Polymers for Second-Order Nonlinear Optics - American Chemical

Chapter 19

Photo-Induced Poling of Polar Azo Dyes in Polymeric Films Toward Second-Order Applications Downloaded by NORTH CAROLINA STATE UNIV on October 14, 2012 | http://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch019

1

1

1,2

Z. Sekkat , E. F. Aust , and W. Knoll 1

2

Max-Planck-Institut für Polymerforschung, Ackermannweg 10, 55021 Mainz, Germany Frontier Research Program, Institute of Physical and Chemical Research (RIKEN), Wako, Saitama, 351-01 Japan

The application of a DC field across a polymer film containing polar azo dye chromophores, at a temperature far below that of its glass transition, leads to an appreciable polar order when the azo dyes undergo eis trans isomerization. This paper reports on room temperature photo-electro poling or photo-induced poling (PIP) of Disperse Red 1 (DR1) molecules in different polymeric environments. We compare the effect of PIP when DR1 molecules are chemically linked to flexible or to rigid main chains polymer, and we discuss the ability of the polymer main chains to move with the photoinduced movement of the azo chromophores they are covalently linked to. The evolution of the polar orientation during PIP is probed by the electro-optic Pockels effect using an Attenuated Total Reflection (ATR) setup. The experimental findings are discussed within the framework of our phenomenological theory of the PIP process. This is based on the enhanced mobility of the azo chromophres during the isomerization process, and allow a real physical insight into the phenomena which appear on PIP.

Much interest has been focused on organic materials for nonlinear optics since Davydov et al. (7) established the correlation between enhanced nonlinear activity and charge transfer character in conjugated molecules. This interest was driven by needs in the field of optical signal processing and telecommunications (2). For second order processes, e.g. second harmonic generation and electro-optical modulation, films of poled molecular polymeric materials take advantage of the high optical quality of polymer glasses and allow the use of organic molecules presenting high molecular optical response (hyperpolarizability) (3). However, centrosymmetry or misorientation of the nonlinear molecules may reduce or cancel the second order coefficient of the bulk material. Orienting polar molecules with a large microscopic second order polarizability within the polymer host, is therefore a necessary step for creating second order susceptibility. This is usually performed by DC electric field poling, in which the temperature of the polymer is elevated above its glass transition temperature so that the nonlinear molecular units are free to rotate in the presence of an external DC electric field (4). On cooling through the phase transition to room temperature, the orientational order of the molecules is preserved after removing the orienting field. Poling at room 0097-6156y95/0601-0255$12.00A) © 1995 American Chemical Society In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

Downloaded by NORTH CAROLINA STATE UNIV on October 14, 2012 | http://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch019

