Nonlinear Optical Properties of Two Thiophene-Based Polyconjugated Systems: Photoinduced Absorption and Third Harmonic G
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NONLINEAR OPTICAL PROPERTIES OF TWO THIOPHENE-BASED POLYCONJUGATED SYSTEMS: PHOTOINDUCED ABSORPTION AND THIRD HARMONIC GENERATION SPECTROSCOPY
R. Zamboni, C. Taliani, G. Ruani, and A.J. Pal Istituto di Spettroscopia Molecolare, CNR, Via de' Castagnoli, 1, 40126 Bologna, Italy F. Kajzar CEA-IRDI, DEIN-LPEM, CEN/Saclay, 91191 Gif-sur-Yvette Cedex, France
ABSTRACT Nonlinear optical properties of two thiophene-based conjugated polymers (PIT and PDTB) were studied by the photoinduced absorption spectroscopy and the third harmonic generation spectroscopy. The results obtained with these two methods are used to estimate the value of x(3) in these materials. Resonant x(3) are (2.0 +_0.2) x 10-11 e.s.u. and (1.0 t 0.1) x 10-11 e.s.u. for PTT and PDTB, respectively. X(3) is shown to be related to the width of the optical gap through a sixth-power law. In PTY a vibronic structure due to the C = C stretching mode appears in the x(3) spectrum.
INTRODUCTION Recent studies on polyacetylene (CH)x and polythiophene (PT) have shown that conjugated ff-electron systems manifest high cubic susceptibilities [1,2] as well as ultrafast response time [3]. Soliton and polaron/bipolaron dynamic has been proposed as the primary factor that gives rise to the optical nonlinearity in (CH)x and PT respectively. With the aim to study the relation between the molecular structure and the nonlinear optical properties (NLO), we have focused our attention on fused-thiophene-ring polyconjugated systems with non-degenerate ground state. Two new conducting polymers have been electrochemically synthesized, polythieno(3,2-b)thiophene (PIT; Fig. 1, left) and poly[1,4-di(2-thienyl)benzene] (PDTB; Fig. 1,right).
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Figure 1: Structure of polythieno(3,2-b)thiophene (left) and poly[ 1,4-di(2-thienyl)benzene] (right).
Mat. Res. Soc. Symp. Proc. Vol. 173. ©1990 Materials Research Society
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The polymers have been synthesized in the form of amorphous thin films and the NLO properties have been investigated by the photoinduced absorption (PA) and wave-dispersive x(3) third harmonic generation (THG) measurements.
EXPERIMENTAL Thin films of PDTB and PIT were prepared and reduced to the neutral state electrochemically [4,5]. High-quality free-standing films were detached from the electrode after swelling with suitable solvents. The thickness of the samples used for the NLO experiments (-i2500 A) was measured by means of a surface profiler TENCOR INSTRUMENTS mod. Alpha-step 200, with an accuracy of approximately 100 A. For PA measurements, the films were scratched off from the electrode, powdered, mixed with KBr, and pressed in pellets. The PA spectra were performed with a modified FTIR spectrometer (Bruker mod. 113v), in which a laser beam with photon energy above the energy gap was used to excite the neutral polymers. THG measurements were performed by transmission as a function of the incident light wavelength (from 1.907 to 1.064 Lm). The laser was a Q-switched Nd-Yag 13 ns pulse-duration coupled with a Quantel dye laser (TDLIII) and wi
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