Pyroelectrical Investigation of Nonlinear Optical Polymers with Uniform

Chapter 22

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Pyroelectrical Investigation of Nonlinear Optical Polymers with Uniform or Patterned Dipole Orientation S. Bauer, S. Bauer-Gogonea, Ş. Ylmaz, W. Wirges, and R. Gerhard-Multhaupt Heinrich-Hertz-Institut für Nachrichtentechnik, Einsteinufer 37, D-10587 Berlin, Germany In this survey, poled nonlinear optical (NLO) polymers are discussed as polymer electrets with molecular dipoles. A range of old and new pol ing techniques are briefly introduced. The pyroelectricity of poled NLO polymers can be exploited for analyzing the relaxation of the oriented chromophore dipoles under isothermal or non-isothermal conditions and for probing the distribution of the dipole orientation in the plane and across the thickness of poledfilms.The available experimental tech niques and some typical results are briefly described. Advantages of pyroelectrical measurement techniques are their compactness, versatil ity, and ease of use. In view of possible waveguide applications of NLO polymers, two-layer systems withinverted-χ structures were prepared from one single NLO side-chain polymer by means of the combination of thermally assisted poling above the glass transition and photo-induced poling under reversedfieldbelow the glass transition as well as from two polymers with different glass-transition temperatures by thermally assisted poling at two different temperatures. The thermal stabilities of the resulting dipole-orientation patterns were studied by means of pyroelectric thermal analysis. (2)

Second-order nonlinear optical T

Corona

Electric Field be tween Electrodes

G

Electric Field from External or Local Surface Charges Heating to T > T

G

Use of Device Electrodes for Poling Poling in Spite of Film Defects

Electron Beam

Electric Field from External or Local Bulk Charges Heating to T > T

Selective Poling across Thickness

Photothermal

Electric Field from Local Heating with Various Sources Light to T > T

Patterned Poling with Continuous Electrodes

PhotoInduced

Electric Field from trans-cis Isomerization Selective Poling at Room Temperature Various Sources of Chromophores

G

G

Pyroelectricity in Poled Polymers The pyroelectric response of poled polymers is characterized by the so-called ex perimental pyroelectric coefficient, which is defined as the temperature derivative of the charge Q induced on the sample electrodes of area A upon heating (or cooling): Pexp = (l/A)dQ/dT. The pyroelectric effect of amorphous polymers results (1) from the dipole libration connected with thermal motion and (2) from the decreasing dipole density which is a consequence of thermal expansion. In poled polymers, dipole libration contributes only little to the overall pyroelectricity as compared to thermal expansion. Therefore, the experimental pyroelectric coefficient is directly related to the frozen-in polarization P [18, 19]: Pexp =

W

where a and are the relative temperature coefficient of thermal expansion and the unrelaxed relative dielectric permittivity of the sample material, respectively. The pyroelectrical investigation of poled polymers is based on this simple relation. x

Lindsay and Singer; Polymers for Second-Order Nonlinear Optics ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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\

T— G

307

Pyroelectrical Investigation of NLO Polymers

(150'C) (80*C)

(Dl

d)|

time

time

*

-Ep-"

(D

9 0 TAP

PIP

time

A 0

TQ1 TG2

TG2

unpolarized light

Figure 1. Preparation of inverted -structures by combining a "hot" thermally assisted poling (TAP) and a "cold"photo-induced poling PIP process (left) and in a double layer of two NLO polymers with different glass-transition temperatures by means of two TAP processes with opposite poling fields (right).

focusing lens

glass substrate modulated poling voltage $

=FtFfc

transparent electrode NLO polymer - metal electrode

Figure 2. Experimental setup for photothermal poling with a fixed light source, a switchable power supply, and a movable sample holder; the polymer sample is sup ported by a glass substrate with a transparent ITO electrode on the top and coated with an aluminium electrode at the bottom.