256

POLYMERS FOR SECOND-ORDER NONLINEAR OPTICS

temperature using CO2 as a swelling agent was reported by Barry and Soane (5). A new method of poling polymeric films containing photoisomerizable polar azo dyes at room temperature, was reported recently (6-10). By applying a DC electric field at room temperature during the photoisomerization process, the centrosymmetry is broken and an important and stable electro-optic Pockels and second-harmonic signals have been obtained using a polymer film of poly(methylmethacrylate) (PMMA) functionalized by Disperse-Red 1 (DR1) chromophores (6,8,9). The photoisomerization reaction allows the polar azo dye molecules a certain mobility, thus making polar orientation in the direction of the DC poling electric field possible. In contrast to flexible main chain polymers, PIP of rigid main chain polymers would be of a particular interest since it may provide valuable information about the capability of the polymer main chains to hinder the movement of the azo dye chromophores induced by the photoisomerization reaction. In the next section we recal briefly some aspects of the isomerization reaction in polymeric environment. The third section reports on PIP and TP of a new rigid main chain polyester and compares the experimental findings with PIP of a PMMA-DR1 copolymer. The evolution of the induced polar order during PIP is probed with the electro-optic Pockels effect obtained by using an ATR setup. The fourth section presents the general equations of our phenomenological theory of the PIP process, and describes the transient properties of the photo-induced polar order. The conclusion and some remarks on the study are given in die last section. CisTrans isomerization The isomerization reaction of azobenzene derivatives is a light or heat induced interconversion of their cis and trans geometric isomers (Figure la). The thermal isomerization proceeds generally in the "cis =>trans" direction because the trans form is generally more stable than the cis form (50 KJ.mol"! in the case of azobenzenes (11)), light induces both "trans =>cis" and "cis trans" isomerizations. When the isomeric forms absorb a photon they are raised to electronically exited states from which a nonradiative decay brings them back to the ground state either in the trans form or in the cis form. Once the molecule is in the cis form it recorvers the trans form either by the back thermal reaction or the inverse photoisomerization cycle. Figure lb shows a simplified model of the molecular states. The photoisomerization of the push-pull azobenzene derivative DR1 occurs efficiently in PMMA thin films at room temperature (12), the quantum yields of the direct and reverse photoisomerizations have been measured and the absorption spectra for the thermally unstable cis form has been deduced (12). As classified by Rau (77), the DR1 molecule is a pseudo-stilbene type molecule which means that the high energy 7C-7C* transition is overlapping the low energy n-n* transition and leads to a large structureless band in the trans isomer with an absorption maximum strongly dependent on the polarity of the host material which may be either a polymer or a solvent. Frequently the geometrical change of the azo molecules during the isomerization process leads to a loss of their initial orientation, then anisotropy can be induced in a system containing anisometrically shaped photosensitive molecules which are known to align perpendicular to the polarization of the irradiating light. The resulting anisotropy depends on the viscosity of the surrounding medium and can be negligible in low viscosity solutions where the reorientationtimeof the molecules is very short and allows the thermal agitation to restore instantaneously the isotropy destroyed by the light. In azo dye doped polymeric films where the mobility of the guest molecules is appreciable, photoisomerization leads to reversible polarization holography (75). In azo dye functionalized polymeric films where the mobility of the azo chromophores is very much reduced, the photoisomerization creates a permanent alignment which may lead to writing erasing optical memories (14,15) or to permanent second order nonlinearoptical effects at room temperature in the presence of a poling DC electric field (6-10).

In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


19.

SEKKAT ET AL.

Photo-Induced Poling ofPolar Azo Dyes

N0

N0

2

257

2

Figure 1. (a) Trans cis isomerization of azobenzenes. (b) Simplified model of the molecular states. Only two excited states have been represented, but each of them may represent a set of actual levels (77): we only assume that the lifetime of all these levels is very short. a> and are the cross sections for absorption of one photon by a molecule in the trans or the cis state, respectively, y is the thermal relaxation rate. ® and O are the quantum yields of photoisomerization. (c) structure of the polymer. t

c t

t e


258



Photo-Electro Poling Experimental

The structure of the DR1-polyester copolymer which we have used for this srudy is shown in Figure lc. The intrinsic viscosity of this polymer obtained from 1,1,2,2tetrachloroethane (TCE) solution at 25 °C is 0.47 dl/g. Details of the polymer synthesis and characterization have been described elsewhere (16). The glass transition temperature (Tg) obtained from differential scanning calorimetry (DSC) at the heating rate of 10 °C/min was 42 °C. This polymer is a random copolymer which means that the terephthaloyl and the 2,5-dihexadecylterephthaloyl comonomers are randomly distributed in the main chain. This random distribution of the comonomers can produce large amount of free volume in the polymer matrix (17-19). Moreover the inclusion of the kink resorcinol unit in the main chain may hinder the packing of the polymer chains in solid state (20). Both of these reasons may cause the low Tg of the present polymer. For the electro-optic measurements by the ATR method, a 2 to 3 nm thick layer of chromium was evaporated on the cleaned glass substrate followed by 30 to 50 nm thick layer of gold as a lower electrode. Polymer films (1-2.5/im) were spin-cast on the gold layer. These films were dried at 65 °C for 30 to 50 hours using a vacuum oven to remove the last traces of solvent from the films. Finally, an upper gold layer of 100 nm thickness was evaporated on the polymeric film. The gold layers act as electrodes. The glass slide was put in optical contact with a prism through which a HeNe (632.8 nm) laser was reflected onto the sample. The power of the HeNe laser was reduced to few microwatts in order to avoid any absorption from the sample and our ATR and electro optic modulation spectra showed perfect shapes (at 632.8 nm the absorption of the DR1 molecules is almost negligible as shown by the UV-VIS aborption spectra in Figure 2, but the waveguiding resonance condition enhances the energy inside the polymer film and may cause some absorption from the sample if the incident intensity is not enough reduced (27)). The waveguiding resonance condition is given by: k

