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Investigation of the Nonlinear Optical Response in Polymeric Azoester Systems

Hasnaa El Ouazzani,† Konstantinos Iliopoulos,† Oksana Krupka,‡ Vitaliy Smokal,‡ and Bouchta Sahraoui†,*

†Institut des Sciences et Technologies Moleculaires d’Angers, MOLTECH ANJOU, CNRS UMR 6200, University of Angers, 2 Bd Lavoisier, 49045 Angers cedex

‡Kyiv Taras Shevchenko National University, 60 Volodymyrska, 01033 Kyiv Ukraine e-mail: hasnaa.elouazzani@etud.univ-angers.fr

ABSTRACT The second and third-order nonlinear optical properties of novel push-pull azobenzene polymers have been investigated using 30 ps laser pulses at 1064 nm. The study of the chromophores has been done before and after corona poling and by employing different polarizations. Very strong nonlinear response, which was different for every investigated system has been found due to enhanced charge transfer within the molecules. Keywords: Second Harmonic Generation (SHG), Third Harmonic Generation (THG), Maker fringes, Push-Pull, azo dyes, Disperse Red 1 (DR1).

1. INTRODUCTION In recent years, azobenzene polymers, have gained a great deal of attention as they are promising candidates for various applications like optical data storage, surface relief gratings, photoswitching, alignment of liquid crystals, optical elements, and so forth [1,2,3].

Especially azobenzene derivatives have gained enormous interest in these research fields, as they can isomerize from the trans- to the cis-state by interaction with light [4].

In this paper, we report some recent results on the NLO properties of various push-pull conjugated polymers based on dispersed Red 1 (Fig. 1a,b), using the Second and Third Harmonic Generation (SHG, THG) techniques. Complete investigation was carried out before and after corona poling.

R NH2NaNO2

HClR N2

+Cl-

N

OH

R N N N

OH

Where R = NO2 , N N NO2

Figure1a.The synthetic route of the initial azobenzene compounds.

N N N N NO2NH3C

O

N N NO2NH3C

O

N N N NO*

CH3

OR

CH3

O OCH3

n m

P1

P2

P3

where R

Figure1b. Structure of the azopolymers.

2. NON LINEAR OPTICAL EXPERIMENTS The nonlinear optical properties of the systems were investigated using Second (SHG) and Third Harmonic Generation (THG) techniques [3,5,6]. For the needs of the investigation a mode-locked Nd:YAG laser, working at 1064 nm with 30 ps duration and 1 Hz repetition rate has been employed. The nonlinearities have been determined using quartz and silica respectively as reference materials.

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3. LINEAR OPTICAL PROPERTIES The UV-Vis spectra of the azopolymers show two absorption bands which are the benzene rings and intense band assigned to the vibronic coupling between n-π∗ and π-π∗ electronic transition of azobenzene polymers. The electron-donor and electron-acceptor substituents in the synthesized polymers are increasing the charge transfer character of the π- π∗ transition and consequently shifting the π- π∗ band to the red. Thereby P3 exhibits a maximum absorption at the longest wavelength among the investigated compounds what can be explained by the highest charge-transfer interaction occurring between the electron-donor group (amino) and electron-acceptor group (nitro) which is characteristic for the “pseudo-stilbene” types in Rau’s classifications.

Figure 2 shows the optical absorption spectrum of a thin film of P1 before (solid line) and after (dashed line) poling. The ordering is observed by the decrease of the thin film optical absorption at normal incidence and the noncentrosymmetry by its red shift.

Figure 2. Optical absorption spectrum of a thin film of P1 before (1) and after (2) poling.

4. RESULTS AND DISCUSSION The nonlinear optical response of thin films of the azobenzene polymers P1-P3 has been investigated by means of SHG/THG Maker fringes measurements. First, SHG measurements were done before and after corona poling of the films for all studied push-pull azobenzene polymers P1-P3 [7]. There is significant difference of the obtained SHG magnitude between the results before and after the corona poling as expected for the three systems, which is attributed to very efficient alignment of the molecules by the applied electric field.

Then, comparative Maker fringes measurements were done between the P1, P2 and P3 (azobenzene polymers) using different excitation-detection polarization configurations s-s, s-p and p-p. For all three investigated systems, the pp configuration has resulted in the strongest nonlinear optical response. Characteristic SHG curve of the P3 system using p-p polarization configuration can be seen in Fig. 3.

-100 -80 -60 -40 -20 0 20 40 60 80 1000.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

SH

G si

gnal

Angle of incidence

Figure 3. Normalized SHG Maker fringes in the case of the P3 sample

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The calculated values of second-order nonlinear optical susceptibility (2)χ using quartz as reference sample can be seen in Table 1.

Table 1. Effective (2)χ Values for All Investigated Systems under s-s, s-p, and p-p Excitation-Detection Polarization Configurations.

Sample χ(2) (pm/V)

s-s s-p p-p P1 3.26 10.79 26 P2 n/a 0.08 0.56 P3 2.16 9 30

The χ(2) values of the P1 and P3 systems are similar but are much higher than that of the system P2. This large enhancement can be attributed to the strong acceptor moieties in the para-position and electron donor group (amino) and results in the highest charge transfer in the polymer system, which has consequently a strong impact on the NLO response.

Then, THG measurements were carried out for the thin films of the same compounds using different polarization and the same excitation source.

The χ(3) values for the polarization configurations p-p, s-s, s-p, and ps of the samples P1, P2, and P3, obtained before the orientation of the chromophores, according to the procedure described in the Experimental Section, are presented in Table 2.

Table 2. Third-Order Nonlinear Susceptibility (χ(3)) under all Polarization Configurations χ(3) (10-22 m2/V2)

Sample polarization configurations p-p s-s s-p p-s

P1 2432 2420 1584 1574 P2 147 148 118 112 P3 618 612 416 432

In all cases, the values were found to be very high, indicative of very efficient third harmonic generation of the azobenzenes, which were up to 3 orders of magnitude higher than the values obtained for the reference material which was silica in this case. This is due to the enhanced charge transfer of these systems [8,9].

4. CONCLUSIONS In this paper, we investigated a novel push pull systems with different sizes of the conjugated system between donor and acceptors and using SHG and THG techniques at the fundamental wavelength 1064 nm in picosecond regime. The second- and third-order nonlinear optical properties have been found to be very high in all cases rendering these materials possible candidates for photonic applications.

ACKNOWLEDGEMENTS The authors would like to thank the COST action MP0702 (Towards Functional Sub-Wavelength Photonic Structures) for financial support.

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