308

POLYMERS FOR SECOND-ORDER NONLINEAR OPTICS

Pyroelectrical Measurement Techniques In order to obtain its pyroelectric response, a sample must be thermally excited. This can be conveniently achieved with light whose energy is converted into heat by means of absorption. However, other excitations such as heating the polymer via convection [19] are also possible. Heating by absorption forms the basis of the so-called photopyroelectrical absorption spectroscopy (PPAS) [20, 21] for measuring weak absorption on thin films. The resulting temperature distribution inside the sample depends on the specific form of the thermal excitation and can be used for determining its thermal parameters. Pyroelectrical microcalorimetry which is based on this principle has been used to measure the thermal diffusivities [22] and the specific heat [23] of films with thicknesses in the /zm or suborn range. Its sensitivity is very high, as a sample mass of only a few fig is sufficient Table n . Pyroelectrical Techniques for Investigating N L O Polymers Measured Physical Properties

Absorption Coefficients

Experimental Technique (and Its Acronym)

Photo-Pyroelectric Absorption Spectroscopy (PPAS)

Specific Heat, Thermal Diffusivity Pyroelectric Microcalorimetry (PMC) Dipole Relaxation

Pyroelectric Thermal Analysis (PTA)

Dipole-Orientation Distribution in the Film Plane

Scanning Pyroelectric Microscopy (SPM)

Dipole-Orientation Profile across the Film Thickness

Pyroelectric Depth Profiling (PDP)

The essential feature of pyroelectrical measurements is the direct electrostatic re lation between dipole polarization and generated signal. It forms the basis for the recently introduced pyroelectrical thermal analysis [13] of dipole-relaxation processes in poled polymers as well as for the various microscopical probing techniques such as the scanning pyroelectrical microscope (SPM) of dipole-orientation patterns in the film plane [24] or the thermal-wave depth profiling of dipole-orientation profiles across thefilmthickness [25]. Since the pyroelectric signal is a linear function of the polarization P, not only the magnitude, but also the direction (up or down) of the dipole orientation can be obtained. The physical quantities that can be measured with pyroelectrical techniques as well as the relevant methods are summarized in Table n. Sample Materials and Preparation The following experiments were performed on the NLO polymers schematically shown in Fig. 3. Fig. 3 (left) depicts a styrene-maleic anhydride copolymer with chemically attached side groups of the azo dye Disperse Red 1 (DR1) (glass-transition tempera ture of 140°C, 60 % weight DR 1, electro-optic coefficient of 15 pm/V at a wavelength


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309

of 1.5/xm with a poling field of around 100V//im); details of its chemical synthe sis can be found in [26]. Fig. 3 (right) shows the chemical structure of the crosslinkable polymer Red-Acid Magly, where thermal cross-linking is possible between a carboxylic-acid functional group (COOH) located on the nonlinear chromophore and an epoxy side group of the polymer, while a chemical reaction between two chromophores is not possible (Liang, J.; Levenson, R.; Rossier, C; Toussaere, E.; Zyss, J.; Rousseau, A.; Boutevin, B.; Foil, E; Bose, D. 7. de Physique, in press.). For the following experiments, approximately 1-5/zm thick films were prepared from suitable solutions of all three polymers by means of spin-coating onto ITO-coated glass substrates. After vacuum evaporation of a top aluminium or gold electrode, the polymer films were poled under an electricfieldof around 50 V//im. Experimental Setup and Typical Results Experimental Setup. The compact and easy-to-use experimental arrangement for pyroelectrical measurements on poled polymers is schematically shown in Fig. 4. It consists of a laser-diode assembly including all electronics necessary for intensity modulation with a waveform generator, the electroded polymer sample and a lock-in amplifier for measuring the pyroelectric signals. Further details are found in previous publications [13, 25]. Isothermal Relaxation of the Dipole Orientation. As the pyroelectric signal is di rectly proportional to the electric polarization, pyroelectrical measurements are ideally suited for the investigation of dipole-relaxation processes. In a recent theoretical and experimental study, it was demonstrated that the relaxation behaviors of the electric polarization and of the pyroelectric and electro-optic responses are almost identical, so that one experimental technique is usually sufficient for characterizing polymer electrets (Bauer, S.; Ren, W.; Yilmaz, S.; Wirges, W.; Gerhard-Multhaupt, R. Nonlin ear Optics, in press.). Fig. 5 shows the isothermal decay of the pyroelectric response — and thus also the dipole orientation — in the side-chain polymer of Fig. 3 (left). Fitting curves according to the stretched-exponential or Kohlrausch-Williams-Watts function H t ) = e M -