zm

d

+

*1

+

^2

=

m

K

(D

where m is the order of the mode, k is the normal component of the wavevector of mode m, d is the film thickness, and and *F ^ phase shifts at reflection on the inner faces of the waveguide. They are given by Fresnel formulae, and depend on light polarization. The prism and the detector were placed on a 0-20 goniometer and the reflectivity of the HeNe probe beam with Transverse Electric (TE or s) and Transverse Magnetic (TM or p) polarizations was studied for incidence angles ranging from 20° to 82° (see Figure 3a). Some of these fulfill the waveguiding resonance condition (equation 1) and dips occur in the reflectivity corresponding to TM (p) and TE (s) guided modes in the polymer film (Figure 3b). The experimental reflectivity curves are fitted by a computer programm which calculates theoretical reflectivities of layered media and gives, with a very good accuracy, the thickness and the indices of refraction n , ny and n in the three principal directions of the polymer film, x and y correspond to the principal directions in the plane of the film and z is the normal to the sample. When an external perturbation is applied to the polymer film, the ATR guided modes shift their angular positions and the reflectivity is modulated (Figure 3c). These angular shifts are very small in the case of electro-optic experiments: they correspond to refractive index variations of the order of 10"*. One has then to modulate the measuring electric field at a low frequency Q (E = EicosQt) and to detect the modulated signal with lock-in amplifiers. The lock-in signals detected at the modulation m

t

n

e

2

x

z



19.

SEKKATETAL.

259


300

400

500

600

700

Wavelength(nm) Figure 2. UV-Visible spectra of (a) DR1 compound and (b) polymer in TCE solution.

Figure 3. (a) The ATR experimental setup and (b) an example of the record of the reflectivity and (c) electro-optic modulation Continued on next page




260

» » —I

30

1

1

30 I

1

40

32

34

I

I

50 e

60

I •

I

70

[deg]

Figure 3. Continued


19.

SEKKAT ET AL.

261

Photo-Induced Poling of Polar Azo Dyes

frequency and its second harmonic give respectively the linear (or Pockels) and the quadratic (or Kerr) electro-optic effects. The amplitude of the modulation of the thickness and the refractive indices is evaluated by a computer fit, and allows the determination of Pockels (r) and Kerr (s) coefficients. These are related to the electro optic susceptibilities by the following formulae (equations 2): 2%fe ® = - r

iz

(3)


IXiizz

nf = 2niAni(0) (d I V\)

= " Hz i n

A

=

4n/An/(2fl) (d I Vi)

(2a) (2b)

2

The factors 2 and 3 in the left hand side of, respectively, equations 2a and 2b refer to the number of permutations of the electric fields in the definition of the macroscopic polarizability when Kleinmann symmetry holds true. Sometimes these factors are included in the definitions of the nonlinear susceptibilities (2). The factor 4 in the right hand side of equation 2b originates from cos Qt (see also equation 3) assuming the general definition for the electro-optic Kerr susceptibility (e.g. 3%()Ei 2«An). Vi is the voltage corresponding to Ei, and An-(Q) and An/(2Q) are the index variations measured from the Q and 2Q lock-in modulated signals, respectively and i refers to x, y and z. The piezoelectric effect and the electrostriction are estimated from the thickness modulation (22), and their contribution to the index variations is almost negligible in our material. For PIP experiments the sample is irradiated by the green light of a frequency doubled laser diode (532nm, at this wavelenght the absorption cross section allows a good penetration in the sample) through the prism and the gold layer as shown in Figure 3a. The power of the pump beam incident on the sample is estimated to be 70 mW/cm^, taking into account the reflection on the gold sample. The variations of during PIP are followed by setting the goniometer at a fixed angle of incidence corresponding to the TM mode at high incidence angles, which provides a high sensitivity to the n refractive index variations, and recording the signal modulated at Q from the lock-in amplifier. When the sample is irradiated the mode shifts its angular position and must be followed manually. This is not a very precise procedure but it allows valuable information about the dynamics of the PIP process to be obtained. The recording of the whole set of TM and TE modes at a stationary state of the pumping or of the relaxation process gives a precise measurement of the electro-optic susceptibilities and the higher the number of the modes the better the precision (23). Figures 4 and 5 show the evolution of the electro-optic Pockels coefficient ^ of our polymeric film (thickness -1.8 pm) during typical PIP cycles at room temperature. These curves were obtained as explained before by recording continuously the lock-in signal modulated at Q near the largest incidence TM mode which gives the direct measurment of An . When the measuring AC (E\) and poling DC (EQ) electric fields are applied simultaneously to the polymer film, the An (z is equivalent to 3 which refers to the symmetry axis of our system) variation is given by (equation 3): 2

