus the absorption o f the sample. The most important effect is the change o f the average angle between the transition dipole moment and the electric field vector o f light. F o r the rod like molecules under investigation the direction o f the transition dipole moment and the electric dipole moment can to be considered to be parallel to the molecular long axis. Because the molecules tend to be parallel to the modulation field the absorption w i l l be lowest in case o f highest field (independend of the direction o f the field). Thus an additional orientalional contribution to the electroabsorption signal at 2 ω with ι

2

UiE/kr)

w i l l be expected for mobile chromophores in the case o f the quadratic effect (eq 1). The term τ](Ί)(υ) describes the magnitude o f the orientational mobility: In case o f maximum reorientation o f the chromophores, i . e. η(Τ,α>) = 1, the contribution (eq 4) is limited by the Boltzmann distribution. If the chromophores are fixed in space the term 7](T,cd) becomes zero.

Experiment The electroabsorption was measured using a spectrometer (Figure 1) based on a monochromator ( T O P A G L M - 0 1 ) and a 100 W halogen lamp as light source. The ac


5 and de part o f the intensity transmitted through the sample ( Δ / and 7, respectively) were measured by a photomultiplier, a lock-in amplifier ( P A R 5210) and a digital multimeter ( H P - 3 4 4 0 1 A ) . The absorbance change was calculated from these measurements according to


ΔΑ = - Δ / / ( / 1 η 1 0 ) .

(5)

Figure 1. Scheme of the setup for the measurement of electroabsorption

spectra.

Applications of Electroabsorption Spectroscopy Electroabsorption spectroscopy offers the possibility to investigate properties, which are important for the optimization o f N L O materials. Basic Physical

different

Properties

The first important step in optimization o f N L O properties is to select the used chromophores.

The

quadratic

or

quasilinear Stark

effect

allow

to

determine

fundamental molecular properties of the chromophores according to the following procedure: From measurements o f the absorption spectra A(v)

the derivatives as

given in eqs. 1 and 2 are calculated. These derivative spectra are used to fit eqs 1 and 2 to the experimental data. Here

AJLI

and Δ α are independent fit parameters because o f

the different wavenumber dependencies o f the related derivative spectra. After fitting, with the knowledge o f those parameters, the hyperpolarizability o f the chromophore can be estimated according to the 2-level model (4). In order to use χ

( 2 )

effects like S H G a polar orientation o f the chromophores must

be present. Therefore, the poling process tends to be the most important step during the preparation o f polymeric systems for photonic applications. In those polar media the linear in field Stark effect is a useful method to investigate the poling efficiency. After determination o f Δμ (using the quadratic effect) it is possible to calculate the 2

2

first polar order parameter < c o s 0 s i n 0 > / < s i n 0 > (where Θ is the angle between the polar

axis and

the

molecular dipole) by

fitting

eq.

3

to

the

experimental


6 electrobsoprtion spectra o f the linear effect. One advantage o f this method is that uncertain parameters such as local field correction factors are cancelled out, because both, Δ μ and /, are determined by the same experimental method and for the same sample (5,6).


2

2

Multilayer Polymer Stacks A s i d e from the characterization o f basic physical properties the E A spectroscopy is also a tool for the investigation o f internal field distributions, especially in case o f multilayer polymer stacks. D u e to the different electrical properties o f the different layers, application o f a voltage to a multilayer system does not produce a uniform electric field i n the system, moreover the distribution has been shown to be time dependent (dc drift phenomenon) (7,8). N o direct method exists for the selective measurement o f the field (9), but it is possible to determine the electric field in the E O active layer v i a the N L O properties o f this layer. However, during application o f a dc electric field the apparent N L O properties are a sum o f second order ( χ ) and third order ( χ ) effects. W e have shown recently that electroabsorption spectroscopy offers the possibility to overcome this problem (10): Both, χ and χ effects, contribute to the E A spectra with different wavelength dependencies, both vanish at different wavelengths within the absorption band. Choosing one o f these wavelengths in the electroabsorption experiment one o f these contributions can be measured selectively. A p p l y i n g this technique to a double layer system with one inactive layer, we could determine the transient behavior o f the electric field in the E O active layer during switching on or off a dc electric field. The behavior could be understood in terms o f a model considering the resistances and capacities o f both layers. However, also single layer samples show a dc drift phenomenon, which gives evidence for formation o f screening charges at the interfaces between electrodes and polymer (10,11,12). ( 2 )

( 3 )

( 2 )

( 3 )

Orientational

Relaxation

of

Chromophores

A s described above, in the case o f orientationally mobile chromophores the E A spectra o f the quadratic effect are different from that case where the chromophores dont reorient in the electric field. The most important change is described by the Η

orientational contribution ΑΑ°™ (2ω)

to the electroabsorption signal, which has a

spectral dependency proportional to the absorbance (in contrast to the linear or the quadratic effect) and is thus well distinguishable from other contributions. A n a l y s i n g the experimental E A spectra by using a fitting procedure gives the value o f η ( Γ , ω ) and thus information about the reorientation mobility o f the chromophores. D o i n g electroabsorption experiments at various modulation frequencies and at different temperatures the study o f the relaxation process o f the chromophores is possible (13). The investigated samples possess a capacitor like structure. T h i n films o f dye doped polymer solution were spin coated onto I T O covered glass substrates. A polymeric matrix poly(methyl-methacrylate) ( P M M A ) was used. The azo dye 4 [ethyl(hydroxy-ethyl)-amino]4-nitroazobenzene ( D R 1 ) was used as electrooptic active dopand. The dye concentration was 5 wt-%. The top electrode was a transparent gold layer sputtered onto the polymer film. T y p i c a l thickness o f those samples was 1.7 μτη.


7


T o demonstrate the difference o f E A spectra o f mobile and immobile systems the quadratic effect was measured with two different modulation frequencies (2 H z and 2 k H z ) at the same temperature (Figure 2). A t 2 k H z , the fitting procedures gives a value o f η ( Γ , ω ) « 0, i . e. the chromophores cannot follow the electric field and are immobile at this time scale. A t 2 H z however, the shape o f the experimental spectrum has changed. T h e fitting procedure gives a significantly higher value o f η ( Γ , ω ) , indicating a certain mobility o f the chromophores at the frequency of 2 H z , but η ( Γ , ω ) doesn't reach the theoretical maximum. The η ( Γ , ω ) values determined from E A measurements at various modulation frequencies at a fixed temperature allow to calculate the relaxation frequency o f the chromophores, e. g. by fitting a C o l e - C o l e function to the experimental data. Figure 3 shows the result for two temperatures. The increase o f the relaxation frequency with increasing temperature is observable. A more detailed study on various systems is presented in (13,14,15). It should be noted, that in comparison to other methods (e. g . dielectric relaxation spectroscopy), the application o f E A spectroscopy is sensitive to the chromophores only and not to the matrix material, which allows for a comparison of matrix and chromophore behavior. A l s o measurements are possible even at very low dye concentrations.

Anisotropic Organic Materials - American Chemical Society

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