7

f

F

)

f

(2)

are also included in Fig. 5. In the equation, r(T) is the temperature dependent mean relaxationtimeand 0 is the stretching parameter. The depicted isothermal decay curves at three different temperatures can be reasonably well fitted with stretchedexponential functions which have the same stretching parameter /? [27]. A different behavior is found for the cross-linkable polymer of Fig. 3 (right). Fig. 6 shows isothermal decay measurements at 90° C after different times of thermal crosslinking at 130°C. From Fig. 6, the enhanced stability after extended cross-linking is obvious. The experiments clearly indicate the presence of two types of dipoles in the cross-linked polymer: Already cross-linked (and thus thermally stable) dipoles and not yet cross-linked (and thus thermally unstable) dipoles [27]. The included fitting curves thus consist of superpositions of two relaxation processes according to


310


N0

2

Figure 3. Chemical structures of the side-chain (left) and the cross-linkable (right) polymer.

waveform generator

laser diode

lock-in amplifier

Figure 4. Experimental setup for pyroelectrical investigations.


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= ^expl-it/r^]

+ * exp[-(t/T ) ], 2

a

A

311 (3)

where $! and $ are the respective fractions of not, cross-linked and cross-linked dipoles, and TI and r are the respective mean relaxation times. For the fast relax ation process of the not cross-linked dipoles, it is possible to derive the parameters Ti and Pu while for the slow relaxation process, the time T is much larger than the time window of our experiment The stretching parameter and the mean relaxation time for the fast, not cross-linked dipoles are comparable to the relaxation time for a similar DR1/PMMA guest-host polymer, which is to be expected because of their similar structural configuration. 2

2

2

Relaxation Behavior after Physical Aging. The thermal stability of the dipole orientation in glassy polymers can be strongly enhanced by aging under an applied electricfieldat temperatures below the respective glass-transition temperature. Fig. 7 shows isothermal relaxations of the pyroelectric response after different times of ag ing. Films of the side-chain polymer according to Fig. 3 (left) were poled under an electric field of approximately 75V//xm at 155°C for 10 minutes and subsequently quenched to 130°C within a few seconds. Immediately after quenching, a relatively steep decrease of the dipole orientation was observed (x x x in Fig. 7). As seen in Fig. 7, isothermal aging under the polingfieldfor time periods of 10 (o o o), 100 ( • • • ) , 1000 ( A A A ) , and 4000 (O O O) minutes yielded more and more stable dipole orientations. It is interesting to note that the unaged sample (x x x ) exhibits better stability at long times (beyond 100 minutes) than the sample aged for 10 min utes. Such a crossover with the isothermal relaxation of the unaged sample can also be expected for the longer aged samples at correspondingly later times (not shown here). These observations can be explained by the narrowing of the relaxation-time distributions during aging, which occurs not only at short relaxation times (where it improves the stability), but also at very long times. Work is in progress in order to further clarify this phenomenon which is of considerable practical interest Pyroelectric Thermal Analysis (PTA). While it is rather time-consuming to collect the necessary relaxation data in isothermal experiments, non-isothermal techniques allow for a relatively fast investigation of the relevant dipole-relaxation processes. During PTA, the polymer is heated at a constant rate, while the pyroelectric response is recorded. As shown in detail in an earlier publication [11, 13], the pyroelectric response during PTA is — to a good approximation — proportional to the frozen-in polarization given by rT

AT

n

P(T) = P exp[-(hj ^n 0

(4)

To7

where P is the frozen-in polarization at temperature T and h is the inverse heating rate. Fig. 8 shows a comparison of the PTA responses after different poling procedures (PIP at 40 and 80°C and TAP) together with theoretical fitting curves according to eq.(4). All the experiments were performed with a heating rate of 5°Omin. From Fig. 8, it is obvious that PIP yields a lower thermal stability of the dipole orientation than TAP [11]. PTA responses similar to that of 40°C PIP-processed samples were 0

0


312


0

20

40 time (min)

Figure 5. Isothermal relaxation of the pyroelectric response, together with fitting curves and parameters based on the stretched-exponential function. p=0.35, T=llmin

T=90°C

56% cross-linked dipoles

20% cross-linked dipoles 60

90 120 time (min)

150

180

Figure 6. Isothermal relaxation at 9 0 ° C of the pyroelectric response after different times of cross-linking at 130°, together withfittingcurves and parameters as above.

^

1 0.8

6

I °-

11

S §0.4

o u -S2 0.2 c

Pyroelectrical Investigation of Nonlinear Optical Polymers with Uniform

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