3

2

=

2

z

z

z

2n An (£l) z

z

=

2x

( ) ( E + EôsQt) + 35C3333 ( ) ( E + EiCOsQt) 2

333

= (2X333

3

0

(2)

2

0

(3)

+ 6%3333 Eo) EiCOsOt 3

2

+ (3/2)X3333 (> E j cos2Qt + unmodulated terms.


(3)

262



Figures 4-5. PIP of a DRl-polyester 1.8 pm thick film, with (4) 40V and (5) 70V as a poling voltage. The moments of turning this latter and the pumping light (70 mW/cm ) on and off are indicated by arrows. The horizontal line gives the estimated value of the Kerr effect contribution (2E S ) to the apparent Pockels effect. The signal above this line characterizes a pure polar orientation of the azo chromophores. 2

0

33


19.

SEKKAT ET AL.

263

Photo-Induced Poling of Polar Azo Dyes

In this case, the apparent r electro-optic coefficient given by the signal modulated at Q contains a contribution from the electro-optic Kerr effect given by %(). This can be measured easily using the signal modulated at 2Q. At the time t=0 where the sample is centrosymmetric (%W = 0), the DC poling field E is applied; then the Pockels effect appears rapidly and grows slowly in the long time range. The contribution of the Kerr effect 2E S (3% ( ) = 110.10" m V~ ) to the apparent Pockels coefficient is indicated by the horizontal line in Figures 4 and 5, and shows the existence of a large and appreciable pure %() signal above this line resultingfroma polar orientation of the nonlinear-optical azo chromophores in the direction of the poling DC field. This is the Electric Field Induced Pockels Effect (EFIPE) which is the analogue of the Electric Field Induced Second Harmonic (EFISH). This indicates that the mobility of the majority of the chromophores is significant even at room temperature ("free molecules"), since they can rotate under the action of the DC poling field alone without heating. The onset and the decay of the EFIPE is fast in the short time range and indicates that the mobility of the "free molecules" is significant at room temperature. The slow effect observed at long times originates from molecules situated in a more viscous environment ("non free" molecules). When the sample is illuminated with a pump beam, the electro-optic Pockels signal increases strongly. When the pump beam is switched off in the presence of the DC field, the %W signal decreases slightly and keeps an r value higher than that obtained with the DC field alone. When this is removed the electro-optic Pockels signal drops rapidly but keeps an appreciable r value corresponding to 1.65 pm.V"l. Using the theoretical expression of the first odd order parameter Aj at the steady-state of the EFIPE, one can compare the experimetal results with Maxwell-Boltzmann statistics (24). The PIP cycle performed on our copolymer shows a quite similar behavior to that obtained with DR1 doped PMMA thin films (7) (see Figure 6) and shows that the photoisomerization process allows a certain mobility to the "non free" molecules since they can experience a polar orientation with the simultaneous application of the DC poling field. This can be seen as an enhancement of the mobility of the azo chromophores during the isomerization reaction and can be described theoretically as we will discuss later. Another important point in these experiments is that the photoinduced polar order obtained with 70V as a poling voltage is at least two times smaller than that obtained with 40V as a poling voltage. This indicates that a certain fraction of the azo chromophores which can not rotate under 40V can rotate under the action of the 70V poling voltage, and demonstrates that the isomerization process lowers the threshold for the viscous polar orientation in the direction of the poling field. This change of the apparent viscosity during the photoisomerization reaction can be introduced formally in a phenomenological theory as we will show in the next Section. Next we discuss the thermal poling results and compare them with the PIP experiments. We poled a 2.4 pm thick film of the copolymer following the TP procedure at 60 °C with 79V/pm poling voltage. The 133 electro-optic Pockel coefficient obtained just after the TP was 15.5 pm.V~l and is much higher than the ^ 3 coefficient obtained just after PIP when the DC field is removed (1.65 pm.V"l). The relaxation process after TP shows the behavior reported in Figure 7 where we can see that 30 minutes after removing the poling DC field, only 10% of the initial signal obtained just after removing the DCfield(15.5 pm.V"!) disappears. In the case of PIP, the majority of the signal drops rapidly when the DC field is removed. Keeping in mind that the copolymer used in this study has rigid main chains, the data suggests that the thermal poling process performed above Tg of the polymer affects the conformation of the main chains and thus can change their orientation, but the PIP affected only the orientation of the azo chromophores through the free volume of the polymer matrix 33

3

0

3

0

33

21

2

2

3333


2

33

33



264


u.u-i0 1

o

1 20

1 40

, 60

1

1

1

10

20

30

, 80

—, 100 mn 1

40 time(mn)

Figure 6. Experimental curve showing the evolution of the Pockels coefficient of a DR1 functionalized PMMAfilmfrom Attenuated Total Reflection electro-optic modulation. The sudden jump up and down of the signal when the DC field is switched on and off originates from the third order nonlinearity %(). The inset shows a similar experiment (ref. 7) performed with DR1 doped PMMA. For more details see refs. 7,8. (Reproduced with permission from ref. 8. Copyright 1992 Gordon and Breach Science Publishers.) 3

Figure 7. Relaxation of the polar order after thermal poling monitored by Pockels effect of a DR1-polyester copolymer (Figure lc) from the ATR electro-optic modulation.



19.

SEKKATETAL.


265

without an appreciable effect on the main chain motion. This may originate from the rigidity of the polymer main chains which hinder the photoinduced motion of the azo dye chromophores. This is in contrast to the results we obtained with a PMMA-DR1 copolymer where the PIP order was stable over several months (8) (see also Figure 6) because the PMMA main chains are flexible and do not hinder the photoinduced motion of the covalently linked azo chromophores. This may originate from a very much reduced free volume in the DR1-PMMA copolymer in which the DR1 chromophores can only produce a change in the conformation of the polymer main chains during the photoisomerization reaction in order to experience an alignment. The free volume distribution seems to be very important for producing a stable polar order either by TP or by PIP, as we can remark from the above behavior of the present DR1 -polyester. These results suggest that further PIP and TP investigations in a high-Tg rigid mainchain polymer should be considered. Theory of Photoisomerization Induced Polar Order

It has been shown in PIP experiments that on applying a DC electric field at room temperature during the photoisomerization process, centrosymmetry was broken and an important and stable %() signal (EOPE and SHG) was obtained using a DR1-PMMA copolymer (6,8,15) (see Figure 6). This implies that the polymer chains are flexible and do not hinder the movement of the covalently linked pendent DR1 group. This conformational change in the polymer chain is induced by the photoisomerization reaction and allows the polar azo dye molecules a certain mobility, which makes possible polar orientation in the direction of the DC poling electric field. This can be seen as a change in the local viscosity of the molecules during the cistrans back thermal isomerization or optical pumping, provided that the memory of the polar orientation in the cis state is not completely lost during these back reactions. Subsequently, an efficient polar orientation may be built after successive cycles of trans cis photoisomerization as shown by the experiments (6-10) (Figures 4-6). One can imagine that polar orientation may be achieved during the transitions, but the relative magnitude of the reorientation time and the time of the optical transition seems to rule this hypothesis. The theory we develop here reproduces exactly the steps of the photoisomerization induced poling and allows a real physical insight into the observed experimental facts. 2



266


The phenomenological theory of photo-induced poling combines the usual thermal poling theory and general equtions which describe the photoisomerization and photo induced reorientation (6,9). When azo molecules are optically pumped by a polarized light beam, the probability of pumping is proportional to the square of the cosine of the angle 0 between the transition dipole moment of the molecule and the pumping light direction of polarization (angular hole burning). Thus the dye molecules which have their transition dipole moment parallel to the direction of polarization of the pump beam present the highest probability for pumping and undergoing a trans =>cis isomerization. This changes the orientation of the molecules (angular redistribution). When the molecules are in the cis state they posses sufficient mobility for alignment in the direction of the DC poling electric field. This creates a polar order and an anisotopy and can be described by the rotational Brownian motion accounting for the thermal agitation in the presence of a polar external torque exerted by a DC poling electric field. To derive the time dependent expression of photo-induced poling, we consider the elementary contribution to the polar order made by the fraction of the molecules dn(Q) whose representative moment of transition are present in an elementary solid angle dQ around the direction Q(6,q>) relative to the different axis of the system. This elementary variation results from the angular hole burning, angular redistribution and rotational diffusion in the presence of an external torque exerted by a poling electric field along the direction 3. We assume that the molecules are anisotropically shaped in both trans and cis configurations, then if the pump light beam is linearly polarized in the 3 direction, the elementary variation is given by: = - M c [ o i + ( ^ - a i ) c o s 0 ] n (Q) 2

^

t

,

2

/

+^JJ«c(^ )[Î+(^//-^l)cos 0 ] c

+

ct

dQ

da

+—\\Q(q^>G)n (Q) + DtR^Rn^Q^

P (a^>Q)

ntiQ^RUtlkT}

(4)

^Jr = ~ t>ct [°l I(

+ (off-oi)cos o]n (Q) 2

c

-

yn (Q) c

+%JJn (r2')[cri + (cJ/ /-î)cos 0 ] P*(a-> Q) da 2

r

/

r

+ D R.{Rn (G,t) c

c

+

n {n,t)RU /kT} c

c

and Q(G'^>Q) are the probabilities that molecule will rotate in the "redistribution" process of trans =>cis and cis =»trans optical transition and cis=>trans thermal recovery respectively. The "angular hole burning" is represented by a probability proportional to cos 0. The last terms in the right hand side of eqs.4 describe the rotational diffusion due to Brownian motion under the action of an external torque exerted by a poling electric field. This is a Smoloshowski equation for the rotational diffusion characterized by a constant of diffusion D (D ) for the cis (trans) configuration (25). This is given by the following Einstein relation. tc

ct

P (Q'->Q)

9

P (Q'^>£2)

2

c

t


19.

SEKKAT ET AL.

267


where £ is the friction coefficient which depends on the viscosity of the polymer and the molecular shape (25). k is the boltzmann constant and T is the absolute temperature. R is the rotational operator. (6)

R = vx V


where V is the nabla operator and v is the unit vector parallel to the long molecular axis which, in turn, is parallel to the permanent dipole moment of the molecule. As a first approximation, the interaction energy U is given by (26): p £cos0

U =-

(7a)

r>c

ttC

where p, and p are, respectively, the permanent dipole moments of the trans and cis forms. If the pump light is unpolarized or circularly polarized in the {1,2} laboratory plane, we change cos 0 to (sin 0)/2 in eqs.4 and we can introduce a parameter e characterizing the polarization state of the pumping light. Indeed the expression between the square brackets can be written in the form c

2

2

c

c

2

a'' [ 1+ e>t> (3cos 0-l)]

,

where

(7b) c

c

0

c

* ' ' = (a*//-a'/j/Ja'f +2a^ ) and a*' = (off +2c^ )/3 represents the molecular anisotropy and the isotropic absorption cross section, respectively, cff and o^ are the absorption cross sections in the parallel and perpendicular directions of the trans and cis long molecular axes respectively, e = 1 corresponds to linearly polarized light in direction 3 and e = - 1/2 corresponds to an unpolarized light or to a circularly polarized light in the {1,2} laboratory plane. I is the intensity of the pumping light expressed in flux of photons per square of centimeter. n {Q)dQ and n (Q)dQ are the number of trans and cis molecules whose representative moment of transition along the long axis is present in the elementary solid angle dQ around the direction Q{0,

Polymers for Second-Order Nonlinear Optics - American Chemical

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